ocean drilling program scientific results volume 123 · proceedings of the ocean drilling program,...

Gradstein, F. M., Ludden, J. N., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 123

7. EVOLUTION OF A MIOCENE CALCICLASTIC TURBIDITE DEPOSITIONAL SYSTEM1

Gregory R. Simmons2

ABSTRACT

A middle to upper Miocene turbidite depositional system exists at Site 765 in the southeastern Argo Abyssal Plain,along the northwest margin of Australia. Turbidites consist predominantly of planktonic calcareous components. Textural,mineralogical, bedding, and downhole logging trends within the system support the idea of a rapid progradation of theturbidite system in the middle to late Miocene. The onset of abundant turbidity currents into the area is attributed tochanges in oceanographic conditions that accompanied the progressive lowering of sea level associated with majorcontinental ice build-up on Antarctica. These data, in conjunction with seismic stratigraphy, support the existence of twomajor depositional pulses. Gradual retrogradation of the system occurred in latest Miocene time, coincident with thesubsequent rising of sea level. Retrogradation of the system was overwhelmed by massive slope failures during thePliocene, presumably triggered by earthquakes associated with the progressive Miocene-Pliocene collision of theAustralian margin and the Sunda-Banda Arc.

INTRODUCTION

Calcareous turbidites are abundant within the Neogene sectionat Site 765, located in the southeastern Argo Abyssal Plain (Fig.1). A turbidite depositional system existed here during middle tolate Miocene time. Turbidity currents were a common sedimen-tary process throughout the Cenozoic and Cretaceous at Site 765,but turbidites within the Miocene series exhibit a systematicpattern having decipherable evolutionary trends. The intention ofthis post-cruise investigation was to document the growth historyof the middle to late Miocene turbidite depositional system at Site765, herein referred to as the "turbidite sequence," and to relatethat history to the outside controlling factors that governed growthof the system. The investigation involves textural, bedding, andmineralogical analyses in conjunction with a synthesis of ship-board results (Ludden, Gradstein, et al., 1990). A LAB-TEC 100particle-size analyzer was used for this investigation. The sameinstrument is available aboard the JOIDES Resolution. Includedhere are some comments about the performance of the LAB-TEC100, which is compared with more traditional techniques alsoemployed in this investigation and aboard the JOIDES Resolution.

REGIONAL GEOLOGYThe Argo Abyssal Plain (Fig. 1) is a thin sliver of old oceanic

crust in the northeastern corner of the Indian Ocean, betweennorthwestern Australia and the Sunda Arc. Subduction of theabyssal plain into the Java Trench is driving convergence ofAustralia and the Sunda Arc. The Neogene sediments of the ArgoAbyssal Plain were likely influenced by the progressive Miocene-Pliocene collision between the Australian margin and the Sunda-Banda Arc (Carter et al., 1976) in influx of volcanic arc-derivedmaterials and tectonic triggering of redepositional events.

Site 765 is located in the southeastern Argo Abyssal Plain ona starved passive margin off northwest Australia (Fig. 1). The siteis situated -75 km north of the continent/ocean boundary and sitsin front of Swan Canyon, a reentrant bisecting Australia^ North-west Sheer and the marginal Exmouth Plateau. Swan Canyon andsimilar reentrants probably funneled mostly marginal marine ma-

1 Gradstein, F. M., Ludden, J. N., et al., 1992. Proc. ODP, Sci. Results, 123:College Station, TX (Ocean Drilling Program).

Department of Oceanography, Texas A&M University, College Station, TX77843, U.S.A.

terials from the Northwest Shelf and the Exmouth Plateau into theArgo Abyssal Plain during the Neogene. Site 765 is thought tohave existed well below the calcite compensation depth (CCD)for most of its history, particularly through the Cenozoic. DuringLeg 122, a transect was cored (Sites 759 through 761 and 764)across the Wombat Plateau, a small subplateau on the northernedge of the Exmouth Plateau (Haq, von Rad, O'Connell, et al.,1990). From this transect, we recovered Upper Cretaceous toCenozoic foraminifer and nannofossil oozes unconformably over-lying Triassic shallow marine and coastal facies.

DESCRIPTION OF TURBIDITE SEQUENCE ANDITS BOUNDARIES

The following is a discussion of shipboard observations (Lud-den, Gradstein, et al., 1990) centered on the upper and middleMiocene calcareous turbidites designated as lithologic SubunitsIIA and IIB, which were recovered from 189 to 460 m below theseafloor (mbsf) at Site 765 (Fig. 2). These turbidites (5 to 200 cmthick) are typically sandy at the base and fine upward to domi-nantly clay- and silt-sized particles. These commonly include a thinquartzose base (<2 cm thick), but are principally composed ofwhole and broken foraminifers, additional calcareous fragments,nannofossils, and clays. Primary sedimentary structures normallyassociated with turbidites and that allow internal differentiation(e.g., facies A, B, C, D, and E; Bouma, 1962) are common onlyin Subunit IIB. The sediments of Subunit IIA are comparativelyunlithified. The relative absence of primary sedimentary struc-tures is attributed more to poor preservation during coding andhandling than to any significant change in sedimentary processesbetween the subunits. Interbedded, clayey pelagic intervals arerare in Subunit IIA, but are common within Subunit IIB. Thepelagic intervals are typically dark gray claystones containing asmuch as 20% to 30% fine sand-sized to coarse silt-sized glassand/or quartz. Common authigenic dolomite was noted as well.Maximum thickness of the pelagic intervals in Subunit IIB in-creases downward from 6 to 20 cm.

As might be expected, reworking includes microfossils olderthan the time of deposition. The oldest planktonic foraminifersare Late Cretaceous. The abundance of calcareous benthic fora-minifers is generally low. Benthic assemblages typically rangefrom upper to middle bathyal depths. Reworking of even oldermaterials occurs within debris-flow deposits immediately under-lying and overlying the turbidite sequence. These debris-flowdeposits are matrix-supported conglomerates that consist primar-

151

G. R. SIMMONS

I2°SINDIAN

OCEAN

20

24

Figure 1. Location of Site 765, Argo Abyssal Plain, and track of BMR seismic Line 56/22. Modifiedfrom Ludden, Gradstein,et al. (1990).

ily of intraformational components, as well as some obviouslyallochthonous pebbles. Nannofossil ages of these pebbles rangefrom Late Cretaceous to Middle Jurassic.

Underlying the turbidite sequence, Subunit IIC (Fig. 2) con-sists of a single turbidite (>225 cm thick) that overlies a disturbed,matrix-supported conglomerate (>350 cm thick). Farther downthe section, in Subunit IIIA, calcareous turbidites (30-500 cmthick) are interbedded with clayey pelagic intervals (20-175 cmthick).

Overlying the turbidite sequence is Subunit IC (Fig. 2), whichconsists largely of calcareous turbidites (5-65 cm thick) similarto those in Subunits IIA and IIB. These turbidites are increasinglyfiner-grained upward, and interbedded pelagic intervals (1-25 cmthick) are increasingly abundant. Concomitant with this apparentretrogradation was an increasing occurrence and magnitude ofmassive redepositional events involving slumps, debris flows,and turbidity currents. Subunit IB consists almost entirely ofmassive sediment gravity-flow deposits up to 42.5 m thick. Far-ther up the section, Subunit IA consists of alternating fine-grainedcalcareous turbidites (10-200 cm thick) and intervals of clayeysiliceous ooze (5-120 cm thick).

The turbidite sequence corresponds with seismic sequences 3,4, and 5 (Figs. 2 and 3). The upper boundary of sequence 3 is neara lower Pliocene hiatus(?) within Subunit IC. This sequenceboundary is locally onlapped, supporting an interpretation of ahiatus. The lower boundary of sequence 5, at the base of SubunitIIC, is a regional seismic unconformity. Sequences 3 and 4 arecharacterized by parallel to subparallel, generally low-amplitudereflections having moderate continuity. Gently mounded featuresexist locally. No obvious lithologic break corresponds to thehigh-amplitude boundary between sequences 3 and 4. The Ship-board Scientific Party speculated that it was a diagenetic boundaryrelated to increased lithification within the turbidite sequence.Here, I identify an apparent depositional break at 290 mbsf, to

which this seismic sequence boundary may tie. Sequence 5 ischaracterized by continuous, parallel, high-amplitude reflectionsrelated to lithologic availability within Subunit IIB.

The total gamma-ray counts fluctuate 10 to 20 API unitslocally in the turbidite sequence (Fig. 2), presumably in responseto lithologic variability; however, only thicker beds tie directly toindividual peaks and troughs. Vertical resolution of the gamma-ray tool is 46 cm (C. Broglia, pers. comm., 1991). Maximum andminimum gamma-ray values through the turbidite sequence occurover a short transition between 360 and 420 mbsf in Subunit IIBand lowermost Subunit IIA. After leveling off, above the shorttransition, the average gamma-ray values gradually increase up-ward through Subunits IIA and IC.

Magnetic susceptibility is generally low through the turbiditesequence (Fig. 2). Distinctive peaks occur primarily above 200mbsf, near the top of Subunit IIA, and below 450 mbsf, near thebase of Subunit IIB. These susceptibility peaks correspond toclay-rich pelagic intervals. Note that zero susceptibility corre-sponds to intervals of no recovery. In a few instances, a one-to-onecorrelation appears to occur between the gamma-ray and suscep-tibility measurements, but recovery was insufficient for a mean-ingful, detailed comparison.

METHODSFor this investigation, 244 samples were selected for carbonate

and grain-size analyses. Between three and seven samples weretaken from each of over 50 selected turbidites proportionatelyspaced through the turbidite sequence (1-2 turbidites/core).

Carbonate percentages were measured with a 5011 Coulometerequipped with a system 140 carbonate carbon analyzer at TexasA&M University. Carbonate samples were dried, ground, andweighed out to between 12 and 20 mg. Each sample was reactedin a 2-mL 2N HC1 solution. The liberated CO2 was reacted in amonoethanolamine solution with a colorimetric indicator to de-

152

EVOLUTION OF A MIOCENE DEPOSITIONAL SYSTEM

100

200

300

400

500

LithologyTotal gamma ray

0 API units 6.0

Clayey calcareousturbidites and clayey

siliceous ooze

Clayey calcareousturbidites, debris flowdeposits and slumps

147.3-Clayey calcareous

turbidites and minorclaystones

189.1

Calcareousturbidites

379.0-

Calcareousturbidites and

minor claystones

459.9-Debris flow deposit

474.1Calcareous turbiditesand dark claystones

-Hoo

-h200

-1-300

+-400

5000 (Kx10"°cgs)1 0 0

Magnetic susceptibility

Figure 2. Site 756 Neogene synthesis chart comparing sub-bottom depths, biostratigraphic ages, seismicsequences, lithology, and magnetic susceptibility from the Site 765 chapter (Ludden, Gradstein, et al.,1990), and total gamma-ray log provided by the Borehole Research Group, Lamont-Doherty GeologicalObservatory.

termine the inorganic carbon content, from which the percentageof carbonate was calculated. Pure (reagent grade) carbonate stand-ards were run twice before starting, and approximately every 15samples thereafter. Carbonate standard percentages ranged be-tween 99.5% and 100.5%.

Grain-size samples were weighed out to -1.5 g and initiallyprepared in 50-mL graduated tubes with screw-on tops. A 30%hydrogen peroxide solution was used to initiate disaggregation for1 hr, bringing the total volume to 5 mL (sediment + 2-3 mLperoxide). Shipboard measurements of total organic carbonthroughout the turbidite sequence were typically low (<0.5%).Some of the coarsest samples were highly reactive with peroxideand required diluting. Shipboard visual core descriptions occa-sionally note coaly fragments within coarse turbidite bases. About

35 to 40 mL of dispersant solution was then added. A 5.5-g/Lsolution of sodium hexametaphosphate (NaPO3)6 was used. Sam-ples were allowed to sit overnight. Periodically, the screw-on topswere tightened and the samples hand shaken. Just prior to grain-size analysis, each sample was placed in an ultrasonic bath for 3min. Finally, the samples were transferred to optically transparentbeakers and dispersant added, bringing the final volume to 100mL. Grain-size analysis consisted of using both a Lasentec LAB-TEC 100 particle-size analyzer (available at Texas A&M Univer-sity) and a modified pipette procedure developed for this investi-gation.

The LAB-TEC 100 used for the investigation is similar to themodel available for use aboard the JOIDES Resolution. Theinstrument has a laser diode light source and measures the

153

sw NESite 76556-23C

Seismicsequence

Site 76556-23C

Seismicsequence

Fiure 3. BMR multifold seismic Line 56/22 across Site 765. Approximate track location is shown in Figure 1. Line drawing shows interpretation of seismic sequences 1 through 7, includingdisconformable relationships at sequence boundaries. From Ludden, Gradstein, et al. (1990).


backscatter of light from particles within a suspension. The sus-pension is maintained within a beaker that has been firmly posi-tioned above a magnetic stirring plate. The size of a particle isdetermined by the length of time it takes for the particle to moveacross the focal point of the light beam. Data were collected fromtwo range settings, each providing eight channels that corre-sponded to half Φ-scale intervals (Table 1). The data (collected ascounts per channel) were converted into percentages of the totalfor each setting and then were adjusted to 100% for a combinedsize range. Finally, the adjusted percentages were summed pro-gressively and plotted as cumulative curves. The cumulativecurves in Figure 4 are from representative coarse- and fine-grained turbidite samples analyzed at concentrations of ~5, 10,and 15 g/L. The LAB-TEC 100 provided similar results over therange of concentrations tested, but increased sample concentra-tions yielded slightly coarser cumulative curves. Sample concen-trations of 15 g/L (corresponding to a 1.5-g sediment sample)were decided upon for this investigation to minimize experimen-tal error associated with weighing in the modified pipette proce-dure.

In Figure 5, LAB-TEC 100 results are compared with tradi-tional sieve and pipette analyses of three samples: relatively fine-,intermediate-, and coarse-grained examples. Except for the coarseend of the fine-grained sample, the LAB-TEC 100 underestimatesboth ends of the grain-size distribution. Traditional proceduresexpress grain size in the weight percent of size classes. TheLAB-TEC 100 detects backscatter of light, which is determinedby surface area of the particles in suspension. Surface area de-creases for a given sediment mass as grain size increases. There-fore, the underestimation of coarse material by the LAB-TEC 100is expected. Leg 124 scientists (1990) pointed out that surface areaabundances are more directly comparable to smear-slide determi-nation, historically the principal method of analyzing grain sizeaboard both the Glomar Challenger and the JOIDES Resolution.In addition, coarser material is less likely to be suspended uni-formly to the level of the light source (2.5 cm above the beakerbottom) by magnetic stirring. Underestimation of the fine end,especially the clay-sized material, is problematic because clayshave a high ratio of surface area to mass. Flocculation is unlikely,as similar methods for dispersal and sediment sample concentra-tions were used for both LAB-TEC 100 and pipette analyses. Itseems more likely that the LAB-TEC 100 is not consistentlydetecting clear backscatter from the fine silt- and clay-sizedparticles. The LAB-TEC 100 does provide a means of calibrationwhereby raw counts for each channel can be weighted by com-pensation factors, but according to Lasentec, "compensation fac-tors may only be good for a specific material at a specific concen-tration," and "entering the correct compensation factors is moreof an art than a science" (Lasentec Operations Manual). The useof compensation factors was not attempted for this investigation.Singer et al. (1988) compared the capability of a number ofgrain-size analysis instruments and found that silt-clay mixturespresent the greatest analytical problem for the instruments tested.

This poses a problem. Turbidites are inherently poorly sorted,clay-rich sediments. Obviously, a procedure for providing grain-size distribution for comparison with LAB-TEC 100 results isdesirable. Standard pipette procedures are time-consuming andrequire large samples. A modified pipette procedure employingStokes settling velocities was developed for this investigation.Total volume of each LAB-TEC 100 sample was 100 mL, andeach contained a known mass of dispersant. Following LAB-TEC100 analysis (while the sample was still stirring and, presumably,a uniform suspension), a 20-mL aliquot was withdrawn; thisrepresented 20% of the total sediment population and dispersant.Decanting the upper 20 mL of the sample remaining in the beaker(after sufficient time was allowed for settling out sand- and

Table 1. Intervals of particle-size data collectedusing size ranges provided by Lasentec LAB-TEC 100 particle-size analyzer.

125-µmsieve

(phi)

>3.03.0-3.53.5-4.04.0-4.54.5-5.05.0-5.55.5-6.0

<6.0

(µm)

>125125-8888-6363-4444-3131-2222-16<16

16-µmsieve

(phi)

>6.06.0-6.56.5-7.07.0-7.57.5-8.08.0-8.58.5-9.0

<9.0

(µm)

>1616-1111-88-66-44-33-2<2

100 r

Sand

5 6 7Phi scale

Silt

9 <9

Clay

Figure 4. Cumulative percent grain-size curves of representative coarse-and fine-grained turbidite samples analyzed using Lasentec LAB-TEC100 particle-size analyzer. Three curves for each sample represent differ-ent sample concentrations (solid circles = ~0.5 g/L, solid squares = ~l.Og/L, solid triangles = ~1.5 g/L).

silt-sized particles) yielded 20% of the total clay population anddispersant. The remaining sample was wet-sieved through a 62-µm screen. Material remaining on the screen represented 80% ofthe total sand population. At this point, one could easily calculatethe total sand, silt, and clay populations. This procedure workedwell, except for the very coarse samples, where the abundant sandtended to clog the pipette during withdrawal of the total sedimentaliquot. For these cases, the LAB-TEC samples were run at -60mL, after which they were wet-sieved with dispersant (~40 mL)until the sample reached 100 mL. Material remaining on thescreen represented 100% of the total sand population. The samplewas re-suspended in the LAB-TEC 100, and the modified pipetteprocedure was run for the silt and clay populations.

Bed thickness represents another important parameter for char-acterizing vertical sequences of turbidites. Upon recovery, cores

155

G. R. SIMMONS

LAB-TEC 100Traditional pipette

Figure 5. Cumulative percent grain-size curves of representative fine-(A), intermediate- (B), and coarse-grained (C) turbidite samples compar-ing LAB-TEC 100 results (solid circles) with standard pipette and sieveresults (open circles).

brought on board the JOIDES Resolution were sectioned into1.5-m lengths to provide easier handling and storage. These sec-tions are convenient units to use when measuring bed thickness.The number of turbidites was tallied for 88 complete sectionsbetween 150 and 475 mbsf, using shipboard visual core descrip-tions. The turbidites/section values (bed abundance) were as-signed the depth to the top of each section. Bed abundance isinversely related to average bed thickness.

Following analyses, data were compiled for the charac-terization of individual turbidites and for the turbidite sequenceas a whole (Table 2).

RESULTSIn Figure 6, the percentages of sand, silt, and clay obtained

from the LAB-TEC 100 are compared with those using the modi-fied pipette procedure. These data are listed in Table 2. In general,a positive, but poor, correlation exists between the two proce-dures. The LAB-TEC 100 exhibited less response to intersample

A100

80

1 60

£40O

20

0B

100

80

feer 40cö

20

0C100

80

JT*

I • I

, : i« ,

c

CO20

<i

20 40LAB-TEC

60100

80(%)

100

Figure 6. Scatter charts comparing percent of clay (A), silt (B), and sand(C) of all samples from LAB-TEC 100 and the modified pipette proce-dures.

variability, over-representing the presence of silt, which is to beexpected in light of comparison to standard pipette procedures(Fig. 3). The percent sand comparison shows the greatest scatter.This is attributed primarily to difficulties in maintaining the sandin uniform suspension. The LAB-TEC 100 appears most sensitivein detecting variations in clay content >~30 wt%, although theactual values were underestimated. At this point, it seems fittingto emphasize the preliminary nature of the methods applied. Thescope of this investigation was concerned more with relative thanwith absolute variations in grain size. Originally, I planned to relyheavily on the LAB-TEC 100 results, but it was soon evident thatintersample variability using the LAB-TEC 100 was insufficientwithout applying weighting factors. An objective and efficientapproach to this problem was not readily apparent. To applyweighting factors successfully requires some prior knowledge ofthe sample being analyzed. Therefore, an additional method wasgoing to be necessary to gain sufficient knowledge of the sampleto apply weighting factors objectively. The modified pipettemethod can provide sand, silt, and clay ratios; these can be

156


matched by undertaking an iterative procedure for applyingweighting factors to raw LAB-TEC 100 counts. However, therepeatability of the modified pipette procedure has not been thor-oughly tested.

To look at textural variation through the turbidite sequence, agrain-size parameter was needed for plotting values vs. depth. Theweight-percent values from the modified pipette procedure dis-play greater intersample variation. The large amount of scatter inthe sand percentage comparison (Fig. 6C) is disturbing and sug-gests significant experimental error in both procedures. Weight-percent clay, or conversely, cumulative weight-percent sand andsilt was selected as the grain-size parameter because it exhibitsthe largest range of values and the least amount of scatter in theLAB-TEC 100 comparison (Fig. 6A).

Grain size varies principally within individual turbidites, butcharacterization of a vertical sequence relates more to variationamong individual turbidites. To plot cumulative weight-percentsand and silt vs. depth in a meaningful way, I had to subdivide thedata scatter that resulted from interturbidite variation. Normally,turbidites are subdivided according to a facies model (e.g., faciesA, B, C, D, and E) on the basis of the primary sedimentarystructures present (Bouma, 1962). The relative absence of primarysedimentary structures in Subunit IIA prevented an objectiveassignment within the context of a "Bouma sequence." To subdi-vide the data (both grain-size and carbonate values) in the depthplots, samples were assigned to upper, middle, and lower turbiditedivisions, based on the spatial location of samples within theturbidites.

In Figure 7, cumulative weight-percent sand and silt and bedabundance are plotted vs. depth. Total gamma-ray and magnetic-susceptibility curves are included for reference and comparison,as with the plot of percent carbonate and bed abundance vs. depth(Fig. 8). The grain-size, carbonate, and bed-abundance data sharea similar overall trend with depth.

All these data are variable in Subunit IIB. Low values ofcumulative sand and silt (<15%) and carbonate (<10%) are fromclay-rich pelagic intervals. Intermediate values are from the topsof turbidites into which pelagic clay has been reworked by bio-turbation. From the relative distribution of upper, middle, andlower turbidite samples, fining- and less calcareous-upwardtrends are evident within individual turbidites. The upper turbiditesamples were almost invariably the finest, but reverse gradingbetween the middle and lower turbidite samples is not unusual.

The variability of all the data decreases over a short transitionthrough upper Subunit IIB and lowermost Subunit IIA. From 420to 360 mbsf, cumulative weight-percent sand and silt valuesincrease (from 7%-75% to 44%-87%), carbonate percentagesincrease (from 6%-87% to 82%-92%), total gamma-ray countsdecrease (from 45 to 15 API units), and magnetic-susceptibilitymeasurements decrease (from 10 to 2 K × 10~6 cgs). Bed abun-dance decreases abruptly from 13 to 6 turbidites/1.5 m section at360 mbsf. Each of these trends levels off somewhat, and then, atleast through the upper half of Subunit IIA, gradually reverses.

Significant bedding breaks occur at 360 and 290 mbsf, wherebed abundance abruptly decreases and then gradually increasesupward. A bedding break at 360 mbsf corresponds to the late tomiddle Miocene boundary in lowermost Subunit IIA and, asdescribed previously, represents a turning point following a shorttransition of grain-size, carbonate, and total gamma-ray values. Abedding break at 290 mbsf possibly ties to the boundary betweenseismic sequences 3 and 4 (Fig. 2) in middle Subunit IIA. Thisbedding break is characterized by gamma-ray and magnetic-sus-ceptibility peaks. Carbonate values (Fig. 8) step down (60%-80%directly above vs. 72%-93% directly below) above the break.Above 290 mbsf, through upper Subunit IIA, average bed thick-ness (inverse of bed abundance), grain size (principally upper

1 5 0

Total gamma ray (API units)and

Bed abundance (turbidites/1.5 m)0 30 60

200 -

450 •

5000 50 100

Sand and silt (Cumulative%)and

- 6 .Magnetic susceptibility (Kx10 cgs)

Figure 7. Bed abundance, total gamma-ray log, cumulative weight percentsand and silt (>8 Φ from modified pipette procedure), and magneticsusceptibility plotted vs. sub-bottom depth for lithologic Subunits IC,IIA, IIB, IIC, and IIIA at Site 765 (dashed line = bed abundance);grain-size samples are from upper turbidite divisions (solid circles),middle turbidite divisions (open circles), and lower turbidite divisions(solid triangles).

turbidite), and percent carbonate gradually decrease. Totalgamma-ray values gradually increase across the same interval.The data in Subunit IC are variable, which, as in lower SubunitIIB, is attributed to increased abundance of clayey pelagic inter-vals.

DISCUSSION

This investigation supports the idea of a late to middle Mio-cene turbidite depositional system having measurable evolution-ary trends in bedding, grain size, and mineralogy. Stow et al.(1985) discussed the principal factors that influence the develop-ment of turbidite systems. These include sediment type and sup-ply, tectonics, and sea level. The setting of Site 765 places

157

G. R. SIMMONS

Table 2. List of core sections for/and bed-abundance measurements and core samples for/and carbonateand grain-size measurements.

Core, section,interval (cm)

123-765B-16H-517H-117H-217H-317H-417H-617H-718H-118H-418H-518H-618H-7, 72-7418H-7, 85-8618H-7, 95-9618H-7, 105-10618H-CC, 4-519X-119X-219X-2, 48-4919X-2, 58-5919X-2, 68-6919X-2, 78-7919X-2, 88-8919X-319X-419X-4, 39-4019X-4, 49-5019X-4, 59-6019X-4, 63-6420X-120X-1,92-9320X-1, 102-10320X-1, 112-11320X-1, 122-12320X-1, 136-13720X-220X-320X-420X-520X-5, 54-5520X-5, 64-6520X-5, 74-7520X-5, 84-8520X-5, 94-9620X-5, 102-10321X-121X-1,64-6521X-1,74-7521X-1, 84-8521X-1,94-9521 X-1, 103-10421X-221X-321X-421X-4, 134-13621X-4. 146-147

Depth(mbsf)

150.2153.9155.4156.9158.4161.4162.9163.6168.1169.6171.1172.7172.8172.9173.0173.1173.3174.8175.3175.4175.5175.6175.7176.3177.8178.2178.3178.4178.4183.0183.9184.0184.1184.2184.4184.5186.0187.5189.0189.5189.6189.7189.8189.9190.0192.7193.3193.4193.5193.6193.7195.7197.2198.7198.5198.7

BA

898105555125

87

129

9

81447

7

659

TO

UUMLL

UMMLL

UMLL

UuMLL

UUMLLU

UuMLL

UU

CO3(%)

29.663.662.969.171.4

3.065.875.478.066.3

10.761.558.17.9

25.077.976.671.957.4

83.081.785.981.380.224.0

58.055.154.960.263.5

77.975.7

Modified pipette proceduresand(wt%)

0.90.00.015.573.8

0.055.756.968.465.8

0.00.169.688.8

0.00.03.535.367.1

0.30.030.532.642.79.3

9.30.20.07.533.1

0.40.7

silt(wt%)

13.822.528.742.610.8

14.117.921.014.912.9

16.453.31.03.2

17.434.845.744.116.1

42.034.947.937.123.628.2

28.222.323.167.447.2

34.737.9

clay(wt%)

85.377.571.341.915.4

85.926.422.116.721.3

83.646.629.48.0

82.665.250.820.616.8

57.765.121.630.333.762.5

62.577.576.925.119.7

64.961.4

LAB-TEC 100sand(%)

0.10.00.02.012.5

0.020.321.822.37.3

0.10.120.65.2

0.00.00.117.625.6

0.00.021.618.419.10.2

0.00.00.012.722.0

0.00.0

silt(%)

68.162.364.775.568.9

68.362.662.961.572.4

65.575.464.272.9

63.560.672.567.260.4

68.364.765.465.466.371.2

70.563.763.971.864.7

65.967.3

clay(%)

31.837.735.322.518.6

31.717.115.316.220.3

34.424.515.221.9

36.539.427.415.214.0

31.735.313.016.214.628.6

29.536.336.115.513.3

34.132.7

significant constraints on these factors. Site 765 exists on amature, starved, passive margin. Low subsidence rates within thereceiving basin, broad sheer widths, and low continental-margingradients are all attributes characteristic of this setting.

Seismic activity tends to influence the frequency and volumeof sediment gravity flows. Mature, massive margins typicallyexperience infrequent, but large, earthquakes that trigger infre-quent, but massive, slope failures. Frequent seismic activity alongactive margins generates more regular, but smaller, events. Themajor tectonic element of the region is the convergent plateboundary along the Sunda-Banda Arc. Site 765 is distant, but wasprobably influenced by earthquakes generated at the convergencezone. The Australian margin experienced a progressive collision

with the Sunda-Banda Arc during the Miocene through Pliocene.It also seems logical that the degree of tectonic influence wouldincrease as the collision progressed.

The profound effect of Neogene glacio-eustatic fluctuations onterrigenous deep-sea sedimentation is well documented, but thenature of the response for calciclastic margins is less clear. Shan-mugam et al. (1985) saw a correlation of both siliciclastic andcalciclastic turbidites with lowstands of sea level throughout therock record. Alternatively, Droxler and Schlager (1985) sug-gested that, at least of active, shallow-water carbonate platforms,calciclastic turbidites are most common during highstands of sealevel, when the platforms are submerged and actively producingand shedding material onto nearby slopes.

158


Table 2 (continued).

21X-5.7-821X-5, 14-1521X-5, 23-2421 X-5, 32-3322X-122X-222X-2, 67-6822X-2, 74-7522X-2, 84-8522X-2, 94-9522X-2, 105-10622X-2, 114-11522X-322X-422X-4, 104-10522X-4, 114-11522X-4, 124-12522X-4, 134-13522X-4, 145-14623X-123X-223X-2, 79-8023X-2, 89-9023X-2, 99-10023X-2, 108-10924X-124X-1, 96-9724X-1,102-10324X-1, 112-11324X-1, 122-12324X-224X-424X-4, 106-10724X-4, 166-11724X-4, 123-12425X-1,49-5025X-1, 59-6025X-CC, 4-525X-CC, 14-1526X-126X-226X-2, 19-2026X-2, 24-2526X-2, 34-3526X-2, 44-4526X-2, 59-6026X-3, 80-8126X-326X-3, 9-1026X-3, 19-2026X-3, 29-3026X-3, 39-4026X-3, 49-5026X-3, 57-5827X-127X-1.9-1027X-1, 29-3027X-1,49-5027X-1, 69-7027X-1,87-8827X-3, 48-4927X-3, 54-5527X-3, 64-6527X-3. 74-75

198.8198.8198.9199.0202.4203.9204.6204.6204.7204.8205.0205.0205.4206.9207.9208.0208.1208.2208.4212.1213.6214.4214.5214.6214.7221.8222.8222.8222.9223.0223.3226.3227.4227.5227.5232.0232.1232.2232.3241.2242.7242.9242.9243.0243.1243.3243.5244.2244.3244.4244.5244.6244.7244.8250.9251.0251.2251.4251.6251.8254.4254.4254.5254.6

109

76

410

9

1010

56

7

3

MMLL

UUUMLL

UUMLL

UUML

UUML

UMLUMML

UUMMLL

UUMLLL

UUMLLULUL

75.174.665.048.8

61.048.759.963.872.569.8

54.769.071.973.167.7

75.073.272.275.8

72.274.170.477M

71.679.062.361.370.970.077.8

52.973.278.480.086.579.1

77.279.773.578.264.460.6

62.870.673.281.467.061.885.059.779.8

5.929.946.865.6

0.00.01.00.343.063.2

0.00.41.214.574.0

0.00.00.441.9

0.00.00.024.9

0.028.667.60.15.13.971.7

0.31.04.812.228.352.0

3.61.865.360.460.771.2

0.10.20.721.955.50.437.60.216.9

33.533.724.415.3

22.427.637.036.128.116.4

30.235.542.644.22.0

8.937.840.932.8

24.544.033.949.5

38.241.311.735.558.350.216.7

38.355.448.152.851.726.1

37.046.616.318.218.012.5

42.941.245.453.522.735.545.042.754.4

60.636.428.819.1

77.672.462.063.628.920.4

69.864.156.241.324.0

91.162.258.725.3

75.556.066.125.6

61.830.120.764.436.645.911.6

61.443.647.135.020.021.9

59.451.618.421.421.316.3

57.058.653.924.621.864.117.457.128.7

0.213.922.72.7

0.00.00.10.021.226.4

0.00.00.74.313.4

0.00.00.017.9

0.00.10.023.6

0.015.214.10.05.32.93.5

0.10.10.83.421.026.9

0.00.116.617.110.913.3

0.00.10.114.828.60.129.90.115.2

70.269.562.273.7

64.164.169.266.863.059.1

65.169.262.174.269.6

70.470.171.166.5

67.376.871.063.0

67.169.570.170.479.776.777.1

74.175.778.379.465.559.6

71.071.667.066.670.868.8

72.775.474.369.558.175.057.575.669.3

29.616.615.123.6

35.935.930.733.215.814.5

34.930.837.221.517.0

29.629.928.915.6

32.723.129.013.4

32.915.415.829.615.020.419.4

25.824.220.917.213.513.5

29.028.316.416.318.317.9

27.324.525.615.713.324.912.624.315.5

The onset of abundant turbidites at Site 765 coincided with aperiod of progressive basinward shifts in coastal onlap (Haq et al.,1987), attributed to the build-up of the Antarctic continental icesheet. Turbidite coarseness, carbonate content, and bed thicknessincreased to near-maximum values at 360 mbsf. This approximatepoint of the upper to middle Miocene boundary coincides with thebase of the second-order TB3 supercycle (Haq et al., 1987), amajor eustatic lowstand. It is unclear how a eustatic event havinga magnitude on the order of 100 m could significantly alter

conditions on the Exmouth Plateau, presumably the source of theredeposited materials, which on average is more than 1500 mdeep. It seems likely that increased turbidite abundance resultedfrom changes in oceanographic conditions, such as increasedproductivity and/or circulation, in response to the onset of glacialbuild-up in Antarctica. An extensive upper Oligocene to middleMiocene progradational cycle for the Northwest Australian Shelfhas been described by Apthorpe (1988). Convincing evidenceexists for at least two depositional episodes within the turbidite

159

G. R. SIMMONS



27X-3, 79-8028X-128X-228X-2, 40-4128X-2, 63-6428X.-2, 88-8928X2, fi 4-11528X^2, 134-13529X-129X-1, 111-11229X-1, 119-12029X-1, 128-12930X-1, 18-1930X-1, 28-2930X-1,38-3930X-1,52-5330X-1,69-7031X-131X-231X-2, 44-4531X-2, 54-5531 X-2, 64-6531 X-2, 67-6831X-431X-4, 104-10531X-4, 114-11531X-4, 124-12531 X-4, 134-13531 X-4, 144-14532X-132X-1, 11-1232X-1,39-4032X-1, 60-6132X-1, 79-8032X-1, 97-9832X-1, 122-12332X-232X-332X-3, 14-1532X-3, 33-3432X-3, 54-5532X-3, 70-7133X-133X-1, 89-9033X-1, 99-10033X-1, 109-11033X-1, 120-12133X-1, 132-13333X-233X-333X-433X-4, 64-6533X-4, 84-8533X-4, 104-10533X-4, 124-12533X-4, 138-13934X-134X-1,74-7534X-1,84-8534X-1,94-9534X-1, 104-10534X-1,118-11935X-135X-1, 103-104

Depth(mbs<)

254.7260.6262.1262.5262.7263.0263.2263.4270.2271.3271.4271.5280.1280.2280.3280.4280.6289.6291.1291.5291.6291.7291.8294.1295.1295.2295.3295.4295.5299.3299.4299.7299.9300.1300.3300.5300.8302.3302.4302.6302.7303.0309.0309.9310.0310.1310.2310.3310.5312.0313.5314.1314.3314.5314.7314.9318.7319.4319.5319.6319.7319.9328.3329.3

BA

42

4

19

6

3

45

4

886

6

3

TO

L

UUMLL

UMLUUMLL

UMLL

UUMLL

UUMMLL

UUML

UUMLL

UUMLL

UuMLL

U

CO3(%)

58.5

64.570.071.173.984.0

60.072.171.764.674.379.975.363.9

45.272.386.576.0

78.089.589.793.090.9

79.278.987.488.284.288.8

82.179.386.273.4

72.873.773.383.583.9

81.579.388.193.891.2

57.671.376.786.340.0

77.2


1.2

0.00.00.30.734.1

2.51.432.20.54.59.832.20.0

5.61.738.328.3

0.036.451.452.048.9

0.00.717.119.243.953.6

0.00.620.976.8

0.20.00.59.135.1

0.31.722.948.337.5

0.10.30.322.268.2

0.5

silt(wt%)

43.0

39.444.148.156.243.1

47.047.643.040.972.058.821.450.0

22.248.738.735.1

55.444.627.732.825.8

49.253.054.457.331.830.5

56.657.352.54.8

50.751.946.459.430.7

50.848.147.126.733.1

46.649.848.358.06.8

49.7

clay(wt%)

55.8

60.655.951.643.122.8

50.551.024.858.623.531.446.450.0

72.249.623.036.6

44.619.020.915.225.3

50.846.328.523.524.315.9

43.442.126.618.4

49.148.153.131.534.2

48.950.230.025.029.4

53.349.951.419.825.0

49.8

LAB-TEC 100sand(%)

0.5

0.00.00.10.724.1

0.20.713.50.113.014.918.30.4

0.20.23.12.8

0.124.321.810.48.5

0.00.512.426.223.18.4

0.00.515.87.6

0.00.00.215.223.9

0.10.420.17.615.1

0.10.10.126.14.1

0.2

silt(%)

77.8

70.773.476.079.361.8

77.775.869.976.573.470.265.480.7

71.177.079.978.2

78.061.363.872.974.8

74.879.372.660.962.974.9

77.080.469.475.3

74.374.279.272.361.7

78.179.166.275.769.5

77.377.875.561.671.3

78.5

clay(%)

21.7

29.326.623.920.014.1

22.123.516.623.413.614.916.318.9

28.722.817.019.0

21.914.414.416.716.7

25.220.215.012.914.016.7

23.019.114.817.1

25.725.820.612.514.4

21.820.513.716.715.4

22.622.124.412.324.6

21.3

depositional system. A significant break, particularly in bed-abundance and carbonate data, occurs within the upper Mioceneat 290 mbsf in proximity to a regional seismic sequence boundary.As mentioned previously, no obvious change in lithology occurs,but recovery across this interval is poor, and useful informationis surely missing. Retrogradation of the turbidite depositionalsystem seems to have accompanied rising sea level through thelower half of the TB3 supercycle. Five third-order eustatic cycles(Haq et al., 1987) occurred during the upper Miocene and lower

Pliocene, but individual responses to the third-order cycles werenot recognized.

Concomitant with retrogradation of the turbidite depositionalsystem, from the latest Miocene into the Pliocene-Pleistocene isincreasing occurrence and magnitude of massive slope failures.The oldest record of such events is that of single turbidites in theuppermost Miocene, up to 630 cm thick, and with intraformationalconglomerates at their base. The largest such deposit in the Plio-cene is more than 42.5 m thick and consists of a single turbidite

160



35X-1, 114-11535X-1, 124-12535X-1, 134-13536X-136X-1, 31-3236X-1,51-5236X-1, 71-7236X-1, 104-10536X-1, 140-14136X-2, 5-637X-1,44-4537X-1, 64-6537X-1, 84-8537X-1,99-10038X-138X-1, 32-3338X-1,39-4038X-1, 50-5138X-1, 59-6038X-1, 69-7038X-1, 79-8038X-1,93-9439X-1, 26-2739X-1, 36-3739X-1, 65-6639X-1, 79-80

123-765C-2R-12R-1, 68-702R-1, 79-802R-1, 89-902R-1, 99-1002R-1, 119-1202R-1, 128-1302R-23R-13R-23R-2, 14-153R-2, 24-253R-2, 34-353R-2, 49-503R-2, 53-544R-14R-1, 63-644R-1, 71-724R-1, 83-844R-24R-3, 63-644R-3, 74-754R-3, 84-855R-15R-25R-2, 73-745R-2, 102-1035R-2, 134-1355R-35R-3, 4-55R-3, 34-355R-3, 74-755R-3, 87-886R-16R-1, 85-866R-1, 95-966R-1, 105-1066R-2

329.4329.5329.6337.9338.2338.4338.6338.9339.3339.5347.9348.1348.3348.5357.1357.4357.5357.6357.7357.8357.9358.0367.0367.1367.4367.5359.6360.3360.4360.5360.6360.8360.9361.1369.3370.8370.9371.0371.1371.3371.3379.0379.6379.7379.8380.5382.6382.7382.8388.6390.1390.8391.1391.4391.6391.6391.9392.3392.5398.3399.2399.3399.4399.8

1

4

6

131311

8

9

97

4

9

11

MLL

UUMMLLUMLL

UUMMLLLUULL

UUuMLL

UUMLU

UML

UML

UUU

MMLL

UML

77.887.089.4

86.681.183.490.880.243.886.892.092.980.1

65.676.580.892.288.591.887.268.180.394.095.6

81.882.484.792.092.389.6

76.875.489.486.39.0

79.475.085.0

51.480.281.5

4.257.857.8

59.362.483.386.1

5.080.182.3

53.426.94.8

1.30.21.054.850.164.82.017.629.134.0

1.01.01.335.869.155.053.75.81.335.729.5

0.33.31.026.126.767.7

1.01.214.639.14.9

1.51.033.9

3.238.543.9

0.02.80.0

0.02.739.850.9

3.00.06.1

29.240.132.6

47.953.251.928.326.215.952.057.050.640.1

39.145.551.344.716.331.132.557.548.550.353.7

43.846.655.854.356.119.3

51.550.244.729.121.8

50.049.242.0

29.827.019.9

5.727.429.0

30.633.028.828.2

4.745.851.0

17.433.062.6

50.846.647.116.923.719.346.025.420.325.9

59.953.547.419.514.613.913.836.750.214.016.8

55.950.143.219.617.213.0

47.548.640.731.873.3

48.549.824.1

67.034.536.2

94.369.871.0

69.464.331.420.9

92.354.242.9

23.118.80.1

0.00.10.110.012.112.50.29.822.010.4

0.10.00.17.921.611.76.82.00.15.43.4

0.10.10.110.819.517.9

0.51.120.116.90.6

0.60.624.8

0.228.025.9

0.00.00.0

0.00.012.213.5

0.00.00.6

60.968.076.3

73.376.177.074.071.771.476.674.264.774.2

75.174.274.976.964.473.674.680.170.178.178.6

77.678.078.274.866.465.8

78.779.767.567.875.5

78.577.462.1

78.758.959.6

61.467.768.9

69.671.972.169.9

63.973.080.4

16.013.223.6

26.723.822.916.016.216.123.216.013.315.4

24.825.825.015.214.014.718.617.929.816.518.0

22.321.921.714.414.116.3

20.819.212.415.323.9

20.922.013.1

21.113.114.5

38.632.331.1

30.428.115.716.6

36.127.019.0

overlying a debris-flow deposit also overlying a slumped unit.The onset of massive slope failures was possibly coincidental tothe progressive Miocene-Pliocene collision between the Austra-lian margin and the Sunda-Banda Arc (Carter et al., 1976), butthey are more likely interrelated.

ACKNOWLEDGMENTSI would like to acknowledge the combined efforts of the

Shipboard Scientific Party, the ODP technical and logistics staff,and the crew of JOIDES Resolution toward the successful com-pletion of Leg 123 and publication of the indispensable Leg 123

Initial Reports. The Borehole Research Group, Lamont-DohertyGeological Observatory, is commended for their prompt transmit-tal of requested logging data.

The Department of Oceanography and Ocean Drilling Pro-gram, both at Texas A&M University, provided the facilities forthis investigation. H. Jensvold contributed excellent technicalsupport for the shore-based analyses.

Reviews by R. Hesse and S. O'Connell greatly improved thismanuscript. Thanks go to A. Meyer for additional review and dis-cussion. An editorial review by S. Stewart was extremely valu-able. Finally, I acknowledge the diligence of F. Gradstein, theEditorial Review Board member assigned to this chapter.

161

G. R. SIMMONS



6R-36R-3. 33-346R-3, 44-456R-3, 54-556R-3, 64-656R-3, 74-757R-17R-27R-2, 59-607R-2, 69-707R-2, 76-777R-2, 89-907R-2, 97-987R-2, 108-1097R-37R-3. 114-1157R-3, 126-1277R-3, 135-1367R-3, 143-1448R-18R-1. 101-1028R-1, 114-1158R-1, 124-1258R-1, 135-1368R-28R-38R-3, 113-1148R-3, 134-1358R-3, 135-1368R-4, 4-58R-4, 29-309R-19R-1, 25-269R-1,32-339R-1, 42-439R-1, 52-539R-1,62-639R-29R-39R-3, 43-449R-3, 52-539R-3, 63-649R-3, 73-749R-3, 83-849R-410R-110R-210R-3, 0-110R-3, 10-1110R-3, 20-2110R-3, 30-3110R-410R-4,11-1210R-4, 22-2310R-4, 33-3411R-111R-1, 101-10211R-1. 110-11111R-1, 122-12311R-1, 146-14711R-1, 149-15011R-211R-311R-4

Depth(mbsf)

401.3401.6401.7401.8401.9402.0408.0409.5410.1410.2410.3410.4410.5410.6411.0412.1412.3412.4412.4417.7418.4418.5418.6418.8419.2420.7421.5421.7421.8421.9422.2427.3427.6427.6427.7427.8427.9428.8430.3430.7430.8430.9431.0431.1431.8436.5438.0439.5439.6439.7439.8441.0441.1441.2441.3446.0447.0447.1447.2447.5447.5447.5449.0450.5

BA

9

711

12

13

97

7

88

161012

7

7

1286

TD

UUMLL

UU

uMLL

UULL

UULL

ULUML

UUMLL

UUMLL

UUML

UML

UUMLU

CO3

(%)

42.059.687.582.992.7

8.257.584.468.777.180.3

9.561.993.190.3

5.961.786.973.7

19.181.150.667.886.0

41.561.758.364.471.1

64.262.165.482.881.1

28.661.662.059.6

20.857.170.2

80.675.580.173.22.7


0.00.016.923.142.6

5.83.13.33.649.045.7

3.43.012.063.7

3.22.863.241.3

2.640.33.23.943.8

2.93.03.19.645.8

2.83.63.332.238.3

3.12.71.227.1

3.62.618.3

8.53.639.251.53.0

silt(wt%)

24.433.741.135.325.2

10.130.230.435.516.722.9

10.835.382.212.6

3.732.511.722.7

8.037.626.840.526.9

22.226.726.636.215.3

37.937.038.235.429.5

25.729.049.930.9

4.230.235.2

53.142.130.520.032.4

clay(wt%)

75.666.342.041.632.2

84.166.766.360.934.331.4

85.861.75.823.7

93.164.725.136.0

89.422.170.055.629.3

74.970.370.354.238.9

59.359.458.532.432.2

71.268.348.942.0

92.267.246.5

38.454.330.328.564.6

LAB-TEC 100sand(%)

0.00.315.520.98.4

0.20.00.00.06.26.0

0.00.09.411.0

0.00.014.611.4

0.17.60.00.626.2

0.00.00.01.814.9

0.00.10.113.314.3

0.00.01.813.6

0.10.010.3

2.40.119.916.728.9

silt(%)

73.878.570.666.075.1

67.871.370.371.676.377.8

66.071.174.174.7

63.670.671.071.2

69.677.073.378.860.0

67.368.170.480.269.0

72.776.276.471.870.8

71.172.180.971.3

63.171.474.1

82.174.665.867.427.9

clay(%)

26.221.213.913.116.5

32.028.729.728.417.516.2

34.028.916.514.3

36.429.414.417.4

30.315.426.720.613.8

32.731.929.618.016.1

27.323.723.514.914.9

28.927.917.315.1

36.828.615.6

15.525.314.315.943.2

Funding for my post-cruise research was provided by JOI-USSAC and was administered by the Texas A&M Research Foun-dation.

REFERENCES

Apthorpe, M., 1988. Cainozoic depositional history of the North WestShelf. In Purcell, P. G., and Purdell, R. R. (Eds.), The North WestShelf, Australia. Proc. Pet. Expl. Soc. Aust. Symp., 55-84.

Bouma, A. H., 1962. Sedimentology of some Flysch Deposits: Amsterdam(Elsevier).

Carter, D. J., Audley-Charles, M. G., and Barber, A. J., 1976. Strati-graphical analysis of island arc-continental margin collision in easternIndonesia./. Geol. Soc. London, 132:179-198.

Droxler, A. W., and Schlager, W., 1985. Glacial versus interglacialsedimentation rates and turbidity frequency in the Bahamas. Geology.13:799-802.

162



11R-4, 7-811R-4.18-1911R-4, 29-3011R-4, 39-4011R-4, 49-5012R-112R-1. 31-3212R-1, 41-4212R-1. 51-5212R-1, 61-6212R-1, 71-7212R-212R-312R-412R-4,18-1912R-4, 69-7012R-4, 119-12012R-5, 19-2012R-5, 70-7113R-1

450.6450.7450.8450.9451.0455.2455.5455.6455.7455.8455.9456.7458.2459.7459.9460.4460.9461.4461.9464.6

7

372

1

UUMLL

LUUML

UU

uML

BA = bed abundance (turbiditθs/1.5 m section);

0.484.482.874.883.2

46.669.577.186.672.9

13.752.953.461.963.5

3.03.73.214.731.0

2.92.83.932.643.1

3.02.72.62.629.5

TD • turbidite division

5.456.152.844.735.9

15.845.750.341.726.0

10.631.432.442.730.1

(U«

91.640.244.040.633.1

81.351.545.825.730.9

86.465.965.054.740.4

upper turbidite,

0.00.30.12.638.0

0.00.00.121.127.1

0.00.00.00.07.4

55.081.178.780.032.0

66.577.981.065.357.6

58.566.367.876.176.1

45.018.621.217.430.0

33.522.118.913.615.3

41.533.732.223.916.5

M « middle turbidite,

Haq, B. U., Hardenbol, J., and Vail, P. R., 1987. Chronology of fluctuat-ing sea-levels since the Triassic. Science, 235:1156-1167.

Haq, B. U., von Rad, U., O'Connell, S., et al., 1990. Proc. ODP. Init.Repts., 122: College Station, TX (Ocean Drilling Program).

Lasentec Operations Manual for the LAB-TEC 100 Particle Size Ana-lyzer: Bellevue, WA (Laser Sensor Technology, Inc.).

Ludden, J. N., Gradstein, F. M., et al., 1990. Proc. ODP, Init. Repts., 123:College Station, TX (Ocean Drilling Program).

Shanmugam, G., Moiola, R. J., and Damuth, J. E., 1985. Eustatic controlof submarine fan development. In Bouma, A. H., Norman, W. R., andBarnes, N. E. (Eds.), Submarine Fans and Related Turbidite Systems:New York (Springer-Verlag), 23-28.

Shipboard Scientific Party, 1990. Explanatory notes. In Rangin, C ,Silver, E. A., von Breymann, M. T., et al., Proc. ODP, Init. Repts.,124: College Station, TX (Ocean Drilling Program), 7-34.

Singer, J. K., Anderson, J. B., Ledbetter, M. T., McCave, I. N., Jones,K.P.N., Wright, R., 1988. An assessment of analytical techniques forthe size analysis of fine-grained sediments. /. Sediment. Petrol.,58:534-543.

Stow, D.A.V., Howell, D. G., Nelson, C. H., 1985. Sedimentary, tectonic,and sea-level controls. In Bouma, A. H., Norman, W. R., and Barnes,N. E. (Eds.), Submarine Fans and Related Turbidite Systems: NewYork (Springer-Verlag), 15-22.

Date of initial receipt: 18 July 1990Date of acceptance: 21 March 1991Ms 123B-147

163

G. R. SIMMONS

Total gamma ray (API units)and

Bed abundance (turbidites/1.5 m)0 30 60

450

5000 50

Carbonate (%)and

100

- 6 .Magnetic susceptibility (Kx10 cgs)

Figure 8. Bed abundance, total gamma-ray log, carbonate percent, andmagnetic susceptibility plotted vs. sub-bottom depth for lithologicSubunits IC, IIA, IIB, IIC, and IIIA at Site 765 (dashed line = bedabundance). Carbonate samples are from upper turbidite divisions (solidcircles), middle turbidite divisions (open circles), and lower turbiditedivisions (solid triangles).

164

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