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LEG SITE CORE

OBSERVER

SECSUB

HOLE

TYPE

SECTION DESCRIPTION

VISUAL CORE DESCRIPTION

SEDIMENTS / SEDIMENTARY ROCKS

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FM7000 These data are to be processed into a computerized data base along with existing standardized data from other legs and will be accessibleto the scientific community at large. RECORD ALL MEASUREMENTS CAREFULLY, COMPLETELY, AND LEGIBLY.

Leg Site Hole Core Type SecTop

Observer

Bottom

Interval (cm)

Sediment/RockName

Smear Slide Thin Section Coarse Fraction Grain Mount Dominant Lithology Minor LithologyPercent Texture

SEDIMENT SMEAR SLIDE/THIN SECTIONWORKSHEET

Sand Silt Clay

Percent

Comments:

Component Percent Component

MINERAL

ROCK

Accessory Minerals

BIOGENIC

Algae

Bioclasts

Coral

Diatoms

Dinoflagellate

Ebridians

Fish Remains

Foraminifers

benthic foraminifers

planktonic foraminifers

Nannofossils

Coccolith

Discoasters

Organic Debris

Pollen

Spores

plant debris

Pteropod

Radiolarians

Silicoflagellates

Sponge Spicules

siliceous sponge spicule

Carbonate

calcite

dolomite

inorganic calcite

organic calcite

Clay

Feldspar

plagioclase

Fe-oxide

Glauconite

Mica

biotite

muscovite

Opaque Minerals

Quartz

Volcanic Glass

Zeolite

Bioclasts

Carbonate Grains

Cement

Clasts

Micrite

Ooids

Rock Fragments

Shell Debris

Volcanic Ash

clinoptilolite

phillipsite

From: http://ship-web.ship.iodp.tamu.edu/publications/201_IR/chap_05/chap_05.htm

Example graphic symbols used by sedimentologists for core description.

CO

RE D

ESC

RIP

TIO

NS

SM

EA

R SL

IDE

S, SIT

E 1256284

Sample Grainsize (vol%) Non-biogenic(vol%)

Biogenic (no%)

CommentsCo

re

Typ

e

Sect

ion

To

p (

cm)

Dep

th (

mb

sf)

San

d

Silt

Cla

y

Gla

ss

Oth

ers

Dia

tom

s

Fora

min

ifer

s

Nan

no

foss

ils

Rad

iola

rian

s

Sili

cofl

agel

late

s

Spo

ng

e Sp

icu

les

Hole A1 H 1 65 0.65 1 49 50 2 98 10 0 85 3 2 0 Nannofossil-rich silty clay1 H 2 50 2.00 2 40 58 2 98 40 3 45 8 4 0 Nannofossil-diatom-bearing silty clay

Hole B1 H 4 60 5.10 5 25 70 1 99 23 1 68 6 2 0 Nannofossil-rich silty clay2 H 2 113 8.70 20 16 64 0 100 9 0 89 1 0 1 Nannofossil-rich silty clay3 H 2 35 17.45 90 8 2 96 4 0 0 0 0 0 0 Clear glass shards, quartz, feldspar, opaque minerals3 H 5 83 22.43 20 16 64 2 98 5 1 90 2 0 2 Nannofossil-rich sandy clay4 H 4 100 30.60 20 20 40 2 85 0 0 10 5 0 Diatom-bearing silty clay5 H 1 122 35.82 30 55 15 2 98 6 0 89 2 2 1 Nannofossil-rich sandy silt5 H 6 93 43.03 30 60 10 1 99 9 0 86 10 2 2 Nannofossil-rich sandy silt6 H 3 60 47.70 40 40 20 1 99 10 0 88 1 0.5 0.5 Nannofossil-rich sandy silt7 H 3 55 57.15 20 60 20 2 98 32 1 62 1 6 0 Nannofossil-rich sandy silt8 H 2 67 65.27 15 60 25 2 98 12 0 83 1 3 1 Nannofossil-rich clayey clay, including pumice, platy and cusfate glass shards9 H 2 80 74.90 10 25 65 1 99 4 0 91 0 2 3 Nannofossil-rich silty clay10 H 4 100 87.60 40 35 25 1 99 52 0 40 1 5 2 Nannofossil-rich sandy clay12 H 5 52 107.62 0 0 0 100 0 20 0 75 0 0 5 Pebble of vesicular brown glass13 H 2 102 112.42 85 0 15 0 0 0 Diatomite13 H 2 106 112.46 78 0 20 0 2 0 Diatomite14 H 4 80 125.40 5 0 95 0 0 0 Nannofossil ooze15 H 3 40 133.00 Diatom-nannofossil ooze. (Not examined)15 H 4 57 134.67 1 60 39 1 99 12 0 86 0 1 1 Diatom-nannofossil-filled burrows. Nannofossil ooze15 H 5 129 136.89 1 39 60 0 100 17 0 77 1 2 3 Contain 1-5% plagioclase crystals. Nannofossil ooze16 H 3 141 143.51 0 15 85 0 100 Opaque minerals (sulfide or pyrite?). Eustress-black at binocular16 H 3 142 143.52 Nannofossil silty clay. (Not examined)17 H 3 112 152.72 5 50 49 tr 100 10 0 87 0 2 1 Phyllosie. Nannofossil ooze19 X 2 53 162.13 5 30 65 0 100 20 0 78 0 1 1 ca. 1% plagioclase. Nannofossil-rich silty clay20 X 5 102 172.22 5 20 75 7 0 92 0 1 0 Nannofossil-bearing clay21 X 2 96 177.26 1 19 80 13 0 83 0 2 2 Nannofossil ooze22 X 2 46 185.96 20 40 40 4 0 96 0 0 0 ca. 1% plagioclase. Nannofossil ooze23 X 3 97 197.67 8 10 82 0 100 Clay24 X 3 80 207.20 Silty clay

SS1

Core Photo

C

ORE

D

ESCRIPTIONS

V

ISUAL

C

ORE

D

ESCRIPTIONS

, S

ITE

1256 3

Site 1256 Hole B Core 2H Cored 6.1-15.6 mbsf

1

2

3

4

5

6

7

8

9

ME

TER

S1

23

45

67

SE

CTI

ON

GR

AP

HIC

LIT

H.

STRUCTURE ICH

NO

.

BIO

TUR

B.

. .

ol GY

BR

ol GY

BR

gy BR

BR

ol GY

BR

ol GY

CO

LOR

DIS

TUR

B.

IW

IW

IW

SS

IW

IW

IW

IW

IW

IW

IW

IW

IW

IW PAL

SA

MP

LE

CLAYEY NANNOFOSSIL OOZE WITH SAND

10YR 5/4

Major Lithology: Slightly to extensively bioturbated CLAYEY NANNOFOSSIL OOZE WITH SAND with minor volcanic glass, diatoms, and radiolarians. The sediment color alternates between predominantly brown (10YR 5/3) and olive gray (5Y 5/2) with substantial mottling due to bioturbation.

General Description:TRACE FOSSILS: Bioturbation generally occurs as solid burrows, some are tentatively identified as Planolites and Skolithos (?).

Concretions occur in Section 2 at 100 cm and in Section 7 at 10 cm.

DESCRIPTION

mess

age openfile IMAGES/1256B2H.PDF1256B-2H

6.1-15.6 mbsf

SHIPBOARD SCIENTIFIC PARTYCHAPTER 3, SITE 1256 22

THE SEDIMENTARY OVERBURDEN(HOLES 1256A, 1256B, AND 1256C)

Lithostratigraphy

Hole 1256A is a single mudline core that recovered the uppermost2.37 m of the sedimentary section. This interval was resampled with asecond mudline core in Hole 1256B, and coring continued in sedimentin this hole down to a depth of 250.70 mbsf, where basement was con-tacted, with roughly 1 m more penetration in and ~5 cm of recovery ofthe uppermost basaltic basement. The lower portion of the sedimentarysection was resampled in the rotary-cored Hole 1256C from a depth of220.10 mbsf down to 245 mbsf. The combination of these three holesprovides a nearly complete sampling of the entire sedimentary se-quence above basement at Site 1256. The sediments are clay rich in theupper 40.6 m and become increasingly calcareous with depth, with cal-careous nannofossils being the dominant component below 40.6 mbsf,except for some minor more siliceous and diatom-rich intervals (Fig.F17).

Visual observations on the recovered core have been integrated withmagnetic susceptibility, density, and color measurements, which areroutinely determined during core processing. These data were com-bined with normal sedimentological criteria (smear slide analysis, sedi-mentary structures, color, bioturbation, and general appearance) and X-ray diffraction (XRD) results to divide the sediment column into litho-logic units. A composite graphical log of the complete sedimentary se-quence at Site 1256 (Fig. F18) shows the relationships between sedi-ments and the continuously measured parameters.

The primary density contrast for biogenic oozes is between biogeniccalcite and biogenic silica. The density (!) of calcareous nannofossil–rich sediment (e.g., ! = 1.4–1.6 g/cm3 in Cores 206-1256B-14H through15H) is greater than that of sediments with a high biogenic silica con-tent (e.g., ! = 1.1–1.3 g/cm3 in Core 206-1256B-10H). Thus, these con-tinuously recorded gamma ray attenuation (GRA) bulk density mea-surements provided a first-order estimate of carbonate content, whichsupplemented our direct observations from smear slides and chemicalanalyses (see “Inorganic Geochemistry,” p. 36).

The sediments from Site 1256 are divided into two principal litho-logic units (Fig. F17; Table T5). Unit I is a clay-rich unit with a few car-bonate-rich intervals, and Unit II is predominantly biogenic carbonate.The unit boundaries are based primarily on the relative clastic and bio-genic component concentrations, as characterized through smear slideanalysis, visual inspection of the core, and changes in color reflectanceand physical properties. The subunit boundary within Unit I was basedon biostratigraphic and magnetostratigraphic constraints, particularlythe location of the Pliocene/Pleistocene boundary, and changes inphysical properties.

Description of Units

Unit I

Interval: Core 206-1256A-1H; Sections 206-1256B-1H-1 through5H-4

Depth: Hole 1256A: 0–2.37 mbsf; Hole 1256B: 0–40.6 mbsfAge: Pleistocene to late Miocene

-

--

IA

IB

II

Dark brown to yellowishbrown nannofossil silty claywith diatoms and traceforaminifers, radiolarians,and volcanic glass

F17. Lithostratigraphic column, p. 119.

IA

IB

II

40 60 80 40 6020 1.2 1.4 1.6 1.8Hol

e 12

56A

Hol

e 12

56B

Hol

e 12

56C

Epo

ch

Age

(M

a)

Lith

. uni

t

Lith

olog

y GRA density(g/cm3)

Susceptibility(x ~10-5 SI)

Color(% reflectance)

F18. Composite summary, p. 121.

T5. Sedimentary overburden units, p. 338.


The clay-rich sediments of lithologic Unit I are divided into two sub-units. Subunit IA consists of Pleistocene dark brown to yellow-brown(7.5YR 3/2 to 10YR 5/4) silty clays. The dominant biogenic componentsare calcareous nannofossils, which compose 10%–25% of the sediment(Fig F19). Subunit IB comprises Pliocene to late Miocene sandy clays tosilts with calcareous nannofossil–rich intervals.

Subunit IAInterval: Core 206-1256A-1H; Sections 206-1256B-1H-1 through

interval 206-1256B-3H-2, 38 cmDepth: Hole 1256A: 0–2.37 mbsf; Hole 1256B: 0–17.48 mbsfAge: Pleistocene

Subunit IA comprises ~17.5 m of nannofossil silty clay and clayeynannofossil ooze. The proportions of clay, silt, and sand are variable,and there is a general decrease from 60% nonbiogenic material at the topof the sequence to 40% at the base of the subunit (see “Site 1256 SmearSlides”). Few radiolarians are found in the subunit, but diatoms general-ly form a minor component (up to 10%). Trace volcanic glass is also ob-served through the subunit. XRD analyses indicate that the clay-rich in-tervals are dominated by smectite (Table T6).

A volcanic ash layer consisting predominantly of colorless volcanicglass with common quartz and feldspar is present in interval 206-1256B-3H-2, 34–36 cm (Fig. F20). This ash layer may possibly be corre-lated with a similar ash layer identified at Site 844 in interval 138-844B-1H-1, 120–126 cm (Mayer, Pisias, Janecek, et al., 1992).

Dominant colors are shades of yellow brown and olive gray that aremottled throughout (10YR 5/4 to 10YR 5/3 and 5Y 5/2). The uppermost1.5 m of the subunit is very dark brown to dark mottled brown (7.5YR2.5/2 to 7.5YR 3/2), which is considerably darker than the rest of thesedimentary sequence. Subunit IA has the lowest reflectance of all ofthe sediments at Site 1256.

Bioturbation is common to abundant throughout. The most com-mon trace fossils are Planolites with tentative identification of Chon-drites and Skolithos. Calcified burrows are present in Sections 206-1256B-2H-2 at 100 cm, 2H-7 at 10 cm, and 3H-1 at 55 cm.

The base of Subunit IA is taken as the Pleistocene/Pliocene boundary(see “Hole 1256B,” p. 27, in “Calcareous Nannofossils” in “Biostratigra-phy”). At this subunit boundary, the observed change in the relativeconcentrations of clastic and biogenic components is also apparent inthe color reflectance and magnetic susceptibility data.

Subunit IBInterval: 206-1256B-3H-2, 38 cm, through Section 206-1256B-5H-4Depth: 17.48–40.6 mbsfAge: Pliocene to late Miocene

Subunit IB consists of clayey nannofossil ooze, sandy silty clay, andsandy silty nannofossil ooze. The subunit is characterized by a coarserclastic component than Subunit IA. Diatoms are a minor component ofthe sediment. As in Subunit IA, radiolarians are sparse, although they aremore abundant at the base of Subunit IB.

Dominant colors vary with lithology. The nannofossil oozes are typi-cally light olive gray grading to pale brown (5Y 6/2 to 10YR 6/3) at thebase of the subunit, with bioturbation resulting in significant mottling.Intervals dominated by clastic components are darker, in particular Sec-

Sand(%)

Silt(%)

Clay(%)

Nannofossils(%)

Diatoms(%)

Foraminifers(%)

Radiolarians(%)

0 5 10100 50 40 800 40 800 400 400 400

50

100

150

200

250

Dep

th (

mbs

f)

Cumulative(%)

0 20 40 60 80 1000

50

100

150

200

250

Dep

th (

mbs

f)

Sand

Silt

Clay

Diatoms

Radiolarians

Nannofossils

Foraminifers

Other

F19. Smear slide data summary, Hole 1256B, p. 123.

T6. XRD results, Hole 1256B, p. 339.

cm

35

40

45

30

25

F20. Ash layer, p. 124.


tions 206-1256B-4H-2 through 4H-7. The clastic-rich sediment is olivegreen to yellowish brown (5Y 5/2 to 10YR 5/4). Localized color banding(olive; 5Y 4/3) is seen in Sections 206-1256B-4H-2 and 4H-4 through4H-CC. The color reflectance in Subunit IB increases gradually downthrough the sequence, with a sharp step increase marking the subunitbase.

Bioturbation is moderate to abundant throughout. Solid burrows,Planolites, Skolithos, and Zoophycos (Fig. F21) are all common. The top ofSections 206-1256B-5H-1 and 5H-4 contain good examples of Zoophycoscutting other solid burrows. A large (4 cm diameter), continuous calci-fied burrow intersects the split surface of Section 206-1256B-4H-1 at 15,35, and 43 cm.

The magnetic susceptibility of Subunit IB is similar in magnitude tothat of Subunit IA, but with much less variability. A decrease in theabundance of clay-hosted magnetite grains associated with an increasein nannofossils marks the Unit I/II boundary, and this is reflected in asharp decrease in chromaticity (a* and b*) (see Fig. F22) at 40.6 mbsfand a decrease (from 30 to 10 in raw SI values) in magnetic susceptibil-ity in the interval 40–45 mbsf.

Unit IIInterval: Sections 206-1256B-5H-5 through 28X-CC; Core 206-

1256C-1R through Section 206-1256C-3R-CCDepth: Hole 1256B: 40.6–250.7 mbsf; Hole 1256C: 220.1–245.0

mbsfAge: late Miocene to middle Miocene

Lithologic Unit II comprises calcareous nannofossil ooze with varyingamounts of clay and other microfossil groups. Diatoms are a significantminor component. Foraminifers and radiolarians are rare throughoutthe unit. Volcanic glass is absent, although fragmented pumice clastswere identified in the smear slide taken from Section 206-1256B-8H-2 at67 cm.

The uppermost 35 m of the unit consists predominantly of diatom-nannofossil ooze, with the diatom component varying between 10%and 25%. These biogenic oozes are much paler than the overlying Sub-unit IB and are dominantly light greenish gray alternating with darkgreenish gray to dark gray (5GY 7/1 and 10Y 4/1 to 7.5YR 4/0), typicallyon a meter-length scale. In some sections there is localized color band-ing in varying shades of gray.

The diatom content of the sediment increases significantly between43 and 85 mbsf, resulting in a siliceous biogenic ooze at 85 mbsf. Thisclayey nannofossil diatom ooze is slightly darker (greenish gray to darkgreenish gray; 10Y 7/2 to 10Y 4/1) than the surrounding nannofossilooze and is lower in density.

With the exception of two intervals, the remainder of the sedimen-tary section is dominated by almost pure nannofossil ooze (up to 97%nannofossils). This ooze is light greenish gray to white (5G 7/1 to 5Y 8/1) with some dark greenish gray (5G 7/1) intervals and rare purple ordark bluish gray (5PB 4/1) to dark greenish gray intervals. Small darkbluish black blebs, which were observed throughout Core 206-1256B-21X (174.8 mbsf) and in all the deeper cores, are interpreted to be ironsulfide, probably pyrite.

Between 111 and 115 mbsf (Sections 206-1256B-12H-6 through 13H-5) is an unusual diatomite with nannofossils. It consists of a ~4-m-thickdiatom mat with a peculiar foliated texture resembling a pile of soggy

cm

10

15

20

5

0

F21. Zoophycos burrows, p. 125.

Carbonate (wt%)b*

Morebluish

MoreyellowishMore

reddish

Moregreenish

SusceptibilityD

epth

(m

bsf)

LithologicUnit IUnit II

0

50

100

150

200

40 1000 0.1250

-8 -4 0 4 8 0 5 10 15 20 0 20 40 0 20 8060

a*Intensity (A/m)

F22. Susceptibility, magnetic intensity, chromaticity, and car-bonate, p. 126.


telephone books (Fig. F23). The XRD analysis for this sediment con-firms it to be dominantly opaline sediment. The interval contains veryabundant tubular diatom tests, as observed in the smear slide. The diat-omite is predominantly light greenish gray to green gray (10Y 8/1 to10GY 5/1). It has a lower reflectance than the nannofossil ooze and ismarked by a significant step decrease in density and an increase in seis-mic velocity, a reflection of the open cage–like structure of diatoms.

The other anomalous interval spans 140–195 mbsf. In this intervalthe nannofossil ooze has significant but variable clay and diatom com-ponents. There is no obvious change in the visual appearance of thesediment in this interval, but the reflectance and density show greatervariability.

Bioturbation is common throughout Unit II. Trace fossils includeboth solid and rind burrows, including Planolites and Skolithos. The ma-jority of cores contain calcified burrows, generally at the top of Section1, and as such their location is considered an artifact of the drilling pro-cess.

Many of the cores also contain chert nodules, again commonly atthe top of Section 1. These chert nodules are generally dark greenishgray (10Y 3/1). Chert was first observed in interval 206-1256B-13H-2, 0–16 cm, at 111.40 mbsf. Distinct chert layers are identified at 111 and158 mbsf in the wireline logs of Hole 1256C and are characterized byincreased resistivity and low porosity (see “Results,” p. 51, in “Down-hole Logging” in “Downhole Measurements”). Only 20 cm of sedimentwas recovered in Core 206-1256B-27X, and it consisted of broken bitsof chert and chert nodules with a nannofossil ooze coating, indicatingthat a chert layer is present somewhere between 230 and 240 mbsf. Re-covery of the same interval in Hole 1256C was also extremely poor. Thechert layer at this level unfortunately could not be confirmed by down-hole logging because a constriction in the borehole prevented the de-ployment of wireline tools to this depth.

Granular glauconitic bands were identified in Sections 206-1256B-9H-3, 9H-7, 25X-4, and 26X-3. These bands are typically up to 5 mmwide and are dark greenish gray with angular mineral grains up to 2mm in diameter. In Section 206-1256B-26X-3 glauconite bands are off-set by a small fault, possibly induced during coring (Fig. F24).

The nannofossil ooze at the base of the sedimentary section has aturquoise hue, reminiscent of the covers of DSDP Initial Reports vol-umes. The X-ray diffractogram of this sediment showed it to be domi-nantly composed of mica, possibly celadonite (Table T6). It is specu-lated that this may relate to the presence of metalliferous sediment,with celadonite formed by the recrystallization of biogenic opal and hy-drothermally derived iron oxides.

In Core 206-1256C-4R, the first core in which igneous basement wasrecovered, two pieces of red-brown chert were cored. The cherts are si-licified recrystallized siliceous sediments with iron oxides, and theymay be recrystallized metalliferous sediments.

The color reflectance of Unit II is much less variable than that of Sub-unit IB, due to the much more homogeneous composition of the sedi-ments and the dominance of calcareous nannofossils. Variations indensity are attributed to variable relative concentrations of diatoms andnannofossils. The magnetic susceptibility throughout Unit II is low be-cause of the very low concentrations of magnetic minerals in calcareousoozes. Point-susceptibility meter measurements were discontinued be-low Core 206-1256B-18H (160.1 mbsf) because the readings were clearlyat the noise level of the meter, which has lower sensitivity than the

cm

106

98

96

100

102

104

F23. Diatom mat, p. 127.

cm

70

65

60

F24. Glauconite bands, p. 128.


whole-core loop meter. Whole-core susceptibility measurements on themultisensor track (MST) were continued down to basement, with read-ings commonly at the noise level of the meter. The presence of greenishblack (10G 2.5/1) blebs throughout Cores 206-1256-21X through 28X,identified as pyrite spots or other iron sulfides, suggests that any mag-netite has been converted to pyrite, which would explain the very lowsusceptibility and magnetic intensity of this lithologic unit (see “Sus-ceptibility,” p. 30, in “Paleomagnetism”).

Trace Fossils

The trace fossils encountered in the sediments are recorded in detailon individual barrel sheets. Bioturbation is common to abundantthroughout the majority of the sediments retrieved from Site 1256. Thetrace fossils typical of Unit I are solid burrows, commonly Planoliteswith minor Zoophycos and rare Chondrites. The carbonate-rich litholo-gies of Unit II contain common Planolites with Chondrites and Skolithos.Carbonate concretions up to 2 cm in diameter in Unit II are identifiedas calcified burrows.

Biostratigraphy

Sediments recovered above basaltic basement at Site 1256 during Leg206 provide a continuous sedimentary record from the Quaternarythrough the middle Miocene. Calcareous nannofossils were used for de-veloping the biostratigraphic framework using the zonation schemes ofMartini and Müller (1986) and Okada and Bukry (1980). Biostrati-graphic assignments were made to core catcher samples and additionalsamples selected to refine the biostratigraphy. The interval (core andsection) and depth (mbsf) constraints for calcareous nannofossil eventsrecognized at Site 1256 are reported in Table T7. The depths of epochboundaries are given in Table T8, along with the same boundaries fromSites 844 and 845. Nannofossil ranges from all core catcher samples aregiven in Table T9.

Calcareous Nannofossils

Calcareous nannofossils are generally abundant and moderately towell preserved at Site 1256. Nannofossil assemblages, however, are af-fected to different degrees by alteration, mainly by etching above Sam-ple 206-1256B-3H-CC, by dissolution and overgrowth below Sample206-1256B-13H-CC, and by fragmentation throughout the sequence.No significant reworking of nannofossils was apparent, and all nanno-fossil events are recognized in normal stratigraphic sequence in thesesediments. More than a dozen nannofossil datums were determinedbased on examination of core catcher samples and other selected sam-ples, providing modest biostratigraphic resolution for the Pleistocenethrough middle Miocene.

Hole 1256AHole 1256A consists of a mudline core only (0–2.17 mbsf) and con-

tains uppermost Quaternary nannofossil assemblages assigned to ZoneNN21. Calcareous nannofossils are common throughout the sequenceand are moderately preserved. The main assemblage is composed of pla-coliths (Emiliania huxleyi and Gephyrocapsa spp.) with a highly diversesubordinate assemblage. Reworked specimens, mainly discoasterids of

T7. Calcareous nannofossils, p. 340.

T8. Epoch boundaries, p. 341.

T9. Calcareous nannofossil distri-bution chart, p. 342.

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