14. lithologic interpretation from geophysical

Barron, J., Larsen, B., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 119

14. LITHOLOGIC INTERPRETATION FROM GEOPHYSICAL LOGS IN HOLES 737B, 738C, 739C,AND 742A1

Gilles Ollier2 and Benoit Mathis3

ABSTRACT

During Leg 119 a full suite of geophysical logs was recorded in four holes drilled in the southern Indian Ocean.Holes 737B and 738C are located on the northern and southern Kerguelen Plateau, respectively, whereas Holes 739Cand 742A are located on the Antarctic Shelf in the Prydz Bay area. The sediments encountered on the Kerguelen Pla-teau consist mainly of fine siliceous or calcareous biogenic deposits accumulated during the past 100 m.y. On the Ant-arctic Shelf, the formations encountered are of glacial origin and consist principally of diamictites.

The geophysical logs were correlated with the core descriptions in order to supplement the lithologic column in thefour holes. The in-situ measured parameters used for lithologic interpretation in Holes 737B, 738C, and 742A are thenatural radioactivity, the neutron porosity, the bulk density, the photoelectric effect, the P-wave sonic transit time, andthe thorium, uranium and potassium content. Only a limited number of geophysical logs (natural radioactivity, />-wavesonic transit time, and resistivity) were recorded in Hole 739C. A lithologic interpretation is, however, proposed for thishole on the basis of the existing logs. The lithology inferred from the logs is determined by comparing the position instandard diagrams of plotted parameters, either untreated or calculated, with the position of mineralogic and lithologicpoles (calcite, quartz, etc.). Also, lithologic information from cores was extrapolated in the non-recovered intervals, ac-cording to the log changes observed between recovered and non-recovered sections.

In the Kerguelen Plateau holes the radioactivity and uranium, thorium, and potassium logs were used to determinethe amount and the nature of clays. Six logging units are recognized in Hole 737B in the depth interval 296.5-701.0 me-ters below seafloor (mbsf) and three in Hole 738C in the depth interval 113.0-455.0 mbsf on the basis of the shalinessand the porosity of the calcareous formations. Almost no clay was found from the logs in Hole 738C, whereas a signifi-cant amount of illite and montmorillonite, up to 38.5% of the dry bulk, was found in Hole 737B.

Unlike the biogenic sediments of the Kerguelen Plateau, the deposits encountered in the holes of the Antarctic Shelfshow a very high radioactivity rate (up to 120 GAPI in Hole 742A) due to the presence of granitic eroded basementrocks. Two kinds of glacial sediments were identified from the log records in Hole 742A and 739C. The first kind corre-sponds to compacted massive diamictite units deposited beneath grounded ice; the second kind corresponds to less com-pacted, stratified diamictite units deposited seaward of the grounding line. The basal glacier deposits are compacted be-cause the weight of the ice has acted upon the sediments since the time of their deposit whereas distal diamictites are lessaffected by ice loading. In both Holes 739C and 742A, the alternation of basal diamictites and distal diamictites sug-gests several cycles of ice advance and recession. At least two cycles seem to have occurred from the Pliocene to thepresent.

INTRODUCTION

Core recovery at the sites drilled during Ocean Drilling Pro-gram (ODP) Leg 119 was only 57.6% on an average for the en-tire cruise (Barron, Larsen, et al., 1989). The present study is anattempt to use the geophysical logs recorded in Holes 737B,738C, 739C, and 742A to supplement the lithologic observa-tions from the cores.

These four holes document two kinds of sedimentary envi-ronments (Fig. 1). Holes 737B and 738C sampled the calcareousand siliceous biogenic sediments deposited during the past 100m.y. on the Kerguelen Plateau (Barron, Larsen, et al., 1989).Prydz Bay Holes 739C and 742A on the Antarctic continentalshelf sampled sediments from glaciomarine origin, of diamictitetype, spanning at least 35 m.y (Barron, Larsen, et al., 1989;Hambrey et al., 1989; Hambrey et al., this volume)

Since the early 1970's and concurrently with the developmentof sophisticated logging tools, the oil companies have developedmethods allowing the interpretation of lithology from geophysi-cal logs in the absence of coring from exploration holes (Schlum-

Barron, J., Larsen, B., et al., 1991. Proc. ODP, Sci. Results, 119: CollegeStation, TX (Ocean Drilling Program).

2 Institut Français de Recherche pour 1'Exploitation de la Mer, Centre de Brest(IFREMER), BP 70, 29280, Plouzané, France.

3 Laboratoire de Géologie des Ensembles Sédimentaires, Faculté des Sciences,Université de Nancy I, BP 239, 54506 Vandoeuvre Les Nancy Cedex, France.

berger, 1972a; Schlumberger, 1972b; Quirien et al., 1982; Serra,1985; Herron, 1986). These methods are mostly based on the useof crossplots of measured parameters such as density and poros-ity, or calculated parameters derived from the former (Burke etal., 1969; Ellis, 1987; Schlumberger, 1988). The methods devel-oped for the oil industry are, however, usually adapted to ce-mented, well-consolidated sediments such as limestone, sand-stone, or dolomite, but have to be employed with special care tothe non-lithified sediment types encountered in the present ODPholes. Nevertheless, applying proper corrections to the loggingmeasurements, some authors have already used the crossplotmethods in the ODP holes (Anderson and Newmark, 1985;Goldberg et al., 1987). Another way is to define a mineralogicmodel on the basis of the sampled intervals, and then to calcu-late the percentage of each mineral at each depth (Quirien et al.,1986; Doveton, 1986; Jarrard et al., 1989; Broglia et al., 1990).

The major difference between the sedimentary formationsdrilled for oil exploration and those recovered in the ODP holesis in the consolidation state of the sampled material. The sedi-ments sampled during Leg 119 commonly reached 60% watercontent and 80% porosity on the Kerguelen Plateau, and 20%water content and 40% porosity in the Prydz Bay area (Barron,Larsen, et al., 1989). By comparison, the consolidated forma-tions mainly exhibit water contents and porosities significantlylower; the water content is mostly equal to a few percent, whereasporosity rarely exceeds 40% (Serra, 1985). Also the hole condi-tions are significantly less favorable for logging acquisition in

263

G. OLLIER, B. MATHIS

3 0°

40°

80° 90"

Figure 1. Location of Holes 737B, 738C, 739C, and 742A. Bathymetry,in meters, is from GEBCO (Hayes and Vogel, 1981; Fisher et al., 1982)

ODP drilling than in commercial drilling. The large size of theODP holes (minimum 11 in., or 27.5 cm, on Leg 119), com-bined with the frequent occurrence of soupy sediments and theinfluence of the drilling fluid on the logging tool responses, in-duce incorrect logging measurements (Lamont-Doherty Bore-hole Research Group, 1988). On the other hand, unlike explora-tion holes where coring is commonly restricted to the reservoir,the ODP holes are continuously cored, and offer an uncom-parable opportunity to match observations from cores and fromin-situ measurements. Accordingly, the core descriptions are usedextensively in this study to calibrate the log responses in termsof lithology. The primary objective of this study is to fill thelithologic gaps of the non-recovered intervals. In addition, weaim to provide quantitative information, such as the porosityand the shaliness of the formations.

The following will successively present the description of theavailable data, the method used to derive the lithologies fromgeophysical logs, the log-inferred lithology in the Kerguelen Pla-teau holes, the log-inferred lithology in the Prydz Bay holes,and finally, a section will be devoted to a discussion of the valid-ity of these results and their implications in the understandingof the local sedimentary environments.

AVAILABLE DATAThe term lithologic unit will be used in the following for the

lithology defined by cores. The term logging unit will be usedfor the units defined by geophysical logs. The data used in this

study are the Schlumberger geophysical logs, the lithologic coredescriptions, and the density and porosity measured on samplespresented in the preliminary leg results (Barron, Larsen, et al.,1989). Information about the mineralogic composition of thesediments recovered is also integrated into the present study(Hambrey, et al., this volume; Bohrmann and Ehrmann, thisvolume)

Hole 737BIn Hole 737B, geophysical logs are available between 124.0

and 701.0 mbsf (Fig. 2). Because of the lack of lithologic logs,such as density and porosity logs, in the upper logged section, alithologic interpretation is only proposed for the depth interval296.5-701.0 mbsf. The log-inferred lithologic column is repre-sented between the radioactivity log (SGR) and the bulk densitylog (RHOB) (Fig. 3). The core recovery between 296.5 and 701.0mbsf reaches 66.6%. This depth interval contains four litho-logic units (Barron, Larsen, et al., 1989): (1) lithologic Unit 3(244.1-306.6 mbsf), (2) lithologic Unit 4 (306.6-321.1 mbsf),(3) lithologic Unit 5 (321.1-677.9 mbsf), (4) lithologic Unit 6,(677.9-711.2 mbsf).

Lithologic Unit 3 consists of middle to upper Miocene dia-tom-nannofossil ooze. Lithologic Unit 4 is a middle Mioceneformation consisting of sandy siltstone mixed with volcanic sandand diatom-nannofossil oozes. Lithologic Unit 5 is an upperOligocene to middle Eocene unit composed of calcareous clay-stone mixed with clayey limestone at its base. Lithologic Unit 6is composed of a middle Eocene clayey limestone with minorchert layers, nodules, and black claystone. Physical propertiesand lithology are further described in Barron, Larsen, et al.(1989) which also contains comparison with the preliminary logresults, and in Bohrmann and Ehrmann (this volume). Thelogging parameters recorded in Hole 737B are either physicalparameters (such as bulk density (RHOB), neutron porosity(NPHI), P-wave sonic transit time (DT), total natural radioac-tivity (SGR) or natural radioactivity (GR), and photoelectric ef-fect factor (PEF) parameters) or chemical parameters deducedfrom radioactivity (such as uranium (U), thorium (Th), and po-tassium (K) content). The previous parameters were acquiredwith the following tools which are commonly used in the oil in-dustry and developed by Schlumberger (Ellis, 1987): lithoden-sity tool (LDT) for RHOB and PEF; dual porosity neutron tool(CNT-G) for NPHI; long-spaced sonic tool (LSS) for DT, natu-ral gamma-ray tool (NGR) for GR; and natural gamma-rayspectrometry tool (NGT) for U, Th, K, and SGR.

Hole 738CIn Hole 738C, logs were acquired between 113.0 and 455.0

mbsf (Fig. 4) and used within this depth interval to supplementthe lithology from cores (Fig. 5). In Figure 5, the log-inferredlithologic column is represented between the RHOB and theNPHI log. The mean core recovery is 57.6% for the entiredrilled section. The core descriptions indicate that the loggedsection covers a calcareous sequence divided in four distinctunits (Barron, Larsen, et al., 1989), (Fig. 5): (1) lithologic Unit3 (17.7 to 120.8 mbsf), (2) lithologic Unit 4 (120.8 to 254.4mbsf), (3) lithologic Unit 5 (254.4 to 418.6 mbsf), and (4) litho-logic Unit 6 (418.6 to 479.7 mbsf).

Lithologic Unit 3 is described as a middle Eocene and lowerOligocene nannofossil ooze unit. Unit 4 consists of lower tomiddle Eocene nannofossil ooze and chalk-bearing chert layers.Unit 5 is a Campanian to lower Eocene chalk unit with chertnodules. Lithologic Unit 6 corresponds to a lower Turonian toCampanian silicified limestone. Further information about physi-cal properties and lithology is described in Barron, Larsen, etal. (1989) and Ehrmann (this volume). The geophysical logsavailable in this hole are the same as in Hole 737B.

264

LITHOLOGIC INTERPRETATION FROM GEOPHYSICAL LOGS

DENSITY CORR. _

Inij^/ n ^ S 3 g/cm3 231 '-3 ppm 71

'TOTAL I N E U T R P O R O I POTASSIUM I URANIUM , DEEP RESISTIVITY

SITE 737°

TOTA

>API un

if c

TRANSIT TIME^δ 5>o c

% 4O.

θ θ

-j o^ _ - ,

tight% 3(-3 ppm 7(0 2 ohm-m 2 O 2 i o u s / f ' UO

7

T IV

4

VI

Figure 2. Core recovery, geophysical logs, lithologic units, and logging units in Hole 737B (modifiedfrom Barron, Larsen, et al., 1989). The geophysical logs consist of the total natural radioactivity, bulkdensity, neutron porosity, potassium content, thorium content, uranium content, deep resistivity, andlong-spacing sonic transit time logs, acquired with the Schlumberger tools described in Ellis, 1987.(Note: Log curve thickness corresponds to underlining in legend at top.)

265


646.6mbsf

701.0mbsf

LITHOFACIES

π Feldspathic andvolcanic siltstones

p~^Tj Silty and clayey calcareous2 I 1 ooze with diatoms and

volcaniclastic fragmentsI-J—JJ Uncemented and clayey

3 i ~ i calcareous ooze

I 1 Uncemented and very4 r J •L\ clayey calcareous ooze

Chert

Moderated cementedclayey calcareous ooze

Moderated cemented veryclayey calcareous ooze

Weakly cemented clayeylimestone

Clayey limestone

Figure 3. Core recovery, log-inferred lithologic column, lithologic units, and ages in Hole 737B. The log-inferred litho-logic column is represented between the total radioactivity log (SGR) and the bulk density log (RHOB). The lithologicunits and the ages are from Barron, Larsen, et al. (1989).

266


PHOTOELECTRICBULK DENSITY EFFECT

TOTAL 'i4 g/cm3 24C boms/β' '.GAMMA RAY iNEUTR.POROk POTASSIUM

THORIUM

SITE 738 5 0

80 %

ppm lö1

URANIUM , DEEP RESISTIVITYwβight% T-2 ppm 3 0 2 ohm-m

TRANSIT T IMERS ~ ?LONG SPACING"? O =I _ IF"" ö D

20'lβO u$/ft βO ‰Sj £ »

f

µ100

-150

-200

-250

-400

ffl

IV

VI

2

Figure 4. Core recovery, geophysical logs recorded, lithologic units, and logging units in Hole 738C (modifiedfrom Barron, Larsen, et al., 1989). The geophysical logs consist of the total natural radioactivity, bulk density,neutron porosity, photoelectric effect, potassium content, thorium content, uranium content, deep resistivity,and long-spacing sonic transit time logs, acquired with the Schlumberger tools described in Ellis, 1987.

267


COMPACTAND CLAYEYLIMESTONE

LITHOFACIES

1 I x i Calcareous ooze

i ^ J Calcareous ooze2 I Wl and chalk

r//s/λ Siliceous ooze3 vfttfλ or chert layer

4 N^H Chalk

5 & ò q Siliceous limestone

i;i;H Ferrugineous6 WTéi limestone

7 \\'. i i | Clayey limestone

8 h1-^1] Compact limestone

Figure 5. Core recovery, log-inferred lithologic column, ages, and lithologic units in Hole 738C. The log-inferredlithologic column is represented between the bulk density log (RHOB) and the neutron porosity log (NPHI). Theages and the lithologic units are from Barron, Larsen, et al. (1989).

268


Hole 742A

The logging tools did not reach the bottom of Hole 742A be-cause of a bridge in the lower part of the drilled section (Fig. 6).Core recovery for the entire drilled section is 53.4%. A litho-logic interpretation from logs is proposed in this hole within thedepth interval 30.8-283.8 mbsf (Fig. 7); this depth interval cov-ers five lithologic units (Barron, Larsen, et al., 1989): (1) litho-logic Unit 2 (5.4 to 115.2 mbsf), (2) lithologic Unit 3 (115.2 to134.4 mbsf); (3) lithologic Unit 4 (134.4 to 172.5 mbsf), (4) lith-ologic Unit 5 (172.5 to 304.3 mbsf), and (5) lithologic Unit 6(304.3 to 316.0 mbsf).

Lithologic Unit 2 is a massive diamictite unit containing theremains of late Pliocene-Quaternary diatoms. Lithologic Unit 3is composed of normally consolidated clayey diamictites con-taining a 80-cm-thick diatomite layer of early to late Plioceneage. Lithologic Unit 4 corresponds to a crudely layered diamic-tite unit with no biostratigraphic control. Diamictites of litho-logic Unit 5 are characterized by pale colors and a minor calcar-eous component. Lithologic Unit 6 consists of diamictites inter-bedded with laminated claystone; for details see Barron, Larsen,et al. (1989) and Hambrey et al. (this volume). The logging re-cords available in this Hole are the same as for Holes 737B and738C.

Hole 739C

Almost all of Hole 739C was logged, between 32.0 and 468.0mbsf (Fig. 8). A lithologic interpretation from the cores and thelogs is proposed for this logged interval (Fig. 9). Core recoveryfor the entire drilled section only reaches 35.3%. The core de-scription allowed the identification of four lithologic units (Bar-ron, Larsen, et al., 1989; Hambrey, et al., this volume): (1) lith-ologic Unit 2 (24.1 to 173.6 mbsf), (2) lithologic Unit 3 (173.6 to267.2 mbsf), (3) lithologic Unit 4 (267.2 to 315.7 mbsf), and (4)lithologic Unit 5 (315.7 to 468.8 mbsf).

Lithologic Unit 2 corresponds to an upper Miocene to lowerPliocene or younger massive diamictite. Lithologic Unit 3 is astratified diamictite unit with a relatively diatom-rich layer ofearly Oligocene age. Lithologic Unit 4 consists of slightly indu-rated but friable diamictites, assigned to a lower Oligocene age.Lithologic Unit 5 is similar to Unit 4 but is of uncertain age(Barron, Larsen, et al., 1989).

Because of time constraints, only a limited number of geo-physical logs were recorded in Hole 739C. The "lithologic logs,"such as density, neutron porosity, photoelectric effect, and ura-nium, thorium, and potassium content logs, were not acquired,so the Hthology interpretation from logs in this hole is mainlybased on the resistivity log, P-wave sonic log, and GR log.The GR log is a radioactivity log which was recorded with theSchlumberger gamma-ray device instead of the gamma-rayspectrometry device used at the other holes (for details see Ellis,1987). Calculated porosities ("PHI SON") (Fig. 10) were de-rived from the sonic transit time log by using the Raymer-Huntequation (Raymer et al., 1980). We chose this method becauseon-core velocity measurements have been completed parallel tothe sonic log record, and a good agreement appears to exist be-tween the two sets of data (Barron, Larsen, et al., 1989). In therange of porosities observed in Hole 739C (mostly less than40%), the Raymer-Hunt equation is:

V = (1 - ΦfVma + Φ Vf (1)

where V - bulk sonic velocity in meters per second (m/s), Φ =porosity of formations in porosity unit (PU), Vma = sonic ve-locity in matrix in m/s, and Vf = sonic velocity of the fluidcontained in the porosity in m/s.

Because the general Hthology is almost identical in Holes739C and 742A and mainly consists of diamictites (Barron, Lar-

sen, et al., 1989), the sonic velocity in the matrix has been in-ferred from the neutron porosity and sonic transit time logs re-corded at Site 742A, as proposed by Pelline (1989). The porosityΦ, computed from the sonic log, will be called "PHI SON" inthe following sections and in the figures.

METHODOLOGY

As mentioned in the "Introduction," the most advanced ac-tual methods for Hthology evaluation from logs consist calculat-ing the normative mineralogy changes of the sedimentary se-quences (Herzog et al., 1987). Unfortunately this method maynot be applied in this study because it requires geochemical logdata such as silicon, iron, calcium, and other elementary com-ponents not recorded in the Leg 119 holes. So the methodologyused for this study is the standard crossplot technique presentedas "facio sequential analysis in complex matrix" by Augier(1980), Mathis (1988), Mathis et al. (1988), and Mathis et al.(1989). The logged sections are divided into logging units on thebasis of gradients or of abrupt changes observed on the log rec-ords. Most of the logging units in the present study correspondto the main lithologic units, with slight depth differences. Eachlogging curve was then carefully sampled in order to divide itinto thin depth intervals characterized by constant log values.These thin selected depth intervals were named electrobank inaccordance with Serra (1979). Each one corresponds to a partic-ular lithology. The thickness of the electrobanks is of the orderof 1 m, sometimes more, depending on the thickness of the lith-ologic layers and on the vertical resolution of the logging tools.Because the sampling rate of the logging tools is about 15 cm,one electrobank averages about 7 log values. The logging pa-rameter values affected by bad hole conditions are systemati-cally discarded, as well as incorrect values resulting from toolfailures, so that the electrobanks are as representative as possi-ble of the lithologic changes. It was not possible to process theincorrect logging data for hole size effect because the caliperdid not work properly during Leg 119 (Barron, Larsen, et al.,1989). The selected logging parameter values of the electrobanksare plotted in diagrams where standard mineralogic or lithologicpoles are already plotted. The lithology of each electrobank isdetermined by comparing its position with the position of thestandard lithologic poles (Augier, 1980). The lithology of anelectrobank may also be inferred from a depth interval whichexhibits similar log characteristics and which was fully sampled.In order to be consistent with the vocabulary commonly em-ployed by the log-analysts, we will call the lithologic facies de-rived from logs "lithofacies" in the following sections (Serra,1985). Two kinds of parameters are used for the final lithologicinterpretation: untreated logging parameters and calculated pa-rameters. The untreated logging parameters used in the presentstudy have been described in the preceding chapter. The loggingresponses are calibrated using physical and chemical parametersmeasured on samples (Barron, Larsen, et al., 1989; Cranston,chapter 20, this volume).

Equations of the M, N, and P parameters (Serra, 1985; Ellis,1987; Schlumberger, 1988) are as follows:

M = (DT/ - DT/RHOB - RHOB/) 0.01, (2)

N = (NPHI/ - NPHI)/(RHOB - RHOB/), and (3)

P = (NPHI/ - NPHI)/(DT/ - DT) 100, (4)

where RHOB/ = drilling fluid density in g/cm3, NPHI/ =drilling fluid neutron porosity in PU, and DT/ = traveltime inborehole fluid in µs/ft.

The calculated M, N, and P parameters used are almost in-dependent from porosity (Burke et al., 1969). At Hole 738C,the N and P parameters were calculated using the corrected

269


PHOTOELECTRIC TRANSIT TIME 5 - -BULK DENSITY, EFFECT , THORIUM , DEEP RESISTIVITY , LONG SPACING | J I %

02 ohm•m 201180 us/ft 801 £ Q ÜT -> O1 u. a

SITE 742TOTAL

GAPIu

i BUL^ UfcWiHI T . bht bUI THORIUMh5 g/cin3 2 5'0 borns/e" 10*5 ppm 301

I POTASSIUM I URANIUMrats 200160 ^S OlO weight % STs pTm 15

r—i—i—i—i—i—i—I—i—i—i—i—i—i—rπ

£< 1Ul ID £

Q « •-,

-100

-150

_ 2 0 0

_ 2 5 0

π

ELI

IV

V

1

2

3

4

Figure 6. Core recovery, geophysical logs, lithologic units, and logging units in Hole 742A (modified from Barron, Larsen, et al., 1989). The geophys-ical logs consist of the total natural radioactivity, bulk density, neutron porosity, photoelectric effect, potassium content, thorium content, uraniumcontent, deep resistivity, and long-spacing sonic transit time logs, acquired with the Schlumberger tools described in Ellis, 1987.

270


LithologicUnits

IV

y • i s

NPHI (%)

60I

1.5I

Rhob (g/cm3)

2.5

742A

Log Unit 1

Compacteddiamictite

Log Unit 2

Clayeystratified

diamictite

Log Unit 3

Highlycompacteddiamictite

Log Unit 4

Stratifieddiamictite

cα5

σ

CD

ro

33.8mbsf

117.7mbsf

140.8mbsf

1

2

3

4

5

6

7

LITHOFACIES

•ΛV;

°/.°

Diatomite

Sand or sandydiamictite

Compacted diami<

Highly compacteddiamictite

Diamictite rich in c

Diamictite rich inboulder

Micritic diamictite

Ferrigeneous diamictite

172.8mbsf

233.3mbsf

Figure 7. Lithologic units, core recovery, log-inferred lithologic column, and ages in Hole 742A. The log-inferred lithologic col-umn is represented between the neutron porosity log (NPHI) and the bulk density log (RHOB). The lithologic units and the agesare from Barron, Larsen, et al. (1989).

271


SITE 734

GAMMA HAY. DEEP RESISTIVITY

GAPl unit ' θ 2 ohm-m 20" u / f t

Figure 8. Core recovery, geophysical logs, lithologic units, and loggingunits in Hole 739C (modified from Barren, Larsen, et al., 1989). Thegeophysical logs consist of the natural radioactivity, deep resistivity, andlong-spacing sonic transit time logs recorded with the Schlumbergertools described in Ellis, 1987.

NPHI and DT parameters for water effect. At Hole 739C, the Nand P parameters were calculated using the PHI SON log valuesand the core bulk density measurements. A given lithology isdetermined by an iterating process between different lithologic ormineralogic crossplots. The diagrams that we used for the lithol-ogy interpretation are standard crossplots commonly used in theoil industry, RHOB-NPHI, RHOB-DT, N-P, PEF-N, PEF-Th/K, SGR-U, and Th-K (Augier, 1980), but modified for thepresent study in order to take into account some particularities ofthe formations encountered, such as their high porosity, whichreaches 85% for some oozes. So, concurrent with the use of thestandard crossplots, we will establish the on-core density-poros-ity crossplots in order to calibrate the log responses for the un-known lithologies, particularly in the high porosity depth inter-vals. The clay and opal poles, kaolinite-smectite pole (clay K-Sm), illite-muscovite pole (clay IU-Mu), and porcellanite pole ofthe different diagrams correspond to uncompacted shaley andopal formations from all around the world (Table 1) (Augier,1980; Serra, 1985; Ellis et al., 1988). The pure mineralogicalpoles are represented in the diagrams by the symbols Ka for ka-olinite, Mu for muscovite, 111 for illite, Cl for chlorite, Mo formontmorillonite, KF for microcline, Q for pure quartz or pure"sandstone," C for calcite or pure "limestone," and D for dolo-mite or pure "dolomite." The numbers 10, 20, 30, and 40 ap-pearing along the lithologic lines in the RHOB-NPHI, RHOB-DT, and density-PHI SON crossplots correspond to 10%, 20%,30%, and 40% porosity values for pure limestone, sandstoneand dolomite lithologies.

RESULTS

Log-Inferred Lithology in Calcareous Sequences of theKerguelen Plateau Sites

Hole 737B (Fig. 3)

According to the core description the dominant lithologywithin the logged section (296.5-701.0 mbsf) is essentially com-posed of calcareous oozes and calcareous claystone with minorcontent of diatoms and volcanic layers (Barron, Larsen, et al.,1989). The carbonate content increases indeed significantly be-low 225 mbsf where it becomes the dominant component in lieuof the opal-A in the upper drilled sequence (Bohrmann andEhrmann, this volume). The density-porosity relationship avail-able for pure calcareous lithology within the range of porosity0%-40% (Schlumberger, 1988) may not be applied to the com-plex high-porosity calcareous formations encountered in Hole737B. A density-porosity crossplot from core measurements al-lows to establish the general linear relationship for the presenthigh-porosity lithology (Fig. 11). The existence of a linear rela-tionship between density and porosity beyond 40% of porosityis also shown on the RHOB-DT crossplot (Fig. 12A). This lin-ear relationship corresponds to a regular decay in porosity withdepth increasing as a result of the compaction of the calcareoussediments. Because of a saturation effect of the neutron device(Serra, 1985) the neutron porosity do not increase above 60%(Figs. 12B and 2). However, the bulk density continues to de-crease, which makes the RHOB-NPHI relationship not as linearas the density-porosity relationship seen from cores (Fig. 12B).On the RHOB-DT and RHOB-NPHI crossplots (Figs. 12A and12B), the plotted values fall between the pole of the clays andthe line of the limestone grain density (2.71 g/cm3), confirmingthe lithology from cores and suggesting the presence of a signif-icant amount of clay in these sediments. The radioactivity is al-most totally related to the potassium content while the contribu-tion from the uranium and thorium content is small, except atthe top of the logged section in Hole 737B (Figs. 12C and 12D).Moreover, the RHOB-DT and RHOB-NPHI crossplots (Figs.

272


LitnoiogicUnits

IV

150 100

SGR (Api) PHI son

739C

Log Unit1

Stratifieddiamictite

Log Unit2

Highlycompacteddiamictite

Log Unit3

Clay-richstratified

diamictite

Log Unit4

Diamictitewith sand

andgravellayers

CDQ_Q .3

CCDOO

α>oΦ

Φ

8LUΦ

32.0mbsf

LITHOFACIES

175.0mbsf

Less compacteddiamictite

Diamictite rich ingranitic clasts

Highly compacteddiamictites

Diamictite rich in clay

π Sandy diamictite orsand layer

1

2

3

4

*

.o•o i• o.• x>

:•:•

—― m J

-315.0mbsf

468.0mbsf

Figure 9. Lithologic units, core recovery, log-inferred lithologic column, and ages in Hole 739C. The log-inferred lithologic column isrepresented between the natural gamma-ray log (GR) and the porosity log (PHI SON) calculated from the long-spacing sonic transittime log using the Raymer-Hunt equation (Raymer et al., 1980). The lithologic units and the ages are from Barren, Larsen, et al.(1989).

273

G. OLLIER. B. MATHIS

Depth (mbsD

2 0 0 . 0 30B.

Figure 10. Comparison between in-situ calculated porosities (NPHI SON) and porosities measured on core. The in-situ calculated porosities

were computed by Pelline (1989) using the Raymer-Hunt equation (Raymer et al. 1980). The matrix velocity of the diamictites was obtained by

comparison with that known for the diamictites of Hole 742A.

Table 1. Standard lithologic and mineralogic poles.

Standard lithologic poles

Quartz 0 (QO)Quartz 10 (Q10)Quartz 20 (Q20)Quartz 30 (Q30)Quartz 40 (Q40)Calcite 0 (CO)Calcite 10 (CIO)Calcite 20 (C20)Calcite 30 (C30)Calcite 40 (C40)Dolomite 0 (DO)Dolomite 10 (D10)Dolomite 20 (D20)Dolomite 30 (D30)Dolomite 40 (D40)Clay Ka-SmClay 111Clay Ka-FePorcellanite

Standard mineralogic poles

OpalK-feldspar (KF)Chlorite (Cl)Muscovite (Mu)BiotiteKaolinite (Ka)Mite (111)Complex clay (Is)Montmorillonite (Mo)Pyrite

RHOB(g/cm3)

2.652.482.32.151.972.712.542.362.22.022.882.692.512.322.132.322.382.671.55

2.162.562.772.933.132.422.53

55

NPHI(PU)

-0 .020.080.180.280.3800.10.20.30.40.010.1150.210.310.410.4120.450.370.465

0.2-0 .02

0.520.30.30.370.3

0.44-0 .03

DT(µS/ft)

55.56982.596.5

10948627689

10543.5587287

101133106112162.5

5856

6565

133106

392

GR(API)

220220270280105280

180

PEF(b/e)

1.81

5.08

3.14

1.833.45

1.15

1.752.866.32.46.271.833.454.32.04

U(ppm)

0.7

Th(ppm)

2

12

18.522

19

K(%)

0

16

0.56.9

1.5

Th/K

0.621924

202.566

M

0.809090910.810810810.819230770.804347830.824742270.82456140.824675320.830882350.833333330.823529410.773936170.775147930.774834440.772727270.778761060.424242420.601449280.461077840.48181818

1.129310340.85256411.067796610.642487050.582159620.39436620.54248366

-1 .891.6875

-0.5075

N

0.618181820.621621620.630769230.626086960.639175260.584795320.584415580.588235290.583333330.588235290.526595740.523668640.523178810.522727270.522123890.445454550.398550720.377245510.97272727

0.689655170.653846150.271186440.36269430.32863850.443661970.45751634

- 10.50.2575

P

0.764044940.766666670.769953050.778378380.7750.709219860.708661420.70796460.70.714285710.680412370.675572520.675213680.676470590.670454551.050.66265060.818181822.01886792

0.610687020.766917290.253968250.564516130.564516131.1250.843373490.529100530.2962963

-0.5073892

Note: After Augier (1980), Serra (1985), and Ellis et al. (1988).

12A and 12B) show that the presence of potassium is not relatedto the existence of non-clay minerals, such as K-feldspar orother non-clay-rich potassium minerals, so the total radioactiv-ity measurement appears as a good clay indicator in this hole.Because no analyses are available for the clay minerals at Site737B, the X-ray analyses provided by Ehrmann and Bohrmannand Ehrmann (both this volume), for similar calcareous forma-

tions encountered at Site 738C, were used in order to calibratethe SGR log in terms of percentage of clay content. Approxi-mately 7 GAPI units were found to correspond to a clay contentrepresenting 10% of the dry bulk in Hole 738C (Fig. 4). Thethorium-potassium plots shown in Bateman (1985) and Schlum-berger (1988), and employed at Hole 737B, suggest that illiteand montmorillonite are trie dominant clay minerals. (Fig. 12D).

274


Hole 737B

-

Limestone line _JJ

m

°a ° a * i

O Lithologic Unit 1• D Lithologic Unit 2

A Lithologic Unit 3H Lithologic Unit 4π Lithologic Unit 5• Lithologic Unit 6

Porosity In Pu0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1.0

1.2

<o 1.4

ε

if 1.6

£ 1.8£ 2.0<Λ

Φ 2.2Q

2.4

2.6

2.8

3.0

Figure 11. Density-porosity crossplot from core measurements in Hole737B. The densities, porosities, and the lithologic units are from Bar-ron, Larsen, et al. (1989).

The sedimentary formations can be differentiated into 9 lithofa-cies in Hole 737B; these nine lithofacies are spread over six ma-jor logging units (Fig. 3).

Logging Unit 1 (296.5-318.5 mbsf)

This logging unit corresponds to the base of lithologic Unit 3and to the top of lithologic Unit 4. The density-porosity cross-plot from core measurements within lithologic Unit 3 (Fig. 11)allows to differentiate two groups of plotted data, one groupwith densities of about 1.4 g/cm3 and porosities of 75% whichcorrespond to diatom nannofossil ooze, and one group withdensities of 1.7 g/cm3 and porosities of 60% which correspondsto volcanic sands or diatom nannofossil ooze mixed with vol-canic sands. Comparatively to the core observations, alterna-tions of relatively high and low density electrobanks, close tothe core density values, are seen on the logging curves (Fig. 2),allowing to extrapolate the previous lithologies within the nonrecovered intervals (Fig. 3). Within logging Unit 1 one lithofa-cies (lithofacies 1) with relatively high bulk density and high ra-dioactivity values and other lithofacies (lithofacies 2) with rela-tively low bulk density and low radioactivity values can be dis-tinguished (Figs. 12E and 2). The high radioactivity values,greater than 120 GAP I units, match a high uranium and potas-sium content and within a Th-K diagram (Fig. 12F) are close tothe field of K-feldspar. This is in agreement with the volcani-clastic layers with a significant content of K-feldspar, as foundin the volcanic sands identified from cores (Morche et al., thisvolume). The low radioactivity values match the general calcare-ous lithology. They fall in the high-porosity region (65%) ofthe RHOB-NPHI crossplot, thus indicating very uncompactedoozes. According to the K-Th crossplot, the uncompacted oozeappears to contain mainly montmorillonite as clay type (Fig.12F). The presence of montmorillonite within this depth inter-val is well correlated with a relatively high carbonate content ob-served by Bohrmann and Ehrmann (this volume).

The following logging units (2, 3, 4, and 5) belong to thesame lithologic Unit 5 that is described as a calcareous claystone(Barron, Larsen et al., 1989).


Logging Unit 2 is characterized by a relatively high carbon-ate content (40% of the dry bulk), (Bohrmann and Ehrmann,this volume). According to the physical properties measurements

completed on cores, this lithologic unit appears as a low-density(1.5 g/cm3), high-porosity (70%) unit (Fig. 11). The geophysicallogs within this depth interval, although underestimating a littlethe density values (1.3 g/cm3), agree with the core measure-ments (Fig. 12B). So the calcareous claystone lithology may beextrapolated to the few nonrecovered intervals of this loggingunit. On the basis of the low rate of radioactivity of this forma-tion (20 GAPI units), one can roughly estimate the clay contentto a maximum of 28.5% of the dry bulk. However, adding here40% carbonate content plus 28.5% clay content, 100% of drybulk is not reached, so the remaining components have proba-bly to be found in an unknown amount of feldspar and zeoliteclinoptilolite as detected by Borhmann and Ehrmann (this vol-ume). Finally, logging Unit 2 is represented totally by lithofacies 3which matches uncemented and clayey calcareous oozes (Fig. 3).


Within logging Unit 3 the radioactivity level is significantlyhigher than within logging Unit 2, with a mean value rangingaround 27 GAPI on the entire unit (Fig. 2). Applying the trans-form defined above to the SGR log, a clay content of 38.5% ofthe dry bulk is estimated within this unit. The higher clay con-tent computed from logs, between 350.0 and 431.5 mbsf, is wellcorrelated with the low in the carbonate content shown by Bohr-mann and Ehrmann (this volume), thus emphasizing a clear de-trital influence confirmed by the presence of illite (Chamley,1989), as clearly shown on the Th-K crossplot (Fig. 13A). Inlogging Unit 3, the core bulk densities range from 1.55 to 1.8g/cm3 whereas the porosities range from 67.5% to 56.6%. Cor-rected NPHI values for the neutron saturation effect would al-most fall within the same area than the core data in the density-porosity crossplot (Fig. 12B). So the general lithology definedfrom the core (calcareous claystone) may be used for the nonre-covered intervals, which are particularly frequent in this depthinterval. However, the term "uncemented and very clayey calcar-eous ooze" will be employed for the major lithofacies (lithofa-cies 4) instead of calcareous claystone, to take into account thehigh porosity and the relatively high clay content of these for-mations. A 2-m-thick layer with relatively low values of porosityand sonic transit time and relative high values of density occursat the lower part of logging Unit 3 (426.5-427.5 mbsf) (Fig. 2).On the RHOB-DT and RHOB-NPHI crossplots (Fig. 13B and13C), the plotted data from this layers are shifted toward theporcellanite pole, indicating the presence of chert layers (litho-facies 5). The sonic velocity and resistivity values are increasingsignificantly below 375 mbsf without appreciable porosity de-crease and density increase. It suggests, however, a beginning oflithification of the oozes in the lower part of the unit (Fig. 2),following the same diagenetic processes described by Moore(1989).


Compared to the previous logging unit, a distinct decrease inradioactivity level is noted in this unit, the SGR log indicates adecay from 25-30 GAPI to 15-20 GAPI between the two units(Fig. 2). This radioactivity rate would be related to a maximumclay content of about 20% of the dry bulk. The decrease in claycontent within logging Unit 4 occurs concurrently with a signifi-cant increase of the carbonate content, up to a maximum of80% of the dry bulk (Bohrmann and Ehrmann, this volume).Thus, the mineralogic composition of logging Unit 4 appearsvery similar to that of logging Unit 2. The main difference be-tween the two units is a difference of compaction which is indi-cated by lower porosities in logging Unit 4 (Fig. 2). Also the na-ture of the clays appears slightly different from that of loggingUnit 2, according to the Th-K crossplot, the plotted data aremostly located in the illite field (Fig. 13D). The lithofacies of

275


DT (microsec./ft) NPHI (pu)40 60 80 100 120 140 160 180 200 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.5

1.7

1.9

2.1

2.3

2.5

2.7

o o

+ Standard lithologic poleso Log* Log

- D Log• Log

• Opale+

1 0 /

KF +#

vy/D

Unit 1Units 2 and 3Unit 4Unit 5

40

J3 0/^7

/

/

Clay

Clay

° Λ°Porcellanite °L Jjàk

•

+ Clay Ka-Sm

III

Ka-Fe

Hole 737B

1.5

1.7

Λ 1.9mEüσ> 2.1

m

S 2-3tr

2.5

2.7

2.9

+ Standard lithologic poles° Log Unit 1A Log Units 2 and 3 PorcellaniteD Log Unit 4• Log Unit 5Δ Log Unit 6

Clay Ka-lron

Hole 737B

140

120

100

80

60

40

20L

n

U (ppm)0 Log Unit 1* Log Units 2 and 3α Log Unit 4• Log Unit 5Δ Log Unit 6

°o °

o

Hole 737B

25K (%)

2 0 •

+ Log Unit 1, 2, 3, 4, 5, and 6.

Kaolinite field

Illite field

10 12

K-Feldsoar field

Eα.o.

Figure 12. Lithological and mineralogical crossplots in Hole 737B. The crossplots were used by Schlumberger (1988) and Mathis (1988). Themineralogic and lithologic poles and fields are from Augier (1980), Serra (1985), and Ellis et al. (1988). A. RHOB-DT crossplot for the entirelogged section. B. RHOB-NPHI crossplot for the entire logged section. C. SGR-U crossplot for the entire logged section. D. Th-K crossplot forthe entire logged section. E. SGR-U crossplot for logging Unit 1. F. Th-K crossplot for logging Unit 1.

this unit is lithofacies 6 which is called moderately cementedclayey calcareous ooze. As for the previous units, the compari-son between the density-porosity crossplot from core and theRHOB-NPHI crossplot allow us to extrapolate the combined li-thology from core and logs to the few nonrecovered intervals.


Logging Unit 5 is characterized by a regular decay in the po-rosity values, from 64% at the top to 42% at the bottom of theunit (Fig. 2), according to the neutron measurements. The on-core porosity measurements agree also with a porosity decay ofabout 20%, from the top to the base of the unit, and with po-rosity values measured, ranging from 50% to 30% (Fig. 11).

The 10% shift between core and log measurements may be ex-plained by the clay "bound-water" content which is seen as po-rosity by the neutron device (Ellis, 1987). Because the log mea-surements appear to be relatively consistent with the on-coremeasurements, the calcareous claystone lithology may be ex-tended within the few nonrecovered intervals. The decay in po-rosity in the calcareous claystone allows a definition of twolithofacies corresponding to the transition from moderately ce-mented and very clayey calcareous oozes (lithofacies 7) to weaklycemented limestones (lithofacies 8) (Fig. 3). According to theSGR log, the uppermost part of the unit, from 517.0 to 550.0mbsf, is corresponding to a clay-rich ooze (lithofacies 7) with amaximum of 27% clay content of the dry bulk. This clay-rich

276


U (ppm)140

120

1 0 0 •

80

6 0 •

4 0 •

20

° Log Unit 1

Lithofacies 1

Lithofacies 2

°o °

o

o

Silts with feldspar andvolcanic elements

/ '

Hole 737B

25

20

15

10

5

n

K

° Log Unit 1

Kaolinite field

I 1

1 /I i l\ Montmorillonite fi1 • / • /

\ \ /

1 o cαa / o o

(%)

eld/

Hole 737B

-

//

Illite field

K-Feldspar field

.—^–1 0 1 2 0 1 2 3 4 5 6 7

Figure 12 (continued).

lithofacies is well correlated with a clear decrease in carbonatecontent observed by Bohrmann and Ehrmann (this volume).The lowermost part of the unit appears to be less clayey than theupper part, which is also in good agreement with the higher car-bonate content observed by Bohrmann and Ehrmann (this vol-ume). As for the preceding unit, the clay composition appearsmainly as illite type (Fig. 13E).


This unit corresponds to the base of lithologic Unit 5 (calcar-eous claystone) and to the top of lithologic Unit 6 (clayey lime-stone). The main characteristics of this Unit, observed from thelog records, are an abrupt decrease in porosity from 58% to46%, and an abrupt increase in bulk density from 2.0 to 2.2g/cm3 at the top of the unit (Fig. 2). A significant increase inbulk density and decrease in porosity is also observed fromcores with the same shift between core and log measurements(Fig. 11). Unlike the preceding high-porosity logging units, thedensity-porosity relationship proposed by Schlumberger (1988)may be applied to the sediments of logging Unit 6. On theRHOB-NPHI crossplot, the plotted data of Unit 6 fall betweenthe line of the limestone and the pole of the clays, confirmingthe term clayey limestone employed in the core descriptions(Fig. 12B). The RHOB-NPHI crossplot and the consistency be-tween core and log measurements in the recovered intervals al-low to extrapolate the clayey limestone to the nonrecovered in-tervals (lithofacies 9), particularly between the top of the unitand 675.0 mbsf where the recovery is particularly poor. The in-crease with depth of the radioactivity rate from 10 to 25 APIsuggests an increase in clay content from the top to the base ofthe Unit. The plotted data of the Th-K diagram for this unit fallmostly within the montmorillonite field (Fig. 13F).

Hole 738C (Fig. 5)

In Hole 738C, the geophysical logs allow a definition ofthree major logging units clearly identifiable on the RHOB-NPHI and RHOB-DT crossplots (Figs. 14A and 14B). In thepresent study, eight lithofacies are defined from both core andlog information, within these three logging units. The core de-scription indicates that the formations encountered within the

logged section consist mainly of a pure carbonate sequence ofcalcareous ooze, chalk, and limestone (Barron, Larsen, et al.,1989). Indeed, the carbonate content measured by Hermann(this volume) is close to 85% on the entire logged interval. How-ever, the density-porosity crossplot from the core shows that thelaboratory density measurements are probably overestimated be-cause the plotted data fall below and not on the limestone line(Fig. 15). It also shows that it is possible to extend the linear re-lationship between density and porosity above 45% of porosity.In the RHOB-NPHI and RHOB-DT crossplots, the plotted val-ues for the logged interval (113.0-455.0 mbsf) were shifted hori-zontally toward the limestone grain density line in order to cor-rect them for water effect (Figs. 14A and 14B). The correctionapplied to the original NPHI values is -0.08 g/cm3 and thecorrection applied to the original DT values is -20 µs/ft. Thelocation of the plotted data from logging Unit 1, slightly abovethe line of the limestone, in the RHOB-NPHI crossplot is due tothe saturation effect of the neutron device. The PEF values in-crease downhole from 2.5 barns/electron (b/e) at the top of thelogged section to 5 b/e at the bottom of the logged section (Fig.4). The PEF factor of water is 0.358 b/e whereas the calcitePEF factor is 5.084 b/e (Lamont-Doherty Borehole ResearchGroup, 1988), so the decrease of the PEF values results from theprogressive loss of water with increasing depth in the calcareoussequence. The level of radioactivity is very low in Hole 738C;the gamma-ray record never exceeds 25 GAPI units, and themean level of radioactivity usually ranges between 5 and 10GAPI units (Figs. 4 and 14C). The low radioactivity rate of theformations in this hole suggests that they contain almost no Th,K, and U, and thus, almost no clay, confirming the pure carbon-ate sequence. The paucity of clay in Hole 738C, suggested bythe SGR log, is in good agreement with the clay determinationby Ehrmann and Bohrmann (this volume). The difference inclay content between Sites 737 and 738 is clearly shown by com-paring the lithologic crossplots from both sites (RHOB-NPHIand RHOB-DT; Figs. 12A, 12B, 14A, and 14B).


The depth interval 113.0-250.0 mbsf defined for loggingUnit 1 almost fits with the depth interval of lithologic Unit 4

277


25K (%)

Log Units 2, and 3

Kaolinite field

Hole 737B

K-Feldspar field

DT (microsec./ft)40 60 80 100 120 140 160 180 200

1.5

1.7

1.9

E P 1Q. 2.1

2.3 •

2.5

2.7

2.9

+ Standard lithologic polesA Log Units 2 and 3

40*

• Opale /

C /

/ D

/

+ Clay

Clay

A

Porcellanite •+A j/t

A

+ Clay Ka-Sm

III

Ka-Fe

Hole 737B

-0,2

1.5

1.7

1.9

2.1

2.3

2.5

2.7

NPHI (pu)-0.1 0 .0 0,1 0,2 0.3 0.4 0.5 0.6 0.7 O.i

25K (%)

2.9

Standard lithologic polesLog Units 2 and 3

Porcellanite

Clay Ka-lron

Hole 737B

α Log Unit 4 Hole 737B

Kaolinite field

Illite field

K-Feldspar field

Eα.Q.

Figure 13. Lithological and mineralogical crossplots in Hole 737B. The crossplots were used by Schlumberger (1988) and Mathis (1988). Themineralogic and lithologic poles and fields are from Augier (1980), Serra (1985), and Ellis et al. (1988). A. Th-K crossplot for logging Units 2and 3. B. RHOB-DT crossplot for logging Units 2 and 3. C. RHOB-NPHI crossplot for logging Units 2 and 3. D. Th-K crossplot for loggingUnit 4. E. Th-K crossplot for logging Unit 5. F. Th-K crossplot for logging Unit 6.

(120.8-254.4), which is described as calcareous nannofossil oozeand chalk. The porosities of this interval are underestimated bythe neutron device, which reads porosity values about 10% lowerthan the predicted porosities for limestones (Fig. 14A). Takinginto account this porosity overestimation, the RHOB-NPHI andRHOB-DT plots (Figs. 14A and 14B) for logging Unit 1 showthat the lithology mainly consists of a highly porous (45% to55%) calcareous sequence, which matches the oozes describedfrom the cores. Some less porous electrobanks are thought tocorrespond to chalk layers. Two lithofacies are defined accord-ing to the degree of consolidation of the calcareous sediments.Lithofacies 1 is the highly porous calcareous ooze of the top ofthe unit, whereas lithofacies 2, which is termed calcareous ooze

and chalk, is the weakly cemented ooze, mostly encountered atthe base of unit (Fig. 4). The lowermost part of the logging unit(180.0-250.0 mbsf) was almost not recovered by coring, but theuniformity of the lithologic log values on the entire Unit 1 sug-gests that no major lithologic change occurs within this depthinterval (Fig. 4). However, a slight increase in density, resistivity,and sonic velocity with increasing depth, is observed in this log-ging unit (Fig. 4). It indicates that more chalk layers occur in thelowermost part of the unit. This conclusion is not confirmed byan increase of the carbonate content toward the base of the log-ging unit (Bohrmann. and Ehrmann, this volume), but the car-bonate content estimation is based only on few samples withinthis unit, and the consolidated carbonate-rich chalk layers were

278


25

20

15

10

5

n

• Log Unit 5

Kaolinite field

I 1

I I\ \ A Montmorillonite

I x / /

K (%)

field -

Hole 737B

•

/

Illite field**

K-FeldsDar field

25K (%)

Log Unit 6

Kaolinite field

Hole 737B

field

K-Feldspar field


probably not recovered by coring. Also, two depth intervals atleast (160.0-161.0 mbsf and 200.0-215.0 mbsf) presents wellcorrelated peaks of high velocity, low density, low porosity, andlow PEF values, which corresponds to physical characteristicsof siliceous matter (Fig. 4). These peaks are interpreted in thepresent sedimentary environment as chert layers (lithofacies 3),and as the indurated chalk layers, they probably contribute tothe bad core recovery within logging Unit 1.


Logging Unit 2 corresponds to lithologic Unit 5, which is de-scribed as calcareous chalk. The plotted data of logging Unit 2,although scattered, fall close to the limestone line of the RHOB-NPHI diagram (Fig. 14A). This plot shows that the material oflogging Unit 2 is more compacted than in Unit 1, with porosityranges from 40% to 50% and density from 1.7 to 2.3 g/cm3.This compaction effect is also seen on the density-porosity cross-plot from cores, which exhibits measured density and porosityvalues significantly lower than in the previous unit (Fig. 15).The resistivity and sonic logs, which are directly related to theporosity, stop rising with increasing depth at the top of the unit(Fig. 4). This signifies that the overburden load is not able to ex-pel water in this unit anymore, probably because of the lithifica-tion of the oozes. The few cored intervals recovered within thisunit correspond to a chalk lithology, and the previous log char-acteristics compared with the physical properties measurementson the core enable us to extrapolate this lithology to the entireinterval. The PEF values appear significantly lower (2.5 b/e) inthe lower chalk interval (325.0-380.0 mbsf) than in the upperchalk interval (3.0 b/e) (Fig. 4). The PEF-N crossplot (Fig. 14D)indicates that the decrease of the PEF values in the lowermostpart of the chalk induces a shift toward the quartz and opalpoles. As multiple peaks of increase in sonic velocity are ob-served correlating with the low PEF value intervals, it suggeststhe presence of chert layers or nodules, as those described fromthe cores. On the lithologic column (Fig. 5), chalk correspondsto lithofacies 4, whereas siliceous chalk or chert layers are litho-facies 3.


The depth interval corresponding to logging Unit 3 is de-scribed as limestone from the cores. The continuity of the logrecords allows us to characterize precisely the boundary betweenlogging Units 2 and 3. Unlike the gradual decrease in porosityvalues between ooze and chalk in the upper logged intervals, theNPHI log exhibits an abrupt decrease in neutron porosity from55% at the base of the chalk unit to 27% at the top of the lime-stone unit, attesting to a major change in the nature of the cal-careous sequence (Fig. 4). The RHOB-NPHI and RHOB-DTcrossplots (Figs. 14A and 14B) show that Unit 3 perfectly fitswith a limestone lithology with porosity values ranging from20% to 30%. The present depth interval was better recoveredthan the previous ones, although about 50% of the material wasnot recovered by cores. Because the log records do not exhibitmajor changes within this depth interval, the limestone lithol-ogy (lithofacies 8) may be extrapolated to the nonrecovered sec-tions (Figs. 4 and 5). The relatively high transit time and poros-ity values visible on the RHOB-NPHI and RHOB-DT diagrams(Figs. 14A and 14B) and the low TV parameter values (Fig. 14E)would suggest a slight increase in clay content, up to 10%within some limestone layers (lithofacies 7). On a PEF-N cross-plot (Fig. 14E), high PEF values—greater than 5.08 b/e—occur,thus indicating the presence of dense elements such as iron andmanganese. On the lithologic column this facies is termed ferru-ginous or manganiferous porous limestone of lithofacies 6 (Fig.5). As with the overlying chalk unit, peaks of decrease in PEFvalues are interpreted as a minor siliceous facies, which is repre-sented on the lithologic column by lithofacies 5.

Log-Inferred Lithology in the Glaciomarine Sequencesof the Antarctic Shelf (Prydz Bay Area)

Hole 742A (Fig. 7)

The lithology proposed from cores within the logged section(30.8-283.8 mbsf) in Hole 742A is described as a very uniformdiamictite sequence of glaciomarine origin (Hambrey et al., this

279


-0.05 0.05 0.15 0.25 0.35 0.45 0.55 0.65DT (microsec./ft)

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

50

45

40

35

30

25

20

15

10:•

5:

* Standard lithologic poles• Log Unit 1° Log Unit 2A Log Unit 3

Porcellanite

Opale

KF

Clay Ka-Fe

Hole 738C

CO

ε2

NPHI calc (pu)

U (ppm)

• Log Unit 1

° Log Unit 2* Log Unit 3

Ms sJ °a°::t:: i ° :io oo o)O O

Hole 738C

>.O

c0.5 1.0 1 .5 2.0 2.5 3.0 3.5 4.0

1.5

1.7

1.9

2.1

CO

O 2.3XGC

2.5

2.7

2"94

6.0

5.5

5.0

4.5

4.0

a.< 3.5KO 3 0

2.5

2.0

1.5

+ Standard lithologic poles• Log Unit 1o Log Unit 2 °* Log Unit 3

Porcellanite

Clay Ka-Sm

Clay Ka-Fe

Hole 738C

60 80 100 120 140 160 180

+

•

oA

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Hole 738C

10°30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

Figure 14. Lithological and mineralogical crossplots in Hole 738C. The crossplots were used by Schlumberger (1988) and Mathis (1988). Themineralogic and lithologic poles and fields are from Augier (1980), Serra (1985), and Ellis et al. (1988). A. RHOB-NPHI crossplot for the entirelogged section. B. RHOB-DT crossplot for the entire logged section. C. SGR-U crossplot for the entire logged section. D. PEF-N crossplot forthe entire logged section. E. PEF-N crossplot for logging Unit 3.

volume). However, the logs appear to be in contrast with respectto the lithology description, thus suggesting a lithology less uni-form than that indicated from the cores (Fig. 6). The diamictitesof the Prydz Bay area are described as unsorted sediments fromvarious origins, usually consisting of gneissic or granitic pebblesand blocks floating in a clayey silty-sandy matrix (Hambrey etal., this volume). The logging tools are influenced by the vari-ous components of the diamictites to give average responses thatintegrate the elementary responses of each component. Figure 7shows the lithologic column which results from the log interpre-tation, and the comparison between logs and core observations.Also shown are the NPHI and RHOB curves which are not cor-rected for hole size effect. This is only a matter of representa-

tion, because we think that these original curves, as a result ofthe cave effect, emphasize the contrast between hard and softlayers and between compacted and less compacted units. How-ever, the data plotted in the various lithologic crossplots, andused to define the final lithofacies, were selected within depthintervals where the hole was not enlarged.

The core density-porosity crossplot shows the linear relation-ship between these two parameters in the diamictite, within the15%-45% porosity interval (Fig. 16). The plotted data, of thediamictites, mostly fall on the dolomite line of the RHOB-NPHI crossplot (Fig. 17A), whereas they fall on and in the vi-cinity of the kaolinite-smectite clay pole in the RHOB-DT cross-plot (Fig. 17B). These data plot close to the montmorillonite

280


N6.0

5.5

5.0

4.5

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3.5

3.0

2.5

2.0

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* standard lithologic poles

* Log Unit 3

Opale

Hole 738C

0°.3O 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80


Porosity in Pu

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1,01.0

1.2

1.4

1.6

1.8

2.0

2,2

2.4

2.6

2.8

3.0

Q Lithologic Unit 1A Lithologic Unit 3o Lithologic Unit 4+ Lithologic Unit 5

• Lithologic Unit 6

A Lithologic Unit 7

Limestone line

A 1Hole 738C

1 . 1

Figure 15. Density-porosity crossplot from core measurements in Hole

738C. The densities, porosities and the lithologic units are from Barron,

Larsen, et al. (1989).

pole and field of the PEF-Th/K diagram and the Th-K dia-grams, respectively (Figs. 17C and 17D). The large amount ofradioactive elements, coming from the erosion of the metamor-phic basement, gives a high general radioactivity to the glacialsediments, which is always greater than 80 GAPI units on theSGR log and usually ranging between 110 and 140 GAPI units(Figs. 17E and 6); it is thus impossible to infer the clay contentof these formations from the SGR log. However, the compari-son between the log records, and particularly the porosity logs,with the granulometry curves from Hambrey et al. (this vol-ume), will in some cases give qualitative information about theclay content of the diamictites. The diamictites have a constantPEF value of 2.5 b/e, except in the lower logged section wherethe lithodensity tool responses were severely influenced by cav-

0.01.8

0.1

Porosity In Pu

0.2 0.3 0,4 0,5

Holθ 742A

Lithologic Unit 1

Lithologic Unit 2

Lithologic Unit 3

Lithologic Unit 4

Lithologic Unit 5

Lithologic Unit 6

Figure 16. Density-porosity crossplot from core measurements in Hole

742A. The densities, porosities, and the lithologic units are from Bar-

ron, Larsen, et al. (1989).

ing (Fig. 6). Granito-gneissic-rich debris layers are characterizedby high thorium and potassium contents that commonly exceedthe theoretical content of the clay fields (Fig. 17D). These layersalso show strong PEF values that are likely related to a signifi-cant iron content (Fig. 17F).

At Hole 742A, logs allow the identification of eight lithofa-cies spread over four logging units (Fig. 7). The RHOB-NPHIand RHOB-DT crossplots for the entire logged section (Figs.17A and 17B) and the shape of the logging curves (Fig. 6) allowdifferentiation of two main kinds of diamictites at Hole 742A(Fig. 7). The first type matches massive highly compacted diamic-tites (lithofacies 5) with porosities ranging from 25% to 35%(logging Unit 1, 33.8-117.7 mbsf, and logging Unit 3, 140.8-172.8 mbsf). The second type of diamictites matches stratifieddiamictites (lithofacies 4) with porosities greater than 35% (log-ging Unit 2, 117.7-140.8 mbsf, and logging Unit 4, 172.8-253.8mbsf). On the RHOB-NPHI and RHOB-DT diagrams the plot-ted data corresponding to logging Units 1 and 3 are grouped,whereas the plotted data corresponding to logging Units 2 and 4are scattered, thus emphasizing the difference between the rela-tively homogeneous composition of the compacted diamictitesand the variable composition of the stratified diamictites (Figs.17A and 17B).

Logging Unit 1

Logging Unit 1 matches lithologic Unit 2, which is describedas homogeneous and massive diamictite by Hambrey et al. (thisvolume). The logging parameters within this poorly recovereddepth interval remain fairly constant, which indicates no majorlithologic change (Fig. 6). So, we proposed to extrapolate themassive diamictite lithology to the entire unit (lithofacies 4).Within the massive diamictite a secondary facies characterizedby high thorium and potassium contents can be distinguished(Fig. 17D). This secondary facies (lithofacies 6) is thought toconsist of diamictites containing numerous pebbles and blocksof metamorphic and granitic origin, as confirmed by the num-ber of clasts found in the few recovered cores.

Logging Unit 2

According to the logs, the stratified diamictite sequence ofUnit 2 is more heterogeneous than that of Unit 1 (Fig. 6). It is ingood agreement with the observations of the cores (Hambrey etal., this volume). With respect to logging Unit 1, a significantincrease in porosity, from at least 10% according to the NPHIlog, occurs within logging Unit 2. We can note that this increasein porosity value influences all the log records (sonic log, den-sity log, resistivity log, and neutron porosity log). By comparing

281


-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

+ Standard lithologic poles• Log Unit 1 +

o Log Unit 2 Porcellanite4 Log Unit 3D Log Unit 4 α

30Opale+ S

/ y

1 0 / / /

0 / / /Q / /

c /D /

G

40 ° ° °

fir• P + Clay Ka-Sm+ Clay III

G

+ Clay Ka-lron

Hole 742A

1.5

1.7

1.9

2 1

2.3

2.5

2.7

2.9

DT (microsec./ft)

+ Standard lithologic poles

• Unit 1o Unit 2^ Unit 3α Unit 4

Opale

Porcellanite

Clay Ka-Sm

Clay Ka-Fe

Hole 742A

NPHI (pu)40 60 80 100 120 140 160 180

Th/K

o

•

σ

Standard lithologic polesUnit 1Unit 2Unit 3Unit 4

III

KF

Mu

IS

ao

MB

O j • f

Cl

L B D Q

1 Mo

Hole

Ka

742A

K (%)

o Granite compositiono

oo m • Log Unit 1Log Unit 2Log Unit 3Log Unit 4

1 0 1 0 0

Figure 17. Lithological and mineralogical crossplots in Hole 742A. The crossplots were used by Schlumberger (1988) and Mathis (1988). Themineralogic and lithologic poles and fields are from Augier (1980), Serra (1985), and Ellis et al. (1988). A. RHOB-NPHI crossplot for the entirelogged section. B. RHOB-DT crossplot for the entire logged section. C. PEF-Th/K crossplot for the entire logged section. D. Th-K crossplot forthe entire logged section. E. SGR-U crossplot for the entire logged section. F. PEF-N crossplot for the entire logged section.

the log curves with the sand/mud-ratio curve provided by Ham-brey et al. (this volume), one notices that the increase in poros-ity is closely related to a decrease in the mean grain size of thediamictites, which appear mainly as muddy diamictites withinthis interval. The increase in porosity related to fine muddy ma-terial suggests a clay-rich-diamictite unit (lithofacies 5). Thepresence of a clay-rich-diamictite facilitates the washout of thehole during drilling, which explains the numerous caves identi-fied within this interval. The unit contains some layers rich inthorium and potassium, thus indicating the presence of higheramounts of granitic components (Fig. 18A). The recovery ofone 62-cm boulder of gneiss and several individual cobbles

within logging Unit 2 (Hambrey et al., this volume), suggeststhat in the nonrecovered interval the rich thorium and potas-sium electrobanks may be interpreted as boulders. Locally, in-durated diamictite layers, having the same characteristics thanthose from Unit 1 and N parameters ranging from 0.5 to 0.6,are also interbedded in this unit (Fig. 18B). Minor lithologic fa-des are also identified within Unit 2. Ferruginous diamictitescorresponding to lithofacies 8 are identified on the PEF-N cross-plot in the region of high PEF values (Fig. 18C). Plotted datafalling close to the opal pole on the N-P, PEF-N, and RHOB-DT crossplot (Figs. 18B, 18C, and 17B) suggest the presence offew diatomite layers (lithofacies 1) within Unit 2.

282


200

180

160

1401

120

I100

80

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40

20

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N+ Standard lithologic poles• Log Unit 1° Log Unit 2* Log Unit 3D Log Unit 4

7 5


Logging Unit 3

Logging Unit 3 corresponds to lithologic Unit 4 (Hambrey etal., this volume), and is described as a massive diamictite. Ac-cording to logs, the compacted diamictite Unit 3 shows very lit-tle lithologic variations; it consists mainly of lithofacies 4 (Fig.7). One can distinguish only one secondary lithofacies (lithofa-cies 6), previously identified as diamictites rich in basement peb-bles (Fig. 18D).

Logging Unit 4

On the logs, the stratified diamictite of Unit 4 looks verymuch like those of Unit 2 (Fig. 6). In particular, the top of theunit (175.0-205.0 mbsf) exhibits a significant increase in poros-ity, well correlated with a decrease of the sand/mud-ratio (Ham-brey et al., this volume), which suggests a clay-rich diamictiteinterval (lithofacies 5). In the lowermost part of logging Unit 4,from 205.0 to 233.3 mbsf, the diamictites have mostly the samelog characteristics as those described for lithofacies 3. Besidesthese compacted diamictites representing the major lithologicfacies, some slight differences appear in the secondary and mi-nor lithologies. Sand or sandy diamictite layers occur here as asignificant lithology and form an important secondary lithofa-cies (lithofacies 2). These sandy layers are characterized by highsonic transit time values, low density values, and high porosityvalues resulting in abnormally low values of TV parameters (Figs.17B and 17F). A good example of this kind of lithology is a 15-m-thick layer between 215.0 and 230.0 mbsf on the lithologiccolumn that was not recovered by coring (Fig. 7). In addition tothe minor lithologies recognized within Unit 2, some slightlycalcareous diamictites appear here, characterized by higher PEFvalues correlated to weaker sonic transit times (lithofacies 7)(Figs. 17B and 17F). No diatomite layers such as those found inUnit 2 were identified within this unit.

Hole 739C (Fig. 9)

As for Hole 742A, the density-porosity crossplot from coredata at Hole 739C indicates a clear linear relationship betweenthese two parameters (Fig. 19). Four logging units were distin-guished in Hole 739C. On these four units, five lithofacies were

defined on the basis of comparisons between the mineralogicand the lithologic crossplots and the core descriptions (Fig. 9).The lithologic column shown on Figure 9 is represented betweenthe GR log and the PHI SON log. In order to provide some lith-ologic information within the nonrecovered intervals, and as nodensity logs are available at this site, the core densities havebeen used in the lithologic crossplots combined with the logdata. On the core density-PHI SON and core density-DT cross-plots (Figs. 20A and 20B), the plotted data of the diamictitesfall in the same regions than those of the RHOB-NPHI andRHOB-DT crossplots at Hole 742A (Figs. 17A and 17B). As inHole 742A, the grouped data in the density-NPHI SON cross-plot and in the density-DT crossplot correspond to massive,highly compacted diamictites (logging Units 2 and 4), whereasthe scattered ones correspond to less compacted stratified diamic-tites (logging Units 1 and 3).


The dominant facies of this logging unit is characterized byrelatively high calculated porosity values (PHI SON), rangingaround 40% (Fig. 10). Within this depth interval, the core po-rosity does not fit at all with the calculated porosity from thesonic log which is systematically about 10% higher. This may bedue to the small number of available core data, which probablyare not representative to the porosity of the entire interval. Inorder to obtain the lithologic crossplots (Figs. 20A and 20B),the few core density measurements available for the presentdepth interval were extrapolated to the entire interval. The datawere plotted on this diagram in the same area as for Units 2 and4 of Hole 742A. For this very poorly recovered section we pro-pose the general term of less-compacted stratified diamictites(lithofacies 1) in accordance with Hole 742A. The term strati-fied is suggested within the lower part of the unit by the alterna-tions visible on the log curves (Fig. 8). Within the alternatingzone, the peaks of high radioactivity suggest the presence ofmetamorphic and granitic blocks and pebbles (lithofacies 2).


According to the core description, logging Unit 2 (110.0-175.0 mbsf), which was almost completely recovered, consists

283


25

20

15

K (%)

o Log Unit 2 ° G r a n ' t e compositiono

00o o

Kaolinite field o coooo o o oo o

o

nπi

7

0.95

0.85

0.75

0.65

0.55

0.45

0.35


° Unit 2

Opale

Porcellanite

Clay Ill-Mu

Clay Ka-Sm

Hole 742A0.25

0.4 0.6 0. 1.0 1.2 1.4 1.6 1.8 2.0 2.2

N

* Standard lithologic poles

° Log Unit 2

KF

Q Opale

Hole 742A

25K (%)

20

Log Unit 3

Kaolinite field A

Granite composition

field

EQ.Q.

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

C D

Figure 18. Lithological and mineralogical crossplots in Hole 742A. The crossplots were used by Schlumberger (1988) and Mathis (1988). The

mineralogic and lithologic poles and fields are from Augier (1980), Serra (1985), and Ellis et al. (1988). A. Th-K crossplot for logging Unit 2. B.

N-P crossplot for logging Unit 2. C. PEF-N crossplot for logging Unit 2. D. Th-K crossplot for logging Unit 3.

Feldspar field

Hole 742A

mainly of massive diamictites. On the logs, and by comparisonwith the previous logging unit, the material of logging Unit 2looks considerably more homogeneous (Fig. 20C). The porosityvalues drop abruptly, suggesting highly compacted diamictites(lithofacies 3) in this unit (Figs. 8 and 10). A minor lithologyidentified at 141.1 mbsf is characterized by high transit time,low resistivity, high porosity values, and an abrupt drop in ra-dioactivity (Figs. 8 and 20C). The log characteristics of thislayer suggest a lithology (lithofacies 5) significantly differentthan the surrounding one, probably consisting of more sandymaterial. The core density-PHI SON and N-P plots for loggingUnit 2 are similar to the ones established for logging Units 1 and3 of Hole 742A (Figs. 20C and 20D).

Logging Unit 3 (175.0-315.0)

In logging Unit 3, the logging curves exhibit shapes corre-sponding to peaks and gradients related to contrasting litholo-gies (Figs. 8 and 10). The general porosity level increases signifi-cantly within this unit, except between 200.0 and 225.0 mbsf(Fig. 4). The increases in porosity within the unit are well corre-lated with the decreases of the sand/mud-ratio observed by Ham-brey et al. (this volume). As for Hole 742A, it suggests a clay-rich diamictite interval (lithofacies 4). Also, the general radio-activity level decreases significantly within the unit, probably in-dicating a major change in the source material of the diamicti-tes (Fig. 8). On the core density-PHI SON plot, the data of Unit

284


ruia/6

c

De

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ty

0.

2.0

2.2

2.4

2.6

2.8

T n

0 0,11

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-

Porosity

0.2 01

* D

o

line

In

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i +a

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0 . 41

<$£Q

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0 . 5

.+° ' α +

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0 . 6

-

-

o

D

+

•

Lithologic Unit 1

Lithologic Unit 2

Lithologic Unit 3

Lithologic Unit 4

Lithologic Unit 5

Figure 19. Density-porosity crossplot from core measurements in Hole739C. The densities, porosities, and lithologic units are from Barron,Larsen, et al. (1989).

3 fall in the same area as those in the Unit 1 (Figs. 20A and20B), which suggests a general lithology similar to the stratified,less-compacted diamictites of Unit 1. However, a compacteddiamictite bank (200.0-225.0 mbsf) is interbedded within un-compacted diamictite in Unit 3 (Fig. 8). The lithologic changesin Unit 3 are due in part to the presence of low-radioactivity lay-ers and low-porosity layers, which are thought to correspond tosandy diamictites or sand layers (lithofacies 5) (Fig. 8).

Logging Unit 4 (315.0-468.0)

Within logging Unit 4, the core density-PHI SON plot (Fig.20E) allows identification of two lithofacies: one major lithofa-cies with low porosity ranging from 25% to 35%, and a secondminor facies with porosities greater than 35%. By comparisonwith the few cored intervals, this major lithologic facies wouldcorrespond to massive unstratified, poorly sorted diamictites(lithofacies 3). A minor facies, with high porosities was notsampled (Figs. 8 and 9) and is thought to correspond to sandydiamictites or sand layers. Strings of high radioactivity at thebase of the unit are interpreted as boulder- or pebble-rich-base-ment pebble layers.

DISCUSSION

Hole 737B

Within logging Unit 1 the shape of the logging curves con-firms the turbiditic origin of the silt layers as suggested by thecores: the sharp contact at the base of the electrobanks is char-acteristic of strong and exceptional sedimentary events (Serra,1985) (Fig. 2).

Only the relatively uniform lithologic Unit 5, described as acalcareous claystone unit, was identified from the cores between321.1 and 677.9 mbsf, whereas four units were distinguishedfrom logs within this depth interval (logging Units 2, 3, 4 and 5;Fig. 3). These four logging units mainly represent gradual stepsin the decrease of the porosity values with increasing depth,which corresponds to the diagenetic transformation of the cal-careous oozes into limestones. This process is usually progres-sive; however, it may also appear abruptly, as within Unit 3(377.5 mbsf), where a jump on the log record is due to anabrupt decrease in the porosity value (Fig. 2). The diageneticfront identified within logging Unit 3 corresponds to the timeboundary between the early Oligocene and the late Oligoceneand may represent a short hiatus (Barron et al., chapter 46, thisvolume). The differences in the clay content and nature of thestudied logging units also reveal information about the detritalinfluences within the calcareous biogenic deposits. The maxi-

mum arrival of detritus seems to have occurred mainly duringthe middle Miocene (logging Unit 1), and during the end of theearly Oligocene and the beginning of the late Oligocene (loggingUnit 3). The minimum of detrital influence seems to have oc-curred in the middle Eocene (logging Unit 6).

Hole 738C

The detrital influence in Hole 73 8C within the logged sectionappears to be minimum, unlike in the previous hole. The log-in-ferred clay content is near 0%. The lithology inferred from logsis divided into three logging units which fit perfectly with thedescription from cores. However, the logs in this hole supple-ment the core description because the recovery was rather in-complete during the drilling operations (57.6%) (Barron, Lar-sen, et al., 1989). This is particularly the case in the lower partof the calcareous ooze unit and in the upper part of the chalkunit. The logs do not enable us, in this hole, to resolve with ac-curacy the siliceous lithology; however, some chert layers or sili-ceous chalk intervals are identified on the lithologic plots, butthe in-situ geochemical information is tremendously lacking fora better interpretation. The vertical resolutions of the tools arealso probably too low to resolve centimeter-scale chert layersand nodules.

Hole 742A

The four logging units defined from logs in Hole 742A cor-respond to the four lithologic units described from the cores(lithologic Units 2, 3, 4 and 5). The log interpretation empha-sizes the lithologic differences between highly compacted diamic-tites (logging Units 1 and 3) and less compacted diamictites (log-ging Units 2 and 4). The comparison between the porosity logand the sand/mud ratio from Hambrey et al. (this volume) con-firms the correlative increase in clay content of the diamictiteswith increasing porosity (Solheim et al., chapter 8, this volume).The clay-rich diamictites mostly correspond to the stratified in-tervals, whereas the diamictites with less clay match the massiveintervals. The "pipe" shape of the log-inferred lithologic col-umn within the highly compacted diamictites indicates a mas-sive deposit, consisting of pebbly mudstone with the same char-acteristics from the base to the top of the unit (Fig. 7). The logswithin the less compacted diamictites exhibit alternations corre-sponding to a varied lithology (pebble layers, diatomites, andindurated diamictites) and to porosity gradients (from non-in-durated to indurated lithologies). So, although the sedimentarysource is identical for both kinds of units (Antarctic, metamor-phic, and granitic basement sediments), as revealed by the uni-formity of the PEF log, the differences in the organization andin the texture of the two kinds of units suggest two kinds of sed-imentary processes. The massive diamictites correspond to whatEdward (1978) calls unstratified till, which is usually thought tohave been deposited in the proximal area of the glacial system atthe base of a grounded glacier. The less compacted ones, termedstratified tillites by Edward (1978), are typically of a glaciomarineorigin and were deposited seaward of the previous deposits (Fig.21). The physical characteristics of the stratified diamictites havealready been reported by Easterbrook (1964), who describedthese as lower-density and higher-porosity sediments than theproximal glacial sediments, which is in good agreement with thepresent log data. From seismic interpretations (Cooper et al.,this volume), lithologic Units I, II, and HI correspond to flatshelf reflectors, whereas Unit IV corresponds to steep slope re-flectors. The presence of sands in logging Unit 4 would confirmthese slope facies. However, it is interesting to note that the seis-mic data only enable us to identify one unit PS1, whereas coresand logs allow the recognition of three distinct units (loggingUnits 2, 3, and 4). The physical characteristics of the sediments,such as density and sonic transit time, change tremendously at

285


1.5

1.7

1.9

2.1

2.3 •

2.5 •

2.7

2-ö.O5

PHI son (pu)

+ Standard lithologic poles• Loα Unit 1° Log Unit 2* Log Unit 3α Log Unit 4

Opale 3 0 ^

Porcellanite

A A

4 0

π/ A *W ^ A A # .

P ÜQV & A^ A3 * / Clay III

Clay Ka-Sm

Clay Ka-Fe

Hole 739C

DT (microsec./ft)1.5

1.7

1.9

2.1

>. 2.3

o° 2.5

0.05 0.15 0.25 0.35 0.45

2.7 •

2.940

0

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Opale

30

20 J//

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A A

A A

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Hole 739C

60 80 100 120 140 160

1.5

1.7

PHI son (pu)

+ Standard lithologic poles0 Log Unit 2

Porcellanite

Clay

Clay Ka-Fe

Hole 739C

0.95

I 0.85

£ 0.75 •

ou| 0.65

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0.15 0.25 0.35 0.45

0.45 •

0.35 •

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o Log Unit 2

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Hole 739C

0 6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Figure 20. Lithological and mineralogical crossplots in Hole 739C. The crossplots were used by Schlumberger (1988) and Mathis (1988). The

mineralogic and lithologic poles and fields are from Augier (1980), Serra (1985), and Ellis et al. (1988). The densities measured on core are from

Barron, Larsen, et al. (1989). A. Density measured on core and porosity calculated from sonic transit time log (PHI SON) crossplot for the en-

tire logged section. B. Density measured on core and DT cross-plot for the entire logged section. C. Density measured on core and PHI SON

crossplot for logging Unit 2. D. N-P crossplot for logging Unit 2. E. Density measured on core and PHI SON crossplot for logging Unit 4.

the top and the base of logging Unit 2 (Fig. 6), causing the ex-pectation of strong seismic reflectors at the boundaries of thisunit. One possible explanation is that logging Unit 2 is not verythick (only 30 m), which is beyond seismic resolution. The litho-logic column inferred from the logs would suggest at least oneepisode of grounding activity, followed by the deposition ofstratified till related to marine conditions during the Plioceneages. From the late Pliocene to Quaternary the region seems tohave been glaciated again as shown by the presence of the mas-sive diamictite of Unit 1.

Hole 739C

Although only few logs were acquired in Hole 739C, they al-lowed us to propose a relatively realistic lithologic interpretationby comparison with the recovered core intervals and the litho-logic column in Hole 742A. As in Hole 742A, the lithologic col-umn consists mainly of a sequence of massive diamictites de-posited at the base of the glacier and less compacted diamictitesdeposited in seawater. Also, as in the preceding hole, the clay-rich diamictites appear to be located in the less-compacted,

286


1.5

1.7

PHI son (pu)

+ Standard lithologic polesD Log Unit 4

Porcellanite

Ka-Sm

0.25 0.35 0.45


stratified intervals. Due to the lack of core recovery within thetop of the drilled section, the core description failed to identifythe lithology in logging Unit 1. This unit corresponds to moder-ately compacted and stratified diamictites (Fig. 9) and was notrecognized by the seismic interpretation (Cooper et al., this vol-ume). In Hole 739C, as in Hole 742A, flat shelf glacial deposits(units 1 and 2) overlie slope deposits (units 3 and 4) (Cooper etal., this volume). Taking into account the relatively short dis-tance between the two holes (30 km), fairly good correlationsmay be proposed between the upper horizontal units between

the logging units. Upper Miocene to Pliocene Units 2 and 1 inHole 739C would correspond respectively to Pliocene Units 3and 2 in Hole 742A. Upper Pliocene to Quaternary loggingUnit 1 in Hole 742A does not appear to exist in Hole 739C.These correlations suggest that grounding ice reached at leastthe location of Hole 739C in the late Miocene-early Pliocene,whereas in the late Pliocene the grounding line did not evenreach Hole 742A or deposits were subsequently eroded away.Between the late Pliocene and the Quaternary the groundingline reached the location of Hole 742A.

CONCLUSIONS

In both the glacigenic and biogenic areas studied, the inter-pretation of geophysical log data compared to the core data al-lowed us to infer comprehensive lithologic columns. Althoughsometimes the log interpretations are still questionable becauseof the lack of certain kinds of logs, or because of the poor qual-ity of some of these logs, these interpretations improve signifi-cantly the knowledge of the sedimentary sequence in the nonre-covered intervals. From a methodologic point of view, the com-parison between the bulk density-porosity crossplots from logsand cores allowed us to establish the relationships between thesetwo parameters in the particular lithologies encountered and forthe poorly known high porosity intervals.

The results of this study confirm and supplement the de-scription from the cores. In the Kerguelen Plateau holes, thelithologic interpretations from the cores already provided an al-most complete lithologic column, but the logging interpretationbrings further quantitative information about the shaliness andthe porosity of the formations. In Hole 737B, six calcareouslogging units, from the middle Eocene to middle Miocene, weredifferentiated on the basis of their shaliness and porosity. Thelogs show that the illite content, which is related to detrital in-fluence (Chamley, 1989), is maximum in the lower to upper Oli-gocene, whereas it is minimum in the middle Eocene. In Hole738C, three lithologic units of the Campanian-late Eocene, con-sisting of calcareous oozes, chalk, and limestones, were recog-nized from both logs and cores. Despite the very bad recovery in

Sediment-laden fresh ΛJK<meltwater

P-».i-.•.;.

\\•:•i ~•• • > I • . V . ' ; . ÷ J V • '•:?*

Unstratified till

Basal till

7* ^ = ~ — ~ * ^ _ ' i / : •- 7"' fh~•>• . y θ j . ^ - , ~• --• K-# w c.*-e — — — •— •~™ — . - ~~^rz^^^

.<*«. Laminated mud cwith dropstones

Figure 21. Depositional environment of grounded and marine tillites (after Edward, 1978).

287


the Maestrichtian to lower Eocene interval, it was possible onthe basis of logs in this hole to follow precisely the transforma-tion of oozes into chalk. In both Kerguelen Plateau holes diage-netic fronts were identified from in-situ physical measurements.These two fronts are located within the very clayey calcareousooze at the boundary between the upper and the lower Oligo-cene in Hole 737B, and at the boundary between lower Eoceneooze and chalk in Hole 738C. In the Antarctic Shelf holes, thelogs allow us to distinguish compacted massive diamictite unitsfrom less-compacted, stratified diamictite units. The compacteddiamictites are seen as lodgement tills, whereas the less com-pacted ones were deposited in seawater (Hambrey et al., thisvolume). The comparison of the logging units with the seismicdata (Cooper et al., this volume) shows that the logging Units 1,2, and 3 in Hole 742A and logging Units 1 and 2 in Hole 739Ccorrespond to flat reflectors (PS1), whereas logging Unit 4 inHole 742A and logging Units 3 and 4 in Hole 739C correspondto slope reflectors (PS2). According to the logs, the depth inter-vals corresponding to the inclined reflectors have a general li-thology different from the horizontal ones; sand layers occur,inter bedded with clay-rich diamictites, suggesting distal deposits,which is in good agreement with a slope environment. The cor-relations between the two holes, based on the logs, indicate thatat least one grounding glacier reached the location of Hole739C in the late Miocene-early Miocene. In the late Pliocene,ice recession seemed to have occurred landward beyond the lo-cation of Hole 742A. Between the late Pliocene and the Quater-nary age, the grounding line again reached Hole 742A.

ACKNOWLEDGMENTS

We wish to thank our colleagues aboard the drillshipJOIDES Resolution for helpful discussions and contributions,particularly B. Larsen, who reviewed this manuscript and pro-posed many improvements, and A. Solheim and A. Pittenger,who provided the physical measurements used in this study. Wealso thank J. Barron, A. Cooper, M. Hambrey, W. Hehrmann,and A. Porat for helpful suggestions and comments. K. Lie-berkind is acknowledged for the useful suggestions she madethrough the review of the present manuscript. We also appreci-ate the help of the Lamont-Doherty Geological ObservatoryBorehole Research Group for useful advice and efficient train-ing, particularly R. Anderson, P. Pezzard, D. Goldberg, R. Jar-rard, C. Broglia, and X. Golovchenko. J. Clermontet and B.Biju-Duval made possible the collaboration between Nancy Uni-versity and IFREMER to realize the present study. E. Le Drezenfrom IFREMER is acknowledged for her contribution to thedata processing, as well as V. Tillot, who provided useful com-ments on the final manuscript. We also thank J. P. Maze for itscontribution to the artwork included in this paper.

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