fluvial architecture element analysis of the brushy basin ... · fluvial architecture element...

Volumina Jurassica, 2014, Xii (2): 69–106 DOI: 10.5604/17313708 .1130130

Fluvial architecture element analysis of the Brushy Basin Member, Morrison Formation, western Colorado, USA

Kenneth G. GALLI1

Key words: Fluvial Architecture Element Analysis, meandering, anastomosing, Brushy Basin Member, Morrison Formation, Kimmeridgian, Colorado.

Abstract. The 85-m Brushy Basin Member of the Morrison Formation in western Colorado, USA, comprises dinosaur-bearing sandstones (architectural element CHR and CH), crevasse-splay deposits and minor levee deposits (architectural element CS), mudstones, marlstones, altered ash beds and minor limestones as well as caliche paleosols and noncalcareous paleosols (architectural element FF). Channel sand-stones occur at five stratigraphic levels at Trail Through Time (TT), eleven levels at Fruita Paleontological Research Area (FP), and at five levels at Echo Canyon (EC). River-channel sandstones hosted by floodplain mudstones tend to have cut down to resistant caliche paleosols. Depositional facies and architectural element analysis show that the rivers were low gradient, mainly anastomosing, with perennial flow, but seasonal with “flashy” peaks in discharge. Dinosaur bone accumulations are found in some floodplain ponds. Isolated bones are present in anastomosing channel sandstones at TT and in channel sandstone 2 at EC. At FP, major accumulations of bones were rapidly buried in the deep pools at three bends in the meandering river resulting in the formation of channel sandstone 2. There is no evidence for a large lacustrine or playa system at the three localities.

1 Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467 USA; e-mail: [email protected]

Most Morrison Formation streams, sourced in the west-ern highlands, flowed eastward across the depositional basin and were anastomosing in nature. Watering holes formed in the locations of deep erosional scours formed by seasonal stream flooding and accessed the water-table groundwater with this input possibly seasonally controlled by periods of wet and dry conditions. Unionid bivalves are found in a few fluvial sandstones and due to their obligate parasitism with fish, this indicates that fish were present in the Morrison ecosystem in perennial streams (Good, 2004; Turner, Peterson, 2004).

The purposes of this study are to: (1) describe the strati-graphic details of the stream channel deposits present in the Brushy Basin Member of the Morrison Formation of western Colorado; (2) show, in detail, their vertical and lateral distribu-tion, architecture and bounding surface hierarchy; and (3) inter-pret the fluvial and associated depositional subenvironments.

INTRODUCTION

The Morrison Formation is world renowned for its as-semblage of dinosaur remains, especially the large her-bivorous sauropods. Much work has been done that at-tempts to place the dinosaurs into a more robustly described ecosystem. A dry climate is thought to have per-sisted during deposition of Morrison strata due to the ba-sin being in the rain shadow of the western highlands and to prevailing westerlies developed due to a subtropical high over the Paleo-Pacific Ocean (Turner, Peterson, 2004). The Morrison Formation of Late Jurassic age is a mo saic of mostly fluvial and minor lacustrine deposits that cover more than 1.5 million km2 of the Rocky Mountain, West-ern Interior and Colorado Plateau regions (Fig. 1; Turner, Peterson, 2004).

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70 Kenneth G. Galli

ZONE

800 km4000

MORRISONFORMATION

ELKO

HIGHLANDS

HIGHLANDS

Study Area

Salt Lake City

ConglomeraticSandstone

MOGOLLON

Denver

SUBD

UC

TIO

NZO

NE

MAG

MAT

ICAR

C

MO

RR

ISO

N F

OR

MAT

ION

Brushy

Basin

Member

Member

Member

Wash

Salt

Tidwell

Burro CanyonFormation

San RafaelGroup

50 m

North

40 km200

CNM

GrandJunction

I-70

Colora

do R

iver

Fruita

FP

TT

EC

Lithofacies Symbol & Code #

5

10

4

6

14

16

21

1211

13

15

17

7

3

98

vcvfCLAY

.0625.004.001

GRAVELf2.0

cSILT m

mm

2120

Bright green nodular calcite in sandstoneBrown shale with barite nodules

SAND

Rock Type

Mudstone with roots, pedotubules, peds

Channel sandstone

Siltstone/very fine sandstone

Green siltstone lens

Red-brown mudstone

Volcanic ash

Ash-rich mudstone

Micritc limestone

Green-gray mudstone; some siltstone

Mottled gray mudstone

Gray marlstoneTan siltstone/fine-grained sandstone

White porcellenite-like rock

Organic-rich, fossiliferous claystone-siltstoneCalcite nodules, pedotubules, peds

Nodular calcite in various hosts

Gypsum-green mudstone interbeds

Fig. 1. Study locus, stratigraphic section, facies

areal extent, source areas, and stratigraphy of morrison Formation, after Brenner (1983) and Hintze (1988). TT – Trail Through Time; FP – Fruita Paleontological research area; Ec – Echo canyon; cnm – colorado national monument. General legend for facies used in all vertical measured sections and architectural panels

71Fluvial Architecture Element Analysis of the Brushy Basin Member, Morrison Formation, western Colorado, USA

RegIONAl SeTTINg

The Morrison Formation was deposited in the most land-ward, back-bulge basin of a foreland basin system developed during Late Jurassic subduction of the paleo-Pacific (Faral-lon) oceanic plate to the west of the preserved Morrison ba-sin (DeCelles, Giles, 1996). Perhaps the relative thinness of the Morrison strata is explained by plate-margin activity dur-ing deposition that prevented the foredeep from subsiding (Miall, 2000). Morrison strata are typically 152 to 183 m thick compared to the tens of thousands of meters of Creta-ceous strata in nearby basins.

Cadigan (1967) showed clearly that the source areas for the Morrison were to the west and the southwest, based on the position of the proximal conglomeratic facies associated with the sandstones and finer facies. The basin was separated from the Paleo-Pacific Ocean by several mountainous re-gions, some of which contributed sediment to the deposi-tional basin. Abundant volcanic ash was sourced from calde-ras in the remnant of an arc-graben depression and in the transtensional rift zone that extended from southern Arizona to central eastern California. The prevailing winds in the Western Interior during the Late Jurassic blew from the southwest. Clastics in the depositional basin were mainly de-rived from highlands that extended from northern Mexico to British Columbia. The highlands are interpreted to have been rift shoulders and uplifts in the back-arc region. A rela-tively minor amount of clastic material was derived from lo-cal topographic highs in the largely buried Ancestral Rockies and perhaps in other local areas farther north (Turner, Peter-son, 2004).

The J-5 unconformity underlies the basal Morrison For-mation of Early Kimmeridgian age. The regional J-5 uncon-formity is present below the base of the Tidwell Member and the regional K-1 unconformity is present at the top of the Brushy Basin strata (Pipiringos, O’Sullivan, 1978; O’Sul-livan, 1980; Peterson, 1994).

Most of the Morrison Formation is Kimmeridgian in age with only the uppermost section being Tithonian in age (Litwin et al., 1998; Schudack et al., 1998). This is based on palynological studies and studies of charophytes and fresh-water ostracods. The Morrison was deposited from 155 to 148 Ma based on isotopic dates of sanidine from smectitic tuffs (Kowallis et al., 1998).

The Brushy Basin is the most widespread member of the Morrison Formation, thinning northward and northeastward from 157 m in southwestern Colorado and eastern Utah, to 79-138 m in the Grand Junction area. It then thickens north-westward to 236 m locally in northwestern Colorado and northeastern Utah (Lohman, 1965), closer to the source areas.

lOCATION OF ThIS STUDy

In the study area in and around Fruita in western Colo-rado, the Morrison Formation consists of 120 to 250 m of mostly terrestrial strata deposited primarily during Kim-meridgian (155 Ma) to Tithonian (148 Ma) time (Kowallis et al., 1998). The 85-m Brushy Basin Member of the Morri-son Formation in western Colorado is dominated by a 30-m lower division (lower Brushy Basin Member (Jmbl)) mostly made of red-brown mudstone and a 55-m upper division (upper Brushy Basin Member (Jmbu)) characterized by varie gated smectitic mudstones of floodplain origin where the smectite is altered silicic ash blown eastward by the pre-vailing winds. This change is discerned in the field by the change from non-swelling red-brown floodplain mudstones typical of the lower Brushy Basin strata to the puffy-weath-ering aspect typical of upper Brushy Basin ash-rich mud-stones. However, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) of lower Brushy Basin mudstones and upper Brushy Basin ash-rich mudstones showed that in most cases, both facies were rich in the swell-ing clay smectite. This color change is the sole basis of the division of the two Brushy Basin lithosomes.

River-channel sandstones are embedded in the mud-stones and tend to have cut downward to resistant caliche paleosol horizons. Also present are thin beds of lacustrine micritic limestone and four major zones of altered silicic ash falls. These strata were studied at three locations, from west to east: Trail Through Time (TT), Fruita Paleontological Re-search Area (FP), and Echo Canyon (EC). This paper pre-sents the analysis of the many hierarchical orders of fluvial architecture observable in the Morrison strata. Description of bounding surfaces and lithofacies details allow the inter-pretation of the fluvial styles of Brushy Basin Member rivers and their associated facies.

locaTion oF sEcTions mEasurEd

The three sections in western Colorado are in the north-eastern portion of the Colorado Plateau Physiographic Prov-ince (Fig. 1). From west to east, they are: Trail Through Time (TT), Fruita Paleontological Research Natural Area (FP), and Echo Canyon (EC). The TT section is adjacent to and east of the Mygatt-Moore Dinosaur Quarry in the Rabbit Valley Research Natural Area. The TT locality is situated on the north side of I-70, 3 km east of the Utah-Colorado bor-der, accessible by exit 2 off I-70. The FP and TT localities are active dinosaur dig-sites. FP is south of the Colorado River approximately 1–2 km southwest of Fruita and is ac-cessible from old uranium roads off Colorado 340, just north

72 Kenneth G. Galli

of the western entrance to Colorado National Monument. FP is approximately 28 km east of the Utah-Colorado border. EC is a side canyon of No Thoroughfare Canyon located near the eastern margin of the Colorado National Monu-ment. The stratigraphy of No Thoroughfare Canyon was de-scribed by Lohman (1965). The EC locality is accessible from Little Park Road, proceeding south from Grand Junc-tion, Colorado. The site is approximately 42 km east of the Utah-Colorado border.

At the Fruita Paleontological Research Natural Area (FP) there is a remarkable abundance and diversity of Upper Ju-rassic terrestrial fossils preserved within an area of approxi-mately one square kilometer. Utah State Paleontologist Dr. James Kirkland has overseen exploration at both FP and TT and graciously invited me to conduct my dissertation re-search in western Colorado at FP and TT. Kirkland identified six facies at FP: (1) ribbon sandstone facies – bank-con-trolled, gravelly, low-sinuosity river channels; (2) levee fa-cies – characterized by clay with well-sorted sandstone of proximal crevasse splay deposits best seen at channel bends; (3) drab floodplain facies – drab, poorly drained alkaline floodplains with carbonate-cemented crevasse splays and poorly developed paleosols; (4) dark red floodplain facies –oxidized well-drained floodplains with well-developed paleo sols containing nodular carbonates; (5) pond/marsh fa-cies – more or less permanent spring-fed marshes that local-ly overlie channel thalwegs; and (6) alkaline pond facies –ephemeral alkaline floodplain ponds with barite nodules. Each facies also preserves a unique suite of fossils (Kirk-land, 2006).

MeThODS

Lithofacies descriptions were assigned to each distinct rock type and its contained features and detailed vertical stratigraphic sections were measured at the three localities. Lateral profiles for architectural analysis were drawn in the field, using photomosaics at and adjacent to measured sec-tions, and other select locations. Sketches were made of the lateral and vertical extent of lithofacies, thickness trends in trough cross-bed cosets, grain-size trends, thickness of cross-bed cosets, surfaces that cut through cosets, gravel at the base of channels, size, type, and alignment of intraclasts and gravel, and alignment of dinosaur bones.

Specimens of many facies were collected at TT and EC. XRD analysis was performed on about twenty specimens at each locality. SEM analysis augmented the petrographic analysis. Twenty-one samples were analyzed from six depo-sitional environments: (1) red-brown mudstone (floodplain deposits), (2) puffy-weathering green or purple silty mud-stone (altered volcanic ash/ashy flood mudstones, (3) green-

ish-gray mudstone (high water-table regions of floodplains), (4) mottled brown mudstone (phase-one paleosols), (5) thin clay lens in channel sandstone, and (6) non-calcareous mot-tled red-brown mudstone with peds and roots (incipient paleosol).

The samples were prepared for clay-mineral analysis fol-lowing the procedures of Gibbs (1971). 1) Samples were broken to pea-size pellets, using mortar and pestle and then crushed to powder in a Spex ball mill. 2) The powders were saturated in a buffered solution of sodium acetate to dissolve calcite cement without dissolving clays and to saturate ex-pandable clays with Na+. 3) The powders were further dis-aggregated with a mechanical shaker and an ultrasonic probe. 4) Liquid was separated from the clays by centrifuga-tion at 2000 rpm for 20 minutes. 5) Powders were washed in deionized water, resuspended, and recentrifuged to remove the sodium acetate. 6) The <2-micron clay fraction was ob-tained by centrifugation at 500 rpm for 8 minutes-37 sec-onds and then pouring off the suspensate containing the clay fraction. 7) Slides were prepared after the method of Gibbs (1971) by evaporating the suspensate to a paste then smeared onto glass slides.

Three slides were prepared for each sample. One was air-dried and untreated. The second was solvated in an ethylene glycol saturated chamber for at least 24 hours. The third slide was heated to 550°C for one hour. Unoriented mounts of 6 samples were prepared by mixing a small quantity of powder with acetone, forming a slurry, which was dropped onto the slide and allowed to dry. All samples were analyzed on a Siemens X-ray diffractometer, using Cu K-α radiation at 35 kV and 20 mA. The goniometer scanning speed was 2° per minute and chart recorder speed 2 cm per minute. All oriented slides were run from 2° to 30° 2θ. Random-mount slides were measured to 50° 2θ to examine the non-basal dif-fraction peaks in illite and to 65° 2θ to examine the non-ba-sal diffraction peaks in smectite.

The identification of smectite was based on expansion of the (001) peak from 12.5 Å to 16.8–17.0 Å on glycolation and col-lapse to 10.0 Å when heated to 550°C. Diffraction traces of random mounts of 6 samples consisting almost entirely of smectite display a (060) peak at 1.53–1.54 Å, typical of triocta-hedral smectite (Dyni, 1976). Adapting the procedure of Greene-Kelley (1973), these 6 smectite samples were saturated in a 3M lithium chloride solution. Then, the slides were heated to 300°C and placed in a glycol-saturated chamber. These sam-ples re-expanded when glycolated after heating; dioctahedral smectites do not re-expand (Green-Kelley, 1973).

To examine interstitial clay in sandstones by scanning electron microscopy, three 9-mm-size samples of sandstone were mounted with double-sided carbon tape onto aluminum stubs and sputter-coated with gold-palladium for 90 to 120 seconds. Samples were analyzed at Mt. Holyoke Col-


lege (South Hadley, Massachusetts) using 20 Kv on a JEOL JSM-35CF with a Tracor-Northern energy dispersive system (Galli, 2003).

FACIeS AND STRATIgRAphy OF The BRUShy BASIN MeMBeR

IntrODuctIOn

In the Brushy Basin Member, 21 facies (19 in the Morrison Formation; facies 18, 19 are in overlying formations) were identified in the field based on lithology, geometry, sedimentary structures, and fossils (Table 1). The TT and FP sections have the largest variety of facies and the best-developed paleosols. The EC section, farthest east, has the fewest number of facies. The three measured sections are annotated with paleocurrent roses of selected channel sandstones, and primary sedimentary structures such as ripple cross-lamination (Sr), planar cross-bedding (Sp), trough cross-bedding (St) and horizontally lami-nated or bedded sandstones (Sh), noted by facies codes and by the sketch of the internal bedding features of each channel sand-stone. The general legend for facies used in the vertical strati-graphic sections, architectural panels and throughout the manu-script, and the lithofacies codes, descriptions, and architectural elements containing these facies, are presented in Table 1 (Galli, 2003).

The base of the TT section was the upper 16 m of ex-posed Salt Wash Member strata topped by the uppermost Salt Wash sandstone. The base of the FP section was the con-tact between the Entrada Sandstone and the overlying Sum-merville Formation, which contained sand crystals and salt casts, and my vertical stratigraphic section closely followed that measured by Kirkland and Rassmussen (J. Kirkland, person. commun., 1989). The base of the EC section was the contact between the Kayenta Formation and the overlying Entrada Sandstone. At TT, the upper Brushy Basin was marked by a silica-replaced limestone, marked as volcanic ash zone IV within the Brushy Basin strata and correlated to the other silica-replaced units at the top of the Brushy Basin at FP and EC. This may mark the disconformity that sepa-rates the Upper Jurassic Brushy Basin strata from the Lower Cretaceous Burro Canyon strata. The top of the section at TT was the lowermost sandstone of the Cretaceous Burro Can-yon Formation. The Burro Canyon sandstone is salt and pep-per (black/white) in color and contains green mudchips.

At FP, the uppermost Brushy Basin is marked by a meter-thick puffy green-weathering, volcanic ash bed (my ash IV) that is overlain by a distinctive 2-m-thick bright green prob-able paleosol of green siltstone with green nodular carbonate and sandstone with bright green matrix, mottled in some

places and botryoidal in other places. At EC, the Brushy Ba-sin/Burro Canyon contact is marked by a partially silica-re-placed micrite of the Brushy Basin, overlain by a meter-thick salt and pepper-appearing black/white pebble conglomerate of the Burro Canyon Formation.

The sandstone to fines (SS : F) ratio, one parameter often used to discern fluvial style, with fines including all flood-plain facies such as mudstones, ashy mudstones, paleosols and tabular very-fine grained sandstones, was calculated for the lower Brushy Basin Member (Jmbl) and the upper Brushy Basin Member (Jmbu) at each section. For each lo-cality, I present the stratigraphy of the uppermost Salt Wash Member to contrast it with that of the lower, middle, and upper Brushy Basin Member. At TT, the SS : F ratio of Salt Wash strata is 29 : 71; at FP, the SS : F ratio of Salt Wash strata is 25 : 75; and at EC, the SS : F ratio of Salt Wash strata is 54 : 46. Clay mineralogy was determined by XRD analysis for various facies at TT and EC. At TT, upper Salt Wash strata contain 85.2% illite, 0.6% smectite and mixed-layer illite-smectite, and 14% chlorite. At EC, upper Salt Wash strata contain 86.9% illite-smectite and illite, 2.1% smectite, and 11.0% chlorite.

lowEr BrusHy Basin mEmBEr (JmBl)

At TT, the 31-m-thick lower Brushy Basin Member is overlain by the 48-m-thick upper Brushy Basin strata (Figs 2A, B). Channel sandstones 1, 3, 4, and 5 are single stories while channel sandstone 2 displays an amalgamated 2-story section locally. Channel sandstones 1, 2, and 3 occur in the lower 20 m of the Brushy Basin within the lower Brushy Basin whereas channel sandstones 4 and 5 (and pos-sibly a thin, discontinuous 6th and 7th channel or lens) occu-py the upper Brushy Basin Member. At TT, the SS:F ratio of the lower Brushy Basin is 23 : 77. The outcrop azimuth of channel sandstone 2 is N45°E and the strike is N50°W. Channel sandstone 3 strikes N40°W. Where channel sand-stone 4 ties into the vertical stratigraphic section, it strikes N75°W. Channel sandstone 4 and/or 5 strike directly toward the main channel sandstone on which the “Trail Through Time” is located. In this area approximately 300 m west of the main stratigraphic section, there exist at least 3 sand-stones, possible small channels or lenses, above the channel sandstone 4 at the base of the “Trail Through Time” and these units strike approximately east-west. The K-1 uncon-formity is at Jmb 79 m.

At FP, the color change (CC) occurs at 31m in the sec-tion, and thus the lower Brushy Basin Member is 31-m thick; overlain by the 70-m thick upper Brushy Basin strata. Lower Brushy Basin strata contain channel sandstones 1–8, where-as upper Brushy Basin strata contain channel sandstones 9–11 (Figs 3A–C). The SS:F ratio for lower Brushy Basin

74 Kenneth G. Galli

Table 1lithofacies codes, descriptions, and architectural elements observed in this study. Architectural elements after Miall (1996)

Facies#

Lithofacies Description Architectural element containing this facies

1 Volcanic ash Green to green-gray, very fine-grained, puffy-weathering FF: Overbank Fines

2 Micritic limestone Brown-gray or mottled red-green, some with chert replace-ment near top, depositional bases and erosional tops FF: Overbank Fines

3 Nodular calcite layers Discontinuous, thin, nodular calcite units, commonly in mudstone, often at tops of caliche paleosols FF: Overbank Fines

4 Red-brown mudstone, locally ash-rich Red-brown mudstone, calcareous or non-calcareous, sandy or silty, locally mottled, interbedded with facies 1, 2, 5, 10, 14 and 16 FF: Overbank Fines

5Tabular, dense, hard, calcareous brown-gray siltstone and fine-grained sandstone, locally mottled

Prominent ledge-formers adjacent to and be-low facies 7 channel sandstones

CS: Minor Sand-stone Sheets

6 Green-gray mudstone and fine-grained sandstone

Greenish gray mudstone, silty mudstone and rare fine-grained sandstones of limited extent FF: Overbank Fines

7 Channel sandstone Channel form with basal scour surfaces overlain by gravel, locally fine upward, often cross-bedded

CHR: Major Sandstone Ribbons (most Ch SSs) CH: Major Sandstone Element (FP Ch 2)

8Red-brown-mottled gray mudstone, weathers black with conchoidal fracture

Gray mudstone, calcareous or non-calcareous, with relict red-brown mottles. Sometimes steel blue-gray, dendritic shrink-age patterns, weathers black and with conchoidal fracture.

FF: Overbank Fines

9 Ash-rich mudstone, puffy weatheringPuffy-weathering green, purple, or gray silty mud-stone, weathers whitish with rusty halos around bio-tite. Interbedded ash beds and ashy mudstone.

FF: Overbank Fines

10Very-fine-grained sandstone, thin-bedded, rippled lo-cally, blocky weathering

Fine- to very fine-grained sandstone, weathers green due to illite and/or chlorite, thin-bedded, lens laterally, shallow 40-cm channels present locally, associated with green, puffy-weathering mudstone

CS: Minor Sand-stone Sheets

11 Tan, fine-grained sand-stone, weathers orange

Tan, fine-grained sandstone, grades laterally into major stream channel sandstones at TT. Facies 11 is absent at FP and EC

CHm: Minor Sand-stone Element

12 Gray mudstone Hard, dense marlstone with locally abundant plant fossils and some calcite-filled root tubes. Associated with facies 8. FF: Overbank Fines

13 White, porcellenite-like rock White, porcellenite-like rock, conchoidal fracture, writes like chalk, horizontal laminae, tan and peach-colored mottles FF: Overbank Fines

14 Reddish-brown mudstone with roots, pedotubules, peds

Facies 14 is a variation on facies 4. Mottled red-brown mudstone, may be siliceous, has peds, pedo-tubules, root casts, sometimes purple

FF: Overbank Fines

15

Organic-rich claystone -siltstone with greater than 4% total or-ganic content with wood, plant, and dinosaur bones, weathers orange

Organic-rich claystone-siltstone with plant fossils and a few small logs. Claystones are dark gray; siltstones are light gray. Siltstones have minor green micaceous lamina, occa-sional calcite nodules and animal fossils. Weathers orange.

FF: Overbank Fines

16 Red-brown mudstone with cal-cite nodules, pedotubules, peds

Calcareous red-brown mudstone with calcite-filled root tubes, well-developed peds, including granular peds, subangular blocky peds, layered calcite nodules, crystallaria, rhizoconcretions, drab halos

FF: Overbank Fines

17 Interbedded gypsum and green mudstone Interbedded gypsum and green mudstone FF: Overbank Fines

20 Brown shale with barite nodules Brown shale, mudstone with barite nodules (aver-age: 1- to 4-cm) but some cobble-sized FF: Overbank Fines

21Green nodular, silica-replaced calcite and sandstone with bright green matrix, botroyoidal in places

This facies is only present as 1.5-m bed at top of Brushy Basin Member beneath Buckhorn Con-glomerate of the Burro Canyon Formation

FF: Overbank Fines


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igra

phic

sec

tion

at t

t. L

ower

Bru

shy

Basi

n st

rata

from

20

to 3

0 m

and

upp

er B

rush

y Ba

sin

stra

ta fr

om 3

0 to

60

m

76 Kenneth G. Galli

brow

n-gr

ay s

iltst

one

25

limes

tone

with

sili

ca-r

epla

ced

top

Cha

nnel

4

303540

puffy

-wea

ther

ing

whi

te, p

urpl

e m

udst

one

whi

telig

ht b

row

n

whi

te p

orce

lleni

te-li

ke

rock

, up

to 2

.1 m

thic

k

gree

n si

ltsto

ne

mot

tled

(in p

lace

s)

gray

mud

ston

e

gray

mud

ston

e

red-

brow

n m

udst

one

puffy

-wea

ther

ing

brow

n m

udst

one

brow

n-gr

ay s

iltst

one

mot

tled

gray

mud

ston

e

gree

n-m

ottle

d re

d m

udst

one

with

ped

otub

ules

and

ped

s,an

d gl

eyin

g

gree

n si

ltsto

ne le

ns

varie

gate

d gr

een

and

purp

le

dom

inan

tly

red-

brow

n co

lor

basa

l cgl

/ss

***

coar

se S

S

4

8

29

10

59

10

7

10

35.3

°13

14

8

Jmbu

Jmbl

Sta

ge II

N

C s

ol4

CO

LOR

CH

AN

GE

CO

RR

ELA

TED

A

SH

HO

RIZ

ON

I E

QU

IVA

LEN

T

silic

a-re

plac

ed s

ands

tone

vcvf

CLA

Y

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5.0

04

GR

AVE

Lf

2.0

cS

ILT

m

mm

Jmb

20

cove

red

inte

rval

Upp

er B

rush

yB

asin

Mem

ber

Low

er B

rush

yB

asin

Mem

ber

gree

n m

ottle

d m

udst

one

w/ p

edot

ubul

es,

peds

, sili

c. b

ranc

hing

rhiz

ocon

cret

ions

5 gree

n m

ottle

d m

udst

one

w/ p

eds

Cha

nnel

5

45505560

gn p

edog

enic

mds

t, w

eath

ers

peac

h, w

hite

puffy

-wea

ther

ing

gree

nish

gra

y vo

lcan

ic a

sh

puffy

-wea

ther

ing

gree

n si

lty m

dst,

wea

ther

s m

ultic

olor

ed, w

hite

, pin

k,

purp

le, g

ray-

gree

n, ta

n, b

row

n, lo

cally

lim

onite

-sta

ined

puffy

-wea

ther

ing

purp

lish

whi

te m

udst

one

red

and

brow

n m

udst

one

puffy

whi

te -w

eath

erin

g gr

een

mud

ston

e

puffy

wea

ther

ing

peac

h/ta

n m

udst

one

over

lain

by

lens

of l

oess

-like

silt

ston

e

gree

n m

udst

one

with

ped

otub

ules

an

d pe

ds a

nd g

leyi

ng

mot

tled

gray

mud

ston

epu

ffy w

hite

-wea

ther

ing

gre

en m

udst

one

gree

n si

ltsto

ne

gree

n si

ltsto

ne

gree

n si

ltsto

ne

red

mud

ston

e w

/ ped

otub

ules

, ped

s, a

nd m

ottle

s.

yel

low

-wea

th d

k gy

cla

yey

slts

t,or

gani

c-ric

h w

/ woo

d fra

gmen

ts

nodu

lar c

arbo

nate

/mds

t w/

pedo

tubu

les,

ped

s, a

nd g

leyi

ng

quar

ry l

evel

vcvf

CLA

Y

.062

5.0

04.0

01

SA

ND

GR

AVE

Lf

2.0

cm

mm

1

5

98

14

9

7

94

5

9

9

14

14

14

3, 1

6Jm

b40

10

puffy

-wea

ther

ing

purp

lish-

whi

te m

udst

one

9

14

15

14

SIL

T

Sta

ge

II N

C s

ol

Sta

geI-I

I NC

sol

Sta

geII-

IIIca

liche

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10

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TED

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OR

IZO

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HO

RIZ

ON

III

Upper Brushy Basin Member, Morrision Formation

1234

Fig.

2A

Fig.

2B

34

Sp

Sr

Fig.

2 c

ont.

B


calic

he c

last

s

354550 40

Cha

nnel

4

Cha

nnel

5

Cha

nnel

6

red

and

gray

cal

care

ous

silts

tone

, som

e re

d m

udst

one,

bad

ly w

eath

ered

light

bro

wn-

wea

ther

ing

gray

peb

bly

med

to v

ery

coar

se @

bas

e to

fine

to

med

at t

op s

ands

tone

up to

.5x1

cm

rd, y

l, bn

, wh

subr

ound

ed c

hert

pebb

les

1x3

cm m

ax m

udst

one

clas

t dia

met

er; m

ean

size

= 1

cm

mf

dese

rt va

rnis

hed,

lt b

row

n -w

eath

erin

g gr

een-

gray

ca

lcar

eous

fine

san

dsto

ne,

prom

inen

t led

ge -f

orm

ing

'mas

sive

' con

tinuo

us s

heet

sa

ndst

one,

Sp

and

St v

isib

le in

pl

aces

clay

dra

pes

gree

n m

ud c

hips

, mea

n si

ze =

2x3

cm

; m

ax. =

5x1

0 cm

calc

areo

us m

udst

one

with

ped

s,

red-

agat

e-ce

nter

ed, s

ilica

-rep

lace

d ca

rbon

ate

nodu

les

sphe

roid

al, l

ight

bro

wn-

wea

ther

ing,

gr

een-

gray

, cal

care

ous

fine

sand

ston

e

angu

lar b

lock

y fin

e pe

ds,

pedo

tubu

les,

fine

root

cas

ts, g

reen

m

ottle

s, a

nd s

licke

nsid

es in

red

mud

ston

e an

d m

uddy

silt

ston

e

brow

n-w

eath

erin

g gr

een-

gray

ca

lcar

eous

fine

san

dsto

ne

gree

nish

whi

te fi

ne

sand

ston

e, S

p in

pla

ces

vcvf

CLA

Y

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5.0

04.0

01

SA

ND

GR

AVE

Lf

2.0

cS

ILT

m

mm

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0

gray

car

bona

te s

oil n

odul

es, r

ed

barit

e &

gyp

sum

nod

ules

, loc

ally

ab

unda

nt in

mud

ston

es o

f Upp

er

Sal

t Was

h M

embe

r

Sta

ge I

calic

he


Lower Brushy Basin Member, Morrison Formation

Cha

nnel

7

Cha

nnel

6

5101520

puffy

gra

y-w

eath

erin

g gr

een

calc

areo

us s

iltst

one

nodu

lar c

arbo

nate

and

mud

ston

e w

/ ped

s5-

cm th

ick

bloc

ky s

ilty

fine

sand

ston

e

brow

n, s

pher

oida

l-wea

ther

ing

gree

n-gr

ay fi

ne

sand

ston

e, (

blue

spl

ay m

arke

r)

puffy

gra

y-w

eath

erin

g gr

een-

dark

gre

en s

iltst

one-

silty

fin

e sa

ndst

ones

, ark

osic

in p

lace

s, (r

ed m

arke

r)

hard

, lig

ht b

uff-g

ray

silts

tone

(y

ello

w s

play

mar

ker b

ed)

red

mud

ston

e, p

oorly

exp

osedba

sal p

lana

r cro

ss-b

edde

d gr

avel

lag,

max

. che

rt cl

ast:

5 m

m

conv

olut

e be

ddin

g, 1

to 2

cm

m

ud c

last

s, a

nd m

ud d

rape

s ne

ar to

p

7 x

30 c

m d

ino

bone

10 c

m b

row

n-w

eath

erin

gca

rbon

ate

nodu

les

Cha

nnel

6S

alt W

ash

Mem

ber

Ch

SS

2 l

evel

165

4

16, 3

9

Jmb0

vcvf

CLA

Y

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5.0

04.0

01

SA

ND

f

2.0

cS

ILT

m

mm

Jmbl

old

pick

for J

mbu

CO

LOR

CH

AN

GE

**

1

Num

ber o

f Pal

eoso

l Zo

nes

peds

, roo

t-tra

ces,

car

bona

te n

odul

es2

peds

, roo

t cas

ts,

carb

onat

e no

dule

s

3

GR

AVE

L

** C

olor

cha

nge

as n

oted

in re

gion

al c

orre

latio

ns (e

.g.,

Pet

erso

n, 1

994)

Gm

CH

SS

1 le

vel

lt gr

ay c

alca

reou

s si

ltsto

ne (o

rang

e m

arke

r; pr

oduc

tive

horiz

on)

ash-

rich,

gre

en-d

ark

gree

n si

tlsto

ne

Low

er B

rush

yB

asin

Mem

ber*

*

“Upp

er B

rush

yB

asin

Mem

ber”

**

12

123456

Fig.

3A

Fig.

3B

Fig.

3C

Sp St

cont

inue

s ne

xt p

age

A

Fig.

3. F

ruita

pal

eont

olog

ical

Are

a (F

p) s

trat

igra

phic

sec

tions

A. t

he lo

wer

str

atig

raph

ic S

how

s th

e up

perm

ost 2

0 m

of S

alt W

ash

stra

ta a

nd th

e lo

wer

20m

of B

rush

y Ba

sin

stra

ta. B

. Fr

uita

Pal

eont

olog

ical

are

a (F

P) m

iddl

e st

ratig

raph

ic s

ectio

n. l

ower

Bru

shy

Basi

n st

rata

from

31

to 6

0 m

. the

col

or c

hang

e (c

c) o

ccur

s at

31

m in

the

sect

ion.

C. F

ruita

Pal

eont

olog

ical

are

a (F

P) u

pper

str

atig

raph

ic s

ectio

n. u

pper

Bru

shy

Basi

n st

rata

from

60

to 1

01 m

, the

top

of th

e m

orris

on F

orm

atio

n. T

erra

ce H

ill is

now

cal

led

“Fla

t Top

” (K

irkla

nd, 2

006)

78 Kenneth G. Galli

10- t

o 30

-cm

thic

k br

own-

wea

ther

ing

nodu

lar l

imes

tone

35 253040

carb

onat

e no

dule

s w

ithin

fin

e si

lty s

ands

tone

brow

n-re

d so

il an

d ca

rbon

ate

nodu

le la

yers

, 2-

cm th

ick

@ b

ase

to >

5-cm

thic

k ne

ar to

p

sphe

roid

al-w

eath

ered

fine

san

dsto

ne

puffy

pur

ple-

wea

ther

ing

mud

ston

e

red

mud

ston

e

calic

he @

149m

"whi

te" m

arke

r be

d; s

mal

l cal

cite

nod

ules

brow

n-w

eath

. nod

. mic

ritic

lim

esto

ne

10-c

m th

ick

or le

ss lt

gn-

wh

fn

sdst

with

mm

-sca

le h

oriz

. lam

.

~3 z

ones

(15,

12,

10-

20 c

m th

ick)

, rd

mds

t w

/ car

b. n

odul

es, c

ryst

alla

ria, r

oots

, ped

s,

root

trac

es

puffy

pur

ple

mds

t; "r

ed" m

arke

r bed

mot

tled

red

mud

ston

e

10-c

m th

ick

nodu

lar l

imes

tone

, "o

rang

e" m

arke

r

gree

n si

ltsto

ne w

ith re

d m

ottle

s

10- t

o 15

-cm

whi

te-w

eath

erin

g ba

nd a

bove

spl

ay s

ands

tone

gree

n-m

ottle

d re

d m

dst-s

ltst

whi

te s

iltst

one

10-c

m th

ick

carb

. nod

ules

, la

min

ated

gy,

rd-b

n si

ltsto

negr

een-

mot

tled

red

mud

ston

e10

-cm

thic

k w

hite

ash

"gre

en' m

arke

r

vcvf

CLA

Y

.062

5.0

04.0

01

SA

ND

GR

AVE

Lf

2.0

cS

ILT

mm

m

4

9

3, 1

6

16

21016

9

4m

3

Jmb2

0

13 4m

4

5

67

8

Jmbu

Jmbl

CO

LOR

CH

AN

GE

Upper Brushy Basin Member Lower Brushy Basin MemberCORRELATED

ASH HORIZON I

45505560

puffy

pur

ple-

wea

th. m

dst,

"blu

e" m

arke

r

whi

te fi

ne s

ands

tone

, "ye

llow

" mar

ker

5-cm

thic

k "fl

aky-

wea

th."

gn s

iltst

one

gree

n-m

ottle

d re

d m

udst

one

lam

inat

ed d

ark

red-

brow

n si

ltsto

ne

w/ o

rgan

ics

and

root

s at

top

bn, b

adly

wea

th.

fine

to m

ed s

dst

purp

le-g

y m

dst,

upr p

art w

. gn

mot

tles

and

pedo

tubu

les

very

har

d, in

dura

ted

pedo

geni

c m

dst

purp

le-g

y m

dst,

upr p

art w

/ gn

mtls

and

ped

otub

ules

volc

anic

ash

bed

s

stra

t. le

vel o

f top

of A

l Loo

k H

illgr

ay-p

ink

volc

anic

ash

whi

te disc

ontin

uous

, bro

wn,

fine

san

dsto

ne w

ith c

arbo

nate

no

dule

s an

d pe

doge

nic

feat

ures

in p

lace

s

puffy

-wea

ther

ing

gree

n-m

ottle

d pu

rple

mud

ston

e

10- t

o 20

-cm

thic

k, n

odul

ar c

alci

te; t

he to

p of

cha

nnel

sa

ndst

one

10 is

to th

e E

at t

his

stra

t. le

vel.

purp

lepu

ffy w

hite

-wea

ther

ing

mud

ston

e

vcvf

CLA

Y

.062

5.0

04.0

01

SA

ND

GR

AVE

Lf

2.0

cS

ILT

m

mm

1

10, 1

6

5

4m

916

5

1

9

9

9

3

Jmb4

08

91011

CO

RR

ELA

TED

AS

H H

OR

IZO

N II

CO

RR

ELA

TED

AS

H H

OR

IZO

N II

I

Upper Brushy Basin Member, Morrison Formation

123456

Fig.

3A

Fig.

3B

Fig.

3C

34

cont

inue

s ne

xt p

age

Fig.

3 c

ont.

B


65707580

puffy

pur

ple-

wea

ther

ing

mud

ston

e

purp

le m

ed. s

dst w

ith y

ello

w-w

hite

w

eath

erin

g rin

ds; t

op is

at s

trat.

leve

l of

low

terr

ace

on e

ast s

ide

of T

erra

ce H

ill

puffy

whi

te-to

ligh

t pur

ple

wea

ther

ing

mud

ston

e w

ith

min

or p

urpl

e

disc

ontin

uous

, nod

ular

cal

cite

red-

brow

n ru

bble

-cov

ered

mud

ston

edi

scon

tinuo

us 5

- to

10-c

m th

ick

nod.

car

b.

bn s

hale

w/ 1

- to

4-cm

bar

ite n

odul

es,

top

uppe

r ter

race

E s

ide

of T

erra

ce H

ill*

plug

ged

horiz

on(s

) of c

arbo

nate

w

ith s

ome

gyps

um

puffy

pur

ple-

wea

ther

ing

brow

nish

mud

ston

e

puffy

pur

ple-

wea

ther

ing

mud

ston

e

red

mud

ston

e, p

uffy

in p

lace

s

puffy

whi

te a

nd g

ray-

wea

ther

ing

mud

ston

epu

ffy p

urpl

e-w

eath

erin

g m

udst

one

puffy

whi

te a

nd g

ray-

wea

ther

ing

mud

ston

e

puffy

pur

ple-

wea

ther

ing

mud

ston

epu

ffy w

hite

and

yel

low

-wea

ther

ing

mud

ston

eno

dula

r cal

cite

und

erla

in b

y gy

psum

puffy

pur

ple-

wea

ther

ing

mud

ston

e

3- to

4-c

m th

ick

gree

n as

h-ric

h si

ltsto

ne ;

10-c

m th

ick

red

mud

ston

e w

ith g

reen

m

ottle

s w

ithin

a n

8- t

o 17

-cm

zon

e

puffy

pur

ple-

gray

-wea

ther

ing

mud

ston

es

vcvf

CLA

Y

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5.0

04.0

01

SA

ND

GR

AVE

Lf

2.0

cS

ILT

mm

m

5

9

9

9

34

3

9

16

(4)

4

9

939

9

5m

9

20

Jmb6

0

12

13


puffy

gra

y-w

eath

erin

g m

udst

one

with

a li

ght y

ello

w la

yer i

n m

iddl

e

859095100

som

e pu

rple

silt

ston

e

dark

pur

ple-

gree

n si

ltsto

ne

sand

ston

e / m

ottle

d re

d m

udst

one

w/

root

s an

d sc

atte

red

carb

. nod

ules

, pe

ds, r

hizo

conc

retio

ns

gree

n fin

e to

med

. san

dsto

ne,

brig

ht g

reen

in p

lace

spu

ffy w

hite

mud

ston

ebrow

n no

d. li

mes

tone

w/ g

n an

d w

h ch

ert

puffy

gra

y m

udst

one

puffy

whi

te m

udst

one

yello

wis

h gr

een

silts

tone

yello

w-g

ray-

wea

ther

ing

tan

mud

ston

edi

scon

tinuo

us fi

ne to

med

. san

dsto

ne

puffy

-wea

ther

ing

gree

n si

ltsto

ne

gree

n si

ltsto

ne w

ith g

reen

nod

. car

bona

te

and

sdst

with

brig

ht g

reen

mat

rix, m

ottle

d in

pla

ces,

bot

ryoi

dal i

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4 c

ont.

B

82 Kenneth G. Galli

strata at FP is 14 : 86. Lower Brushy Basin strata here are dominated by 3–5-m thick facies 4 red-brown mudstones, whereas the lower portion of upper Brushy Basin strata (Jmb 31–54 m) are characterized by finely intercalated variegated purple-gray, puffy-weathering, ash-rich mudstones and cre-vasse splays, paleosols, and volcanic ash zones. The upper-most 47 m of Brushy Basin strata (Jmb 54–101) exhibit much thicker facies 9 ashy mudstones, averaging approxi-mately 4–5 m in thickness as well as thicker paleosol zones. The K-1 unconformity is at Jmb101m.

At EC, the color change occurs at 24 m in the section, and so the lower Brushy Basin Member is 24 m thick and overlain by the 52-m-thick upper Brushy Basin Member (Figs 4A, B). The K-1 unconformity is at 76 m, marking the top of the Brushy Basin strata. The SS:F ratio for lower Brushy Basin strata is 15 : 85. At EC, even the facies 4 red-brown mudstones of the lower Brushy Basin Member are smectitic, varying from 74% to 97.3% smectite. Thus, it ap-pears that even though all variegated purple-gray puffy-weathering ashy mudstones are smectitic, not all non-puffy-weathering, red-brown mudstones are low in smectite. Clay mineralogy of the lower Brushy Basin strata here, from the base-up trends from 85.4% illite, 1.7% mixed-layer illite-smectite, and 12.9% chlorite at 0.6 m in the matrix of a facies 5 brownish-gray siltstone, to 92.4% smectite, 3.4% mixed-layer illite-smectite-illite, and 4.2% chlorite from matrix of channel sandstone 2 at ~7 m in section to nearly pure (99.7%) smec-tite in matrix of channel sandstone 3 at 19 m in the section.

The two dominant facies of the lower Brushy Basin are red-brown mudstones (facies 4) and puffy-weathering ash-rich mudstones (facies 9). Major facies include channel sandstones (facies 7) and their associated crevasse-splay de-posits composed of siltstone/very-fine sandstone (facies 5). These crevasse splays are found adjacent to and under their related channel sandstone bodies and the splays are found intercalated with facies 4 floodplain mudstones and ashy mudstones. Caliche paleosols are also a major facies and in-clude mottled incipient caliche developed in floodplain mud-stones as well as ashy mudstones. Caliche paleosols with calcite nodules, pedotubules and peds (facies 16) are a major component of the strata as are nodular calcite in various hosts (facies 3), and noncalcareous mudstone with roots, pedotubules and peds (facies 14). Minor facies include thin micritic limestones (facies 2).

The Brushy Basin strata at TT and EC have light gray to tan, tabular, fine-grained sandstones of variable thickness. These sandstones are calcite-cemented and sometimes have desert varnish. Small- to medium-scale cross-bedding is pre-sent just above the contact with the Salt Wash Member, com-monly over a thin interval of mudstone-rich strata. The ma-jority of the channel sandstones in the Brushy Basin are in the lower Brushy Basin at all localities, with dinosaur bones

present in all TT channel sandstones, in channel sandstone 2 at FP and in channel sandstone 2 at EC. At TT, channel sand-stones 1, 2, and 3 are in the lower Brushy Basin, whereas channel sandstones 4 and 5 are in the upper Brushy Basin at TT. At FP, channel sandstones 1–8 are in the lower Brushy Basin, channel sandstone 9 lies near the boundary between the lower and upper Brushy Basin Members, and channel sandstones 10 and 11 are in the upper Brushy Basin Member.

At EC, as at TT, channel sandstones 1 and 2 are in the lower Brushy Basin and channel sandstones 3, 4, and 5 are in the upper Brushy Basin. Lower Brushy Basin strata have a much higher sandstone-to-fines ratio than the upper Brushy Basin at all localities. At EC, the thickest and only dinosaur-bearing channel sandstone (channel 2) at EC is in the lower Brushy Basin strata at 6 meters in Brushy Basin Member. The 24-m lower Brushy Basin section at EC is characterized by reddish-brown mudstones, locally ash-rich. These are in-terbedded with channel sandstones 1 and 2, thin siltstone/fine sandstone splay units, and caliche horizons, including one thick zone with 16 distinct carbonate nodule layers in a 1-m interval.

uPPEr BrusHy Basin mEmBEr (JmBu)

The upper Brushy Basin at TT is 49 m thick. It has a channel-sandstone-to-fines ratio of 10 : 90; ashy mudstone makes up 40% of the strata. The upper Brushy Basin silt-stones and mudstones at EC and TT are predominantly smectitic, although a few are mainly chlorite.

A chalk-white porcellenite-like rock (facies 13) up to 2.1 m thick at Jmb39 appears to be a silica-replaced ash or mudstone bed. Approximately 2 to 3 m of caliche and silica-replaced caliche are above this in the section, overlain by the Mygatt-Moore dinosaur quarry level at Jmb43 followed by the thickest ash unit (3 m).

At the FP section, the 85-m-thick upper Brushy Basin has a channel-sandstone-to-fines ratio of 8 : 92; ash-rich mudstone makes up 52% of the strata. The greenish mud-stones in the lower Brushy Basin and green siltstone at FP possibly suggest illite and/or chlorite, although no clay anal-yses were performed on these strata. The upper Brushy Ba-sin strata at FP are dominated by thin to thick, ash-rich mud-stones; siltstones and mudstones with superimposed paleo sols; and continuous, massive, tabular siltstone/fine-grained sand-stones. Thick, cross-bedded channel sandstones make up only 7 m of the section. Superimposed sandstone channels are present in at least three locations at FP, including channel sandstones 6 and 7 that are within the stratigraphic section.

Unique at FP are three crevasse-splay deposits interca-lated with levee deposits present at the bends of the second channel sandstone. These can be traced away from the chan-


nel for 10s of meters. The three crevasse-splay/levee se-quences contain abundant dinosaur bones. Two- to 10-m-thick lacustrine micrites occur predominantly in the southeastern and northeastern part of FP. An especially thick lacustrine interval in the southeastern FP is associated with channel sandstones 3, 4, and 5. Agate Hill in the northeastern FP region is surrounded by another thick lacustrine mud-stone interval. Fossils include broken, disarticulated dino-saur bones, fish fossils, conchostrachans, and abundant plant debris, as well as red agatized snails in the Agate Hill area (Kirkland, 2006). Some lacustrine intervals include radial barite and calcite nodules.

FlUvIAl ARChITeCTURe eleMeNT ANAlySIS

oBsErVaTions

Introduction

In river systems, the lithofacies (Fig. 1, Table 1) combine to form structures at three scales: mm to cm microforms formed in minutes to hours, like ripples; cm to m mesoforms formed in days to years, such as dunes; and m to 10s of m macroforms, formed in and persisting for years to thousands of years, like point bars and crevasse splays. All levels in the hierarchy may be active simultaneously in a river. Architec-tural-element analysis uses lithofacies to describe fluvial strata. A hierarchy of physical scales also exists, for exam-ple, from the cross-bed foreset to basin-fill. Ideally, at least 14 orders of magnitude are represented, from a few square centimeters in a ripple foreset to the tens of thousands of square kilometers of a basin (Miall, 1996).

First- and second-order surfaces are within microform and mesoform deposits, respectively. First-order surfaces bound ripple sets and represent the continuous sedimentation of a train of ripples. Second-order surfaces bound cosets with different lithofacies above and below the boundary; the surfaces indicate changes in flow conditions or direction, but commonly not a significant time break. A third-order surface is erosional and dips at low angle within macroforms, with the lithofacies the same above and below the surface. These surfaces are sometimes called “reactivation surfaces” (Fig. 2A, channel sandstone 2). A fourth-order surface bounds the upper surfaces of macroforms, and is usually flat to convex upward. Underlying bedding surfaces and first- to third-order surfaces are truncated at low angle, or are locally parallel with, a fourth-order surface, indicating the fourth-order surfaces are due to lateral or downstream accretion. Mud drapes are commonly present along the surface. The basal scours of minor channels, such as chute channels, are fourth-order surfaces.

Fifth-order surfaces bound major sand bodies, such as channel-fill complexes. The surfaces are flat to concave up-ward, but can have local cut-and-fill relief and lag gravels. Sixth-order surfaces define groups of channels, paleovalleys, or members of formations. Seventh-order surfaces bound depositional systems, commonly members of formations. Eighth-order surfaces bound major basin-fill complexes (Fig. 2A, channel sandstone 2).

The proportions of primary sedimentary structures ob-served and measured at FP and TT are shown in Table 2.

Table 2proportions of sedimentary structures at Fruita paleontological Area (Fp)

and the Trail Through Time (TT)

Location Sp % St % Sh % Sr %

Fruita Paleontological Area

Channel 2 23 58 6 13

Channel 8 40 60 0 0

Trail Through Time

Channel 1-1 77 0 23 0

Channel 1-2 100 0 0 0

Channel 1 mean 88 0 12 0

Channel 2-1 73 0 27 0

Channel 2-2 64 0 36 0

Channel 2-3 15 0 80 5

Channel 2-4 46 19 35 0

Channel 2-5 46 4 50 0

Channel 2-6 32 23 45 0

Channel 2 mean 46 8 46 <1

Channel 3-2 44 50 0 6

Channel 3-3 81 19 0 6

Channel 3-4 0 83 16 1


Channel 4-1 11 50 39 0

Channel 4-2 26 74 0 0

Channel 4-6 100 0 0 0


Grand mean 52 26 22 <1

Sp – planar cross-bedded sandstone, St – trough cross-bedded sandstone, Sh – horizontally laminated sandstone, Sr – rippled sandstone

84 Kenneth G. Galli

These include the percentage occurrence of Sp, St, Sh, and Sr within the channel sandstones.

Eight architectural elements, defined by their geometry, bounding surfaces, size, orientation with respect to paleo-flow, and containing distinctive lithofacies assemblages were described by Miall (1985) after Allen’s (1983) original description of the element in fluviatile sediments. These ele-ments are thought to contain all possible lithofacies in a flu-vial sequence.

The Brushy Basin Member strata in the study area are dominated by architectural element FF, Overbank Fines. This overbank fines element contains facies 1, 2, 3, 4, 6, 8, 9, 12, 13, 14,15, 16, 17, 20, and 21 (Table 1).

The next most abundant element is the Major Sandstone Ribbon (CHR) element. This element contains all facies 7 sandstones except channel sandstone 2 at FP, which is con-tained within the Major Channel Sandstone element (CH). Minor Sandstone Sheet (CS) element contains facies 5 and 10 tabular sandstone bodies that are best observed associated with channel sandstone bodies at FP and to a lesser extent at TT. Rare CHm, Minor Sandstone Element, contains facies 11 tan, fine-grained sandstone bodies.

Fluvial system at Trail Through Time (TT)

The sandstones at TT are ribbon sandstones, based on their map patterns (Figs 5, 6). The channel sandstones are commonly single-story, for example, channels 1, 3, 4, and 5. Channel sandstones are typically 1-3 m thick, but up to 5 m or more locally. Occasionally, there is a multi-story sand-stone, as in channel sandstone 2, but even then, this is a local feature. The east-west-oriented Frontage Road along the southern perimeter of TT, exposes the east-west lateral ex-tent of channels 2 and 3 within the lower Brushy Basin strata and channel 4 in the lower portion of the upper Brushy Basin strata (Fig. 5A) and the thin ribbon aspect of the channel bodies. Channel sandstone 2 was traced and measured for its entire 67-m horizontal breadth or lateral extent. Its thickness varies from 0.25 m at its feather edge at the western end of Frontage Road to a maximum of 7.1 m and then to nothing to the east where it “wings out” which is characteristic for a ribbon sandstone body. From 0 to 9.1m channel sandstone 2’s outcrop azimuth is N80°E. This includes the two-story channel segment from 5 to 10 m, where a 0.4 m to 1.2 m lower story is overlain by a 0.8 m upper story. From 9.1 to 16.5 m, its azimuth is due east and thickness is 1.2 m, from 16.5 to 19.5 m, it’s azimuth is N30°E and thickness is 1.2 m, from 19.5 to 22.5 m, its azimuth is due east, with thickness of 2.7 m, and then from 22.5 to 67 m its azimuth is N25°E and thickness varies from 2.4 m to 6.7 to 7.1 m and then to 5.5 m and finally 0.9 to 0.7 m near its easternmost edge.

Paleoflow in lower channel 2 is toward 98.2° (to the east) and 101.8° in the upper amalgamated channel (to the east). Paleoflow in channel 3 is toward 220° (to the southeast), and paleoflow in channel 4 is toward 35.3° (to the northeast). The channel sandstones pinch out into associated floodplain facies (Figs 5A, 6). The thalweg gravel of rounded caliche and mudstone small pebbles and granules is present at the base of channel 1 (Fig. 7A). Locally, channel 2 is a two-story amalgamated channel (Figs 6; 7, panel 2). Definition of a macroform story is based on: (1) erosional scour base, (2) gravel lag, (3) laterally extensive, and (4) significant mud top (Miall, 1996). Channel sandstone 2 exhibits oversteep-ened cross-beds (Fig. 7, Panel 2). Following a rapid drop in river stage, the excess pore pressure of the water in the sand caused down-slope deformation of the cross-beds. Planar cross-bedded sandstone (Sp) overlain by trough cross-bed-ded sandstone (St), which is overlain by coarser-grained horizontally laminated sandstone (Fig. 7, panel 4) indicates either an increase in flow velocity of the river or more likely, a drop in the river stage, which caused increased flow from lower flow regime features (Sp and St) to upper flow regime features (Sh). Panel 6 photograph shows Sp overlain by St topped by a 4th order bounding surface and then upper flow-regime horizontally laminated medium sandstone. Thus, Fig. 7B depicts a non-systematic fill of the channel over time.

Channel 2 is single-story, with a non-systematic fill of 15-48-cm-thick planar cross-beds. After the channel was scoured out, migration of trains of linguoid bars at or near bankfull discharge constructed the cosets of planar cross-bed sets, with flow consistently to the southeast. The small vari-ation of individual paleocurrent azimuths from the mean suggests an anastomosing pattern. Reactivation surfaces within the cosets reflect separate floods or fluctuations in a flood.

Paleoflow was to the northeast (47.1°) for channel sand-stone 1; to the east (98.2°) for the lower story of channel sandstone 2; toward 101.8° for channel sandstone 2’s upper story; to the southwest (220°) for channel sandstone 3; to the northeast (35.3°) for channel sandstone 4 (Fig. 6). Channel sandstone 3 displays Sp overlain by a thin interval of ripples (Sr), followed by St. The bedload of the channel contains a 2.0 × 4.5-cm Stegosaurus vertebrae (Fig. 8A). The mean of 30 paleocurrent readings show that the paleoflow of channel sandstone 3 was to 220° (southwest) which is different than all the others at TT. The mean flow direction for channels 1, 2, and 4 are to the northeast or east (Fig. 6). Channel 4 is 2.3 to 3.0-m thick and the otherwise friable sandstone is partial-ly silica-replaced in patches, especially along slanting cross-bed strata (Fig. 8B). Where channel sandstone 4 is thinnest, the lower portion exhibits a coset of Sp with divergent paleo-current flow directions.


55

54

4

4

Cha

nnel

2C

hann

el 3

Cha

nnel

4

A

B

Fig.

5. p

hoto

mos

aics

of B

rush

y Ba

sin

stra

ta a

t Tra

il Th

roug

h Ti

me

A. P

hoto

mos

aic

show

ing

the

arch

itect

ure

of c

hann

el s

ands

tone

s 2,

3, a

nd 4

and

enc

losi

ng fl

oodp

lain

mud

ston

es a

nd a

ssoc

iate

d fa

cies

. Vie

w is

to th

e no

rth

and

show

s th

e ea

st-w

est l

ater

al e

xten

t of

chan

nels

2 a

nd 3

with

in th

e lo

wer

Bru

shy

Basi

n st

rata

and

cha

nnel

4 in

the

low

er p

ortio

n of

the

uppe

r Bru

shy

Basi

n st

rata

. not

e th

e th

in ri

bbon

asp

ect o

f the

cha

nnel

bod

ies.

Pal

eoflo

w in

low

er c

hann

el

2 is

to th

e ea

st (9

8o ), a

nd to

the

east

-sou

thea

st (1

02o )

in th

e up

per a

mal

gam

ated

cha

nnel

. Pal

eoflo

w in

cha

nnel

3 is

tow

ard

the

sout

heas

t (22

0°),

and

in c

hann

el 4

is to

war

d th

e no

rthe

ast (

35.3

°). n

ote

that

the

chan

nel s

ands

tone

s pi

nch

out i

nto

asso

ciat

ed fl

oodp

lain

faci

es. B

. cha

nnel

san

dsto

ne 2

86 Kenneth G. Galli

Ch

2 Lo

wer

Sto

ry

N =

10

98.2

°

calic

he

flood

plai

n m

udst

one

(4)

flood

plai

n m

udst

one

(4)

calic

he (1

6)

flood

plai

n m

udst

one

(4)

flood

plai

n m

udst

one

2-1

2-5

2-6

2-2

2-3

2-4

Sto

ry 1

Sto

ry 2

0

1040 20

3-1

3-2

3-3

3-4

4-1

f c

4-2

4-6 f -

m

m - v

c

silic

a-re

plac

ed

1-1

1-2

0

30

2040

6080

100

ash-

rich

mud

ston

ecr

evas

se-s

play

silt

ston

epo

orly

-dev

elop

ed n

onca

lcar

eous

pal

eoso

l (14

) gr

een-

mot

tled

red-

brow

n si

lty m

udst

one

ash-

rich

mud

ston

e (9

)

47.1

°N

101.

8°

35.3

°

Ch

2 U

pper

Sto

ry

WE

ST

EA

ST

App

roxi

mat

e lin

e of

ver

tical

stra

tigra

phic

sec

tion

N =

32

N =

30

N =

5

N =

7

calic

he

220°

met

ers

Cha

nnel

San

dsto

ne4

Cha

nnel

San

dsto

ne3

Cha

nnel

San

dsto

ne2

Cha

nnel

San

dsto

ne 1

Lower Brushy Basin MemberUpper BrushyBasin Member Fi

g. 6

. Arc

hite

ctur

e of

cha

nnel

san

dsto

nes

(ele

men

t Ch

R) i

n lo

wer

40

m a

t Tra

il Th

roug

h Ti

me

(TT)

and

the

line

of s

ectio

n fo

r con

stru

ctio

n of

the

vert

ical

str

atig

raph

ic s

ectio

n

Blan

k ar

eas,

cal

iche

, mud

ston

es, a

sh-r

ich

mud

ston

es a

re c

onta

ined

with

in o

verb

ank

fines

ele

men

t (FF

). cr

evas

se-s

play

dep

osits

are

ele

men

t cs.

som

e th

in c

reva

sse

spla

ys a

ssoc

iate

d w

ith c

hann

el

2 ar

e no

t sho

wn.

des

igna

tions

2–1

and

4–6

ref

er to

loca

tions

of v

ertic

al s

ectio

ns s

how

n in

arc

hite

ctur

e pa

nels

. on

pale

ocur

rent

ros

e ar

row

sho

ws

aver

age

pale

oflo

w d

irect

ion,

n –

sam

ple

num

ber.

myg

att-

moo

re d

inos

aur Q

uarr

y le

vel i

s ~2

–3 m

abo

ve to

p of

cha

nnel

san

dsto

ne 4

. Top

of B

rush

y Ba

sin

mem

ber i

s at

79

m


Low

er s

tory

of c

hann

el s

ands

tone

2 a

t TT

red

mud

ston

eP

anel

4 Sh

Sp

St

26

25

1233.7

6

24

48 c

m

34 c

m

19 c

m

Sp

cose

ts

0m2

40m

1

0mS

h

4gr

een-

purp

le p

uffy

gro

und

0m

Sh

Sp

cose

ts16 15

171819

20

15 c

m35

cm

36 c

m

26 c

m

med

ium

- co

arse

sa

ndst

one

2

Pan

el 2

2S

h

Und

erla

in b

y 31

-cm

-thic

k re

d ca

liche

121

8

1.8234

8

red

mud

ston

esa

ndst

one

ShS

p

St

Sp

0 m

24

6 10

1

2 4

22

6 5

9

14

1011

7

13

5S

p

med

ium

san

dsto

ne

0 m

4

35

12345

24

6

cove

red

Sh

Sp Sh

Sp

Sh

St

Sp

1

2

34

5

0m0m5.

2

cf

Upp

er s

tory

of c

hann

el s

ands

tone

2th

at o

verli

es lo

wer

sto

ry

Pan

el 2

pho

to:

Pan

el 4

pho

to:

calic

he

1.3

Sp c

oset

(4)

(16)

(16) (4)

123456

red-

brow

n pu

ffy m

udst

one

Sx

and

Sh

Red

mud

ston

e0

m

1

0 mPan

els

1 an

d ph

oto

of c

hann

el s

ands

tone

1 a

t TT

24

Pan

el 5

CH

2,P

anel

6

Pan

el 1

Sp

St

Sh

A B

Fig.

7. A

rchi

tect

ure

pane

ls a

nd p

hoto

grap

hs o

f cha

nnel

s 1

and

2 at

Tra

il Th

roug

h Ti

me

(TT)

A. A

rchi

tect

ure

pane

ls a

nd p

hoto

grap

hs fo

r cha

nnel

san

dsto

ne 1

at t

t. B

. arc

hite

ctur

e pa

nels

and

pho

togr

aphs

of c

hann

el s

ands

tone

2. P

anel

2 p

hoto

grap

h is

of o

vers

teep

ened

cro

ss-b

eds.

see

Fig

. 6 fo

r loc

atio

n of

pho

togr

aphs

and

sec

tions

. cha

nnel

1, p

anel

1 il

lust

rate

s m

udst

one

clas

ts a

t bas

e, p

lana

r cro

ss-b

ed s

ets

up to

14

cm th

ick,

car

bona

te n

odul

es w

ith p

edot

ubul

es. P

anel

2 s

how

s pl

anar

cro

ss-b

eds

up to

13

cm

thic

k. P

anel

4 s

how

s sa

ndst

one,

with

min

or li

mon

itic

stai

ns a

long

bed

ding

. cha

nnel

2, p

anel

6 s

how

s th

at o

verly

ing

red

mud

ston

e is

cut

out

to w

est w

here

cha

nnel

2 h

as tw

o st

orie

s

88 Kenneth G. Galli

Fig.

8. A

rchi

tect

ure

and

phot

ogra

phs

of c

hann

els

3 an

d 4

at T

rail

Thro

ugh

Tim

e (T

T)

A. a

rchi

tect

ure

pane

ls a

nd p

hoto

grap

h of

cha

nnel

san

dsto

ne 3

at T

T. P

anel

2 a

nd th

e ph

otog

raph

dep

ict t

he b

ase

of th

e ch

anne

l with

a 2

× 4

.5-c

m S

tego

saur

us v

erte

brae

, with

sp

over

lain

by

a th

in

inte

rval

of r

ippl

es, f

ollo

wed

by

st. B

. arc

hite

ctur

e pa

nels

and

pho

togr

aphs

of c

hann

el s

ands

tone

4 a

t TT.

Pan

el 1

and

pho

togr

aph

show

s th

e en

tire

3-m

thic

knes

s of

cha

nnel

4. n

ote

silic

a-re

plac

emen

t-fr

ont v

isib

le a

s da

rk, b

ette

r-ce

men

ted

area

s of

cha

nnel

san

dsto

ne 4

abo

ve r

ight

. cha

nnel

3 p

anel

1 il

lust

rate

s co

mm

on fo

ssil

bone

s an

d bo

ne fr

agm

ents

up

30-

× 45

-cm

. mud

ston

e an

d ch

ert p

ebbl

es

occu

r thr

ough

out c

hann

el. B

urro

ws

are

0.25

to 0

.5 c

m. c

hann

el 3

, Pan

el 2

illu

stra

tes

isol

ated

trou

gh c

ross

-bed

s co

ntai

ning

whi

te m

ud c

hips

and

abu

ndan

t din

osau

r bon

e fr

agm

ents

. cha

nnel

4, p

anel

1

show

s tr

ough

cro

ss-b

eds

cont

aini

ng g

rave

l lag

. san

dsto

ne is

par

tially

repl

aced

by

silic

a in

pat

ches

Pan

els

1 an

d 2

of c

hann

el s

ands

tone

3 a

t TT

Pan

els

1 an

d 2

of c

hann

el s

ands

tone

4 a

t TT

f c

Red

mud

ston

e

123 2.8

0 m

12

3

2Sp

Sh

St

Sh

St

Sh

Sx

Sh

St

Sh

13

1

3

7- x

23-

cm

10- x

20-

cm

dino

bon

e

0 m

St

Sp

5

0 m

24

22.3

1

69 8

Sp

set t

hick

ness

-7

, 8, 1

3 cm

60-c

m p

uffy

gre

en m

udst

one

Din

osau

rbo

ne0

m

dino

saur

bon

e

Ch

4 P

anel

1

Ch

4 P

anel

2

4

321

St

St

Sh

21

0 m

0 m1

3

Rib

& F

urro

w

St b

ecom

esm

alle

r set

s

Sr S

h

Sp35

-cm

poo

rly d

evel

oped

cal

iche

1.76

5

Ch

3, P

anel

1

1

0 m

0 m

24

6

6

7 St

Sp

SpSt

Sp

Sr

1.6

Cha

nnel

3 P

anel

2C

h 3,

Pan

el 2

Pho

to:

Cha

nnel

San

dsto

ne 4

A B


Much of the upper Brushy Basin at TT is typified by a 2–3-m thick marlstone that makes up much of the Mygatt-Moore dinosaur quarry (Fig. 9A) within predominantly floodplain and associated strata with a very low sandstone-to-fines ratio. The upper portion of the measured stratigraph-ic section is to the north of the main outcrop belt (Fig. 9B).

Fluvial system at Fruita Paleontological research area (FP)

The south side of FP exposes a 7th-order bounding sur-face between the Salt Wash and overlying Brushy Basin Members. 5th- and 4th-order bounding surfaces bracket the bottom and top of fluvial channel sandstones, respectively (Fig. 10A). The 7th-order bounding surface between the

Brushy Basin and Salt Wash members is near the break in slope of Al Look Hill (Fig. 10A). Erosive, flat to concave-up surfaces below major channel sandstones in the Salt Wash Member are 5th-order surfaces visible near the base of the mosaic in Figure 10A. Flat to convex-up surfaces at the top of these sandstone bodies are 4th-order surfaces. Note the difference between the more laterally continuous, thicker sandstones of the Salt Wash and the thinner, less laterally continuous sandstones in the Brushy Basin. Bounding sur-faces on the south side of Al Look Hill mostly continue through to the north side, which is the active side of FP pale-ontological investigations. Here, one can observe the lateral continuity of tabular-body, crevasse-splay siltstone/very fine-grained sandstones. George Callison’s small mammal quarry is in the mound of purple-gray mudstones and silt-stones in the left foreground (Fig. 10B).

A

B

Fig. 9. photographs of upper Brushy Basin Member strata from the section at TT

A. mygatt-moore dinosaur Quarry in 2-m thick marlstone and associated facies at 43 m. B. upper stratigraphic section. Arrow points to top of measured stratigraphic section. note channel sandstone (probably channel 3), approximately on strike from the east in middle foreground and the low sandstone to fines ratio of the strata

90 Kenneth G. GalliA

B

Fig.

10.

pho

togr

aphs

of b

oth

sout

h si

de a

nd m

ain

(nor

th) s

ide

of F

ruita

pal

eont

olog

ical

Are

a (F

p)

A. V

iew

look

ing

nort

h to

war

d th

e Bo

ok c

liffs

, fro

m th

e so

uth

side

of F

P. a

sev

enth

-ord

er b

ound

ing

surf

ace

lies

betw

een

salt

was

h an

d Br

ushy

Bas

in m

embe

rs, m

orris

on F

orm

atio

n. F

ifth-

and

four

th-

orde

r bo

undi

ng s

urfa

ces

brac

ket t

he b

otto

m a

nd to

p of

fluv

ial c

hann

el s

ands

tone

s, r

espe

ctiv

ely.

Ver

tical

line

s sh

ow th

e lo

catio

n of

the

mea

sure

d se

ctio

n. B

. FP

, loo

king

sou

th w

ith a

l loo

k H

ill in

the

cent

er. B

ound

ing

surf

aces

on

the

sout

h si

de o

f al l

ook

Hill

mos

tly c

ontin

ue th

roug

h to

this

sid

e. n

ote

the

late

ral c

ontin

uity

of t

abul

ar-b

ody,

cre

vass

e-sp

lay

silts

tone

/ver

y fin

e-gr

aine

d sa

ndst

ones

. G

eorg

e ca

lliso

n’s

smal

l-mam

mal

qua

rry

is in

the

mou

nd o

f pur

ple-

gray

mud

ston

es a

nd s

iltst

ones

in th

e le

ft fo

regr

ound

that

are

at t

he o

rang

e cr

evas

se-s

play

leve

l


The fluvial architecture of the 101 meters of the Brushy Basin strata at FP demonstrates that channel sandstones 1 to 8 lie within the lower Brushy Basin Member; channel sand-stone 9 lies near the contact between the lower and the upper Brushy Basin and channel sandstones 10 and 11 are within the upper Brushy Basin Member (Fig. 11). Note also that channel sandstone 2 is the most laterally extensive of all at a bit over 1 kilometer. Channel sandstones 1, 2, and 6/7 are

only separated by a few meters stratigraphically where they crop out at what is called the Parking Lot, in the eastern part of FP, but channel sandstone 2 occupies a lower stratigraphic level in western FP. Channel sandstone 6 occupies a higher stratigraphic level to the southwest in FP. Most other channel sandstones appear to lie at just one stratigraphic level, al-though, this point should be confirmed.

Channel Sandstone 2

Channel Sandstone 9

Channel SS 4ChannelSS 5

Channel SS 8

Channel SS 11

Channel SS 10

Channel SS 3

Channel SS 7/6

Channel Sandstone 1

Approximately 1 km

0

20

10

30

80

100

101m

40

60

met

ers

abov

e B

rush

y B

asin

/Sal

t Was

h C

onta

ct

Sr: flow to N70°E

Sp: flow: N30°W 27°NE

St: flow 12° to S70°ESp: flow: N50°W 24°E2 Cross-bed sets

1 Rib-and-Furrow

Xv = 67°N = 3L = 95%P = 0.001

66.9°

2 Cross-bed sets1 Ripple

Xv = 62°

N = 12

L = 77%

P = 0.001 62.2°

1 Pillow axis

Upper Brushy BasinMember

Lower Brushy Basin Member

Insert Map:

Fig. 11. Schematic Architecture of Brushy Basin channel sandstones at Fruita paleontological Research Area (Fp).

Blank areas are floodplain mudstones and associated facies (Element FF). channel sandstone 2 (Element cH); all other channel sandstone (Element cHr); thin crevasse splay deposits (Element cs) not shown in this schematic. insert map: FP channel sandstones, arrows show inferred paleoflow directions for channel sandstones 1-11 (Kirkland, 2006). usGs topo 7 ½ minute quadrangle map for mack, colorado

92 Kenneth G. Galli

Cha

nnel

San

dsto

ne 1

Lev

ee d

epos

its w

ith c

evas

se s

play

s

coar

se sa

ndsto

ne

Joint

1

1.02.0

23

45

5.5 m

Sp

brow

n coa

rse ss

brow

n coa

rse ss

very

coar

secg

l- vc s

s

cong

lomer

atic s

sco

nglom

erati

c f-m

ss

cgl c

-vc ss

cgl-s

s

fine-

mediu

m ss

cgl-c

ssbn

m-c

ss

c

m-c s

sSh

Shcg

l-ss

fine s

s

mediu

m-co

arse

sand

stone

mediu

m-co

arse

sand

stone

SpSx

m-vc

ss w

ith gr

anule

s

mediu

m ss

fine s

ands

tone

mediu

m sa

ndsto

nefin

e-me

dium

ss

set b

ack

gran

ule cg

lc-v

ery c

oarse

ss

0.51.5

cgl-s

s cong

lomer

atic

coar

se-ve

ry co

arse

ss

med s

sme

d-co

arse

sand

stone

fine s

s

cong

lmer

atic c

-vc sa

ndsto

ne

N50°

E, 22

°NW

N70E

S70W

5

5flood

plain

muds

tone

flood

plain

muds

tone

Fig.

12.

Fru

ita p

aleo

ntol

ogic

al A

rea,

Bru

shy

Basi

n ch

anne

l san

dsto

ne 1

and

ass

ocia

ted

leve

e de

posi

ts w

ith c

reva

sse

spla

ys

“5”

mar

ks th

e fif

th-o

rder

bou

ndin

g su

rfac

es a

t the

bas

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The sedimentary structures of channel sandstone 1 (Blue Channel) at FP are largely obscured by bioturbation from plant roots and burrows. Only one or two fining-upward se-quences were observed and this channel is medium-very coarse-grained sandstone, coarser than fine-medium-grained channel sandstone 2. Small channel forms are all but absent in channel sandstone 1. The map patterns of channel 1 and all other channels at FP except channel sandstone 2 are rela-tively straight. Sandstone fills the channel form and two 5th-order bounding surfaces mark the bases of two channels within channel sandstone 1. No lateral-accreting surfaces (LAS) were observed; the channel trends N70°E. No mud drapes are present but some sandstone bodies are topped by fine- to very fine-grained sandstone. Channel scours are nu-merous with relief on the order of 10–15-cm. Large, mostly horizontal burrows or roots are common, mostly filled with coarse-grained sandstone but some with dark mudstone. The thicknesses of the channel fills within channel 1 decrease up-section; from the base up, the thicknesses are 0.54, 0.43, 0.42, 0.18, 0.12 to 1.0 m. Grain-size trends are hard to dis-cern. The sandstone is medium to coarse-grained, silica-re-placed and chert-cemented in places. Poikilitic chert occurs in places, mostly in the middle and upper part of the channel. Most units thin to the east; upper units pinch-out to the east. A paleocurrent measured from Sp in the basal, conglomer-atic, medium-grained sandstone yielded an azimuth towards 320° (Fig. 12). Most of the lower Brushy Basin strata at FP show extensive floodplain mudstones and ribbon channel sandstones (Fig. 13A).

Channel sandstone 2, near one of its bends, is oriented to the west and then turns north where the channel takes a 90° turn and has associated well-developed levee deposit with

crevasse-splay deposits cutting through. The color change (CC) marking the division between lower and upper Brushy Basin strata is just below the base of the channel. The azi-muth of paleoflow of channel 8 was 66.9° (to the northeast) (Fig. 14). The architecture of channel sandstone 8 is similar to many of the channel sandstones at FP, which range in thickness from 1.5 to 4.5 m (channel 4), averaging 2.5 m. Sedimentary structures are heavily bioturbated. The fill of channel 8 is similar to channel 1 and no fining-upward se-quences are present. Eight of the 11 channel scour fills with-in channel 8 are single story with non-systematic fills of 7–30-cm thick, planar cross-beds produced by downriver migration of linguoid bars that prograded at or near bankfull discharge. Paleoflow direction for channel 8 is to N67°E based on three readings (Fig. 14).

As with channels 1 and 2 at TT, each FP channel, except channel 2, has a consistent within-channel paleocurrent flow direction; in most cases the channel sandstone form is a rib-bon. Table 2 shows the proportions of sedimentary structures within FP and TT channel sandstones. Channel 2 at FP has a lower proportion of planar cross-bed sets than most chan-nels at TT. FP channel 2 also has a much higher proportion of ripple cross-lamination than any of the TT channels. The architecture of channel sandstone 2 differs from channel 1 by: (1) finer grain size, (2) abundant fining-upward sequenc-es, (3) abundant small channel scours filled by fining-upward sequences, (4) abundant dinosaur bones, including a long bone lying on a bedding plane oriented parallel to the flow, (5) common mud drapes, (6) multi-colored chert granule and fine pebbles at the base of many channels, and (7) sandstone pillows in a few places. Twelve paleocurrent readings indi-cate that flow was variable, averaging towards 62° (north-

A B

Fig. 13. lower Brushy Basin floodplain features at Fruita paleontological Area (Fp)

A. Extensive exposure of floodplain mudstones and ribbon channel sandstones, with low sandstone to fines ratio. B. crevasse splay-levee system associated with channel sandstone 2

94 Kenneth G. Galli

Chan

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east). Within the area of study, channel sandstone 2 con-tained ten sizable dinosaur bones, many lying along bedding surfaces and several as part of a coarse sand-gravel lag de-posit, while others are within fine- to medium-grained sand-stone. Bones range in size from 2 × 2 cm to as large as 7 × 88 cm. The basal 40–60 cm of the channel contains most of the bones (Fig. 14A).

No mudcracks were observed, which may argue against the climate being semi-arid or alternatively they may be missing due to low preservation potential during flash floods. Some channel sandstones, such as channel 7 at FP, contain mud drapes near their tops as well as the usual mud clasts in the basal lag.

As each flood-cycle waned, the mud mantled the lower-flow-regime bedforms. This probably indicates seasonal variations in discharge. At times a channel shifted abruptly, cutting into older channel sands, producing an amalgamated channel body, for example in channels 6 and 7 at FP.

Upper Brushy Basin floodplain strata from 46.5 to 50.4 m in the Brushy Basin Member contain a crevasse-splay deposit and ash zone II (Figs 15A, B). The thick puffy-weathering ash-rich floodplain mudstones above the cre-vasse splay suggests a retreat of the river channel away from the section occurred.

Blue

Red

Orange

Channel 7

Channel 6

Yellow

Ash II

A

B

Fig. 15. Typical architecture of the lower Brushy Basin Member at Fruita paleontological Area (Fp)

A. Photograph showing lowermost lower Brushy Basin strata, with several flagged crevasse-splay deposits noted (blue, red, orange) and a faint yellow splay barely visible, with an amalgamated channel sandstone 6/7 at the top. Field assistant John Bailey for scale. B. Floodplain strata from 46.5 to 50.4 m in the Brushy Basin member at FP. yellow-flagged crevasse splay is indicated by the arrow. ash ii is at top of photo. Thick puffy-weathering ash-rich floodplain mudstones above the crevasse splay suggest a retreat of the river channel away from the section. Jacob staff is 1.5 m

96 Kenneth G. Galli

CC

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Fig.

16.

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Fluvial system at Echo canyon (Ec)

Echo Canyon shows the largest scale bounding surfaces seen in this study (Fig. 16A). Morrison Fm. strata are brack-eted by 8th-order surfaces of discontinuity: (1) the regional J-5 unconformity over the Jurassic Summerville Formation and (2) the regional K-1 unconformity beneath the Creta-ceous Burro Canyon Formation. 7th-order surfaces are the boundaries between the Tidwell, Salt Wash, and Brushy Ba-sin members of the Morrison Formation; these surfaces typi-cally form over intervals of 105 to 106 years (Miall, 1996) and are traceable over the extent of the members, an area slightly smaller than the total Morrison Formation.

A possible paleovalley observed within Brushy Basin strata high above the EC dry wash, oriented approximately N 20° E, contains several lens-shaped channel bodies that sit in a larger scale (possibly 7th order boundary) concave-up surface that is potentially a paleovalley with the long axis parallels the dry wash. The sandstone bodies indicate that the streams that produced them flowed most likely eastward.

The author discovered an Apatosaurus femur in the 3 to 4-m thick channel sandstone 2 at EC (Fig. 16B). This bone is silica-cemented into the sandstone and the central portion is filled with sand and gravel. There is a 1-m scour at the base of Channel 2, indicating that the river scoured deep into the underlying floodplain mudstones.

The color change between lower Brushy Basin and upper Brushy Basin strata a third of the way from the base. The color change occurs just above a crevasse splay zone. The puffy-weathering aspect of the volcanic ash-rich mudstones is well displayed. The “clay change” between the red-brown floodplain mudstones below and the gray, puffy-weathering, ash-rich, smectitic mudstones above occurs twenty-four me-ters within the Brushy Basin Member middle to upper Brushy Basin strata (Fig. 16C).

inTErPrETaTions

Fluvial complex at Trail Through Time (TT)

Channel 2 is single-story, with a non-systematic fill of 15–48-cm-thick planar cross-beds. After the channel was scoured out, migration of trains of linguoid bars at or near bankfull discharge constructed the cosets of planar cross-bed sets, with flow consistently to the southeast. The small vari-ation of individual paleocurrent azimuths from the mean suggests an anastomosing pattern. Reactivation surfaces within the cosets reflect separate floods or fluctuations in a flood. The second story of channel 2 is also filled with a non-systematic fill of planar cross-strata and trough cross-strata as well as upper flow regime horizontally laminated sandstone.

Commonly, the base of a horizontally-laminated sand-stone cuts into a coset of cross-beds, implying shallower flow depth and/or higher velocity. Evidently, there was “flashy” discharge, probably due to seasonal precipitation. With no grasses present, flash floods swept across the flood-plains toward the channels. Lacking depth-to-width ratios for the channels, as well as laterally accreting (LA) macro-forms, quantitative estimates of the hydrology and morphol-ogy of the rivers are not possible. However, because gravel is transported at velocities above about 1.0 m per second, the flow rate was mainly lower than this, assuming the availabil-ity of gravel, particularly intraclasts. Gravel lags can occur when flow is 0.4–0.7 m/s (Einsele, 1992).

An analysis of the channel sandstones at TT and all but channel sandstone 2 at FP show the following features in abundance: (1) low sandstone to fines ratio, (2) ribbon sand-stone geometry, (3) highly variable paleocurrents, (4) evi-dence of stable vegetated islands, and (5) single-channel form. Additionally, mature caliche paleosols are common. Features sometimes present are: (1) multi-story channels, (2) crevasse-splay deposits, (3) fining-up channel fills. Features absent for the most part, are: (1) LA or downstream-accret-ing macroforms (DA) elements, (2) levee deposits, point-bar sequences, (3) sheet sandstone geometry, and (4) large-scale sandy bedforms and gravel bedform elements (Table 3).

The rapid changes between lower flow regime (LFR) and upper flow regime (UFR) structures in the channel sand-stones imply that the rivers were “flashy,” low-gradient channels that reached maximum discharge several times per year during the wet season. The rapid alternation of struc-tures of the LFR and UFR can be seen in Figures 5 to 8, which contain St, Sp, Sr, and Sh.

Channel sandstone 1 allows the following interpreta-tions. The paleoflow was to the northeast (47°) in a small, fairly fast-flowing river, with a medium-sandstone to fine-gravel basal thalweg lag of rounded floodplain and caliche clasts. Most flow was in the lower flow regime, with current velocities from 0.4–0.7 m/s (Ashley, 1990). The 23% upper flow regime structures in channel 1 indicate some upper flow regime transport at 0.7–1.2 m/s, for example, when the rip-up clasts were produced, probably by bank collapse. Water depth was shallow as indicated by the 1.3-m thickness of the channel sandstone.

The rivers at TT and at FP (excepting channel 2) were narrow and shallow. During floods they streams overflowed the floodplain and deposited crevasse-splay silts and very fine sands near the channels. Crevasse splays are particularly well developed at FP, where many of them can be correlated to specific channel sandstones (J. Kirkland, person. comm., 1992, 2006).

In summary, the fluvial style at TT has features of both meandering and anastomosing rivers, but tends more toward

Fig.

16.

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of B

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he w

ay fr

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e ba

se

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e ph

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98 Kenneth G. Galli

anastomosing. Only panel 2 of channel sandstone 4 (Fig. 8B) is close to an ideal point-bar sequence. River discharge was flashy, with rapid changes in velocity and/or depth, produc-ing interbedded upper flow regime beds and lower flow re-gime planar and trough cross-beds with rare ripples. River depths were variable, based on the heights of individual St and Sp sets (4 to 36 cm) and thicknesses of the channel sand-stone bodies (1.0 to 5.2 m). Avulsions were common, with the hard, underlying caliche horizons contributing to a ten-dency to avulse. The few fining-up channel sequences reflect progressive filling, but also may be due, in part, to decelerat-ing flow during a major flood. The anastomosing rivers probably had some meander reaches. Paleocurrent flow vari-ability is much higher in channel sandstones 3 and 4 than in channel sandstones 1 and 2, which might indicate a change in the fluvial regime through time at TT. These sandstones bracket the lower Brushy Basin-upper Brushy Basin contact and may indicate that the style of rivers changed in response to the increasing volcanic ash input.

Migrating subaqueous two-dimensional lingoid bars un-der lower flow regime conditions formed the planar cross-bed sets. Three-dimensional sinuous-crested dunes formed the larger trough cross-bed sets. Horizontal laminations in me-dium- to coarse-grained sandstones, some with parting-step lineation, are upper flow regime forms. A braided river at TT is unlikely because of a low ratio of sandstone to mudstone (31 : 69), the common to locally abundant caliche paleo sols, lack of downstream-accreting macroforms and gravel, the ab-sence of large-scale sandy or gravelly bedforms, and especially, the lack of numerous channel cross-sections.

The Brushy Basin Member anastomosing river system (Fig. 17) is similar to the Rio Flumen section of the Oligio-Miocene Huesca Formation, Ebro Basin, Spain (Hirst, 1991). Hirst (1991) describes a ribbon-dominated alluvial architecture of a section along the east side of the Rio Flu-men approximately 3 km south of the basin margin. The cross section of this sequence is similar to that at TT. Ribbon sandstones are mired in floodplain mudstones. Some channel

Table 3Qualitative presence of sedimentologic and architectural features in Brushy Basin Member channel sandstones at Trail Through Time (TT),

and Channel Sandstone 2 at Fruita paleontological Area (Fp) compared to generalized anastomosing, meandering, and braided rivers

Item #

Possible features Features observed Typical features (from literature)

Channel SSs at TT

CH 2 at FP

Anastomosing Meandering Braided

1 Low sandstone to fines ratio + + + + + + + + –

2 Ribbon sandstone + + + / – + + – –

3 Highly variable paleocurrents + + + + + + + + –

4 Stable vegetated islands + + + + + – + / –

5 Abundant mature paleosols + + + + + –

6 Abundant LA or DA elements – – – + + + +

7 Multi-story channels + / – – + + + +

8 Crevasse-splay deposits + / – + + + + + –

9 Single channel + + + + + / – + + –

10 Levee deposits – + + – + + –

11 Point-bar deposits – + + – + + –

12 Fining up channel fill + / – + + + / – + + –

13 Sheet sandstone – – – + + –

14 Large-scale SB and GB elements – – – – + +

15 Low channel width to depth ratio ? ? + + –

16 High sinuosity ? + + + + + –

+ + abundant, + common, + / – sometimes present, – absent


stacking or amalgamation occurs, but most sandstone bodies are single-story or two-stories at most. The section at TT could possibly be part of a larger fluvial system that has nearly the dimensions and large-scale architecture of the Rio Flumen system. The Rio Flumen (Hirst, 1991) shares two things with the strata at TT: (1) the proximal strata are tectonically disrupted, and/or eroded away, and (2) regional correlation is imprecise, with the age of the sequence deter-mined by a few vertebrate horizons (Hirst, 1991). Until ash beds were recently dated (Kowallis et al., 1991), the same was true for TT. To this day, some of the Brushy Basin se-quence is not well-constrained and time-markers are hard to find.

Typical of anastomosing systems, the central, thick sand-stones commonly “wing-out,” a geometry present in channel sandstones 2 and 3 at TT (Fig. 5A). A sheet sandstone with poorly channelized flow, in a course with poorly defined banks, has a channel form that wedges out laterally without clear cutbanks (Hirst, 1991). Channels 2 and 4 at TT may be

of this type. This type of river channel commonly is filled with horizontally-laminated sandstones, as at the tops of channel 2 and part of channel 4 at TT. Hirst’s sheet sand-stones are the product of unconfined overbank flow, equiva-lent to the thin crevasse-splay sheet sandstones at TT and FP. These are wide relative to their thickness and lack cutbanks (Hirst, 1991).

The sequence at TT has predominantly ribbon-sand ge-ometry due to the stable river banks cut in mudstone. Bank collapse shed mud and caliche clasts that were incorporated in the basal lags during flood events (Fig. 7A). The moder-ately to well-developed caliche paleosols locally prevented river downcutting, resulting in avulsion from the shallow channel to a new one. This typically occurs when the chan-nel is filled to about 50% of the depth (Parker, pers. comm., 2002). At TT, small ponds and lakes dotted the landscape, yielding micritic limestone. Episodically, ash covered the landscape, so that at shallow burial depths some caliche and other paleosols were hardened to the silicified paleosols

Crevasse-splays (facies 10)

Channel sandstone

Volcanic ash bedCrevasse-splay deposit

Crevasse-splay deposit

2

MicriteCS

Channel sandstone (facies 7)Floodplain mudstone (facies 4)

Organic-rich, fossiliferous claystone-siltstone (facies 15)

Marlstone (facies 13)

Micrite (facies 2)

Carbonate nodules and caliche paleosols (facies 3 and 16)

Volcanic ash horizons (facies 1)

Facies developed on the floodplain Facies developed in/near the river

7

1

10 16

10

Caliche paleosol

Fig. 17. paleogeographic reconstruction of Brushy Basin anastomosing rivers at Trail Through Time (TT) and Fruita paleontological Area (Fp) and probably at echo Canyon (eC) (after Miall, 1985, river model 8)

100 Kenneth G. Galli

common at TT. Later, silica became more abundant, as evi-denced by the silica-replacement front(s) in channel sand-stone 4 (Fig. 8B) where the friable sandstone was converted to a hard silica-replaced sandstone and thus increasing the likelihood of preservation of dinosaur bones.

The anastomosing fluvial system of Figure 17 is similar to one for the Morrison strata in southwest Montana, where Cooley and Schmitt (1998) retrodict “a north flowing, low-gradient, mud and sand-dominated fluvial system.” Channel sandstones in their study area are “encased in extensive overbank-derived mudstone” as at TT and most of FP. They also interpret Morrison rivers as having rapid fluctuations in discharge.

Fluvial complex at Fruita Paleontological research area (FP)

Analysis of channel sandstone 2 at FP show the follow-ing abundant features: (1) low sandstone to fines ratio, (2) highly variable paleocurrents, (3) crevasse-splay deposits, (4) single channel geometry, (5) levee deposits, (6) fining-upward point-bar sequences, and (7) fining-upward channel fill. Common channel sandstone 2 features are: (1) mature paleosols, and (2) high sinuosity, and (3) highly bioturbated (stable vegetated islands) (Table 3). Even with FP’s excel-lent outcrop exposure, no lateral accreting or downstream accreting elements were observed within channel sandstone 2. This system is similar to the model for the Miocene Lower Freshwater Molasse of Switzerland (Platt and Keller, 1992).

I envision a small meandering river similar to the model 6, sand-bed meandering stream of Miall (1996) (Fig. 18). Abundant mudstone and caliche rip-up clasts in the basal lag deposits of channel sandstone 2 indicate that bank collapse was an important process. Although caliche paleosols are not as abundant at FP as at TT, the base of channel sandstone 2 directly overlies a 13-cm well-developed stage III-IV cali-che. The lowermost part of the FP channel sandstone 2 in Figure 14 contains most dinosaur bones.

Facies of channel sandstone 2 in common with the Low-er Freshwater Molasse are: levee, crevasse-channel, and crevasse-splay deposits, floodplain fines, and abandoned channels. Organic-rich, fossiliferous claystone and siltstone (facies 15) is locally abundant, and interpreted as a standing water body, such as an oxbow lake. The levee deposits de-scribed by Platt and Keller (1992) are a little larger than those at channel 2, which are on the order of centimeters thick, versus the 2–6 m-thick deposits of the Lower Fresh-water Molasse. The lateral extent is also smaller at FP.

It is possible that the north- to northeastward-flowing, meandering channel sandstone 2 represents a longitudinal trunk stream developed during the period of most persistent

humid climate and that the other channel sandstones at FP represent transverse streams that flowed directly away from the north-south oriented source area. Most of these other channel sandstones trend more or less from west to east. This arrangement puts channel 2 parallel to the north-south strike of the Morrison depositional basin. Kirkland (2006) interprets that the channel 2 river re-occupied the position of the channel 1 river. This is similar to a partially abandoned or avulsed channel of the Rio Colorado, Altiplano, Bolivia, that is seen to diverge away from the present-day channel (Donselaar et al., 2013). This is interpreted as a channel that was the direct precursor of the present-day active channel and was abandoned by avulsion.

It is possible that some of the other anastomosing streams may have fed other meandering streams that were either not preserved or were eroded away. Indeed, there is a “Ghost Channel” interpreted by Kirkland (2006) based on a map pattern of desert varnished individual boulders arranged in a pattern interpreted to be a ribbon river channel active be-tween the times of deposition of ribbon channel sandstones 9 and 10. These boulders possibly weathered out and are now found “high and dry” on the recent landscape.

Crevasse-splay deposits (facies 5, 10) were identified by concave-up channel floor (4th order surface) or nearly flat base, proximity to the main channel sandstone unit, and sheet-like bodies tens to hundreds of meters across and in some cases up to a kilometer, and approximately 10 or so cm thick that thin and pass laterally into element FF, overbank/floodplain fines. Both upward-coarsening and upward-fining successions may be present, indicating progradation or grad-ual abandonment, respectively (Miall, 1996). Therefore, channel sandstone 2 was deposited by a small, meandering, highly sinuous, river as inferred by the size of the oxbow-lake deposits, the crevasse-splays, and the spacing of the bends and their associated levee/crevasse-splay deposits.

The mass concentrations of dinosaur bones in channel sandstone 2 are present at the bends of the meandering river, evidently produced by a sequence similar to that described by Hubert and Panish (2000) for the formation of the dino-saur-bearing sandstone channel in the Morrison Formation on Dinosaur Ridge at Morrison, Colorado: (1) accumulation of bones along dry river bed during prolonged drought; (2) heavy rainfall produces flood waters that fill and overfill the channel and scour sand from its floor and the dinosaur bones and sand are transported down the river, along with pebble-sized mud and paleosol rip-up clasts; (3) rapid deposition of the sand and bones in the deeper parts of the channel, more likely to be preserved; (4) an interval of hours to a day of deposition results in the bones being covered within cross-bedded and/or horizontally-laminated sand, thus protecting them from decay or scavenging. Mass accumulations of di-nosaur bones in channel sandstones in the Morrison basin


occur at: (1) the outcrop at Dinosaur Ridge (Hubert, Panish, 2000); (2) the Carnegie quarry at Dinosaur National Monu-ment where there were two pulses to a flood or two closely-spaced floods (Bilbey et al., 1974; Lawton, 1977); (3) Dry Mesa Quarry 60 km south of Grand Junction, Colorado (Richmond, Morris, 1998); 4) the Bone Cabin quarry 16 km north of Como Bluff, Wyoming (Osborn, 1904); and 5) the Felch quarry at Garden Park, Colorado (Carpenter, 1998). A prolonged drought prior to deposition of channel sand-stone 2 is supported by the stage IV caliche that underlies the channel. This caliche has a nodular carbonate-plugged hori-zon about 13-cm thick, and required thousands of years of overall semi-aridity, a slow sedimentation rate, and seasonal precipitation.

The Brushy Basin Member at FP is unique in that it con-tains both dinosaur body fossils and theropod tracks (Kirk-land, 2006; Eriksen, 1976). The tracks are associated with the Main Callison Microvertebrate Quarry. Dinosaur tracks are also found at the bases of some channels (Engelmann, Hasiotis, 1999). Commonly, dinosaur-bearing formations contain either dinosaur tracks (as in the Portland Formation, Hartford Basin) or dinosaur bones, but not both. Possibly, many of the Brushy Basin subenvironments such as the pond/marsh facies (Kirkland, 2006) developed within is-lands that existed between the strands of the anastomosing

channels (Makaske, 2001). For example, the morphology of alluvial islands is a feature found in the anastomosing river system of the Solimes River of the Amazon basin (Baker, 1978) where there was a “saucer-like” geometry due to the bounding natural levees. Kirkland (2006) found his pond/marsh facies to occur at FP, always accompanying an under-lying ribbon sandstone in the case of channel sandstones 1, 2, 3, 4, 7, 9, and 10 and that the facies association typified the lower two-thirds of the Brushy Basin Member at FP. Pos-sibly, this facies records evidence of islands that were subse-quently eroded away.

The Brushy Basin rivers, with the exception of channel 2 at FP, may have belonged to the organo-clastic system sub-type (Smith, Smith, 1980) of the type-1 cohesive-sediment anabranching rivers category of Nanson and Knighton (1996) and Knighton (1998). In this type of river system, shifting of channels occurs as a result of long-term, large, floodplain-scale via avulsion instead of the deposition or dis-section of within-channel bars (Knighton, 1998). The orga-no-clastic type of river was first described by Smith and Smith (1980) with respect to alluvial valleys near Banff, Alberta, Canada.

Ash-fall events were common throughout Brushy Basin time, especially during the deposition of the upper Brushy Basin Member. The possible correlation among the three

Oxbow lake in abandonedmeander (future clay plug) (facies 15)

Red-brown mudstone (facies 4)

Channelsandstone(facies 7)

Crevasse splays(facies 5, 10)

N

5

00 10

M

M

Facies 3, 16

Mud and caliche intraclasts form by bank collapse

Overbank muddeposits

Channel sand deposits

Crevasse splay/levee deposits

Ferns

Cycads

Roots

Soil horizon

Scouring

Conifers

Burrows

Carbonate nodules

2-D dunes (Sp)

3-D dunes (St)

Scours

Fig. 18. Meandering river reconstruction of Brushy Basin Member Channel Sandstone 2 at Fruita paleontological Area (Fp) (after McInerney, 1993, similar to Miall, 1996, river model 6: sand-bed meandering stream)


TrailThrough

Time

Ch 1 level

Ch 2

Ch 3

Ch 4

Ch 5

K-1

green-gray /variegatedred-brown

0

10

20

30

40

50

60

70

79

3

5

C I

EchoCanyon

C I

0

10

20

30

50

60

70

40

Top - 76K-1

FruitaPaleontologicalResearch Area

Ch 7

Ch 2

Ch 1

Ch 6

0 meters

10

20

30

50

60

40

70

80

90

100Top - 101K-1

LL

LL

L

LL

LLL

III

I L

II NC

NCNC

NC caliche

Thick

er, m

ore m

ature

calic

he pa

leoso

ls

- III

I

I

I

I

II

II

II

II

II

II

I - II

I - II

ASH I

silica-replaced limestones

ASH IV

HORIZON EQUIVALENT

Uppe

r Bru

shy

Basin

Mem

ber

COLORCHANGE

WEST EAST

16 km 23 km

ASH III

ASH II

ASH IV

ASH IV ASH IV

ASH III

ASH I

COLORCHANGE

North

60 km40200

CNM

GrandJunction

I-70

Colorad

o Rive

r

Fruita

FP

TT

EC

ASH II

567

8

910

11

12

11

10

13 13

9

8

7

432

1

14

5

3

2Lowe

r Bru

shy

Basin

Mem

ber

112

3

4

67

8

10

5

4

6

ASH III ASH II

12II - III

II - IIIIASH HORIZON I

~ quarry level

I

Covered intervalCI

L Micritic lake beds

II Stage of paleosol developmentNumber of paleosol horizon11

Noncalcareous paleosolNC

9

Fig. 19. Correlation among stratigraphic sections in this study

see Figure 2 for legend to facies patterns. The color change at 31 m at FP is the basis for correlation of: (1) color change, (2) zones of the channel sandstones, (3) paleosol horizons, (4) volcanic ash beds, and (5) positions of some thin lake beds


sections using the distinctive four volcanic ash bed zones is significant. It is probable that the capacity of some of the Brushy Basin rivers was taxed due to their becoming over-whelmed with fine ash. The distance west to the volcanic belt was about 800 km (Turner, Peterson, 2004). The signifi-cant increase in both the number of and the thickness of ash beds and puffy-weathering ashy mudstones of the upper Brushy Basin strata probably reflect an increase in volcan-ism at that time (Galli, 2003). Most bentonites similar to those observed are produced due to diagenetic alteration of the silicic vitric fallout ash (Slaughter, Earley, 1965). Indeed, Newell (1997) interpreted Brushy Basin sandstones of FP as being formed by suspended-load rivers where their carrying capacity was overwhelmed due to the large amount of vol-caniclastics coming in as both air-fall ash on floodplains and channels and that carried by the streams.

Avulsions were an important process by which the Brushy Basin rivers sculpted their floodplains and spread their load, especially during flooding events. Kirkland (2006) and Newell (1997) both retrodict that major flooding events were important in producing the observed levee-cre-vasse-splay systems associated with channel 2 at FP. Cre-vasse splay deposits were produced as sediment-choked stream water overtopped the levees and spread across the floodplain. In the process, they formed basal erosional sur-faces as well as built up the levees so that, eventually, the rivers stood higher the adjacent floodplain (Kirkland, 2006). Some aspects of the present-day Rio Colorado, Bolivia seem to be quite similar to those observed or inferred for the Brushy Basin rivers. In the Rio Colorado, near an avulsion point in the main river, the height of the abandoned-channel floor is 30- to 50-cm above the present-day river and it has a covering of sand with current ripples (Sr) that indicate a flow direction that is away from the present-day river (Donselaar et al., 2013). It is quite possible that the anasto-mosing rivers of the Brushy Basin Member are dryland riv-ers like the Rio Colorado. These rivers can occur in a wide range of climates, from arid to sub-humid (Donselaar et al., 2013). The low vertical accommodation space of the Brushy Basin Member is similar to that observed for the Rio Colo-rado, Bolivia, where it is thought to have caused a thin series of successive meandering-channel belt deposits, forming lat-erally to each other due to repeated channel avulsions (Don-selaar et al., 2013). At FP, perhaps all channel sandstones formed in a similar manner.

Fluvial complex at Echo canyon (Ec)

Echo Canyon channel sandstones are less accessible than those present at TT and FP. A channel sandstone that may be channel 1 within the Brushy Basin Member or the top sand-

stone of the Salt Wash Member is located high above the dry wash at the base of Echo Canyon. The lens shape and the channel width is estimated to be 3–5-m at its thickest, thin-ning to 1.5–2.5-m thick at either end. The channel is under-lain by floodplain mudstones and three crevasse-splay de-posits that are most likely related to flood events associated with this channel. There is a possible paleovalley that has two to three channel sandstones within it. The lens shape and low sandstone to fines ratio at EC also argue for an anasto-mosing stream system. However, the shape and size of this channel could also be produced perhaps by a meandering stream oriented longitudinally with respect to the viewer.

regional Interpretations and probable correlations

The three localities provide a nearly east-west cross-sec-tion orthogonal to the N–S-trending source area to the west. The correlations shown in Figure 19 for TT and FP are con-sistent with those of Kirkland (2006).

pAleOgeOgRAphIC ReCONSTRUCTION

FLuvIAL StrAtA

Most of the channel sandstones formed as anastomosing rivers that crossed muddy floodplains. These sandstone bod-ies are typically 1–3 m thick but reach 5 m; most are single story and do not meander where plan view can be seen. The abundant pebbles of floodplain red-brown mudstone and caliche paleosols along the channel floors show that erosion and collapse of the mud banks were common. No lateral ac-cretion surfaces were found and the channels have a non-systematic fill of lithofacies. Meander-channel sandstones are rare, but channel sandstone 2 at FP is the most prominent example. This sandstone body has a sinuous outcrop pattern and small channel forms that typically fine up from medium/coarse to fine gravel size to fine/very fine-grained sand. Lev-ee and crevasse-splay units are associated with the channel bends.

In both the lower and upper divisions, the mudstones can be definitively assigned a floodplain origin rather than playa/playa-lake origin because: (1) they are mostly non-sandy and rarely laminated; (2) where sandy, the sand content decreas-es away from channel sandstones and levee deposits; (3) the levee bodies thin and fine away from the channel sandstones; and (4) ripple cross-lamination is rare.

The Brushy Basin paleo-landscape was an open plain with scattered vegetation and distinct wet and some pro-longed dry seasons, and where the trees and shrubs were concentrated along the river courses. Dinosaurs roamed the


riparian ecosystems and rare scattered bones are preserved in anastomosing-river channel sandstones. Major accumula-tions of bones are restricted to the pools at three bends of meandering-river channel sandstone 2 at FP where preserva-tion potential was enhanced.

PalEoclimaTE

In general, the paleoclimate during accumulation of the Brushy Basin strata was semi-arid to sub-humid, but there undoubtedly were many short and long-term fluctuations in precipitation. In Late Jurassic time, the study area was slightly south of its present latitude, with strong westerlies. The variegated floodplain mudstones of the upper division contain numerous non-calcareous and calcareous paleosols that vary from incipient scattered calcite nodules to stage III caliche. The caliche horizons are best developed at TT and FP, and to a lesser extent at EC. The caliche paleosols imply semi-aridity and seasonal precipitation, particularly for the upper division.

A 70-cm cherty limestone near the top of the Brushy Ba-sin at Dinosaur National Monument was cited as an occur-rence of Magadiite-type chert (Bilbey, 1991). However, this unit contains stromatolites and ostracods, and is evidently a lacustrine limestone with volcanic ash that altered to smec-tite. A 50- to 70-cm silicified limestone near the top of the Brushy Basin sections examined in this study is also a mic-ritic limestone partly replaced by silica. Furthermore, the clay-mineral assemblages in the floodplain mudstones, cali-ches, and micrites do not contain evaporite-type minerals, for example, zeolites, Mg-smectite, palygorskite, and sepio-lite. Due to the small amount of illite, the proportion of the 1Md soil illite was not determined (Srodon, Eberl, 1984). However, most illite is degraded, presumably 1Md. Detrital kaolinite was a minor detrital clay. No arid-region, Maga-diite-type chert was found in any of the strata. This precludes an arid climate interpretation.

CONClUSION

The 85-m Brushy Basin Member of the Morrison Forma-tion in and around Fruita, Colorado is dominated by a 30-m lower division dominated by red-brown mudstone and a 55-m upper division dominated by variegated smectitic mudstones of floodplain origin. The fluvial system may have been similar to a dryland river system that commonly is de-posited under conditions of very low gradient and accomo-dation space, within a back-bulge basin. River-channel sand-stones are embedded in the mudstones and tend to have cut

downward to resistant caliche paleosol horizons. Four zones of altered volcanic ash appear at approximately the same stratigraphic level in all three sections; these may allow correlation among Brushy Basin strata within western Colorado.

Abundant pebbles of floodplain red-brown mudstone and caliche paleosols along the channel floors show that erosion and collapse of the mud banks were common. No lateral ac-cretion surfaces were found and the channels have a non-systematic fill of lithofacies. In both the lower and upper di-visions, the mudstones are of floodplain origin rather than playa/playa-lake origin. Features diagnostic of playa/playa-lakes are absent.

The overbank fines (FF) architectural element is the dominant element at all localities and contains the majority of the observed lithofacies. Major sandstone ribbon element (CHR) is the next most prevalent element and contains all lithofacies of the channel sandstones with a straight geome-try and exhibiting very few fining upward cycles. Most of these channel sandstones are found in the lower Brushy Ba-sin Member. Channel sandstone 2 at FP is the only instance of the major channel sandstone element (CH). Both CHR and CH elements grade or pinch out into the associated FF element. Minor sandstone sheet element (CS) as seen by crevasse-splay and levee deposits are common at FP an fair-ly common at TT and less so at EC.

The Brushy Basin paleo-landscape was an open plain with scattered vegetation and distinct wet and potentially prolonged dry seasons, and where the trees and shrubs were concentrated along the river courses. Dinosaurs roamed the riparian environments and rare scattered bones are preserved in anastomosing-river channel sandstones. Major accumula-tions of bones are restricted to the pools at three bends of meandering-river channel sandstone 2 at FP where preserva-tion potential was enhanced. In general, the paleoclimate during accumulation of the Brushy Basin strata was semi-arid to subhumid.

Acknowledgments. The author thanks John F. Hubert and Richard Yuretich (University of Massachusetts Am-herst), and James I. Kirkland, Harley Armstrong and David Wolny for assistance in the field. Funding for this research came from the American Federation of Mineralogical Socie-ties and the Radke Award, Fierman Award and Hall Awards from the University of Massachusetts Amherst, Department of Geosciences. Chief Rangers Hank Schok and Patrick Per-rotti provided logistical support at Colorado National Monu-ment. This manuscript was markedly improved by careful editorial guidance from Lawrence Tanner and an anonymous reviewer.


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