Dziewonski, A., Wilkens, R., Firth, J., et al., 1992Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 136

2. EXPLANATORY NOTES1

Shipboard Scientific Party2

INTRODUCTION

In this chapter we have assembled information that will help thereader understand the basis for our preliminary conclusions and alsohelp the interested investigator select samples for further analysis. Thisinformation concerns only shipboard operations and analysesdescribed in the site reports in the Initial Reports volume of the Leg136 Proceedings of the Ocean Drilling Program. Methods used byvarious investigators for shore-based analysis of Leg 136 data will bedetailed in the individual scientific contributions published in theScientific Results volume.

AUTHORSHIP OF SITE CHAPTERS

The separate sections of the site chapters were written by thefollowing shipboard scientists (authors are listed in alphabetical order,no seniority is necessarily implied):

Site Summary (Dziewonski, Wilkens)Operations (Firth, Harding)Lithostratigraphy (Naka, Tribble)Biostratigraphy (Firth, Hull)Paleomagnetism (Briden, Helsley)Inorganic Geochemistry (DeCarlo)Physical Properties (Carson, Collins, Janecek)Igneous Petrology (Alt, Garcia, Waggoner)Logging (Goldberg)Appendix (Shipboard Scientific Party)

At the end of the volume are summary core descriptions ("barrelsheets" and igneous rock visual core descriptions) and photographsof each core.

SURVEY AND DRILLING DATA

Geophysical survey data collected during Leg 136 include mag-netic, seismic reflection profiler, and bathymetric data acquired duringthe site survey immediately prior to drilling. These data are discussedin the "Underway Geophysics and Seismic Stratigraphy" chapter (thisvolume). During the Leg 136 JOIDES Resolution survey, single-chan-nel seismic, 3.5- and 12-kHz echo sounder, and magnetic data wererecorded across the planned drill site to aid site confirmation prior todropping the location beacon.

The single-channel seismic profiling system used two 80-in3 waterguns as the energy source and a Teledyne streamer with a 100-m-longactive section. These data were recorded digitally on tape using aMasscomp 561 computer, and were also displayed in real time inanalog format on two Raytheon recorders using a variety of filtersettings (commonly 30-150 Hz) and various vertical scales, depen-dant on the water depth.

Bathymetric data collected using the 3.5- and 12-kHz PrecisionDepth Recorder (PDR) system were displayed on two Raytheon

'Dziewonski, A., Wilkens, R., Firth, J., et al., 1992. Proc. ODP, Init. Repts., 136:College Station, TX (Ocean Drilling Program).

Shipboard Scientific Party is as given in the list of participants preceding the contents.

recorders. The depths were calculated on the basis of an assumed 1500m/s sound velocity in water. The water depth (in meters) at each sitewas corrected for (1) the variation in sound velocity with depth usingCarter's (1980) tables, and (2) the depth of the transducer pod (6.8 m)below sea level. In addition, depths referred to the drilling-platformlevel are corrected for the height of the rig floor above the water line(see Fig. 1).

Magnetic data collected using a Geometries 801 proton precessionmagnetometer were displayed on a strip chart recorder, and wererecorded on magnetic tape for later processing.

DRILLING CHARACTERISTICS

Because water circulation downhole is open, cuttings are lost ontothe seafloor and cannot be examined. Information concerningsedimentary stratification in uncored or unrecovered intervals may beinferred from seismic data, wireline-logging results, and from anexamination of the behavior of the drill string as observed andrecorded on the drilling platform. Typically, the harder a layer, theslower and more difficult it is to penetrate. A number of other factorsmay determine the rate of penetration, so it is not always possible torelate the drilling time directly to the hardness of the layers. Bit weightand revolutions per minute, recorded on the drilling recorder, alsoinfluence the penetration rate.

Drilling Deformation

When cores are split, many show signs of significant sediment distur-bance, including the concave-downward appearance of originallyhorizontal bands, haphazard mixing of lumps of different lithologies(mainly at the tops of cores), and the near-fluid state of some sedimentsrecovered from tens to hundreds of meters below the seafloor. Coredeformation probably occurs during cutting, retrieval (with accompany-ing changes in pressure and temperature), and core handling on deck.

SHIPBOARD SCIENTIFIC PROCEDURES

Numbering of Sites, Holes, Cores, and Samples

Drill sites are numbered consecutively from the first site drilled by theGlomar Challenger in 1968. A site number is used for one or more holesdrilled while the ship was positioned over one acoustic beacon. Multipleholes may be drilled at a single site by pulling the drill pipe above theseafloor (out of the hole), moving the ship some distance from the previoushole, and then drilling another hole. In some cases, the ship may return toa previously occupied site to drill additional holes.

For all ODP drill sites, a letter suffix distinguishes each hole drilledat the same site. For example, the first hole drilled is assigned the sitenumber modified by the suffix A, the second hole takes the site numberand suffix B, and so forth. Note that this procedure differs slightlyfrom that used by DSDP (Sites 1 through 624), but prevents ambiguitybetween site- and hole-number designations. It is important to distin-guish among holes drilled at a site, because recovered sediments orrocks from different holes usually do not come from equivalentpositions in the stratigraphic column.

The cored interval is measured in meters below seafloor (mbsf);sub-bottom depths are determined by subtracting the drill-pipe meas-

SHIPBOARD SCIENTIFIC PARTY

DRILLED(BUT NOT

CORED

Sub-bottom bottom

Represents recovered material

BOTTOM FELT: distance from rig floor to seafloor

TOTAL DEPTH: distance from rig floor to bottom of hole(sub-bottom bottom)

PENETRATION: distance from seafloor to bottom of hole(sub-bottom bottom)

NUMBER OF CORES: total of all cores recorded, includingcores with no recovery

TOTAL LENGTHOF CORED SECTION: distance from sub-bottom top to

sub-bottom bottom minus drilled(but not cored) areas in between

TOTAL CORE RECOVERED: total from adding a, b, c, and d indiagram

CORE RECOVERY (%): equals TOTAL CORE RECOVEREDdivided by TOTAL LENGTH OF COREDSECTION times 100

Figure 1. Diagram illustrating terms used in the discussion of coringoperations and core recovery.

urement (DPM) water depth (the length of pipe from the rig floor tothe seafloor) from the total DPM (from the rig floor to the bottom ofthe hole; see Fig. 1). Note that although the echo-sounding data (fromthe precision depth recorders) are used to locate the site, they are notused as a basis for any further measurements.

The depth interval assigned to an individual core begins with thedepth below the seafloor where the coring operation began, andextends to the depth where the coring operation ended for that core(see Fig. 1). For rotary coring (RCB and XCB), each coring intervalis equal to the length of the joint of drill pipe added for that interval(though a shorter core may be attempted in special instances). The drillpipe in use varies from about 9.4 to 9.8 m. The pipe is measured as itis added to the drill string, and the cored interval is recorded as thelength of the pipe joint to the nearest 0.1m. For hydraulic piston coring(APC) operations, the drill string is advanced 9.5 m, the maximumlength of the piston stroke.

Coring intervals may be shorter and may not necessarily beadjacent if separated by drilled intervals. In soft sediments the drillstring can be "washed ahead" with the core barrel in place, withoutrecovering sediments. This is achieved by pumping water down thepipe at high pressure to wash the sediment out of the way of the bitand up the annulus between the drill pipe and the wall of the hole.If thin, hard, rock layers are present, then it is possible to get"spotty" sampling of these resistant layers within the washed inter-val, and thus to have a cored interval greater than 9.5 m. In drillinghard rock, a center bit may replace the core barrel if it is necessaryto drill without core recovery.

Cores taken from a hole are numbered serially from the top of thehole downward. Core numbers and their associated cored intervals inmeters below seafloor usually are unique in a given hole; however,this may not be true if an interval must be cored twice, because ofcaving of cuttings or other hole problems. Maximum full recovery fora single core is 9.5 m of rock or sediment contained in a plastic liner(6.6-cm internal diameter) plus about 0.2 m (without a plastic liner)

Fullrecovery

—— I I . •i I — j — Top

Partialrecovery

Partialrecoverywith void

Sectionnumber

1

2

3

4

5

6

7

Sectionnumber

1

2

3

4

5

6

_££.

Voi

d Emptyliner

Top

Sectionnumber

1

2

3

4

~cc

Voi

d

7rr.

&

T3O

Sr

T

^ Tv: ~ ~ to T°P

Core-catchersample

Core-catchersample

Core-catchersample

Figure 2. Diagram showing procedure used in cutting and labeling core sections.

10

EXPLANATORY NOTES

in the core catcher (Fig. 2). The core catcher is a device at the bottomof the core barrel that prevents the core from sliding out when thebarrel is being retrieved from the hole. For sediments, the core-catchersample is extruded into a short piece of plastic liner and is treated asa separate section below the last core section. For hard rocks, materialrecovered in the core catcher is included at the bottom of the lastsection. In certain situations (e.g., when coring gas-charged sedimentsthat expand while being brought on deck) recovery may exceed the9.5-m maximum.

A recovered core is divided into 1.5-m sections that are numberedserially from the top (Fig. 2). When full recovery is obtained, thesections are numbered from 1 through 7, with the last section possiblybeing shorter than 1.5 m (rarely, an unusually long core may requiremore than seven sections). When less than full recovery is obtained,there will be as many sections as needed to accommodate the lengthof the core recovered; for example, 4 m of core would be divided intotwo 1.5-m sections and one 1-m section. If cores are fragmented(recovery less than 100%), sections are numbered serially and inter-vening sections are noted as void, whether shipboard scientists believethat the fragments were contiguous in-situ or not. In rare cases asection less than 1.5 m may be cut to preserve features of interest (e.g.,lithologic contacts).

By convention, material recovered from the core catcher is placedbelow the last section when the core is described, and labeled corecatcher (CC); in sedimentary cores, it is treated as a separate section.The core catcher is placed at the top of the cored interval in cases wherematerial is only recovered in the core catcher. However, informationsupplied by the drillers or by other sources may allow for more preciseinterpretation as to the correct position of core-catcher material withinan incompletely recovered cored interval.

Igneous rock cores are also cut into 1.5-m sections that are num-bered serially; individual pieces of rock are then each assigned anumber. Fragments of a single piece are assigned a single number, andindividual fragments are identified alphabetically. The core-catchersample is placed at the bottom of the last section and is treated as partof the last section, rather than separately. Scientists completing visualcore descriptions describe each lithologic unit, noting core and sectionboundaries only as physical reference points.

When, as is usually the case, the recovered core is shorter than thecored interval, the top of the core is equated with the top of the coredinterval by convention to achieve consistency in handling analyticaldata derived from the cores. Samples removed from the cores aredesignated by distance measured in centimeters from the top of thesection to the top and bottom of each sample removed from thatsection. In curated hard rock sections, sturdy plastic spacers are placedbetween pieces that did not fit together to protect them from damagein transit and in storage; therefore, the centimeter interval noted for ahard rock sample has no direct relationship to that sample's depthwithin the cored interval, but is only a physical reference to thelocation of the sample within the curated core.

A full identification number for a sample consists of the followinginformation: leg, site, hole, core number, core type, section number,piece number (for hard rock), and interval in centimeters measuredfrom the top of section. For example, a sample identification of"136-842B-5H-1, 10-12 cm" would be interpreted as representing asample removed from the interval between 10 and 12 cm below thetop of Section 1, Core 5 (H designates that this core was takenduring hydraulic piston coring) of Hole 842B during Leg 136.

All ODP core and sample identifiers indicate core type. Thefollowing abbreviations are used: R = Rotary Core Barrel (RCB); H= Hydraulic Piston Core (HPC; also referred to as APC, or AdvancedHydraulic Piston Core); P = Pressure Core Sampler; X = ExtendedCore Barrel (XCB); B = drill-bit recovery; C = center-bit recovery; I= in-situ water sample; S = sidewall sample; W = wash-core recovery;and M = miscellaneous material. APC, XCB, RCB, and W cores werecollected on Leg 136.

Core Handling

Sediments

As soon as a core is retrieved on deck, a sample is taken from the corecatcher and given to the paleontological laboratory for an initial ageassessment. The core is then placed on the long horizontal rack, and gassamples may be taken by piercing the core liner and withdrawing gas intoa vacuum-tube. Voids within the core are sought as sites for gas sampling.Some of the gas samples are stored for shore-based study, but others areanalyzed immediately as part of the shipboard safety and pollution-prevention program. Next, the core is marked into section lengths, eachsection is labeled, and the core is cut into sections. Interstitial-water (I)samples are then taken. In addition, some headspace gas samples arescraped from the ends of sections immediately after cutting, and sealed inglass vials for light hydrocarbon analysis. Each section is then sealed atthe top and bottom by gluing on color-coded plastic caps, blue to identifythe top of a section and clear for the bottom. A yellow cap is placed on thesection ends from which a whole-round sample has been removed, andthe sample code (e.g., IW) is written on the yellow cap. The caps areusually attached to the liner by coating the end liner and the inside rim ofthe cap with acetone, and then the caps are taped to the liners.

The cores then are carried into the laboratory, where the sections areagain labeled, using an engraver to permanently mark the full designationof the section. The length of the core in each section and the core-catchersample are measured to the nearest centimeter; this information is loggedinto the shipboard CORELOG database program.

Whole-round sections from APC and XCB cores are normally runthrough the Multisensor Track (MST). The MST includes the GRAPE(gamma-ray attenuation porosity evaluator) and P-wave logger devices,which measure bulk density, porosity, and sonic velocity, and also includesa meter that determines the volume magnetic susceptibility. At this point,whole-round samples for physical properties (PP) and structural analysis aretaken. In well-lithified sedimentary cores, the core liner is split and the top halfremoved so that the whole-round core can be observed before choosing thesamples. Relatively soft sedimentary cores are equilibrated to room tempera-ture (approximately 3 hr) and thermal conductivity measurements are per-formed on them.

Cores of soft material are split lengthwise into working and archivehalves. The softer cores are split with a wire or saw, depending on thedegree of induration. Harder cores are split with a band saw or diamondsaw. The wire-cut cores are split from the bottom to top, so investigatorsshould be aware that older material could have been transported up thecore on the split face of each section.

The working half of the core is sampled for both shipboard andshore-based laboratory studies. Each extracted sample is logged intothe sampling computer database program by the location and the nameof the investigator receiving the sample. Records of all removedsamples are kept by the curator at ODP. The extracted samples aresealed in plastic vials or bags and labeled. Samples are routinely takenfor shipboard physical property analysis. These samples are sub-sequently used for calcium carbonate (coulometric analysis) andorganic carbon (CNS elemental analyzer) and the data are reported inthe site chapters.

The archive half is used for the visual core description. Smearslides are made from samples taken from the archive half and aresupplemented by thin sections taken from the working half. Mostarchive sections are run through the cryogenic magnetometer. Thearchive half is then photographed with both black-and-white and colorfilm, a whole core at a time. Close-up photographs (black-and-white)are taken of particular features for illustrations in the summary of eachsite, as requested by individual scientists.

Both halves of the core are then put into labeled plastic tubes,sealed, and transferred to cold-storage space aboard the drilling vessel.At the end of the leg, the cores are transferred from the ship inrefrigerated airfreight containers to cold storage at the Gulf Coast

11

SHIPBOARD SCIENTIFIC PARTY

Repository (GCR) at the Ocean Drilling Program, Texas A&MUniversity, College Station, TX.

Igneous and Metamorphic Rocks

Igneous and metamorphic rock cores are handled differently fromsedimentary cores. Once on deck, the core-catcher is placed at the bottomof the core liner and total core recovery is calculated by shunting the rockpieces together and measuring to the nearest centimeter; this informationis logged into the shipboard core-log database program. The core is thencut into 1.5-m-long sections and transferred into the lab.

The contents of each section are transferred into 1.5-m-long sec-tions of split core liner, where the bottom of oriented pieces (i.e., piecesthat clearly could not have rotated top to bottom about a horizontalaxis in the liner) are marked with a red wax pencil. This is to ensurethat orientation is not lost during the splitting and labeling process.The core is then split into archive and working halves. A plastic spaceris used to separate individual pieces and/or reconstructed groups ofpieces in the core liner. These spacers may represent a substantialinterval of no recovery. Each piece is numbered sequentially from thetop of each section, beginning with number 1; reconstructed groupsof pieces are assigned the same number, but are lettered consecutively.

SITE 842 HOLE A CORE 999H CORED 0.0-0.0 mbsf

GraphicLith. Structure

Q_

COoü

Description

0.5"

1.0-

Figure 3. Core description form ("barrel sheet") used for sediments and sedimentary rocks.

12

EXPLANATORY NOTES

Pieces are labeled only on external surfaces. If the piece is oriented,an arrow is added to the label pointing to the top of the section.

The working half of the hard-rock core is then sampled forshipboard laboratory studies. Records of all samples are kept by thecurator at ODP.

The archive half is used for the visual core description, thenphotographed with both black-and-white and color film, one core ata time. Both halves of the core are then shrink-wrapped in plastic toprevent rock pieces from vibrating out of sequence during transit, putinto labeled plastic tubes, sealed, and transferred to cold-storagespace aboard the drilling vessel. As for the other Leg 136 cores, theyare housed in the GCR.

SEDIMENT CORE DESCRIPTION FORMS

The sediment core description forms (Fig. 3) or "barrel sheets,"summarize the sedimentological data obtained during shipboardanalysis of each sediment core. The barrel sheets for Leg 136 wereproduced using the new Visual Core Description program (Bernstein,unpubl. data) and therefore differ somewhat in format from those ofprevious ODP legs. The following discussion explains the ODP con-ventions used in compiling each part of the core description forms andthe modifications of these procedures adopted by Leg 136 scientists.Barrel sheets for each core are included in Appendix A.

Core Designation

Cores are designated using leg, site, hole, core number, and coretype as previously discussed (see "Numbering of Sites, Holes, Cores,and Samples" section, this chapter). The cored interval is specified interms of meters below seafloor (mbsf). This depth is based on thedrill-pipe measurements (DPM), as reported by the SEDCO coringtechnician and the ODP Operations Superintendent.

Graphic Lithology Column

The lithologic classification scheme of Mazzullo et al. (1987),accepted for shipboard use by the JOIDES Sediments and OceanHistory Panel, was used in a slightly modified version on Leg 136.The classification adopted is outlined in the "Sedimentology" sectionof this chapter. Sediment type is represented graphically on the coredescription forms using the symbols illustrated in Figure 4.

In the graphic lithology column a maximum of three differentlithologies can be represented within the same interval of core. Forhomogeneous sediments or sedimentary rocks, the constituent com-ponents are separated by a solid vertical line. Interbedded lithologiesare separated by a dashed vertical line. Percentages are rounded to thenearest 10% and only lithologies that constitute at least 10% of thecore are shown. Only lithologic units that are 16 cm or greater in

PELAGIC SEDIMENTSCalcareous

Nanno-foram orNannofossil Ooze Foraminiferal Ooze foram-nanno ooze Calcareous ooze Nannofossil chalk

J. J. JL J. J- J.

r T T T T T TT T T T T T ηT T T T T T TT T T T T T •P T T T T T TT T T T T T "1

» * + + +-#- •+• •+ •+• -i- - i -

VOLCANICLASTIC SEDIMENTS

Volcanic ash/tuff Volcanic lapilli Volcanic breccia

B B B B B B Ea a a a a a

B B B B B B E1 B B B B D BB B B B B B E

a a a B a aB B B B B B E

B a a a a aCB1 CB3CB2

Nanno-foram orForaminiferal chalk foram-nanno chalk Calcareous chalk

.:• & J > Λ .÷ .*

* > * * > A* > **J > .<- Λ rfr Jp sl

iWWWÉVΛVΛiWWW

CB4

Limestone

j-α-o-o-α-α-ö ' . ' . ' . ' . ' . ' . '

CB6

SiliceousCB7 CB8

Diatom-radiolarian

CB9

Diatom ooze

^^-•^- j—-i—-i—-r-

Radiolarian ooze or siliceous oozei> i> i> i><i> i> i> i><i> i> i> i>i> i> i> i>i> i> i> i>i> i> i> i> '

Diatomite

SB1

Radiolarite

SB2

Porcellanite

SB3

Chert

SB4

Spiculite

Jj.Jj.JJ.JJ.Jj.JJ.

JJ.JJ.JVJJ.JJ.JJ.JV JV JV JJ. JV JV J

JV JV JV JV JV JV J!k. jk jk jk Jk. jk jk.

SB5

V V V V V V VV V V V V V

V V V V V V VV V V V V V

SB7 SB8

CB5 V2 V3

SILICICLASTIC SEDIMENTSClay/claystone Shale (fissile) Sand/silt/clay Silt/siltstone

T1

Sand/sandstone

1111T3

Silty sand/sandy silt

• .V • .V • .V • .V • .V • .V •" "•.""?."'?.""?.""?."'?."• • ' • • • ' • • ' • • . ' • • • ' • • . ' • •

• .V • .V • -V • .V • .V • .V •> • • • . • • • • • • • • • • • •• .V • .V • .V • .V • .V • .V'

l-•

-l-

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• 1 •

1 •

1

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•-

•-

•-

• 1

1 •

1 • 1

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1

• 1

•-

l-•

-•

-i

• i

• i

• i

•

T4

Silty clay/clayey silt

[-L-D•ü-il-il•v

T5Sandy clay/clayey sand

T6

Gravel

T7 T8Conglomerate Breccia

• • • • • • •• • • • • •

• • • • • • •• • • • • •

• • • • • • •• • • • • m

SR1 SR2 SR3

Chert

Jk.Jk.JbJk.JkJk,.A Jk. Jk Jk. Jk Jk Jl

jk.Jh.Jb.Jk.Jk.Jk,

Jk Jk Jk. Jk. JL Jk j |A A A A A A .

Jk. Jk. A A A A ,

Porcellanite

•.JJ.JJ.JJ.JJ.JJ.JJ.JJ.JVJJ.JJ.JJ.JJ..

JJ.JJ.JVJJ.JJ.JJ..

v A A A A A A

Jolomite

J. J. J. J. J. J. U. jlXi i i jlJ. J. J. J. J. J. J'. J. J. J. J. J. JL

J. J. J. J. J. J. 4

CHEMICAL SEDIMENTS

Concretions

Mn=Manganese

B = Barite P = Pyrite Z = Zeolite

SB7 SB6 SR7Drawn circle with symbol (others may be designated)

Figure 4. Key to symbols used in the graphic lithology column on the core description form shown in Figure 3.

13

SHIPBOARD SCIENTIFIC PARTY

thickness are represented. Lithologies present in abundances too smallto be represented in the graphic lithology column are described in thedescription section of the barrel sheet.

Age

The chronostratigraphic age, determined by micropaleontologicalcriteria, is shown in the age column. Shipboard paleontologistsgenerally base their age determinations on core-catcher samples,although additional samples from other parts of the core may beexamined when required. Boundary indicators for the ages indicatedon the core description forms are:

1. Sharp boundary: Straight line2. Unconformity or hiatus: Wavy line3. Uncertain: ? (question mark)

Sedimentary Structures

Sedimentary structures are indicated in the structure column of thecore description forms. A key to the symbols used on Leg 136 is givenin Figure 5. Note that the positions of ash layers, too thin to be includedon the graphic lithology column, are indicated on the structure column.The symbols for bioturbation are used here to reflect degree ofhomogenization of the sediment as indicated by lack of bedding orlaminations and by mottling of the sediment where volcanic ash ispresent. No definite burrows or other evidence of the role of animalsin this homogenization were noted.

Sediment Disturbance

The coring techniques used may result in varying degrees ofdisturbance of the recovered core material. Observations of drilling-related disturbance over an interval of 20 cm or more are recorded inthe disturbance column on the barrel sheet. The symbols for thevarious types of drilling disturbance are shown in Figure 5.

The following disturbance categories are recognized for unlithifiedand partially lithified sediments:

1. Slightly deformed: bedding contacts are slightly bowed towardtheir edges.

2. Moderately deformed: bedding contacts have undergone ex-treme bowing.

3. Highly deformed: bedding is completely disturbed, sometimesshowing symmetrical diapir-like structures ("flow-in").

4. Soupy: intervals are water-saturated and have lost all traces oforiginal bedding.

The following categories are used to describe the degree of frac-turing in lithified sediments and igneous rocks:

1. Slightly fractured: core pieces are in place and have very littledrilling slurry or breccia.

2. Moderately fractured: core pieces are in place or partly dis-placed, but original orientation is preserved or recognizable. Drillingslurry may surround fragments.

3. Highly fractured: pieces are from the interval cored and probablyin correct stratigraphic sequence (although they may not represent theentire section), but original orientation is totally lost.

4. Drilling breccia: core pieces have completely lost their originalorientation and stratigraphic position and may be completely mixedwith drilling slurry.

Sedimentary structure symbols Drilling deformation symbols

Soft sediments-A-

A

Ash layer

Sharp contact

Gradational contact

Wavy laminae/beds

Planar laminae

Slight bioturbation

Moderate bioturbation

Heavy bioturbation

Fining-upward sequence

Concretions/nodules

Zeolite disseminated

Ash or pumice pods

Vugs

>

>

X×X

Slightly disturbed

Moderately disturbed

Highly disturbed

Soupy

Hard sediments

Slightly fractured

Moderately fractured

Highly fractured

Drilling breccia

Figure 5. Symbols used for drilling disturbance and sedimentary structures oncore description forms shown in Figure 3.

Samples

The positions of samples taken from each core for analysis areindicated by letters in the samples column of the core description form,as follows:

S: Smear slideT: Thin sectionP: Physical properties sampleM: Micropaleontology sampleX: Paleomagnetic sampleI: Interstitial water sampleO: Organic geochemistry sampleD: X-ray diffraction (XRD) sampleF: X-ray fluorescence (XRF) sample

14

EXPLANATORY NOTES

Color

Colors of sediment were determined by comparison with theMunsell Soil Color Chart (1975). This was done immediately afterthe cores were split and while they were still wet, to minimizealteration due to exposure to air. Colors of indurated sedimentswere determined from wet samples. Color, hue, and chroma codesare shown in the color column of the core description form.Intervals of 34 cm or greater are represented by a single colorcode. Intervals of 80 cm or greater may be represented by twocolor codes, indicating either two colors or a gradation betweentwo colors. The color of thinner units (less than 34 cm) may begiven in the description column.

Lithologic Description

The lithologic description that appears on each core descriptionform consists of a brief summary of the major lithologies observed,followed by a summary of the minor lithologies. Color, composition(determined from the analysis of smear slides, in some cases aug-mented by XRD analysis), sedimentary structures, or other notablefeatures of each lithology are described and, where possible, thedistribution of these features in the core is indicated.

Smear-Slide Summary

A table summarizing data from smear slides appears in each sitesummary chapter. The table includes information on sample location,whether the sample represents a dominant ("D") or minor ("M")lithology in the core, and the estimated percentages of sand, silt, andclay, together with all identified components.

SEDIMENTOLOGY

Classification of Sediments and Sedimentary Rocks

Leg 136 used a slightly modified version of the sedimentclassification scheme of the Ocean Drilling Program (ODP; Maz-zullo et al., 1987), which defines granular and chemical sedimenttypes. Granular sediment consists of discrete organic (e.g.,foraminifer tests, mollusc shells) or inorganic (e.g., quartz grains,rock fragments, volcanic ash) grains. Some examples of granularsediment are nannofossil ooze, quartz sandstone, fine vitric ash,and bioclastic grainstone. Chemical sediment consists of mineralsformed by inorganic processes, such as precipitation from solutionor colloidal suspension, deposition of insoluble precipitates, orrecrystallization of detrital grains. Some examples of chemicalsediment are coal, halite, pyrite, and gypsum.

The portions of the ODP sediment classification schemerelevant to Leg 136 and our modifications of the scheme aredescribed below.

Granular Sediments

Variations in the relative proportions of pelagic, neritic, siliciclas-tic, and volcaniclastic grain types define four major classes of granularsediments: pelagic, neritic, siliciclastic, and volcaniclastic (Fig. 6).The intermediate sediment class called mixed sediment in the ODPClassification Scheme has been eliminated for Leg 136 use. Pelagicgrains are the skeletal remains of open-marine siliceous and calcareousmicrofauna and microflora (e.g., radiolarians, planktonicforaminifers, nannofossils) and associated organisms. Neritic grainsinclude calcareous grains and skeletal remains (e.g., bioclasts, shal-low-water benthic foraminifers, peloids, etc.) of nonpelagic origin.Siliciclastic grains are mineral and rock fragments derived fromplutonic, sedimentary, and metamorphic rocks. Volcaniclastic grainsinclude those of pyroclastic (direct products of magma degassing),

Ratio of siliciclastic to volcaniclastic sediments

<1:1 1:1 >1:1100

50

Volcaniclasticsediments

Neriticsediments

Siliciclasticsediments

Pelagicsediments

50

100<1:1 1:1 >1:1

Ratio of pelagic to neritic grains

Figure 6. Diagram showing classes of granular sediment (modified fromMazzullo et al , 1987).

hydroclastic (products of granulation of volcanic glass by steamexplosions) and epiclastic (detritus derived from erosion of volcanicrocks) origins.

Pelagic and neritic sediments are composed of >50% pelagic andneritic grains and are differentiated by the predominance of eithergrain type. Similarly, siliciclastic and volcaniclastic sediments contain>50% siliciclastic and volcaniclastic grains, with the relative propor-tions of the two grain types determining the sediment type.

Classification of Granular Sediment

A granular sediment is classified by designating a principal namewith additional major and minor modifiers. The principal name of agranular sediment defines its granular-sediment class. The major andminor modifiers describe the texture, composition, fabric, or round-ness of the grains themselves (Table 1).

Principal Names

Each granular-sediment class has a unique set of principal names. Asummary of these principal names for the sediments recovered during Leg136 (pelagic, siliciclastic, and volcaniclastic) is presented below.

Pelagic Sediment

For pelagic sediment, the principal name describes the compositionand degree of induration using the following terms:

1. Ooze: unlithified calcareous and/or siliceous pelagic sediments;2. Chalk: partially lithified pelagic sediment predominantly com-

posed of calcareous pelagic grains;3. Limestone: lithified pelagic sediment predominantly composed

of calcareous pelagic grains;4. Radiolarite, diatomite, and spiculite: partially lithified pelagic

sediment predominantly composed of siliceous radiolarians, diatoms,and sponge spicules, respectively;

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SHIPBOARD SCIENTIFIC PARTY

Table 1. Outline of granular-sediment classification scheme (modified from Mazzullo et al., 1987).

Sediment class

Pelagic sediment

Neritic sediment

Siliciclastic sediment

Volcaniclasticsediment

Major modifiers

1. composition of pelagic andneritic grains present inmajor amounts.

2. texture of clastic grainspresent in major amounts

1. composition of neritic andpelagic grains present inmajor amounts

2. texture of clastic grainspresent in major amounts

1. composition of all grainspresent in major amounts

2. grain fabric(gravels only)

3. grain shape(optional)

4. sediment color(optional)

1. composition of allvolcaniclasts present inmajor amounts

2. composition of all pelagicand neritic grains presentin major amounts

3. texture of siliciclasticgrains present in majoramounts

Principal names

1. ooze2. chalk3. limestone4. radiolarite5. diatomite6. spiculite7. chert1. boundstone2. grainstone3. packstone4. wackestone5. mudstone6. floatstone7. rudstone1. gravel2. sand3. silt4. clay(etc.)

1. breccia2. lapilli3. ash/tuff

1.

2.

1.

2.

1.

2.

1.

2.

3.

Minor modifiers

composition of pelagic and neriticgrains present in minor amountstexture of clastic grains present inminor amounts.

composition of neritic and pelagicgrains present in minor amountstexture of clastic grains present inminor amounts.

composition of all grains present inminor amountstexture and composition ofsiliciclastic grains present as matrix(for coarse-grained clasticsediments)

composition of all volcaniclastspresent in minor amountscomposition of all neritic andpelagic grains present in minoramountstexture of siliciclastic grainspresent in minor amounts

5. Chert: lithified pelagic sediment predominantly composed ofsiliceous pelagic grains.

Siliciclastic Sediment

For siliciclastic sediment, the principal name describes the textureand is assigned according to the following guidelines:

1. The Udden-Wentworth grain-size scale (Wentworth, 1922)defines grain-size ranges and names of the textural groups (gravel,sand, silt, and clay) and sub-groups (fine sand, coarse silt, etc.) thatare used as the principal names of siliciclastic sediment. Note that theterm "clay" is used only as a grain-size designation and does not denoteor imply mineralogical composition.

2. Principal names are listed in order of increasing abundance whentwo or more textural groups or subgroups are present in a siliciclasticsediment (Shepard, 1954).

3. The suffix -stone is affixed to the principal names sand, silt, andclay when the sediment is lithified.

4. Conglomerate and breccia are used as principal names of gravelswith well-rounded and angular clasts, respectively. Degree of lithifica-tion is designated by a modifier before the principal name.

Volcaniclastic Sediment

Volcaniclastic sediments are subdivided into three groups:pyroclastic, hydroclastic, and epiclastic, with the principal name ineach group describing the texture. The following names and ranges ofthree textural groups for pyroclastics (from Fisher and Schmincke,1984) are used for all volcaniclastic sediments:

1. Volcanic breccia: pyroclasts >64 mm in diameter.2. Lapilli: pyroclasts between 2 and 64 mm in diameter; when

lithified, described as lapillistone.3. Ash: pyroclasts <2 mm in diameter; when lithified, described as tuff.

Major and Minor Modifiers

The principal name of a granular-sediment class is preceded bymajor modifiers and followed by minor modifiers (preceded by"with") that describe the lithology of the granular sediment in greaterdetail (Table 1). Major and minor modifiers are used most commonlyto describe composition and textures of grain types present in major(>25%) and minor (10%-25%) proportions. In addition, majormodifiers can be used to describe degree of lithification, grain fabric,grain shape, and sediment color. The nomenclature for major andminor modifiers is outlined as follows:

Composition of pelagic grains can be described with the major andminor modifiers: diatom(-aceous), radiolarian, spicule(-ar), siliceous,nannofossil, foraminifer(-al), and calcareous. The terms siliceous andcalcareous are used generally to describe sediments composed ofsiliceous or calcareous pelagic grains of uncertain origins.

Texture of siliciclastic grains is described by the following majorand minor modifiers: gravel(-ly), sand(-y), silt(-y), and clay(-ey).Composition of siliciclastic grains can be described by:

1. Mineralogy: using modifiers such as "quartz," "zeolitic,""lithic" (for rock fragments), or "calcareous" (for detrital clasts ofcalcium carbonate).

2. Provenance: source of rock fragments (particularly in gravels,conglomerates, and breccias) can be described by modifiers such asvolcanic, sed-lithic (contains clasts or grains of sedimentary rock),basaltic, etc.

Volcaniclastic grain composition is described by the followingmajor and minor modifiers: lithic (rock fragments), vitric (glass andpumice), volcanic sed-lithic (contains clasts or grains of volcaniclasticrock), and crystal (mineral crystals and cleavage fragments), or bymodifiers describing the composition of lithic grains and crystals (e.g.,basaltic or feldspar).

16

EXPLANATORY NOTES

Sediment fabric can be described by the major modifiers: grain-supported, matrix-supported, and imbricated. Generally, fabricdescriptors are applied only to gravels, conglomerates, and breccias.

Degree of consolidation is described using the following majormodifiers: "unlithified" designates soft sediment that is readily defor-mable under the pressure of a finger, "partially lithified" designatesfirm sediment that is incompletely lithified, and "lithified" designateshard, cemented sediment that must be cut with a saw.

Grain shapes are described by the major modifiers: rounded,subrounded, subangular, and angular. Sediment color is determinedwith the Munsell Chart, a standard color-comparator, and can beemployed as a major modifier.

X-ray Diffraction Analysis

A Philips ADP 3520 X-ray diffractometer was used for the X-raydiffraction (XRD) analysis of bulk samples. Cu K-alpha radiation witha Ni filter was used with a tube voltage of 40 kV and a tube current of35 mA. Bulk samples were freeze-dried and ground using the Spex8000 Mixer Mill or with an agate mortar and pestle, pressed into asample holder and scanned from 2° to 60° 29, with a step size of 0.02°2θ and a count time of 1 s per step.

BIOSTRATIGRAPHY

Calcareous nannofossils, radiolarians, and ichthyoliths wereanalyzed during Leg 136 for biostratigraphic purposes. Age assign-ments are based mainly on core-catcher samples that were taken fromthe bottom of each core that contained sediment. Additional sampleswere studied when core-catcher samples were barren or when boun-daries or unconformities occurred.

Calcareous Nannofossils

The calcareous nannofossil zonal schemes used during Leg 136are those of Martini (1971) and Sissingh (1977).

Smear slides were prepared for each sample using Norlan OpticalAdhesive as a mounting medium. The calcareous nannofossils wereexamined in the slides using light microscopy techniques (plane-polarized light, phase contrast, and cross-polarized light) at ap-proximately 1250× magnification. Abundance of nannofossils wasestimated by traversing up to 100 fields of view across a slide andassigning the following categories:

R = rare (<l specimen per 10 fields of view).F = few (1-10 specimens per 10 fields of view).C = common (1-10 specimens per field of view).A = abundant (10-100 specimens per field of view).

The state of preservation was estimated as follows:

G = good (little or no evidence of dissolution or secondary over-growth of placoliths and discoasters).

M = moderate (dissolution and/or secondary overgrowth partiallyalter primary morphological characteristics, but nearly all specimenscan be identified at the species level).

P = poor (severe dissolution, fragmentation, and/or secondaryovergrowth with primary features largely destroyed; many specimenscannot be identified at the species and/or generic level).

Radiolarians

Age determinations and zonal assignments of Quaternaryradiolarians were derived from Nigrini (1971) and Sanfilippo et al.(1985). Cretaceous identifications were based on Pessagno (1976).Samples were disaggregated by boiling for 2-4 min in a 50:50 mixture

of water and hydrogen peroxide. Calgon was also added to break upextremely clay-rich samples. After boiling, samples were sieved andgently washed through 60, 100, and 230 mesh stainless steel sieves.Residues from each sieve were collected into filter paper and dried ina standard laboratory oven. Radiolarians were picked from theresidues and concentrated on separate cardboard slides for studyunder a binocular scope at 70×to 125× magnification.

Abundances of radiolarians were estimated by counting the num-ber of specimens observed per tray (9 × 8 cm) of residue. Theabundances were recorded as follows:

R = Rare (1-2 specimens per tray).F = Few (3-4 specimens per tray).C = Common (5-10 specimens per tray).A = Abundant (more man 10 specimens per tray).

Preservation of radiolarians was documented in the following manner:

Good = radiolarians free of clay, pore structures and ornamentationeasily visible.

Moderate = Some clay in pores of radiolarians; overall structurestill visible.

Poor = radiolarians completely or nearly completely covered inclay; pore structure largely obscured.

Ichthyoliths

Age determinations of ichthyoliths were based on Doyle andRiedel (1979, 1985). Samples were processed for ichthyoliths in thesame manner as for radiolarians (above). Ichthyoliths were pickedfrom residue and mounted on glass slides, with cover slides fastenedwith Norland Optical Adhesive. The slides were studied using trans-mitted light microscopy at 50× magnification. Absolute numbers ofeach taxon were recorded per slide.

PALEOMAGNETISM

Paleomagnetic studies performed on board JOIDES Resolutionduring Leg 136 included routine measurements of natural remanentmagnetism (NRM) and of magnetic susceptibility of sedimentarycores and discrete samples. Because cores are commonly affected bysecondary magnetizations, alternating field (AF) demagnetization wasalso systematically performed. Where magnetic cleaning sufficientlyisolates the primary component of remanence, paleomagnetic inclina-tions are used to assign magnetic polarity to the stratigraphic columnand to correlate polarity reversal zones with the magnetic polarity timescale of Berggren et al. (1985).

Remanent Magnetization Measurements

The JOIDES Resolution maintained two magnetometers formeasurement of magnetic remanence during Leg 136, a 2-GEnterprises (model 760R) pass-through cryogenic superconduct-ing rock magnetometer and a Molspin spinner magnetometer. Weused the spinner only for cross-calibration. An AF demagnetizer(Model 2G600) capable of alternating fields up to 20 mT is on-linewith the cryogenic magnetometer. Both the cryogenic mag-netometer and the AF coils are encased in a Mu-metal shield, andan automated sample-handling system moves the core sectionsthrough the AF coils and the magnetometer sensor region. Thecryogenic magnetometer, AF demagnetizer, and sample-drivesystem are controlled by a multiserial communication board(FASTCOM 4) in an IBM PC-AT compatible computer. Measure-ments using the cryogenic magnetometer were controlled by amodified version of a University of Rhode Island BASIC pro-gram. Prior to receiving core, we conducted various calibration

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SHIPBOARD SCIENTIFIC PARTY

experiments to establish the accuracy of the basic measurementapparatus and to check the operation of the processing software.

The superconducting quantum interference device (SQUID) sen-sors in the cryogenic magnetometer measure magnetization over aninterval approximately 20 cm long. The widths of the sensor regionsuggest that as much as 150 cm3 of core contributes to the outputsignal. The large volume of core material within the sensorregion allows accurate determination of remanence for weaklymagnetized samples despite the relatively high background noiserelated to the motion of the ship. The cryogenic magnetometer isincapable of measuring archive-halves whose magnetization isgreater than 1 A/m, but this limitation did not affect measurement ofLeg 136 cores.

The output from the cryogenic magnetometer during continuousmeasurement assumes a uniformly magnetized core, and providesreliable directional data when a core section larger than the sensingregion is measured. Tests conducted during Leg 131 indicated that incases where the core is not uniformly magnetized, either throughnatural processes or artifacts (voids in the core, or differential rotationof segments within the core liner), the values of declination, inclina-tion, and intensity should be treated with caution.

Remanence measurements of sediments were performed by pass-ing continuous archive-half core sections through the cryogenicmagnetometer. NRM measurements were taken at intervals of 5 cmalong the core and after AF demagnetization at 2, 4 or 5, 7, and 10mT. Selected sections were treated to 12 and 15 mT, the maximumAF demagnetizing field allowed by the Information Handling Panel(IHP) for archive-half sections. We should report that in our searchfor the stable remanence we overlooked the other IHP limitation(median destructive field) but as that is more dependent on con-tamination than on any paleomagnetic property of these cores, we donot expect this violation to handicap future workers. (Indeed muchof the remanence is so soft that it would probably decay naturally inthe course of a few years).

Discrete samples in soft sediment were taken with oriented plasticsampling boxes, both of standard size (7 cm3) and miniatures (1 cm3).With the exception of NRM of continuous sections of miniaturesamples across the 5-cm intervals of whole-core taken for pore-waterchemistry, measurements of discrete samples were deferred for shore-based study.

We did not use the GSD1 AF demagnetizer above 15 mT, so thatthe problems of anhysteretic and gyromagnetic remanence reportedon Leg 134 did not affect us.

Perturbations Produced by Drilling

Interpretations of measurements with the pass-through cryogenicmagnetometer often are difficult because of mechanical disturbancessuch as twisting of unconsolidated sediments during XCB or rotarycoring. A persistent problem is the pervasive overprint produced bydrilling, as noted on previous legs. Exposure to large magnetic fieldson the rig floor and contamination by rust have often been suggestedas sources for these spurious magnetizations. During Leg 115,Schneider and Vandamme (in Backman, Duncan et al., 1988) sug-gested that one particular source of remagnetization was the remanentfield of the core barrel.

We made a number of observations on these spurious magnetiza-tions (see "Paleomagnetics" Section, Site 842 chapter) and also tookdiscrete samples for shore-based work to extend the investigationsmade on Leg 134.

Magnetic Susceptibility Measurements

Whole-core magnetic susceptibility (MS) measurements weremade on all APC and selected XCB cores using a Bartington Instru-ments magnetic susceptibility meter (model M.S. 1) with an M.S.

1/CX 80-mm whole-core sensor loop set at 0.47 kHz. Measurementswere routinely made at 2-cm intervals. The susceptibility meter is partof the multisensor track (MST) that also contains a gamma-ray at-tenuation porosity evaluator (GRAPE) and P-wave logger (see"Physical Properties" section, this chapter).

Initial magnetic susceptibility per unit volume, k, is a dimension-less scalar quantity relating the applied magnetic field vector, H, andthe magnetization, M, that it induces in a material:

M = k•H.

In sediments and rocks, MS is usually a measure of the amount ofmagnetizable material (usually magnetite), but it is also sensitive tomagnetic grain size and shape. Because the shipboard MS data gather-ing and processing computer programs are set up for cgs units, theseare used in the site reports. A conversion to standard S1 units is easilymade by the multiplication of these values by a constant,

The Bartington MS meter has a sensitivity of 1 × I0"7 cgs, but onLeg 136 the weakest sediments measured had MS values one to twoorders of magnitude higher.

Core Orientation Tool

Two APC cores were oriented for paleomagnetic work by usingthe Eastman-Whipstock multishot camera tool. This unit is a smallcamera that takes photographs at pre-determined intervals of an orien-tation unit that consists of a compass and pendulum inclinometer. It isinstalled in a pressure housing on the sinker bar assembly that attachesthe sandline to the APC core barrel. The multishot camera yields theorientation of the APC relative to magnetic north. The APC and coreliner are aligned so that the camera records the angle between magneticnorth and the double lines on the core liner (the working half side).

INORGANIC GEOCHEMISTRY

The inorganic geochemistry program for ODP Leg 136 included(1) analysis of interstitial water for pH, alkalinity, salinity, chlorinity,calcium, magnesium, potassium, strontium, sulfate, silica, nitrate,ammonia, phosphate, and (2) X-ray mineralogical analysis of intersti-tial water squeeze cakes.

Interstitial water was obtained from the sediment by a modificationof the standard method described in detail by Manheim and Sayles(1974). Whole-round (5- or 10-cm-long) core samples were collectedfor interstitial water analyses immediately after the core arrived ondeck. Because of the interest in obtaining trace-element profiles,whole-round cores and subsequent treatments of the interstitial water(except actual squeezing) were performed in a Class 100 laminar flowbench. The sediment was extruded from the core liner, the ends andouter layer of the sediment (in contact with the core liner) werecarefully removed by scraping, with a plastic spatula, the sampleplaced in titanium squeezers, and then placing it in a CarverLaboratory Press for removal of interstitial water. The press wasoperated to a pressure of about 30,000 psi (2.11 kg/m2). The sedimentsample remained under pressure until water could no longer besqueezed from it. Interstitial water for shipboard determinations wascollected in Nanopure water-rinsed plastic syringes, filtered through0.2 µm Acrodiscs, and stored in plastic vials until used. Interstitialwater for shore-based trace-element analysis was collected in 50-mLacid-cleaned syringes, filtered through acid-cleaned 0.22 µmAcrodiscs into 25-mL acid-cleaned linear polyethylene bottles, andacidified with 25 µL Ultrex HNO3. International Association for thePhysical Sciences of the Ocean (IAPSO) standard seawater P99 wasthe primary standard for the shipboard water analyses.

18

EXPLANATORY NOTES

Individual inorganic constituents were analyzed according to pro-cedures outlined by Gieskes and Peretsman (1986) or modificationsthereof as described below. Results are expressed in milli- or micro-molar units. Alkalinity and pH were determined using a metrohmautotitrator with a Brinkmann combination pH electrode. Alkalinityreproducibility is better than 5%; data are given in milli-equivalentsper liter. Salinity was determined using a Goldberg optical hand-heldrefractometer measuring the total dissolved solids.

Chlorinity was measured by a silver nitrate titration of a 200 µLsample to which 500 µL of a 5 M NaNO3 solution was added prior todilution to approximately 25 mL with 18 Mücm distilled deionizedwater (DD H2O). The end point of the Cl~ titrations was determinedelectrometrically with an Ag electrode combined with a double junc-tion reference electrode connected to a Brinkman Model 686titroprocessor. Reproducibility of the IAPSO standard is better than0.3% relative.

Calcium was determined by complexometric titration of 500 µLaliquots with EGTA (ethylene-bis-(oxyethylenenitrilo)-tetra-aceticacid) using GHA (2-2'-ethane-diylidine-dinitrilo-diphenol) as an in-dicator. To enhance the determination of the end point, the calcium-GHA complex was extracted onto a layer of butanol (Gieskes, 1973).No correction was made for strontium because it is compensated forby inclusion in the standardization vs. IAPSO.

Magnesium was determined by a total alkaline earth titration of250 µL aliquots with EDTA (di-sodium ethylenediamine-tetraacetate)(Gieskes, 1973). Subsequent use of the calculations recommended byGieskes in the shipboard chemistry manual yielded the magnesiumconcentration in the interstitial water sample.

Sulfate was determined by ion-chromatography using a Dionex-2120 Ion Chromatograph. Reproducibility on different dilutions of theIAPSO standard is better than 2% relative. Determinations for NH4

+,NOj, PO4

3- and SiOH4 were performed by colorimetric methodsdescribed by Gieskes and Peretsman (1986).

Atomic absorption spectroscopy (AAS) using a Varian SpectraAA-20 spectrophotometer was used for the determination of K and Sr.All measurements were made using an air-acetylene flame. The emis-sion mode was employed for K whereas the absorption mode was usedfor Sr.

Potassium was determined on 1:500 sample dilutions; five stand-ards ranging from 1:420 through 1:12,000 were prepared fromvolumetric dilutions of IAPSO with DD H2O. The reproducibility ofK determinations by flame emission is approximately 2% relative.

Strontium was determined on 1:10 sample dilutions. A releasingagent was used (LaCl3 -5,000 ppm) in samples and standard solutions.Five standards were prepared by diluting 0.25, 0.50, 0.75, 1.00, and1.5 mL of IAPSO standard seawater to 10 mL with 5,000 ppm La asLaCl3 in a volumetric flask. The reproducibility of determinations wasapproximately 3%-4% relative.

PHYSICAL PROPERTIES

Index Properties

Water content, bulk density, grain density, and derived physicalproperties on discrete samples were determined by methods detailedin Kate Moran's 25 November 1990 memorandum: "Recommendedmethods for the discrete measurement of index properties on theJOIDES Resolution:'

Determinations of sediment mass (in standard aluminum beakers)were performed on the matched, top-loading Scitech microbalances.Repeated measurements (n = 53) using 1000 counts indicates anoperational standard deviation of 0.043 g in the weighing procedure.The Quantachrome Penta-Pycnometer helium displacement pyc-nometer was used to determine wet and dry volumes. Three determi-nations were made on each sample; evaluation of 95 suchdetermination sets yield a standard deviation of 0.037 cm3. Sampleswere dried in the mechanical convection ovens at 105° C for 24 hr prior

to measuring dry weights and volumes. Grain densities were deter-mined on eight samples in which the dry volume was determined firstwithoutcrushing, and again afterphy sical disaggregation. There wasno discernible difference between the two techniques, and we areconfident that the dry volume determinations are repeatable. A saltcorrection assuming 35 ppt interstitial fluid salinity was applied(Hamilton, 1971). Calculations wereperformedbythePHYSPROPSprogram on the ODP database, using the beaker calibrations deter-mined on Leg 130.

Despite our concerted efforts to accurately calibrate the balan-ces and pycnometer, bulk density determinations on discretesamples consistently fall 0.05 g/cm3 below the GRAPE results. Asthat unit is calibrated directly to density, we suspect a systematicerror in either the sample weight or volume determinations on theindividual sediment samples.

Shear Strength

Shear strength was measured on the Wykeham-Farrance ap-paratus, as the automated vane shear instrument could not becalibrated. All determinations were made on freshly split cores byinserting a 1.28-cm vane (#8; 1:1 blade ratio) about 1 cm beneaththe sediment surface. The test was conducted at a rotation rate of607min in the motorized mode. Springs 1 and 2 were used toproduce shearing within 20-70° rotation; existing calibrationswere used although they were apparently last checked in 1973(Boyce, 1973). Calculations were performed on the PHYSPROPSprogram within the ODP database.

Vane tests assume that a sediment cylinder is sheared in anundrained condition, and that the shear strength is attributableprimarily to cohesion. These assumptions may be invalidated bycracking, dissipation of local pore pressure by drainage, non-uniform failure, and frictional effects induced by large grains. Thesediments in the upper 20 m of Site 842 include silts andradiolarians that may exhibit less than ideal behavior. For thisreason, and because some of the core has been disturbed bydrilling, the minivane test probably does not provide a true sedi-ment shear strength. Nevertheless, the test does provide an estimateof downcore strength.

GRAPE

All cores were run through the gamma-ray attenuation porosityevaluator (GRAPE), and associated detectors on the multisensortrack prior to splitting, and after thermal equilibration. Scans weremade at a rate of 0.763 cm/s and counts were accumulated for 2 s,so that the interval scanned by a single determination is ap-proximately 1.5 cm. An air and aluminum standard was used tocalibrate the beam. The standard error of the bulk density deter-minations is 10% (Boyce, 1976), but because the number of deter-minations per core section (approximately 90) is large, the resultsare probably more accurate than the discrete determinations, par-ticularly when a moving average is used to discriminate againsthigh-frequency variability.

Sonic Velocities

Compressional-Wave Velocity Logger (P-wave logger)

Compressional-wave (P-wave) velocities were obtained at along-core intervals of about 2 cm using the P-wave logger (PWL) on themultisensor track (MST) (Boyce, 1976). The measurements weretaken on whole-round sections lying inside the core liner. The soundsource on the MST is a 500-kHz impulse repeated at a frequency of 1kHz. The source and receiving transducers are mounted along an axisperpendicular to the core axis; the separation between thesetransducers is measured using a pair of displacement transducers.

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SHIPBOARD SCIENTIFIC PARTY

Before any measurements were taken, the PWL was calibratedusing samples of distilled water, glycerol, and ethylene glycol asstandards. These samples fill a short length of core liner. Calibrationresults in a single correction factor for the combined effects of physicalwear on the transducers and traveltime through the core liner. Thethickness of the core liner is assumed constant.

Hamilton Frame Velocimeter

In addition to the data provided by the PWL, P-wave velocitieswere also measured using the screw-press Hamilton FrameVelocimeter (Boyce, 1976). The traveltime of a 500-kHz source pulsewas measured using an oscilloscope. Sample thickness was measuredby using a variable resistor attached to the calipers that hold the samplebetween the transducers on the frame. Nominal measurement errorswere 0.4 mm and 0.2 us. For the soft sediments cored at Site 842,velocities were measured about every 75 cm by mounting the split-core section and core liner between the jaws of the velocimeter; allvelocity measurements were taken perpendicular to the core axis.Seawater was used to improve the acoustic contact between the sampleand the transducers. Routinely, velocity and index properties sampleswere taken from adjacent positions in the core.

The Hamilton Frame transducers and calipers were calibrated withlucite and aluminum standards of known velocity and thickness.Thickness and traveltime corrections (i.e., "zero" thickness and "zero"time) were calculated by performing a linear regression between theactual and measured times and the actual and measured distances. Forthe traveltime calibration, the slope and intercept of the best-fit straightline were 0.98 and -2.46 , respectively. For the distance calibration,these parameters were 1 and -0.21 mm, respectively. Application ofthese correction factors to the measurements made on the aluminumand lucite standards resulted in errors of up to 2.5%.

The traveltime and thickness of the core liner were also measuredusing the Hamilton Frame. Corrected velocities were calculated byapplying the traveltime and thickness corrections to the measured dataand subtracting the effects of the core liner. In practice this results inthe cancellation of both intercept factors discussed above. An inde-pendent measurement of the core liner traveltime would allow a moreaccurate estimate of sample velocity. As mentioned in the previoussection, the traveltime correction for the core liner could not beextracted from the software that controls the MST.

For Site 842, the velocities measured using the Hamilton Frame wereabout 3% larger than the MST values. This discrepancy is probably dueto inaccurate correction of the sample thickness and traveltime measure-ments. An additional source of discrepancy is the difference in thecorrection factors for the core liner. Accordingly, the PWL data areprobably more reliable than the Hamilton Frame measurements.

Thermal Conductivity

Thermal conductivity (k) was measured by monitoring the changein temperature of the sample as a function of time when the sample isheated at a known rate by means of a needle probe. The probe containsboth a heater wire and a calibrated thermistor. Prior to taking themeasurements, the cores were allowed to equilibrate at room tempera-ture for at least 4 hr. The criterion for thermal equilibrium was thatthermal drift of the sample prior to the thermal conductivity measure-ment be less than 0.004 °C/min. For each sample, the temperaturevariations were recorded for 6 min. A correction factor for each probeused was calculated by performing a linear regression between theconductivities measured for a set of standards and the actual conduc-tivities of the standards.

Conductivity measurements were made using both the "full space"method described by Von Herzen and Maxwell (1959) and the "halfspace" method described by Vacquier (1985). The former techniquewas used on whole-round sediment samples cored at Site 842, and thelatter technique was used to measure the conductivity of the basaltscored at Site 843. The probes used for the full space method werecalibrated using the red rubber (k = 0.96 W/m °C) and black rubber{k = 0.54 W/m °C) standards. In addition to the above standards,the probes used for the half space method were calibrated using asample of basalt of known conductivity (k = 2.05 W/m °C). Foreach probe, about three measurements were made on each standard.Applying these correction factors to the measured values of thestandards results in errors of about 5% in the case of the full spacemethod and errors of up to 20% for the half space method.

The full space method, which assumes a medium of infinite extent,predicts that the temperature variation is a linear function of the naturallogarithm of the time. The conductivity is estimated by calculating theslope of the best-fit straight line to a plot of temperature versus the naturallogarithm of time; the value of the slope is inversely proportional to theconductivity. Specifically, the data are fit to the following equation:

T=(q/4ric)ln(t)+At + B,

where T is the temperature, q is the heat input to the sample perunit length per unit time, and k is the thermal conductivity. Thelinear term in the variable t accounts for the temperature drift ofthe sample during the measurement. The procedure for the halfspace method is similar except that the slope is multiplied by aprobe-specific constant that has a value of 2 in the ideal case of noheat loss to the insulating material that supports the sample. Inpractice, the assumptions of infinite and semi-infinite media breakdown after a few minutes. This is readily recognized on inspectionof the temperature vs. time plot by a decrease in the rate of changeof temperature as a function of the logarithm of the time. Conse-quently, for both methods, the estimated conductivity is dependenton the time interval over which the slope is calculated.

This dependency of the conductivity estimate on the time interval isespecially pronounced for the case of the half space method. For thistechnique, factors in addition to edge effects lead to a nonlinear relation-ship between temperature and the logarithm of the time (ShipboardScientific Party, 1989). Forthe half space measurements, it was impossibleto choose a consistent sample interval over which to estimate the conduc-tivity. Inclusion or exclusion of a couple of points resulted in changes inthe estimate of thermal conductivity of up to 20%. Repetition of theexperiment using either the same or a different probe sometimes resultedin a similar variability. This sensitivity to the time interval over which theconductivity was estimated was not observed for the full space measure-ments made on the sediment samples.

The precision of the full space measurements is estimated to beabout 5%. It is difficult to estimate the precision of the half spacemethod. A crude estimate was derived by calculating the standarddeviation about the mean of about ten measurements, each measure-ment being based on either a different time interval or a different run.A measurement was deemed acceptable if (1) the standard deviationabout the best-fit straight line to the plot of temperature vs. thelogarithm of the time was less than 0.015, and (2) the linear driftcoefficient was less than 0.04°C/min. This approach results in aprecision of about 10%. However, this value is only an estimatebecause it was not feasible to measure all the values that met the abovecriteria. Based on the sensitivity of the conductivity estimate to thetime segment, it seems more likely that the real precision is closer to20% than 10%.

20

EXPLANATORY NOTES

The thermal conductivity data were not corrected to in-situ tempera-ture and pressure. All measurements are reported in units of W/m °C.

IGNEOUS ROCKS

Core Curation and Shipboard Sampling

To preserve important features and structures, core sections con-taining igneous rocks were examined prior to splitting. During corehandling and splitting, core orientation was preserved by carefullymarking the original base of individual pieces. Each piece was num-bered sequentially from the top of each core section and labeled at thetop surface. Pieces that could be fitted together were assigned the samenumber and were lettered consecutively (e.g., 1A, IB, and 1C), evenif they occupied more than one section. Plastic spacers were placedbetween pieces with different numbers. The presence of a spacer,therefore, may represent a substantial interval of no recovery. If it wasevident that an individual piece had not rotated about a horizontal axisduring drilling, an arrow was added to the label pointing to the top ofthe section. As all pieces were free to turn about a vertical axis duringdrilling, azimuthal orientation of the core was not possible. After thevertical core orientation was noted, the pieces were split with adiamond saw into archive and working halves, with care taken toensure the division of key features.

After the core was split, the working half was sampled for ship-board physical properties, magnetic studies, X-ray fluorescence, X-ray diffraction, and thin-section studies. Nondestructive physicalproperty measurements, such as magnetic susceptibility, were madeon the archive half of the core. Where recovery permitted, sampleswere taken from each lithologic unit for XRF analysis for majorelements. The archive half was described on the visual core descriptionform and was photographed before storage.

Visual Core Descriptions

Visual core description forms were used in the documentation ofthe igneous rock cores (see site summary appendixes). The leftcolumn is a graphic representation of the archive half. A horizontalline across the entire width of the column denotes a plastic spacer.Oriented pieces are indicated on the form by an upward-pointingarrow to the right of the piece. Shipboard samples and studies areindicated in the column headed "shipboard studies," using thefollowing notation: X = X-ray diffraction analysis; F = X-rayfluorescence analysis; T = petrographic thin section; P = physicalproperties analysis; and M = paleomagnetic analysis.

To ensure consistent and complete descriptions, the visual coredescriptions were entered into the computerized database HARVI. Thedatabase is divided into separate data sets for fine-grained rocks andcoarse-grained rocks. Each record is checked by the database program forconsistency and completeness, and is subsequently printed in a format thatcan be directly pasted onto the barrel sheet for curatorial handling.

When describing sequences of rocks, the core was subdivided intolithologic units on the basis of changes in texture, grain size, mineraloccurrence, and abundance, rock composition, and rock clast type. Foreach lithologic unit and section, the following information wasrecorded in the database system:

A. The leg, site, hole, core number, core type, and section number.B. The unit number (consecutive downhole), position in the sec-

tion, number of pieces of the same lithologic type, the rock name, andthe identification of the describer.

C. The Munsell color of the dry rock and the presence and characterof any structural fabric.

D. The number of mineral phases visible with a hand lens and theirdistribution within the unit, together with the following information foreach phase: (1) abundance (volume %); (2) size range {mm}; (3) shape;(4) degree of alteration; and (5) further comments if appropriate.

E. The groundmass texture: glassy, fine grained (<l mm), mediumgrained (1-5 mm), or coarse grained (>5 mm). Grain-size changeswithin units were also noted.

F. The presence and characteristics of secondary minerals andalteration products.

G. The abundance, distribution, size, shape, and infilling materialof vesicles (including the proportion of vesicles that are filled byalteration minerals).

H. The rock structure: determining whether the flow is massive,pillowed, thin or sheetlike, brecciated, or a hyaloclastite.

I. The relative amount of rock alteration, expressed both in thedescription and next to the graphic description of the archive half.Alteration was graded as fresh (<2%); slightly altered (2%-10%);moderately altered (10%-40%); highly altered (40%-80%); veryhighly altered (80%-95%); and completely altered (95%-100%).These grades of alteration were expressed in columnar form as F, S,M, H, V, and C, respectively. The type, form, and distribution ofalteration were also noted.

J. The presence of veins and fractures, including their abundance,width, mineral fillings or coatings, and orientation. The orientation ofveins and fractures was measured with a protractor, such that the topof each core is considered 0 degrees.

K. Other comments, including notes on the continuity of the unitwithin the core and on the interrelationship of units.

Basalts and diabases are termed aphyric (<1%), sparsely phyric(l%-2%), moderately phyric (2%-10%), or highly phyric (>10%),depending upon the proportion of phenocrysts visible with the handlens or binocular microscope. Basalts are further classified byphenocryst type (e.g., a moderately phyric Plagioclase olivine basaltcontains 2%-10% phenocrysts, mostly Plagioclase, with subordinateolivine). Volcanic rock names were initially assigned from megascopicphenocryst assemblages. Where chemical analyses or thin sectionswere available, more specific rock names were given.

Lavas erupted on the seafloor typically show a sparsely crystallineglassy margin overlying a variolitic (synonymous with spherulitic) zoneof quench phases, which may then grade into a coarser microlitic zonetoward the interior of the flow. In many samples, these zones are not welldefined, and may be expressed as alternating layers or areas. Variolescommonly show a sheaf-like or radial form, and microlites commonlyshow crystal clustering networks in areas of microlitic texture. We followBryan (1972) and Natland (1978) in interpreting these textures as quench-growth features. Finally, we use the term subophitic to refer to groundmasstextures where individual Plagioclase grains are partially surrounded byclinopyroxene grains with little to no glass.

Visual core descriptions of igneous rocks are given in the sitesummary appendixes, and descriptions of each rock unit are availablefrom the computerized database at the ODP repositories.

Thin-Section Descriptions

Thin sections of igneous rocks were examined to complement andrefine the hand-specimen observations. The percentage of variouscomponents present in a thin section was determined by counting 500points using an automatically advancing stage with an attachedcounter. The percentages and textural descriptions of individualphases were reported in the computerized database HRTHIN. Thesame terminology was used for thin section descriptions as was usedfor the megascopic descriptions. Thin-section descriptions are in-cluded in the site summary appendixes and are also available from theODP computerized database.

X-Ray Fluorescence Analysis

Prior to analysis, samples were crushed in a Spex 8510 shatterbox usinga tungsten carbide barrel. Where recovery permitted, at least 20 cm3 of

21

SHIPBOARD SCIENTIFIC PARTY

material was ground to ensure representativeness of the sample.Sample contamination with Nb, as a result of grinding in the tungstencarbide barrel, is kept below 1 ppm by grinding the samples for notmore than 2 min.

A fully automated wavelength-dispersive ARL8420 XRF (3 kW)system equipped with an Rh target X-ray tube was used to determinethe major oxide and trace- element abundances of whole-rocksamples. Analyses of the major oxides were carried out on lithiumborate glass disks doped with lanthanum as a "heavy absorber"(Norrish and Hutton, 1969). The disks were prepared from 500 mg of rockpowder that had been ignited for 2 hr at about 1030°C and mixed with6.000 g of preweighed (on shore) dry flux consisting of 80% lithiumtetraborate and 20% L a ^ . This mixture was then melted in air at 1150° Cin a Pt-Au crucible for about 4 min with constant agitation to ensurethorough mixing. It was then poured into a Pt-Au mold using a ClaisseFluxer. The 12:1 flux-to-sample ratio and the use of the lanthanumabsorber made matrix effects insignificant over the normal range ofigneous rock compositions. Hence the relationship between X-ray inten-sity and concentration becomes linear and can be described by:

where C; = concentration of element / (wt%); / - peak X-ray intensity ofelement i; m = slope of calibration curve for element i (wt%/cps); and bi

= apparent background concentration for element i (wt%).The slope m was calculated from a calibration curve derived

from the measurement of well-analyzed reference rocks (BHVO-1,G-2, AGV-1, JGB-1, JP-1, BR, and DRN). The analyses of thesestandards derived from the calibration curves used are given inTable 2. The background b was determined by regression analysisfrom the calibration curves.

Systematic errors resulting from short-term or long-term fluctua-tions in X-ray tube intensity and instrument temperature were ad-dressed by counting a standard disk among no more than six unknownsin any given run. The intensities of this standard were normalized toits known values, providing correction factors to the measured inten-sities of the unknowns. To reduce shipboard weighing errors, two glassdisks were prepared for each sample. Accurate weighing was difficulton board the moving platform of the JOIDES Resolution, and wasperformed with particular care as weighing errors could be a majorsource of imprecision in the final analysis. To maintain internalconsistency, all weighing was done by Joan Perry (Leg 136) or DonSims (Leg 137), the XRD/XRF technicians. Loss on ignition wasdetermined by drying the sample at 110°C for 8 hr and then byweighing before and after ignition at 1030° C in air.

Replicate analyses of rock standards show that the major-elementdata are precise within 0.5%-2.5%, and are considered accurate to

1% for Si, Ti, Fe, Ca, and K, and to between 3% and 5% for Al, Mn,Na, and P. Analytical conditions for the XRF analyses are given inTable 2 and the results of measurements of the internal rock standardBHVO-1 are reported in Table 3.

DOWNHOLE MEASUREMENTS

Tool Strings

Downhole logs can be used to characterize the geophysical,geochemical, and structural properties of a drilled sequence. Logmeasurements have the advantage over core-based measurements inthat they represent continuous and in-situ measurements of theborehole. After coring is completed at a hole, a tool string is lowereddownhole on a seven-conductor cable, and each of several tools in thetool string continuously monitors some property of the borehole.Three Schlumberger tool strings were used on Leg 136: (1) theQuad-tool (seismic-stratigraphic and litho-density tools), (2)geochemical, and (3) formation microscanner (FMS) combinations.The Lamont-Doherty temperature tool (TLT) was attached to the base

of each Schlumberger tool string. The analog borehole televiewer(BHTV) was also used in the basement interval of Site 843.

The Quad-tool combination used on Leg 136 comprised a digitalstring, consisting of long-spaced sonic transducer/receiver with adigital sonic cartridge (LSS), high-temperature lithodensity tool(HLDT), dual induction tool (DIT), compensated neutron porositytool (CNT), mechanical caliper (MCD), and natural gamma-ray tool(NGT). This tool combination measures compressional-wave velocityand provides indicators of the two variables that most often controlvelocity: porosity, as indicated by density or resistivity, and claycontent, as indicated by the natural gamma tool. The Quad-toolcombination can be broken down into two strings: the seismicstratigraphic combination and the litho-porosity combination. Theseismic stratigraphic string comprises the LSS/SDT, DIT, and NGT.The litho-porosity string combines the HLDT, CNT, and NGT.

The geochemical tool combination used during Leg 136 consistsof the NGT, aluminum clay tool (ACT), gamma-ray spectrometry tool(GST), and compensated neutron porosity (CNT). This tool stringmeasures the relative concentrations of 12 elements: silicon, calcium,aluminum, iron, titanium, sulfur, hydrogen, chlorine, potassium,thorium, uranium, and gadolinium.

The formation microscanner tool string includes both the FMS anda general purpose inclinometer tool (GPIT) that spatially orients theFMS resistivity image of the borehole wall. The tool string alsocontains a natural gamma-ray tool (NGT) to allow depth correlationof the FMS data with other logs.

Logs

A condensed description of the physical principles of the variouslogging tools follows; more detailed descriptions have been prepared bySchlumberger (1989), Serra (1984,1989), and Timur and Toksoz (1985).

Electrical Resistivity

The dual induction tool (DIT) provides three different measure-ments of electrical resistivity, each one capable of penetrating adifferent depth into the borehole wall. Two induction devices: deep(ILD), and medium (ILM, resistivity) send high-frequency alternatingcurrents through transmitter coils, creating magnetic fields that inducesecondary (Foucault) currents in the formation. These ground-loopcurrents produce new inductive signals, proportional to the conduc-tivity of the formation, which are recorded by the receiving coils.Measured conductivities then are converted to resistivity. A thirddevice (spherically focused resistivity, SFL) measures the currentnecessary to maintain a constant voltage drop across a fixed intervalbetween electrodes. Vertical resolution is around 2 m for the ILM andILD and about 1 m for the SFL.

Water content and salinity are by far the most important factors control-ling the electrical resistivity of rocks. To a first order, resistivity respondsto the inverse square root of porosity (Archie, 1942). Other factors influenc-ing resistivity include the concentration of hydrous and metallic minerals,vesicularity, and the geometry of interconnected pore space.

Sonic Velocity

The long-spaced sonic (LSS) tool uses two acoustic transmittersand two receivers to measure the time required for sound waves totravel over source-receiver distances of 2.4, 3.0, and 3.6 m. The rawdata are expressed as time required for a sound wave to travel through0.31 m of formation; these traveltimes are then converted to sonicvelocities. First arrivals for the individual source-receiver paths areused to calculate the velocities of the different waves traveling in theformation (compressional, shear, etc.). Only compiessional-wave ve-locity is determined during data acquisition, but the recorded full-waveforms may be processed post-cruise to improve the compressional-wave

22

EXPLANATORY NOTES

Table 2. Leg 136 XRF analytical conditions.

Element

SiO2

TiO2

A1,O3

Fc~2O3MnOMgOCaO

Na7OK 2 O

P 2O 5RhNbZrYSrRbZnCuNi

CrFcVTiO2

CeBa

Line

KαKαKαKαKαKαKαKαKαKαK-CKαKαKαKαKαKαKαKαKαKαKαKαLα

Lß

Crystal

PET(002)LiF(200)PET(002)LiF(200)LiF(200)TLAPLiF(200)TLAPLiF(200)Ge(lll)LiF(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(220)LiF(220)LiF(200)LiF(220)LiF(220)

Detector

FPCFPCFPCFPCKrSCFPCFPCFPCFPCFPCScintScintScintScintScintScintScintScintScintFPCFPCFPCFPCFPCFPC

Collimator

CoarseFineCoarseFineFineCoarseCoarseCoarseFineCoarseFineFineFineFineFineFineFineFineCoarseFineFineFineFineCoarseCoarse

Peakangle(deg.)

109.1086.16

144.4957.5363.0344.88

113.1854.73

136.66141.00

18.6021.3922.5423.8325.1526.6041.8145.0248.6469.3885.73

123.2086.16

128.35128.93

Backgroundoffset(dcg.)

00000

± 0.800

-1.20000

-0.

t 0.35t 0.35t 0.40t 0.41t 0.60t 0.40t 0.40t 0.60t 0.5040+0.70-0.50t 0.50t 1.50

± 1.50

Total count time

Peak

4040

1004040

20040

20040

10010020010010010010060606060406040

100100

(seconds)

Background

00000

4000

200000

20010010010010060606060406040

100100

Total Fe as Fe2C>3FPC: Flow proportional counter using P l o gasKrSC: Scaled krypton gas counterScint: Nal scintillation counterElements analyzed under vacuum using both goniometers at generator settings of 60 kV and 50 mA.

velocity or for determination of shear-wave velocities in hard formations.The vertical resolution of the tool is 0.61 m. Compressional-wave velocityis dominantly controlled by porosity and lithification; decreases in porosityand increases in lithification cause the velocity to increase.

Natural Gamma Ray

The natural gamma-ray tool (NGT) measures the natural radioactivityof the formation. Most gamma rays are emitted by the radioactive isotope*°K and by the radioactive elements of the U and Th series. The gamma-rayradiation originating in the formation close to the borehole wall ismeasured by a scintillation detector mounted inside the sonde. Theanalysis is achieved by subdividing the entire incident gamma-rayspectrum into five discrete energy windows. The total counts recorded ineach window, for a specified depth in the well, are processed to give theelemental abundances of K, U, and Th.

Because radioactive elements tend to be more abundant in clayminerals relative to other common sedimentary minerals, the gamma-ray curve is commonly used to estimate the clay or shale content. Otherrock or sediment types may also have radioactivity values rangingfrom moderate to extremely high, because of the presence of volcanicash, potassium feldspar, or other radioisotope-containing minerals.

Lithodensity Tool

The high-temperature lithodensity tool (HLDT) uses a 137Cegamma-ray source and measures the resulting flux at fixed distancesfrom the source. Under normal operating conditions, attenuation ofgamma rays is caused chiefly by Compton scattering (Dewen, 1983).Formation density is extrapolated from this energy flux by assumingthat the atomic weight of most rock-forming elements is approximate-ly twice the atomic number. A photoelectric effect index is also

provided. Photoelectric absorption occurs in the energy windowbelow 150 keV and depends on the energy of the incident gamma ray,the atomic cross section, and the nature of the atom. Because thismeasurement is almost independent of porosity, it can be used directlyas an indicator of matrix lithology. The radioactive source and detectorarray are placed in a tool that is pressed against the borehole wall bya strong spring arm; position of this spring arm indicates hole diameter.Excessive roughness of the hole will cause some drilling fluid toinfiltrate between the skid and the formation. As a consequence,density readings can be artificially low. Approximate corrections canbe applied by using caliper data. The vertical resolution is about 0.30 m.

Compensated Neutron Porosity

A radioactive source mounted on the compensated neutronporosity tool (CNT) sonde emits fast neutrons (4 MeV) into theformation, where they are scattered and slowed by collisions withother nuclei. When the neutrons reach a low energy level (0.025MeV) they are captured and absorbed by atomic nuclei such ashydrogen, chlorine, silicon, and boron. The scattering cross sectionis the quantity that describes the rate at which neutrons are slowed.Because the scattering cross section for hydrogen is about 100times larger than for any other common element in the crust, mostenergy dissipation is caused by collisions with water molecules.Therefore, a change in the number of neutrons detected at areceiver can be related to porosity. In practice, an array of detectorsis used to minimize hole size or drilling fluid effects. Because wateris present both in pores and as bound water (e.g., clay minerals),porosity measurements made in the presence of hydrous mineralsare overestimates of the true porosity. The vertical resolution of thetool is theoretically about 0.25 m, but low signal-to-noise ratiodegrades this potential resolution.

SHIPBOARD SCIENTIFIC PARTY

Table 3. Leg 136 analyses of standards derivedfrom the calibrations used for whole-rock analysis.

Element

SiO2

TiO2

A12O3Fe2O3

MnOMgOCaONa2OK2OP 2 O 5Total

Given

49.622.68

13.6712.230.177.21

11.322.170.530.27

99.87

A

50.202.76

13.6612.370.177.11

11.602.220.540.27

100.90

B

49.752.73

13.9812.250.177.12

11.482.240.530.27

100.52

Mechanical Caliper Device

The mechanical caliper device (MCD) provides a two-dimensionalcaliper log of the borehole by means of a bowspring-mounted meas-urement system. The hole diameter (HD) log is used to detect washoutsor constrictions. Borehole diameter significantly affects many of theother logging measurements, and the hole diameter is an importantsource of data for log correction routines. The MCD caliper tool issubject to sticking when formation mud gets into its mechanical parts,resulting in bimodal (fully open or nearly fully closed) readings. Incontrast, the hole diameter measurement produced by the high-temperature lithodensity tool is much more reliable. Consequently, onLeg 136 the MCD tool was used primarily to provide tool centralizationand to improve the quality of the sonic log, rather than to measurehole diameter.

Gamma Spectrometry Tool

The induced gamma-ray spectrometry tool (GST) consists of apulsed source of 14-MeV neutrons and a gamma-ray scintillationdetector. A shipboard computer performs spectral analysis of gammarays resulting from the interactions of neutrons emitted by the sourcewith atomic nuclei in the formation (Hertzog, 1979). Characteristicsets of gamma rays from six elements dominate the spectrum, permit-ting calculation of yields from six elements: Ca, Si, Fe, Cl, H, and S.The tool normalizes their sum, so they do not reflect the actualcomposition but only relative abundances of these elements. There-fore, ratios of these yields are commonly used in interpreting thelithology, porosity, and salinity of the formation fluid. Shore-basedprocessing is used to derive actual elemental abundances from this andthe other logs.

Aluminum Clay Tool

Aluminum abundance as measured by the aluminum clay tool(ACT) is determined by neutron-induced (Cf nuclear source) lategamma-ray spectrometry. By placing Nal detectors both above andbelow the neutron source, contributions from natural gamma-rayactivity can be removed. Calibration to elemental weight percent isdone by taking irradiated core samples of known volume and densityand measuring their gamma-ray output while placed in a jig attachedto the logging tool, usually after logging.

Formation Microscanner (FMS)

The FMS produces high-resolution microresistivity images of theborehole wall that can be used for detailed sedimentological or struc-tural interpretations, and for determining the orientation of fracturesand breakouts. The tool consists of sixteen electrode "buttons" on eachof four orthogonal pads that are pressed against the borehole wall. The

electrodes are spaced about 2.5 mm apart and are arranged in twodiagonally offset rows of eight electrodes each. The focused currentthat flows from the buttons is recorded as a series of curves that reflectthe microresistivity variations of the formation. Shipboard or shore-based processing converts the current intensity measurements intocomplete, spatially-oriented images. Further processing can also pro-vide measurements of dip angle and dip direction or azimuth ofbedding surfaces.

Applications of the FMS images include: detailed correlation ofcoring and logging depths; orientation of cores; mapping of fractures,faults, foliations, and formation structures; and determining strikesand dips of bedding. An important limitation is the restriction to a holediameter of less than 41 cm (16 in.); thus little useful information canbe obtained from washed-out hole sections.

General Purpose Inclinometer Tool (GPIT)

This tool provides a measurement of borehole inclination, theorientation of the tool with respect to the earuYs magnetic field usinga three-component magnetometer, and tool motion using an ac-celerometer. It is run with the FMS to provide spatial orientation ofthe borehole pads.

Borehole Televiewer

The borehole televiewer (BHTV) produces an ultrasonic acousticalimage of the borehole wall. A transducer emits ultrasonic pulses at afrequency of 1.3 MHz that are reflected at the borehole wall and thenreceived by the same transducer. The amplitude and traveltime of thereflected signal are determined and stored on video tape or in the loggingcomputer. A continuous rotation of the transducer and the upward motionof the tool produces a complete map of the borehole wall.

The amplitude depends on the reflection coefficient of the boreholefluid-rock interface, the position of the BHTV tool in the borehole, theshape of the borehole, and the roughness of the borehole wall. Thechange of the borehole wall's roughness (e.g., at fractures intersectingthe borehole) is responsible for the modulation of the reflected signal;therefore fractures or changes in character of the drilled rocks caneasily be recognized in the amplitude image. On the other hand, therecorded traveltime image gives detailed information about the shapeof the borehole: knowing the velocity of the ultrasonic signal in theborehole fluid allows calculation of one caliper value of the boreholefrom each recorded traveltime. As 128 points are typically recordedper transducer revolution, the BHTV can be regarded, in effect, as a"multi-arm caliper log."

Amplitude and traveltime are recorded together with a referenceto magnetic north by means of a magnetometer, permitting orientationof images. If features (e.g., fractures) recognized in the core areobserved in the BHTV images, orientation of the core is possible. TheBHTV can also be used to measure stress in the borehole throughidentification of borehole wall breakouts. In an isotropic, linearlyelastic rock subjected to an anisotropic stress field, breakouts form inthe direction of the axis of least principal horizontal stress.

Initial data analysis (breakouts, azimuth, hole radius, fracture/folia-tion azimuth, and inclination) can be performed using the Masscompand Macintosh computers after the data have been transferred fromvideotape (or for the digital tool, from the data acquisition PC).

LDGO Temperature Tool

The LDGO temperature tool is a self-contained tool that can beattached to any Schlumberger tool string. Data from two thermistors(one taking measurements once every second and the other once every5 s) and a pressure transducer (sampling every 5 s) are collected andstored in a Tattletale computer within the tool. Following the loggingrun, data are dumped from the Tattletale to the Masscomp computer

24

EXPLANATORY NOTES

for analysis. The fast-response, lower accuracy thermistor is able todetect sudden, very small temperature excursions caused by fluid flowfrom the formation. The slow-response, higher accuracy thermistorcan be used to estimate heat flow if one knows the history of drilling-fluid circulation in the hole and if one has at least two temperature logs(Jaeger, 1961). Data are recorded as a function of time; conversion todepth can be based on the pressure transducer or, preferably, onsimultaneous recording of an elapsed time vs. depth, usually providedby Schlumberger.

Log Data Quality

The quality of the log data may be seriously degraded in exces-sively large sections of the borehole by high wall rugosity or by rapidchanges in the hole diameter. Resistivity and velocity measurementsare less sensitive to changes in borehole size, whereas the nuclearmeasurements (density, neutron porosity, and both natural and inducedspectral gamma ray) are most seriously impaired because of thelarge attenuation by the borehole fluid. Corrections can be appliedto the original data to reduce the effects of these conditions and,generally, any departure from the conditions under which the toolwas calibrated.

Different logs may have small depth mismatches, caused byeither stretching of the cable or heaving of the ship during record-ing. Small errors in depth matching can impair the results in zonesof rapidly varying lithology. To minimize such errors, a wirelineheave motion compensator (HMC) adjusts for rig motion. Precisedepth matching of logs with cores is not possible in zones wherecore recovery is low because of the inherent ambiguity of placingthe recovered section within the interval cored.

Log Analysis

During logging, incoming data are observed in real time on amonitor oscilloscope and simultaneously recorded on digital tape inthe Schlumberger logging unit. After logging, these tapes are read bythe Masscomp computer system or the Vaxstation 3200 in thedownhole measurements laboratory and reformatted to a file formatcompatible with the Terralog log-interpretation software package.Terralog is an interactive system permitting many log manipulationand plot options. Thus, the log analysis and interpretation varies induration and procedure for each site. Most of the log interpretation iscarried out aboard ship; further analysis and interpretation are doneafter the cruise using a companion system in the Borehole ResearchLaboratory of Lamont-Doherty Geological Observatory.

Reprocessing of Logs

Sonic logs obtained in real time are not based on full-waveformanalysis, but on a threshold-picking technique that detects the com-pressional-wave arrival by a first-break criterion. Occasionally thistechnique fails because the threshold is exceeded by noise, or the firstcompressional arrival is attenuated. These phenomena, noise precur-sors and cycle skipping, create spurious spikes on the sonic log. OnLeg 136, raw traveltimes were reprocessed when necessary with analgorithm designed to reject cycle skips (Shipboard ScientificParty, 1987).

Raw geochemical logs for Leg 136 are shown following the barrelsheets for each site at which the geochemical tool string was run. Rawcount rates for six elements (Ca, Si, Fe, S, Cl, and H) are obtained inreal time by the Schlumberger data acquisition software. Post-cruisereprocessing utilizes a Schlumberger Elite 1000 workstation andproprietary Schlumberger software. The gamma spectrum at eachdepth is inverted for titanium, gadolinium, and potassium in additionto the six elements in the shipboard inversion. Though gadolinium ispresent in concentrations of only a few parts per million, its neutron-

capture cross section is so large that it can account for 10%-30% ofthe total gamma spectrum. Inclusion of these additional elementsimproves the quality of the overall inversion, particularly improvingthe accuracy of calculated calcium abundance, by converting sourcesof unaccounted variance to signals. Aluminum concentrations fromthe aluminum clay tool are adjusted for variations in cable speed.Changes in logging speed affect the time lag between neutronirradiation of the formation and recording of the induced gammaspectrum because the number of induced gamma rays decreasesrapidly with time. However, the small amount of ship heaveduring logging may make this correction less reliable in ODPholes than in land wells.

When the lithodensity tool is also run at the same site where thegeochemical tool string is run, further reprocessing of geochemicallogs is possible. The relative abundances of Ca, Si, Fe, Ti, Al, K, S,Th, U, and Gd are used to calculate a log of predicted photoelectriceffect. The difference between this log and the actual log ofphotoelectric effect can be attributed to the only two major elementsnot directly measured, Mg and Na. Major elements are expressed interms of oxide dry weight percent, based on the assumption thatoxygen is 50% of the total dry weight.

REFERENCES

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Ms 136A-102

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