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Scale 1:24,000

Contour Interval 40 feetSupplementary Contour Interval 20 feet

National Geodetic Vertical Datum of 1929

STATE OF CALIFORNIA – EDMUND G. BROWN JR., GOVERNORNATURAL RESOURCES AGENCY – JOHN LAIRD, SECRETARY FOR NATURAL RESOURCES

DEPARTMENT OF CONSERVATION – DAVID BUNN, DIRECTOR CALIFORNIA GEOLOGICAL SURVEYJOHN G. PARRISH, Ph.D., STATE GEOLOGIST PRELIMINARY GEOLOGIC MAP OF THE SANTA MARGARITA 7.5’ QUADRANGLE, CALIFORNIA

Copyright © 2015 by the California Department of Conservation,California Geological Survey. All rights reserved. No part ofthis publication may be reproduced without written consent of theCalifornia Geological Survey.

"The Department of Conservation makes no warranties as to thesuitability of this product for any given purpose."

PRELIMINARY GEOLOGIC MAP OF THE SANTA MARGARITA 7.5' QUADRANGLESAN LUIS OBISPO COUNTY, CALIFORNIA

VERSION 1.0By

Mark O. Wiegers and Earl W. Hart

Digital Database by

Mark O. Wiegers and Marc Delattre

2015

This geologic map was funded in part by the USGS National Cooperative Geologic MappingProgram, Statemap Award no. G14AS00006

35°30'120°30''

35°22'30''120°30'

35°30'

Projection: Universal Transverse Mercator, Zone 10N, North American Datum 1927.

Topographic base from U.S. Geological SurveySanta Margarita 7.5-minute Quadrangle, 1993.Shaded relief image derived from USGS 1/3 arc-second National Elevation Dataset (NED).

120°37'30”

120°37'30”35°22'30''

Professional Licenses and Certifications: M.O. Wiegers - PG No. 4157, CEG No. 1506

Preliminary Geologic Map available from:http://www.conservation.ca.gov/cgs/rghm/rgm/preliminary_geologic_maps.htm

(Paso Robles) - Adjacent 30’ x 60’ quadrangles

135

58

41229

227

166

46

1

101

SantaMaria5

Kilometers5

Miles

CAYUCOS

MORROBAYNORTH

MORROBAYSOUTH

PORTSAN LUIS

SAN LUISOBISPO

PISMOBEACH

ARROYOGRANDE NE

LOPEZMTN

TARSPRINGRIDGE

CALDWELLMESA

LOSMACHOSHILLS

BRANCHMTN

MIRANDAPINEMTN

HUASNAPEAK

NIPOMO

OCEANO

CHIMNEYCANYON

SANTAMARGARITA

SANTAMARGARITALAKE

WILSONCORNER

CAMATTARANCH

POZOSUMMIT

LA PANZARANCH

LA PANZA

CALIFORNIAVALLEY

LA PANZANE

ATASCADERO

(Pa(Pa(Pa(Paso sss Roblesles)))(Paso Robles)

(Paso Robles) - Adjacent 30’ x 60’ quadrangles

(T(T(T(T(TTaft)Taft)

(Sa(S(Sa(Sa(Sa(Sa(S((S(S(Santantntntantantaa MaMaMMaria)(Sa(Santan MaM ria)

Mapping completed under STATEMAP

Previous projects since 2008

Current FY 2014-15

121°0’0” 120°30’0” 120°0’0”

35°3

0’0”

35°0

’0”

REFERENCES

Chipping, D. H., 1987, The Geology of San Luis Obispo County, A Brief Description and Field Guide: unpublished field guide by Cal. Poly. geology instructor.

Clark, D. G., Slemmons, B., Caskey, J. S. and dePolo, D. M., 1994, Seismotectonic framework of coastal central California, in Alterman, I. B., McMullen, R. B., Cluff, L. S., and Slemmons, D. B, eds. Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292, p. 9-30.

Dibblee, T. W., 1974, Geologic Map of the San Luis Obispo 15-Minute Quadrangle, California: U. S. Geological Survey Open File Report 74-223, scale: 1:62,500.

Hall, C. A. , and Prior, S. W., 1973, Geologic map of the Cayucos – San Luis Obispo Region, San Luis Obispo County, California: U.S. Geological Survey Miscellaneous Field Studies, Map MF-686, scale 1:24,000.

Hart, E. W., 1976, Basic Geology of the Santa Margarita Area, San Luis Obispo County, California; California Division of Mines and Geology Bulletin 199.

McClean, H., 1994, Geologic Map of the Lopez Mountain Quadrangle, San Luis Obispo, California; U. S. Geological Survey Geological Quadrangle Map GQ-1723.

Page, B. M., 1972, Oceanic Crust and Mantle Fragment in Subduction Complex near San Luis Obispo, California: Geological Society of America Bulletin, v. 83, p. 957-972.

30

s30

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30

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Contact between map units - Solid where accurately located; dashed where approximately located; dotted where concealed

Fault - Solid where accurately located; dashed where approximately located; short dash where inferred; dotted where concealed; queried where identity or existence is uncertain. U = upthrown block; D = downthrown block

Thrust fault - Solid where accurately located; dashed where approximately located; dotted where concealed. Barbs on upper plate.

Synclinal axis - Solid where accurately located; dashed where approximately located; dotted where concealed. Arrow shows plunge direction.

Anticlinal axis - Solid where accurately located; dashed where approximately located; dotted where concealed. Arrow shows plunge direction.

Strike and dip of bedding plane.

Approximate strike and dip of bedding plane.

Strike and dip of overturned bedding

Vertical bedding

Shell bed

Line of cross section

MAP SYMBOLS

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DESCRIPTION OF MAP UNITS

SURFICIAL UNITS

Artificial fill (Historic)—Mapped locally, primarily small dam embankments for livestock ponds.

Alluvial flood plain and channel deposits (Historic to late Holocene)—Active and recently active flood-plain and channel deposits mapped along the Salinas River and the Middle and East Forks of Huerhuero Creek. Consist of unconsolidated, gray to brown silty sand and sandy gravel with cobbles, scattered boulders with occasional lenses of silty clay. Clasts include granitic rocks, volcanic porphyry, Franciscan greenstone and greywacke, quartzite and Monterey rocks. Bedding is interfingered and lenticular.

Landslide deposits (Holocene)—Small landslide deposits, widely scattered. Includes debris slide deposits consisting of fragmented mixtures of soil and rock and block slides consisting of relatively intact bedrock displaced along rotational or translational slip surfaces.

Young alluvial flood-plain deposits, undivided (Holocene to late Pleistocene)—Unconsolidated brown silty sand and sandy gravel with cobbles and interbedded lenses of silt and clay deposited on flood-plains and along valley floors. Clasts include granitic rocks, volcanic porphyry, Franciscan greenstone and greywacke, quartzite and Monterey rocks. Bedding is interfingered and lenticular.

Old alluvial flood-plain deposits (late to middle Pleistocene)—Fluvial sediments preserved above active flood plains and channels. Consists of brown interfingering beds and lenses of weakly-consolidated silty sand and sandy gravel with cobbles. Deposits contain abundant silt and clay in the matrix with clay coatings on clasts. Terrace surfaces preserved along the Salinas River and other drainages are slightly dissected and capped by moderate to well-developed pedogenic soils. Locally divided by relative age (2 = youngest, 1 = oldest):

Old alluvial flood-plain deposits, Unit 2

Old alluvial flood-plain deposits, Unit 1

TERTIARY ROCKS

Paso Robles Formation (Pliocene to lower Pleistocene)—Poorly-sorted conglomerate lenses set in a sandy and muddy matrix. Clasts range from sand-sized to boulders, are derived primarily from the Monterey Formation and include siliceous shale, chert, porcelinite, calcareous sandstone and dolomite. Clasts of volcanic, sedimentary and granitic rocks from other sources are also present. Some of the beds are poorly sorted with large boulders and are probably debris flow deposits. The unit also contains interbedded sandstone, claystone and mudstone. Some of the claystone contains freshwater diatoms and mollusks, indicating a probable lake origin (Hart, 1976). Pholad-bored clasts are present near the base, indicating the unit was deposited near a former marine shoreline. Locally includes:

Basal Member—A basal conglomerate member containing mostly well-rounded, well-sorted clasts of volcanic porphyry is mapped in the Chalk Hill area and in the region to the south (Hart, 1976). The sand beds and conglomerate matrix are arkosic and are almost devoid of silt and clay. The unit also contains large, angular pholad-bored clasts. The unit grades rapidly upward into typical Paso Robles sediments that consist primarily of Monterey debris.

Santa Margarita Formation (upper Miocene)—White, weakly-consolidated, coarse arkosic sandstone. Includes interbedded mudstone, siltstone and diatomite, mostly in the lower part of the unit. Resistant shell beds contaning pecten and oyster shells are locally present, possibly representing storm-constructed masses similar to the chenier ridges of the modern Gulf Coast (Chipping, 1987). Sandstones contain pebble and small cobble lenses with clasts of volcanic rocks, siliceous Monterey debris, granitic rocks, diabase, chert and quartz. Mostly confined to the synclinal trough on the west side of the the Rinconada Fault (Hart, 1976).

Monterey Formation, undivided (upper to middle Miocene)—Predominately siliceous or calcareous shale, mudstone, siltstone, and diatomite. In the mapped area, Hart (1976) subdivided the Formation into an upper siliceous member and a lower, more calcareous member:

Upper Member—Light gray and tan, often rhythmically-bedded porcelinite, opaline chert, mudstone shale, siltstone, diatomite and tuff. Porcelinite is hard and brittle with a dull luster. Opiline chert is subvitreous with a waxy luster. Unit includes white to light-gray arkosic, locally pebbly sandstone. Nearly white diatomite is present near the top of the unit north of Santa Margarita.

Lower Member—Well bedded foraminiferal and calcareous shale, mudstone and siltstone with some interbedded siliceous shale, porcelinite and sandstone. Contains minor amounts of conglomerate, limestone and dolomite. Rocks are generally gray-brown where fresh and weather to lighter shadows of buff and pale gray. Contains some arkosic sandstone beds up to 20 feet thick.

Vaqueros Formation (middle Miocene to Oligocene)—Predominantly coarse-grained marine sedimentary rocks exposed on the east side of the Rinconada Fault. The unit varies in thickness and lithology from north to south in the map area. The northern facies is 20 to 100 feet thick and consists of hard, thick-bedded calcareous sandstone and pebbly sandstone that grades downward into a basal conglomerate. The southern facies is 200 to 700-plus feet thick and includes poorly-stratified sandstone, conglomerate, siltstone, mudstone and limestone. Sandstone beds in the southern facies typically consist of white to orange arkose that is generally friable or uncemented except for silicified zones close to the Rinconada Fault. Contains fossiliferous zones with broken mollusk shells and large oysters.

Simmler(?) Formation (lower Miocene to Oligocene)—Nonmarine, gray to brown, poorly-bedded conglomerate composed almost entirely of granitic clasts derived from granitic basement rocks on the east side of the Rinconada Fault. Locally includes sandstone, siltstone and claystone interbedded with conglomerate near the base of the unit. Much of the unit is massive and unsorted and was probably deposited by ancient debris flows. The sands are poorly sorted and likely are alluvial deposits. Some of the clay beds may have been deposited in a lacustrine environment. Tentatively correlated to the Simmler Formation mapped in the La Panza Range and Caliente Range east and southeast of the map area (Hart, 1976). The type section of the Simmler is about 60 miles to the southeast.

CRETACEOUS AND OLDER ROCKS

Atascadero Formation (Upper Cretaceous)—Light gray to dark olive gray, thin to thick-bedded sandstone with interbedded siltstone, mudstone and conglomerate interpreted to be turbidite deposits. Unit structurally overlies Franciscan rocks and the Toro Formation and is internally disrupted by faulting and shearing. Sandstones typically consist of quartz (30-40%), feldspars (30-50%), volcanic and lithic debris (10-30%) and biotite (2 -10%) Hart (1976). Conglomerates contain silicic and basic volcanics, quartzite and granitics and lack Franciscan debris. Also includes:

Lower unit—Dark mudstone and sandstone mapped in the southwest corner of the map area.

Toro Formation (Lower Cretaceous to Upper Jurassic)—Thin-bedded, greenish brown to brown micaceous shale interbedded with thin sandstone beds. Limited to a small outcrop area in the southwest corner of the quadrangle. Present only on the west side of the Rinconoda Fault. Buchia fossils indicate Late Jurassic to Early Cretaceous age.

Franciscan Complex mélange (Cretaceous to Jurassic)—Chaotic mixture of fragmented, fault-bounded rock masses embedded in a penetratively sheared matrix of argillite and crushed metasandstone. Individual rock masses include altered mafic volcanic rocks (greenstone), graywacke , chert, serpentinite and high grade blueschist. Forms the basement complex on the west side of the Rinconada Fault.

Unnamed granitic rocks (Cretaceous)—Primarily biotitic granodiorite and adamellite cut by large dikes of biotitic aplite and abundant small resistant dikes of aplite and pegmatite. Forms the basement complex on the east side of the Rinconada Fault. Rocks are generally deeply weathered and decomposed except where exposed in stream beds and in large quarries north of Santa Margarita. Most of the rocks are coarse-grained and porphyritic except aplite which is finer and more even grained. The dike rocks are abundant and represent various late stages of emplacement (Hart, 1976).

Unnamed metamorphic rocks (pre-Upper Cretaceous)—Dark gray foliated metamorphic rocks exposed in a small pendant in the northeast part of the quadrangle. The metamorphic rocks are weakly schistose to gneissic, enclosed by granitic rocks and cut by aplite and pegamatite dikes that also cut the surrounding granitics.

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QUATERNARY

TERTIARY

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JURASSIC

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

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CORRELATION OF MAP UNITS

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SEA LEVEL

1,000

2,000

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FEET

SEA LEVEL

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2,000

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FEETA A’Santa Margarita Syncline

RinconadaFault Zone

Nacimiento Fault Zone

HORIZONTAL & VERTICAL SCALE = 1:24,000

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