14. ANATOMY AND EVOLUTION OF THE INNER ......sediments correspond to a phosphate-manganese crust...

Haggerty, J.A., Premoli Silva, I., Rack, F., and McNutt, M.K. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 144

14. ANATOMY AND EVOLUTION OF THE INNER PERIMETER RIDGE (SITES 874 AND 877)OF A CAMPANIAN-MAASTRICHTIAN ATOLL-LIKE STRUCTURE

(WODEJEBATO GUYOT, MARSHALL ISLANDS)1

Gilbert F. Camoin,2 Annie Arnaud Vanneau,3 Douglas D. Bergersen,4 Michel Colonna,5 Philippe Ebren,2

Paul Enos,6 and Jim G. Ogg7

ABSTRACT

The inner perimeter ridge of Wodejebato Guyot (Marshall Islands) is the subject of a detailed sedimentologic, seismic, andgeochemical study. The deposition of the shallow-water carbonate sequence, 177.7-190.2 m thick, is related to a two-stagetransgression leading to the development of fringing sand shoal facies overlain by early cemented rudist-coralgal "reef" units thatconstituted barrier-like features. Wodejebato Guyot exhibited, at least temporarily, an atoll-like morphology. The irregularities of theupper surface of platform carbonates, the occurrence of a trough separating the inner perimeter ridge from the outer one, and theformation of deep solution cavities in platform carbonates suggest karstification processes before the final drowning of the carbonateplatform during Maastrichtian time. Drowning events were probably related to the combination of a rapid sea-level rise and climaticchanges that characterize Maastrichtian time. Following the demise of the carbonate platform, subsidence possibly combined witheustatic sea-level changes carried rapidly the platform carbonates in a basinal setting below the aragonite saturation depth, resultingin dissolution of aragonite and subsequent in place precipitation of calcite cements possibly throughout the Cenozoic. Post-drowningsediments correspond to a phosphate-manganese crust that formed from the late Paleocene to the middle Eocene.

INTRODUCTION

Wodejebato Guyot (12.0°N, 164.9°E; formerly Sylvania Guyot) islocated approximately 74 km northwest of Pikinni Atoll in the north-ernmost part of the Ralik Chain in the Marshall Islands (see site mappreceding title page). A volcanic ridge 20 km long and about 1500 mdeep connects the two edifices. The summit of Wodejebato Guyot isabout 43 km long and increases in width from less than 12 km in thesoutheast to more than 25 km in the northwest. Four lobes project fromthe edifice and, along with the volcanic spur attaching WodejebatoGuyot to Pikinni Atoll, give the guyot a distinct "starfish" appearance.The flanks of the guyot may be divided into a steep upper slope (20°-24°) and a more gently inclined lower slope (about 7°); the transitiondepth between these two zones is around 2500 m. Sidescan images andseismic profiles collected during a pair of site survey cruises (MoanaWave cruises MW8805 and MW9009) show that two perimeter ridgeslie along the shelves formed by the northern and northeastern flankridge, and along the ridge extending southeast towards Pikinni Atoll(Bergersen, 1993). These perimeter ridges broaden across areas wherethe shelf margin widens (e.g., adjacent to the flank ridges).

The first observations on Wodejebato Guyot come from the late1940s Operation Crossroads (Emery et al., 1954). Early dredges on thisguyot recovered only basaltic rocks and tuff breccias (Hamilton andRex, 1959; Duennebier and Petersen, 1982). Initial interpretations ofthe subsurface of Wodejebato were based on a limited number of

1 Haggerty, J.A., Premoli Silva, I., Rack, R, and McNutt, M.K. (Eds.), 1995. Proc.ODP, Sci. Results, 144: College Station, TX (Ocean Drilling Program).

2 Centre de Sédimentologie et Paléontologie, U.R.A. 1208 du CNRS, Université deProvence, 3 place Victor Hugo, F-13331 Marseille Cedex 3, France.

3 Institut Dolomieu, Université J. Fourier, Rue M. Gignoux, F-38031 Grenoble cedex,France.

4 University of Sydney, Department of Geology, New South Wales, 2006 Australia.5 Département de Géochimie du B.R.G.M., GCH/TIE, B.P. 6009, 45060 Orleans

cedex 2, France.6 The University of Kansas, Department of Geology, 120 Lindley Hall, Lawrence, KS

66045^974, U.SA.7 Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette,

IN 47907, U.S.A.

analog single-channel seismic lines crossing the summit (Schlangeret al., 1987). During a 1988 Moana Wave cruise (MW8805) muchof the site survey work on Wodejebato Guyot was performed (seeBergersen, 1993), and four more dredges attempted across the guyotrecovered basalt, basalt breccia, basalt conglomerate, and limestone.From the study of these limestone samples, it was assumed that therewere two distinct sequences of shallow-water carbonate accumulation,which were Albian and Campanian-Maastrichtian in age (Lincoln,1990; Lincoln etal., 1993).

During Leg 144, two holes were drilled at Site 874 (Holes 874Aand 874B) and one hole at Site 877 (Hole 877A) on the inner perime-ter ridge of northeastern Wodejebato Guyot. Site 874 was located onthe seaward summit of this ridge and Site 877 was in a slightly moreinward position, 0.7 km northeast of Site 874 (Figs. 1-2).

The major objective of this paper is to reconstruct the depositionalhistory of the shallow-water carbonate sequence that documents thesubmergence of a volcanic island followed by the development andthe demise of a Campanian-Maastrichtian carbonate platform or atoll.Data used in this study include seismic, logging, sedimentologic, andgeochemical methods.

METHODS

Geomorphic and stratal pattern interpretations are based on side-scan images, swath bathymetry, 3.5 kHz and 12 kHz echo sounderprofiles and single and 6-channel digital seismics. During Leg 144,seismic data were recorded digitally on tape using a Masscomp 561super minicomputer and also were displayed in real time in analogformat on Raytheon electrostatic recorders using a variety of filtersettings and scales.

In Hole 874B, suites of geophysical (two passes), geochemical,and Formation MicroScanner (FMS) (two passes) downhole logswere obtained from approximately 164 mbsf (clay weathering pro-files developed on basalt substrate) to 30 mbsf (base of drill pipe). Thegeochemical logging record was extended to the sediment surface bymeasuring the neutron activation of various elements through the irondrill pipe.

271

G.F. CAMOIN ET AL.

12° 15

2120 Z 2130 Z

1 Site 874 •1'45'South Rank

RidgeL V . l Pelagic sediments

Exposed carbonate sedimentsChannels

165° 00'

Figure 1. A portion of the migrated six-channel seismic profile. This line crosses the ridges obliquely. The strong reflector underlying the ridges marks the base ofthe carbonate sediments.

The logs were used for interpreting the sedimentary succession andpossible discontinuities in accumulation within the carbonate plat-form. Core recovery within platform carbonates averaged only 10% inHole 874B and 13.3% in Hole 877A. In particular, the high-resolutionmicro-resistivity borehole-wall imagery from the FMS tool is excellentfor determining the thickness and vertical succession of lithologiesdisplaying similar "resistivity textures," which can be related to car-bonate facies. However, interpretations of sedimentological featuresfrom FMS images have varying degrees of reliability (Bourke et al,1989; Harker et al., 1990). Stratigraphic contacts, bedding thicknessesand dips, slumps, crossbedding, fractures, and rock textures, such asconglomerates, can generally be recognized on the FMS imagery.Other downhole logs, such as natural gamma ray, resistivity, porosity,and geochemical logs, were used for calibration of general composi-tional and textural trends. However, standard wireline logs do notprovide the resolution of the FMS. Therefore, interpretation of fine-scale compositional contrasts, diagenetic variations, and other detailsnoted on the images requires calibration with core data.

The geochemical records from the induced-gamma spectrum tool(GST) were converted by BRG-Lamont to relative elemental contri-butions: Fe (assuming an equal contribution by magnetite and hema-tite), Ca, Cl, Si, Su, Ga, Ti, and H (the last two elements werecomputed during processing) with corrections for hole diameter asrecorded by calipers on the density tool (CALI on the HLDT) and onthe FMS string (Cl and C2).

We used the Ca-abundance log at Site 874 to examine the relativechange in calcium-carbonate weight-percent in the borehole outside ofthe drill pipe. These values were scaled to be compatible with the resis-tivity measurements and serve as a proxy to identify porosity trends.

Thin sections were prepared from cored samples and studied usingpetrographic microscopes coupled with cathodoluminescence. Scan-

ning electron microscopy and electron microprobe (WDS) were alsoused. In order to obtain correlations between sites and general envi-ronmental gradients, we used correspondence analysis (CA) and thencluster analysis. First, we distinguished 13 grain types (variables) inthin sections (observations). Those grain types were red algae, greenalgae, benthic and planktonic foraminifers, corals, rudists and otherbivalves, gastropods, ostracodes, echinoderms, lithoclasts, peloids,and unidentifiable bioclasts. To keep only significant biologic compo-nents, planktonic foraminifers, lithoclasts, peloids and unidentifiablebioclasts were excluded and the remaining component values recalcu-lated to 100%. The data were introduced into the correspondenceanalysis program as logarithmic values [y = log10 (x + 1) ] to avoidstretched point cloud. From coordinates of observations obtained withthe CA, cluster analysis classifies thin sections into groups interpretedas paleoenvironmental sets.

Carbonates were sampled for geochemical analyses using a mini-ature lath fitted with a 0.6-mm-diameter dental drill. The drill wasguided for geochemical subsamples during observation through abinocular microscope. Stable isotope analyses (carbon and oxygen)on samples of carbonates were performed at the Département deGéochimie du B.R.G.M. (Orleans, France), using FINNEGAN MAT262 mass spectrometers. Reproducibility is 0.07‰ for δ'8O and 0.05‰for δ13C. The δ 1 8 θ and δ'3C isotopic compositions are expressed as:

and

δ 1 8θ = [(18O/16O) sample/(18O/16O standard"1)] 103

δ13C = [(13C/12C) sample/(13C/12C standard"1)] 103.

The results are reported in the conventional delta notation (in per mil),relative to the Chicago PDB isotopic standard.

272

ANATOMY AND EVOLUTION OF THE INNER PERIMETER RIDGE

Site 877Inner ridge

Site 876

> f Outer ridge

1 7 2 0 z Time (GMT) 1 7 3 5 z

I H S S S A Outer ridge

Time (GMT)

1715 z Time (GMT) 1655 z

« ^ Outer Ridge

1930 z Time (GMT) 1915 z

Site 874^ / Inner ridge

Φ/f), Outer ridge

1 6 1 5 z Time (GMT) 1 6 3 O z 1820 z Time (GMT) 1800 z

Outer ridge

Inner ridge/

h1

f> ;•:.:;, Outer ridge

1 6 0 0 2 Time ( G M T ) 1 5 5 O z1840z 1850z

Time (GMT)

Figure 2. Profiles of 3.5-kHz echo sounder across the perimeter ridges of Wodejebato Guyot. Profiles are arranged from northwest (a-a') to southeast (h-h')• Thelocation of Sites 877 and 874 are shown on profiles a-a' and g-g', respectively. The location of profiles on the guyot is shown in Bergersen (this volume).

G.F. CAMOIN ET AL.

a E

s ε

s ε~ o3 (50

CQ 3

•3 o ~ . S

J2 •S '6o ^ S

3..

_o" °


Table 2. Slope comparisons (in degrees).

Profile

a-a'b-b 'c-c'd-d'e-e'f-rg_g-h-h'

Note: NA =

Inner perimeter ridge

Upper slope Lower slope

6.71.60.83.32.26.92.40.3

not analyzed.

0.92.45.19.85.93.29.59.9

Outer perimeter ridge

Upper slope

3.75.63.23.21.62.83.27.2

Lower slope

3.710.26.2

15.314.58.59.67.2

Lagoon reflector

Maximum

NANANA1.40NA1.631.781.33

Minimum

NANANA0.95NA1.410.850.40

0-0.11 mbsf; Site 877—Unit I: Hole 877A, 0-0.03 mbsf), (2) rudist-algal-foraminifer shallow-water limestone (Site 874—Unit II: Hole874A, 0.14-7 mbsf, and Hole 874B, 0.11-162.8 mbsf; Site 877—UnitII: Hole 877A, 0.03-182.9 mbsf), (3) ferruginous clay, claystone, andextremely altered vesicular basalt (Site 874—Unit III: Hole 874B,162.8-177.7 mbsf; Site 877—Unit III: Hole 877A, 182.9-190.2 mbsf),and (4) basalt and volcanic breccia. Depth limits among the differentlithologic units are discussed herein on the basis of wireline log andFMS data at Site 874. Although the uppermost 25 m were not loggeddirectly, the calibrated geochemistry log of relative calcium abundancewas used as proxy for carbonate density or resistivity.

Biostratigraphy

At both sites, argillaceous limestone and dark clay (Unit III p.p.),which overlay the weathered volcanic rocks, contain a nannofos-sil assemblage of late Campanian age (Shipboard Scientific Party,1993a, 1993b).

Biostratigraphic analysis of platform carbonates relies mainly onlarger benthic foraminifers. These constitute successive assemblagesincluding Pseudorbitoides trechmanni, Sulcoperculina sp., and As-temrbis sp. and then Omphalocyclus macroporus Lamarck. The twofirst assemblages (VI, V, and II on Fig. 3) are late Campanian in agewhereas Omphalocyclus (I on Fig. 3), which dominates the last as-semblage, characterizes the Maastrichtian (see Premoli Suva et al.,this volume). The rudist assemblage recorded in these limestones in-clude radiolitids {Distefanella mooretownensis [Trechmann], Diste-fanella sp.) and caprinids (Mitrocaprina sp., Coralliochama orcuttiWhite, Coralliochama sp., Plagioptychus aff. fragilis Chubb, P. aff.minor Chubb, Antillocaprina sp.), which are reported in Campanian-Maastrichtian strata from Jamaica, Mexico, Cuba, and California(Chubb, 1971; Kauffman and Sohl, 1974).

Cavities in platform carbonates are partly filled by pelagic sedi-ments that contain Maastrichtian, early late Paleocene, and earlyEocene planktonic foraminifers (Shipboard Scientific Party, 1993a,1993b). The manganese crust and manganese-phosphate coated con-glomerate which cap the shallow-water limestones contain pelagicsediments of various ages ranging from the late Paleocene to themiddle Eocene (Shipboard Scientific Party, 1993a, 1993b).

WEATHERING PROFILES OF VOLCANICSUBSTRATE AND INITIAL MARINE DEPOSITS

The first sedimentary deposits directly overlying the volcanic sub-strate consist of claystone breccia, claystone, organic-rich clay, andargillaceous limestone (Figs. 3-4), 14 and 7.3 m thick at Sites 874 and877, respectively (Site 874: Unit III—interval from Section 144-874B-21R-1, 2 cm, through Core 144-874B-22R, 162.82-177.7 mbsf; Site877: Unit III—interval from Section 144-877A-20R-1, 20 cm, toSection 144-877A-20R-4, 110 cm, 182.9-190.2 mbsf). The top ofthe clay of lithologic Unit III is at 161.4 mbsf on the BRG-Lamont-processed FMS imagery. This compares to 162.8 mbsf on the ship-board preliminary processed FMS imagery, which was adjusted ac-cording to the position of this clay-to-limestone boundary (162.4 mbsf)in the resistivity logs from the geophysical tool. The top of the clay unit

is at the top of Core 144-874B-21R (162.8 mbsf), although it mayactually be at a higher level near the unrecovered base of overlyingCore 144-874B-20R. Based upon this comparison, it seems that thedepths of the BRG-Lamont-processed FMS imagery are about 1 mhigher than the other records. The sharp contact between clays andoverlying platform carbonates at 161.4 mbsf dip slightly (a few de-grees) toward the southeast as other beds within the Clay Unit.

The claystone breccia (interval 144-877A-20R-4, 0-110 cm) iscomposed of dusky red to reddish brown angular clasts ranging in sizefrom 1 mm up to 4 cm. The overlying claystone is dark, brownish andgray within an olive green clayey matrix (interval from Section 144-877A-20R-2, 132 cm, to -20R-3, 90 cm). Pyrite is common in thematrix (up to 4%) and forms small aggregates. Roots are preserved ininterval 144-877A-20R-2,46-120 cm. The dusky red to gray clay andclaystone, which form the bulk of Unit III in both sites, is interpretedas a subaerial weathering profile of the basalt. The contact betweenthe basalt and the clay is gradational and the degree of alterationdecreases downward. This is consistent with the enrichment in ironand silica recorded by the gamma spectrometer log (Fig. 3). Thetexture of the former igneous breccia (interval 144-877A-20R-4,100-110 cm) or altered basalt, including laths of plagioclase andvesicles filled by white patches of zeolites (e.g., interval 144-874B-22R-3,85-127 cm) is locally preserved in the clay and claystone. Thesubaerial weathering profile formed by clay and claystone is thinnerat Site 877 (6.5 m thick; Sections 144-877A-20R-2 through -20R-4;depths, 183.7-190.2 mbsf) than at Site 874 (14.5 m thick; intervalfrom Section 144-874B-21R-1,41 cm, through Core 144-874B-22R),possibly as a result of the higher substrate relief of Site 877.

The formation and preservation of the peat reported in interval144-877A-20R-2, 0-20 cm, may represent the first influences of ma-rine waters. The high sulfur content of this peat is probably related tothe bacterial reduction of sulfates provided by marine waters; preser-vation of plant debris in this peat suggests the prevalence of low-energy conditions. The organic-rich clay (intervals 144-874B-21R-1,2-41 cm; 144-877 A-20R-1,20-32 cm; and 144-877 A-20R-1,77-100cm) is gray to black with preserved kerogen-type woody material andalso includes calcareous nannofossils and larger foraminifers. Theargillaceous limestone that forms the top of Unit III at Site 877 (interval144-877A-20R-1, 32-77 cm) are intercalated with laminae of blackclay and contains few thin-shelled mollusc fragments and rare benthicforaminifers. The depositional environment corresponds to a quietshallow-marine environment, as suggested by the preservation of clayand the lack of any reworking at the base of marine deposits.

SHALLOW-WATER PLATFORM CARBONATES

Facies Model

The Campanian-Maastrichtian platform carbonates (161.4 and182.6 m thick at Sites 874 and 877, respectively) consist of interbeddedgrainstone shoal, rudist-coralgal framework, and "lagoonal" wacke-stone complexes (Figs. 3-4). The greater thickness of the carbonatesequence at Site 877 (18.6 m thicker than at Site 874) results from thethickening of the lower "reef" unit that makes up most of the upper halfof the platform carbonate sequence (Hole 874B: Subunit IIC, 42 m

275

G.F. CAMOIN ET AL.

lithologyII 8.

Bdst. Gst., rdsl.

11.1

For,

3 8 -

lifer-rudist

packstoπe /

grainstonβ

and Gastropod

wackβstone

Algal-rudist

graiπstonβ/rudstoπe

and

algal-coral

boundstonβ

Skθlβlal-coral

packstonβ

Reddish yellov

grainstone

42.2

Rhodolith-

foraminifβr

grainstone

Clay and

claystoπβ

caliper density

total natural

resistivity gamma

• • • l • • • l • • • l • • • l •

10 12 14 16 18 20 1.4 1.8 2.2 2.6 0 10 20 30 0 10 20 30 40

3(inches) (g/crrT (Ohm-m)

= Major, minor Gamma peaks

Figure 3. Stratigraphy of Hole 874B from selected geophysical logs as compared with cored intervals, lithostratigraphy, and biostratigraphic ages. Resistivity isfrom the medium-penetration phasor-induction (IMPH) tool. Total natural gamma measurements are from the FMS run. Caliper or apparent hole diameter doesnot record values greater than 48 cm.

thick; Hole 877A: Subunit IIC, 77.3 m thick). This elevational differ-ence between the two sites may result either from the differentialorganic growth or from late erosion processes.

The following summary of the carbonate facies integrates the ob-servations from the cored sediments (typically much less than 1 m ofrecovery from each 10-m-cored interval), the quantitative resistivitymeasurements, the natural-gamma intensities, and the detailed FMS"textures" (darkness on gray-scale imagery) expressed in a short-hand of "low" (5ohm-m) resistivity. Division of the lithologic succession into subunitsis based initially on the facies and nomenclature of shipboard-definedlithologic units. Grain counting data and subsequent correspondenceanalysis data are shown in Figures 5 and 6, where an environmentalgradient appears with a clear component relay on Axis 1.

Sand Shoals

Occurrence

This facies constitutes Subunits IID to IIF at Site 874 (intervalfrom Core 144-874B-10R to Section 144-874B-21R-1, 2 cm, 83-162.82 mbsf) and Subunits IID and HE at Site 877 (interval from Core144-877A-12R to Section 144-877A-20R-1, 20 cm, 105.7-182.9mbsf) (Fig. 4).

In Subunit IIF, 19 m thick (142.4-161.4 mbsf), the resistivitycurve displays three main features (Fig. 3): (1) a basal peak in resis-tivity followed by a sharp drop, (2) a multipeaked interval of mediumresistivity, and (3) a major high-resistivity peak. Natural gamma is ata relatively high intensity throughout this subunit, with maximumvalues at 149.0-153.0 mbsf. This high gamma value is associatedwith a facies of algal flats and concretions. The top of Subunit IIF maycorrespond to a cemented horizon.

The boundaries of reddish yellow grainstone from Subunit HEwere originally placed at major peaks in natural gamma-ray intensity,which were thought to coincide with surfaces of cementation. How-ever, later BRG-Lamont depth corrections indicate that the lowermajor peak in uranium concentration is centered at about 140 mbsf,2.5 m above the pronounced cementation surface at 142.2 mbsf, andappears to coincide with an algal-rich interval. Subdivisions weremade relative to the lower peak in uranium concentration. The caliperof borehole diameter progressively opens between these two uraniumpeaks, and is wider from 123 to 92 mbsf (Fig. 3).

Description

Subunit IID is essentially a featureless low-resistivity facies, ex-cept for the uppermost 7 m where resistivity gradually increases (Fig.3). The contact between these two logging subunits is marked by aminor peak in gamma-ray intensity. Based on well-log data, the upperboundary of this lithologic Subunit IID is placed at the top of themedium-resistivity facies (89.1 mbsf), where it is sharply overlain bya low-resistivity interbedded facies that is more typical of a basalportion of the "upward-shallowing cycles" that characterize the over-lying lithologic Subunit IIC.

Sand shoal facies primarily consist of white, pink, yellow, andreddish yellow, fine-grained skeletal packstone and grainstone (PI.1A). The FMS signature of this facies is a very poorly cemented,massive grainstone to "rubble-speckled" deposit without significantbedding or cementation levels. Grain size averages 0.1-1.5 mm, andthe sorting, usually poor, may be locally bimodal (mixing fine sandsand coarse sands or gravels); grains are usually angular to subangular.Major components of these skeletal packstone and grainstone includebenthic foraminifers (orbitoidids, Sulcoperculina and miliolids, litu-olids, textulariids), fragments of red algae (corallinaceans, few frag-


1374 -

1414 -

1454 -

1494 -

1534 -

1574 J

•S 11.1— c f^jηt ^ - ' _ _Ske[etal grainstone / rudstone

System tracts

Depositionalenvironments

and facies

Coralgal rudist boundstone

Foraminifer-rudist packstone/grainstoneGastropod wackestone

-38.0

Algal-rudist grainstone/rudstoneand

Algal-coral boundstone

- 8 3

Skeletal-coral grainstone and packstone

No Recovery

-123

142.2Reddish yellow grainstone

Rhodolith-richforaminifer-algal-grainstone

h 162.4

175

200

Clay and claystone

Basalt

HST

mfs

TST

LST

HST

TSTmfs

Rudist coralgal frameworks

Lagoonal deposits

Rudist coralgalframeworks

Sand shoalfacies

Figure 4. Stratigraphy of Hole 874B with lithostratigraphy, biostratigraphic ages, systems tracts, and inteΦretation of depositional environments and facies.

ments of solenoporaceans and peyssonneliaceans), rudists (radio-litids), and other bivalves (inocerames and pycnodonts) (Fig. 5 andTables 3-4). Large fragments of corals and rhodoliths, 1.0-1.5 cm indiameter, are locally common. There are few fragments of green algae(dasycladaceans, codiaceans), gastropods, bryozoans, and echino-derms. Calcisphaerulids and scarce planktonic foraminifers are notedlocally. A few lithoclasts consist of micrite with benthic foraminifersand comminuted fragments of rudists and red algae.

Interpretation

The fossil content and sedimentologic criteria imply a shallow-marine depositional environment with moderate-to-high energy con-ditions. The vague oblique and low-angle cross laminations notedlocally on cores (e.g., intervals 144-874B-17R-1, 36^4 cm, and144-874B-17R-1,47-53 cm) as well as the imbrication of large clastssuggest the prevalence of a sedimentary regime dominated by cur-rents and the occurrence of sedimentary slopes. The changes in tex-ture and in the relative abundance of organisms (increasing abun-dance of benthic foraminifers coeval with a decrease in the abundanceof rudists and red algae) recorded at the Subunit IIE-Subunit IIDtransition at Site 874 suggest a slight decrease in wave energy. Thelocal dusky red color is primarily related to irregular impregnationsof iron oxides both in matrix and in cavities, which could result fromeither a very slow sedimentation and associated oxidation or perva-sive weathering. The possibility of reddish coloration caused by sur-ficial weathering and "Terra Rosa" soil development was not sup-ported by microfacies (no vadose-style cement development), nor by

isotopes (no vadose signature), nor by chemistry (no thorium orsilica/aluminum increase suggesting soil clays).

The abundance of red algal balls and nodules reported at the baseof the carbonate sequence (e.g., Section 874B-18R-21R-1, 2 cm) is acommon feature in the initial steps of development of Campanian-Maastrichtian isolated carbonate platforms (Camoin et al., 1988) andCenozoic atolls (Bourrouilh, 1979; Collot, Greene, Stokking, et al.,1992). As recent rhodolites, these nodules seemingly played a signifi-cant role in the stabilization of previously mobile substrates.

Rudist-coral Frameworks and Associated Bioclastic

Shoals Occurrence

These facies constitute two distinct "reef" intervals at both sites(Fig. 4): a lower one comprises Subunit IIC (Cores 144-874B-5R to-9R, 38.1-83 mbsf; Cores 144-877A-4R to -11R, 28.4-105.7 mbsf)and an upper one constitutes Subunit IIA (interval from Section 144-874B-1R-1, 11 cm, to Section 144-874B-2R-1, 140 cm, 0.11-11.1mbsf; and Core 144-877A- 1R, 0.03-9.5 mbsf). Although the existenceof a true framework is difficult to ascertain in cores, the facies involvedconsist of closely associated rudist-coralgal frameworks and bioclasticunits. The FMS shows a series of stacked horizontal elongate depositsof high resistivity (coral-algal-rudist frameworks), alternating withless-resistant material (skeletal grainstones and rudstones) (Fig. 3).

Description

The lower interval is composed of algal-rudist grainstone/rudstone, with a few packstone layers and algal-coral boundstone.

277

G.F. CAMOIN ET AL.

Table 3. Grain-counting data of skeletal components of shallow-water carbonates at Site 874.

Subunit

Ha[la[la[laHa[laHaHaliblibliblibliblibliblibliblibliblibliblibliblibIklielieIkHelieHelieHeHeHe-IIe-HelieHe-IIe-Helie-HelieHeHeHeHelieHe-IIe-HelidHdHdlidHelielieI k[IfIlf[IfIlfIlf

Core, sectioninterval (cm)

1R-1,25-291R-1,35-391R-1, 56-611R-1,83-901R-1, 122-1282R-1,25-302R-1,69-702R-1,96-1002R-2, 12-172R-2, 47-55 A2R-2, 47-55 B3R-l,0-43 R-1,43^493R-1,84-963R-1, 130-1343R-2, 0-33R-2, 48-523R-3, 8-113R-3, 19-22 A3R-3, 19-22 B3R-3, 31-334R-1, 16-184R-1,23-314R-1,66-755R-1, 17-225R-1,28-325R-1,39^4A5R-1, 39^14 B5R-1,49-555R-1,80-835R-1, 145-1495R-2, 20-245R-2, 85-885R-2, 102-1086R-1, 16-196R-1,47-526R-1, 61-636R-1,64-676R-1,82-866R-1,97-1036R-1, 103-1066R-1, 122-1266R-1, 138-1416R-2, 24-267R-1.5-107R-1,26-317R-1,59-628R-1, 18-219R-1,3-79R-1,50-539R-1, 107-1109R-2,4-510R-l,7-10HR-1,4-716R-1, 16-2016R-1,44-4717R-1,47-5517R-1,95-9918R-1, 12-1618R-1,53-5519R-1. 16-2019R-1,31-3519R-1,93-9919R-1, 145-14920R-1,5-7

Depth(mbsf)

0.270.370.590.861.259.98

10.4010.6811.2611.7211.7219.2219.6620.1020.5220.6921.1722.2022.3022.3022.4228.8728.9729.4038.3038.4038.5238.5238.6238.9239.5739.8140.4640.6447.7848.1048.2248.2548.4448.6048.6448.8449.0049.3257.2857.4957.8067.1076.6577.1177.6878.1286.2891.25

116.98117.25126.90127.37136.14136.54143.68143.83144.46144.97153.16

Texture

GGRRBFPPFP

wwG

W/PPPP

P

wP

W/MWG

G/RBBBP

G/PPPP

P/FGCiG

B/PG/PP/FBBFBB

G/BB/G/P

BBB

G/PP/GB/PPGGGGGGPBBGG

RDA(%)

000000.20

13.80

13.16.3000000000000.800004.60001.20.501000000000000000000000000.50.83.4000.60

GRA

(%)

6.75.51.01.71.2

32.239.030.022.028.840.031.822.751.357.770.531.062.253.066.133.465.568.022.328.715.019.014.327.016.042.037.033.41 1.038.030.017.010.022.8

7.21.86.0

10.86.17.0

43.017.916.218.916.074.653.8

4.734.0

9.59.3

17.317.518.014.530.6

7.16.0

39.035.1

BTF(%)

20.828.231.015.632.0

0.70.50.2

29.00006.00.50.702.10006.200.2

10.75.24.54.1

10.29.4

10.52.100.2

12.06.09.0

11.741.0

8.04.52.60

31.566.483.632.826.308.0

17.500

92.318.00.20.300000.26.88.703.4

COR(%)

20.818.663.044.0

1.014.039.846.927.826.319.067.463.036.035.425.832.024.123.0

2.053.8

1.414.754.053.057.057.046.840.040.534.928.047.060.337.029.340.033.053.069.377.083.023.7

1.31.69.42.10.72.10.23.0

18.00.2

14.557.066.735.321.531.018.0

1.104.72.33.4

RDT(%)

01.900.51.01.502.300.23.701.12.4500003.101.402.7000003.00004.0000.500000.80.50000"0000000003.07.04.40.501.000

BIV(%)

34.09.10(i0

32.08.70.74.7

12.512.004.000001.70.3

10.80.25.77.24.04.58.08.200.500.7

16.70.78.22.02.7001.7000.800.70000000001.700000004.20.201.0

GTP(%)

0.500001.20.5001.2

16.00.802.2000.50.20

17.00.2

253.70

00000000.70.50000000000

00

0II

00000000

0000

OST(%)

0.55.00001.2

10.02.04.56.33.001.02.50.73.7

10.54.5

13.31.02.50.55.22.05.57.52.58.79.4

25.210.715.912.52.5

10.010.020.8

8.54.25.04.53.05.22.55.08.54.52.28.22.7

11.212.2

1.015.79.5

18.025.120.016.58.88.203.76.70

ECH(%)

0.55.00001.2

10.02.04.56.33.001.02.50.73.7

10.54.5

13.31.02.50.55.22.05.57.52.58.79.4

25.210.715.912.52.5

10.010.020.8

8.54.25.04.53.05.22.55.08.54.52.28.22.7

11.212.2

1.015.79.5

18.025.120.016.58.88.203.76.70

Notes: RDA = red algae, GRA = green algae, BTF = benthic foraminifers, COR = corals, RDT = rudists, BIV = other bivalves, GTP = gastropods, OST = ostracodes, ECH = echinoids,G = grainstones, P = packstones, W = wackestones, M = mudstones, B = boundstones, R = rudstones, and F = floatstones.

Core recovery and FMS imagery suggest that these facies are inter-bedded and constitute eight major cycles, with a general "saw-tooth"pattern of resistivity caused by differential cementation. These cyclesare characterized by a progressive upward increase in resistivity (i.e.,gradational change from porous grainstone and rudstone to morecemented boundstone), probably reflecting minor fluctuations in en-vironmental conditions (i.e., hydrodynamic conditions) and/or thedistribution of building organisms. Intervals with sustained moderatenatural gamma-ray intensity correspond to algal-rich facies (78.8-85.0 mbsf). On the basis of well-log data, the upper limit of SubunitIIC at Site 874 should be placed at 41.3 mbsf instead of at 38.1 mbsf,as reported by the Shipboard Scientific Party (1993a).

Bioclastic facies corresponds to poorly sorted skeletal grainstone/rudstone (PI. IB) or packstone/floatstone, depending on the occur-rence of secondary peloidal micrite, which frequently exhibits a geo-petal pattern. Grain size generally ranges from 0.1 mm up to a fewmillimeters, and grains are well rounded. Major skeletal componentsinclude rudists (radiolitids: Distefanella mooretownensis [Trechmann],Distefanella sp.; caprinids: Mitrocaprina sp., Coralliochama sp., Pla-gioptychus sp.), red algae (Corallinaceans, Solenoporaceans, Peysson-neliaceans), and benthic foraminifers (Sulcoperculina, Asterorbis;miliolids: Triloculina and Quinqueloculina; discorbids; lituolids: Am-mobaculites; rotaliids; and small textulariids) (Fig. 5 and Tables 3-4).Rudists are generally fragmented, except the small radiolitids, which

278

Table 4. Grain-counting data of skeletal components of shallow-water carbonates at Site 877.

Core, section, Depth RDA GRA BTF COR RDT BIV GTP OST ECHSubunit interval (cm) (mbsf) Texture (%) (%) (%) (%) (%) (%) (%) (%) (%)

[la 1R-1, 13-14 0.13 B 5.8 1.7 3.5 81.5 4.6 0.6 0 0 2.3Ha 1R-1,31-34 0.32 P 38.3 0 16.6 4.4 20.0 0 2.2 0 18.5Ha 1R-1,96-100 0.98 B/G/P 2.7 0 2.4 92.0 1.6 0 0 0 1.3Ila 1R-1, 124-128 1.26 P/W 20.8 0 19.3 2.1 50.8 1.8 1.6 1.0 2.6Ha 1R-1, 144-148 1.46 B/F 7.6 0 8.2 39.5 37.0 0 6.7 0.8 0.2Ila 1R-2, 18-21 1.68 W 10.4 0 13.0 8.5 63.4 0.5 0.7 0.7 2.8Ila 1R-2,38—41 1.88 P 24.6 0 23.2 0 42.2 0 1.3 0 8.7Ha 1R-2,76-79 2.26 P 26.7 0 17.2 7.1 41.0 0 2.4 0 5.6Ila 1R-2,88-90 2.37 G 28.0 0 8.1 29.0 27.1 2.4 2.7 0 2.7lib 2R-l ,0-5A 9.53 W 0 1.4 21.7 0.3 34.0 3.6 17.5 18.0 3.5lib 2R-1.0-5B 9.53 W/P 0 0 7.8 0 8.3 0 0 69.5 14.4lib 2R-1,29-33 9.80 W/P 1.5 0 1.8 0 57.9 14.5 6.7 9.3 8.3lib 2R-1,49-52 10.00 W 2.0 1.7 11.5 5.1 50.0 1.2 7.0 16.4 6.1lib 3R-1.4-8 19.06 W 1.2 0 0.5 1.2 77.0 0 0.5 2.6 17.0lib 3R-1, 11-15A 19.13 M 18.0 2.8 8.5 4.7 55.2 0.5 4.9 0.2 5.2lib 3R-1, 11-15 B 19.13 P 0 0 0 0 32.5 6.5 0 53.0 8.0lib 3R-1, 11-15 B' 19.13 P 24.5 3.7 6.7 3.4 57.0 0.6 0 0 4.1lib 3R-1,27-32 19.30 W/P 0.7 4.6 54.0 0 0 1.7 5.0 33.0 1.0lib 3R-1,52-57 19.55 W/M 0 0 73.0 0 0 0 1.2 25.4 0.4lib 3R-1,58-64A 19.61 W/M 0 0 57.0 0 0 0 0.7 42.0 0.3lib 3R-1,58-64 B 19.61 W 0.3 0 42.6 0 0 0 4.3 52.8 0lib 3R-1.88-92A 19.90 W O O 50.2 0 0 0 3.7 46.1 0lib 3R-1,88-92 B 19.90 W 1.0 0 15.3 0 71.5 0 0 5.1 7.1lib 3R-1, 111-115 20.13 P 18.4 0 17.6 0 53.8 0 0.5 0.7 8.7He 4R-1, 11-14 28.52 B 0 0 0 0 92.0 0 0 0 8.0lie 4R-1,55-60 28.98 P 16.8 0 14.7 0 58.9 4.7 0.7 0 4.2He 4R-1,79-83 29.21 G/R 11.8 0 9.6 0 76.0 0 1.3 0 1.3He 4R-1, 103-107 29.45 R/F 10.5 0 10.2 31.4 42.0 0 0 0 2.8He 4R-1, 109-112 29.50 R/F 9.1 0 10.2 28.0 50.5 0.2 1.5 0 0.5He 4R-1, 112-114 29.53 R/F/B 7.0 0 8.9 0.2 82.5 0.6 0 0 0.8Ik 4R-2, 30-37 30.25 G/R 13.6 0 17.4 27.0 29.0 0 9.3 0 3.7He 4R-2,55-59 30.48 G/R 17.1 0 33.9 9.3 23.7 0.7 0 0 1.3He 4R-2,86-88 A 30.78 B 12.8 0 9.5 4.5 71.7 0.2 0 0 1.3lie 4R-2,86-88 B 30.78 B 15.6 0 19.1 15.4 45.0 1.2 0 0 3.7lie 4R-2,89-93 A 30.82 G/R 10.8 0.6 25.8 7.1 50.5 0.2 2.0 0 3.0He 4R-2,89-93 B 30.82 G/R 13.6 0 14.0 12.7 52.2 1.3 2.9 0 3.3lie 4R-2, 110-112 31.02 G 16.9 0 30.5 17.0 30.5 0.5 1.3 0 3.3He 4R-2, 128-132 31.21 B/G 8.7 0 9.0 61.2 19.5 0 0 0 1.6lie 4R-2, 143-146 31.35 G 7.1 0 12.6 17.2 54.4 4.2 3.7 0 0.8He 4R-3,4-8 31.47 G/R 14.8 0 16.0 8.8 58.0 0.2 0 0.2 2.0lie 4R-3,31-33 31.73 R 8.0 0 7.5 0.5 83.5 0 0 0 0.5lie 4R-3,43^5 31.85 G 28.8 0 8.3 3.1 57.0 1.1 0 0 1.7He 5R-l ,3-8 38.05 R 37.0 3 25.0 4.3 27.0 0 1.3 0 2.4He 5R-1.46-50A 38.48 R 8.8 0 9.1 0 79.7 0 0 0 2.4He 5R-1,46-50 B 38.48 R 11.9 0 15.0 0 70.5 0 1 0 1.6He 5R-1,80-87 38.83 R 17.2 0 13.5 16.3 50.6 0 0 0 2.3He 5R-1.91-93A 38.92 R 15.2 0 15.8 25.9 38.6 0.8 0 0 3.7lie 5R-1,91-93 B 38.92 R 1.3 0 0.2 49.0 49.5 0 0 0 0He 5R-1, 106-110 39.08 R 23.9 0 16.4 1.3 55.7 0 1.9 0 0.8He 5R-1, 135-140 39.37 F 14.0 0 21.0 5.6 61.0 0 0.8 0 3.6He 5R-2, 14-17 39.60 G/R 16.3 0 44.2 7.5 28.0 0 0 0 4.0He 5R-2,91-96 40.38 G/R 21.0 0 25.5 7 33.0 1.8 4.6 0 7.1He 5R-2, 112-117 40.59 G/R 37.4 0 13.0 8.1 36.0 0 2.3 0 2.8lie 5R-2, 122-127 40.69 G/R 8.0 0 11.0 0 76.5 1.1 0.5 0 2.9He 5R-3,4-7 41.00 R 9.2 0 21.5 0.5 65.0 0.5 1.6 0 2.2He 5R-3, 12-17 41.10 R 4.6 0 13.8 0 78.9 0.8 1.1 0 0.8lie 5R-3,31-36 41.29 R 4.1 0 7.0 0.5 83.3 0.2 0.7 0 4.2He 5R-3,37-39 41.33 R 5.3 0 9.0 0 84.0 0 0 0 1.7He 5R-3,44-50 41.41 R 6.5 0 14.8 0 77.4 0 0 0 1.3He 5R-3,46-50 41.43 R 4.4 0 12.1 0 81.4 0 0 0 2.1He 5R-3, 51-57 41.49 B 33.2 0 20.0 0 45.7 0 0 0 1.1He 5R-3,72-74 41.68 R 5.8 0 7.4 0.5 85.7 0 0.5 0 0lie 5R-3,81-84 41.77 R/F 4.6 0 10.4 0 83.3 0 0 0 1.7lie 5R-3,86-91 41.83 R 6.0 0 11.5 0.7 79.5 0 0 0 2.3He 5R-3, 126-129 42.23 G 17.1 0 19.4 6.6 53.9 0 0 0 3.0He 6R-1,30-34 47.92 G 17.2 0 35.6 8.0 36.2 0 0 0 3.0lie 6R-1,90-96 48.53 P 15.1 0 29.0 1.1 51.9 0 0 0 2.9lie 6R-1,97-100 48.58 P 10.1 0 30.2 4.2 50.6 0 2.1 0 2.8He 6R-1, 104-106 48.65 B/P 9.1 0 14.4 0 75.2 0 0 0 1.3He 7R-l ,4-9 57.36 P/F 15.0 0 32.8 11.1 25.8 0 3.5 0 2.2He 7R-1,24-28 57.56 P 8.3 0 56.6 0 24.4 0 0 0 10.7He 7R-1, 136-140 58.68 F 11.0 0 18.0 15.1 46.0 0.2 2.7 0 7.0He 7R-2,4-8 58.86 F 13.6 0 32.5 0.7 43.0 0 0 0 10.2He 7R-2,23-27 59.05 F 14.0 0 32.7 11.5 32.6 0 3.8 0 5.4lie 7R-2, 100-106 59.83 F 10.6 0 29.4 14.1 37.7 0 4.1 0 4.1He 7R-2, 120-126A 60.03 G/R 14.6 0 32.1 9.0 36.4 0.6 3.4 0 3.9He 7R-2, 120-126 B 60.03 P 14.0 0 18.0 1.6 54.6 0 0 0.6 11.2He 8R-1,4-8 67.06 R/F 28.7 0.2 21.0 5.6 37.0 0 1.3 0 6.2He 8R-1,45-48 67.46 B 40.0 0 10.4 5.2 41.2 0 0 0 3.2He 9R-l,3-9 76.66 G/P 15.3 0 29.3 0 32.3 3.8 0.5 0 18.8He 10R-l,0-3 86.32 G 13.5 0 30.0 0 1.3 0 0 0 56.2

Notes: RDA = red algae, GRA = green algae, BTF = benthic foraminifers, COR = corals, RDT = rudists, BIV = other bivalves, GTP = gastropods, OST = ostracodes, ECH = echinoids,G = grainstones, P = packstones, W = wackestones, M = mudstones, B = boundstones, R = rudstones, and F = floatstones.


279

G.F. CAMOIN ET AL.

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either tightly packed various skeletal allochems in the associated sed-iment or encrustations, up to a few millimeters thick, on corals and/orrudists in association with encrusting foraminifers (Nubecularia) andmicrobial fabrics {Bacinella, Girvanella). Borings in framebuildingorganisms are related to the activity of clionid sponges and worms.The abundance of algal bindstones composed of closely stacked redalgal crusts (peyssonneliaceans and, to a lesser extent, corallinaceans)at the base of the subunit (interval 144-877-5R-3,51-57 cm) suggeststhat these organisms may have played a role in the stabilization ofpreviously mobile substrate, thus promoting larval attachment offrame builders and initiating framework development. Sedimentsassociated to the boundstones consist of moderate to poorly sortedskeletal grainstone to packstone.

In the upper interval, the dominant facies include skeletal grain-stone-rudstone with few algal-octocoral-rudist boundstone, which areapparently interlayered with, or enclosing, initially unconsolidatedsediment. Boundstone intervals are seemingly more abundant at Site877 than at Site 874. Downhole logging data show that, at Site 874, thelowest 2 m of Subunit IIA consists of very porous poorly cementedgrainstone (4.3-6.5 mbsf), whereas the uppermost 4 m has a highweight-percent calcium-carbonate (0.1^.3 mbsf) and corresponds tocemented rudist-algal-stromatoporoid rudstone-boundstone facies. Onthe basis of well-log data at Site 874, the upper boundary of SubunitIIA should be placed at 6.5 mbsf instead of at 11.1 mbsf, where it wasinitially placed by the Shipboard Scientific Party (1993a).

Skeletal grainstone and rudstone are generally poorly sorted, me-dium, coarse to very coarse grained, with various amounts of gravel-sized fragments; a few packstone beds occur at Site 877. Major com-ponents include rudists (caprinids and radiolitids), corals (hexacoralsand octocorals), calcareous sponges (chaetetids), and red algae (coral-linaceans, peyssonneliaceans: Poly strata alba [Pfender], along withmillimeter- to centimeter-sized bushes of solenoporaceans [Pycnopo-ridium ) (Fig. 5 and Tables 3-4). Large skeletal fragments are usuallymicritized and display millimeter-sized borings. Benthic foraminifers(orbitoidids, rotaliids, and miliolids) are moderately abundant. Othergrains are scarce and include echinoids, gastropods, green algae (a fewfragments of Terquemella). Calcisphaerulids and small ammonitesoccur in trace abundance. Few intraclasts consist of micritic elementswith rudist and algal fragments.

The boundstone includes encrusting laminar colonies of octocor-als (Polytremacis) and hexacorals, rudists (radiolitids: Distefanellamooretownensis [Trechmann], Distefanella sp.; and caprinids: Mitro-caprina sp., Coralliochama orcutti White, Coralliochama sp., Pla-gioptychus aff. fragilis White, P. aff. minor White, Antillocaprinasp.), and red algal (solenoporaceans) clumps that are heavily en-crusted (0.3-1 mm average thickness) by red algae (corallinaceansand peyssonneliaceans: Polystrata alba [Pfender]) or, to a lesserextent, by foraminifers and microbial fabrics {Bacinella). The matrixis similar to the associated skeletal grainstone and rudstone.

Interpretation

Fossils and sedimentologic criteria suggest that the two "reef"units grew in a shallow-marine environment, probably less than 10-20 m deep. Associated bioclastic shoals acquire all sizes of debrisfrom the carbonate factory by means of normal waves and storms, andpass some of the accumulation into the lagoonal zone during storms.Further evidence of turbulent waters includes the abrasion of shellfragments and the prevalence of grainstone and rudstone textures.

The frameworks are successively dominated by coralgal or rudistcommunities in the first "reef" episode and coralgal communities inthe second "reef" episode. The abundance of grainstone intervalswithin the recovered cores and corresponding downhole logging sig-natures strongly suggests that the "rim" consisted of interbeddedorganic frameworks and shoals of wave-redepositedbioclastic grains.The rudist-coralgal framework probably was not composed of "wave-resistant" structures, as shown by their weak organic frame; rather, it

was probably restricted to a terrace-bench at the fair-weather wavebase (ca. 10 m depth). In addition, the rigidity and stability of theframework must have been provided by early cementation (see "Dia-genesis" section). Vertical zonation of rudist assemblages in the first"reef" episode may reflect either an evolutionary sequence in reefdevelopment or slight changes in environmental conditions. The de-velopment of a more open framework with loosely packed assem-blages of recumbent large caprinid rudists may result from the smoth-ering of the framework by increased sedimentation. Furthermore, thedominance of caprinid rudists over erect forms could be related totheir better adaptability to high-energy environments and unstablesubstrates, such as shifting sands, where exposure of hard surfaces forattachment is likely to be limited in time and space (Skelton, 1979;Camoin et al., 1988). However, as noted by Kauffman and Sohl(1974) in the Caribbean, such recumbent rudists barely attained thegregarious habit. Relative to the coeval "reef" episode at Site 874,which includes algal-coral boundstone, the development of rudistcommunities at Site 877 suggests a lateral zonation of the framework,the rudist settlements being in a more lagoonward position withrespect to the coral communities.

In contrast to the first "reef" unit, the second one does not displayany clear lateral zonation, either because of the short duration of"reef" development or the late erosion processes.

Lagoonal and Peritidal Deposits

Site 874: Subunit IIB (Section 144-874B-2R-2 through Core 144-874B-4R,11.1-38.1 mbsf); Site 877: Subunit IIB (Core 144-877A-2R throughCore 144-877A-3R, 9.5-28.4 mbsf) (Fig. 4)

Occurrence

On the basis of well-log data at Site 874, the top of this lagoonal-dominated lithologic Subunit IIB should be placed at the top surface ofa well-cemented, 1-m-thick layer at 6.5 mbsf. This boundary assign-ment contrasts with the shipboard placement at 11.1 mbsf, because theinterval from 11.1 to 6.5 mbsf appears to be a continuation of theunderlying cyclic facies, whereas 6.5 mbsf is a major discontinuity.The lowest 10 m of Subunit IIB seemingly consists of two majorsequences, similar in logging aspects to the underlying Subunit IIC.

Description

Typical facies consist of poorly sorted very pale brown to whiteskeletal grainstone, packstone to floatstone and light gray to pinkishgray burrowed dolomitic wackestones (PL 1C). The contact betweenthese two facies usually corresponds to an unconformity with bor-ings, local erosion, and reworking of intraclasts, implying early lithi-fication processes (interval 144-877A-1R-1,124-128 cm). Radiolitidclusters {Distefanella mooretownensis [Trechmann]) are reported ininterval 144-874B-3R-3, 0-18 cm. The signature on the FMS ischaracterized by beds of high resistivity (skeletal packstones to float-stone) alternating with beds of low resistivity (lagoonal foraminifer-wackestone to -packstone with leached gastropod molds).

Major components of skeletal packstone to floatstone includemicritized fragments of usually leached gastropods, rudist fragments(radiolitids and scarce caprinids), inoceramids, and other bivalves(probable pycnodonts) (Fig. 5 and Tables 3—4). Benthic foraminifersinclude miliolids, rotaliids, discorbids, and, to a lesser extent, orbi-toidids, lituolids, and encrusting forms on large bioclasts. Other grainsare clearly subordinate, including (in order of decreasing abundance)fragments of green algae (dasycladaceans: Terquemella), red algae(corallinaceans and peyssonneliaceans), corals (hexacorals and octo-corals), and echinoids. Calcisphaerulids and planktonic foraminifers{Rugoglobigerina sp.) occur in trace abundance. The micritic matrixis locally peloidal and may display a geopetal distribution in inter-granular pores. Average grain sizes are 0.5-2 mm and up to 1 cm infloatstone beds.

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Dolomitic wackestone beds are characterized by abundant fenes-trae, which may form irregular networks of vertically elongated tinycanalicules reminiscent of fluid escape structures or horizontal stackedalignments ("Laminoid Fenestral Fabric" sensu Tebbutt et al., 1965).The micritic matrix is brownish and may display locally a reticulateaspect. The skeletal content is poor and limited to small gastropods(cerithids), benthic foraminifers (miliolid, discorbids, textulariids,fragments of Dicyclina and orbitoidids), smooth-shelled ostracodes,green algae (Terquemella), and a few abraded fragments of corals, redalgae, and rudists. These wackestones may display undulating lami-nations, probably microbial in origin, which are commonly cut ir-regularly because of burrowing (common millimeter-sized tubes),loading, and/or fluid escape.

Interpretation

A shallow-marine depositional environment, probably a few me-ters deep, may be inferred from the fossil content and the sedi-mentologic criteria detailed in the foraminifer-gastropod wackestone.Among these criteria, the fine-grained matrix and the scarcity, or eventhe lack, of conventional current indicators imply quiet-water condi-tions. The occurrence of laminar microbial mats and the abundanceof fenestrae are consistent with very shallow depths. Temporary peri-ods of emergence may be deduced from the early dolomitization ofwackestone beds, the early dissolution processes of shells, and thelocal reworking of caliche lithoclasts. The local prevalence of pauci-specific assemblages of very small foraminifers, smooth-shelled os-tracodes, and gastropods (cerithids) may indicate temporary episodesof restriction and rather unfavorable ecologic conditions such as lowillumination or oxygenation, temperature fluctuations, poor nutrientavailability, or turbidity.

Skeletal packstone and floatstone with abraded bioclasts inter-layered with wackestone beds correspond to redeposited "wash-overs" or "storm beds" from the outer debris shoals caused by peri-odical storms, changes in current patterns, or short-term relative sea-level fluctuations.

Diagenesis

Sediments on guyots generally display a complex diagenetic his-tory involving carbonate cementation, transformation of carbonatephases, dissolution processes, cavity fillings, phosphatization, and sec-ondary mobilization of Fe-Mn oxides. The data presented herein rep-resent an overview of the diagenetic features reported in sedimentsfrom the inner perimeter ridge of Wodejebato Guyot to record the suc-cessive changes of physicochemical conditions controlling diagenesis.

Porosity and Cementation

The sediments from the guyots still exhibit a high percentage ofporosity in contrast to carbonates influenced by burial diagenesis. OnWodejebato Guyot, sediments are strongly cemented in the upper 40m of the carbonate sequence (Subunits IIA and IIB, and the top ofSubunit IIC), with a clear diagenetic boundary occurring in Cores144-874B-5R and 144-877A-5R, coeval with a sharp increase inporosity around 65 mbsf downward caused by poor cementation inthe underlying units. In skeletal shoal facies, intense leaching of theoriginal grains resulted in high moldic, vuggy, and solution-enlargedinterparticle porosity, ranging from 30% to 40%. These sediments aregenerally poorly lithified, although isopachous crusts of equant cal-cite cements may line intra- and interskeletal voids. In contrast, poros-ity generally ranges from 1% to 10% in the upper part of the carbonatesequence because of extensive cementation by multiple generationsof banded calcitic cements, up to a few millimeters thick. The remain-ing pores are intragranular, moldic (especially in lagoonal beds), andsheltered, with a few solution-enlarged, intra- and interparticle pores.Additional fenestral porosity is present in peritidal carbonate beds.

In the upper part of the carbonate sequence, the general diageneticsequence within inter- and intragranular pores includes the following:

1. Isopachous fringes of amber to brownish bladed, radiaxial tofascicular-optic calcites that exhibit a typical sweeping extinction andinclude equant, bladed, and fibrous morphologies. These calcitesappear to be zoned by variations of inclusion density, with the outer-most zone being generally clear (i.e., inclusion poor; Plates 2-3). Thecrystals generally thicken away from cavity walls; boundaries be-tween adjacent crystals are sharp- These cements may display a botry-oidal habit in large intergranular cavities. These cements partly fillsyndepositional primary porosity and are completely absent in sec-ondary pores, especially moldic pores.

2. Prismatic to scalenohedral, inclusion-free calcites overgrow-ing radiaxial to fascicular-optic calcites (Plates 2B and 3A-B). Thecontact between these clear calcites and inclusion-rich, bladed, radi-axial to fascicular-optic calcites mimics crystal terminations.

3. Translucent scalenohedral to equant blocky calcite cements.These cements may crosscut the isopachous fringes of fascicular-optic calcitic cements (Plates 2B and 3C-D). Equant blocky calcitecements also fill the chambers of foraminifers and cavities in theoverlying Eocene pelagic cap.

Although calcite cements generally attest the phreatic nature of thediagenetic environment, petrographic features alone are not conclu-sive as diagenetic indicators, and it is of prime importance to integratealso geochemical approaches. However, the present isotopic compo-sitions of carbonate cements reflect a mixture of the original compo-sition of carbonate cements and late diagenetic products. The stablecarbon and oxygen isotopic compositions of the successive cementphases are plotted in Figure 7.

Radiaxial and fascicular-optic calcites are generally interpreted asresulting from the lateral coalescence of previous radial fibrous ce-ments (Kendall, 1977), either aragonitic (Bathurst, 1977) or highMg-calcite (Lohmann and Meyers, 1977), or from direct precipitation(Sandberg, 1985; Kendall, 1985). The stable isotope composition(carbon and oxygen) of Wodejebato radiaxial and fascicular opticcalcites ranges, respectively, from +1.81‰ to +3.03%o PDB (aver-age: +2.34‰) for δ13C and from -1.46‰ to -0.34%o PDB (average-0.83‰) for δ 1 8θ. These values are slightly lighter than other modernmarine cements, including high Mg-calcite cements from Pikinni(Gonzalez and Lohmann, 1985). Stable carbon isotope compositionsare within the theoretical field of δ13C values for calcite precipi-tated from modern Pacific seawater (i.e., +2.0‰ to +2.5%o PDB;Kroopnick et al., 1977). Thus, petrographic observations and stableisotope data support a precipitation penecontemporaneous with depo-sition in a shallow-marine environment. Stable carbon and oxygenisotope values presented herein are similar to those reported fromradiaxial and fascicular-optic calcite cements in the Bahama Escarp-ment (Freeman-Lynde et al., 1986) and comparable to those occurringin the Miocene of Anewetak Atoll (Sailer, 1986). In contrast, they areslightly higher than those measured on Early Cretaceous radiaxialcalcites from Mexico (Moldovanyi and Lohman, 1984).

The δ'3C and δ 1 8 θ values measured in clear, inclusion-free,scalenohedral calcites range, respectively, from +1.85%o to +2.63%oPDB (average: +2.22%o PDB) and from -1.56‰ to +0.07‰ PDB(average: -1.03‰ PDB). Average δ'3C and δ 1 8 θ values measured onblocky calcites are, respectively, +1.46%e and -1.62‰ PDB. Despitepartial overlap between these data and those reported on bladed, radi-axial, and fascicular optic calcites, the shift between values measuredon these two types of cements is slight, ranging from O.l‰ to 0.8‰for δ13C and from 0.2%o to 0.8%c for δ 1 8θ. Carbon isotope values forthese cements are slightly lighter than most modern shallow-marinecements (see Gonzalez and Lohmann, 1985), suggesting precipitationin relatively deep-marine waters where the δ13C of marine bicarbonateis l‰ to 2‰ lighter than in surface-marine waters (Kroopnick et al.,

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318OFigure 7. Stable carbon and oxygen isotopic composition of successive cement phases and skeletal components in the two "reef" units at Sites 874 and 877 (leftcolumn = lower unit, and right column = upper unit). Cements: black circles = scalenohedral calcite, black squares = clear equant spars, and black triangles =radiaxial to fascicular-optic calcite cements. Skeletal components: open circles = aragonitic rudist shells, open squares = calcite replacing originally aragoniticlayers with relict microstructures in rudist shells, and open triangles = calcite replacing originally aragonitic layers without relict microstructures in rudist shells.

1977). Blocky and scalenohedral calcite cements produced in a phre-atic meteoric environment should have lighter δ'3C and 618O compo-sitions. Cements precipitated from cold deep waters are calcite with-out appreciable magnesium (Schlager and James, 1978; Hine andSteinmetz, 1984). The stable carbon and oxygen compositions aresimilar to those reported from cements of the same type in Maas-trichtian deposits from the Bahama Escarpment (Freeman-Lynde etal., 1986) and other Caribbean areas (DSDP Leg 15; Anderson andSchneidermann, 1973). In contrast, values reported from Wodejebatocalcite cements are lighter than those reported in coarse crystallinecalcite cements (δ 1 8θ = +3%e; δ13C = +2.5‰) from Limalok (formerlyHarrie) Guyot by Lincoln (1990), suggesting precipitation in warmerwaters than on Limalok Guyot.

Skeletal Diagenesis

Former aragonitic skeletons or parts of skeletons (corals, rudistsand gastropods) may exhibit a differential preservation throughout thecarbonate sequence. Coral and gastropod skeletons are either totallyreplaced by blocky calcite or dissolved and preserved only as micriticenvelopes. Such a fragile pore network could only have been devel-oped by dissolution in situ, either by meteoric waters or by deep-marine waters undersaturated with respect to aragonite. The generallack of internal sediment and meteoric cements, filling moldic pores,suggests that the aragonite dissolution probably occurred away fromthe rock-water interface. Modern Pacific Ocean water is undersatu-rated with respect to aragonite at depths below 300-350 m (Scholle et

al., 1983; Sailer, 1986) resulting in dissolution of aragonite and highMg-calcite. Similar sequences of aragonite dissolution and subsequentcalcite precipitation by marine waters undersaturated with respect toaragonite have been reported in Jamaica (Land and Moore, 1980), inthe Bahamas Escarpment (Freeman-Lynde et al., 1986), and on otherPacific seamounts (Haggerty, 1982). These processes have been de-scribed as "subsolution" in the Calcareous Alps (Schlager, 1974) andare obviously related to the duration of nonsedimentation.

In some intervals, rudist and inoceramid shells or parts of shellsdisplay a gradational sequence of transformation, from fragmentstotally replaced by clear, inclusion-free, blocky calcites (Plates 4A,4B, and 4D) to fully preserved aragonitic parts of shells (e.g., interval144-877A-5R-3, 103-109 cm) through the partial recrystallizationwhere relics of aragonitic skeletal structures are preserved in inclu-sion-rich, coarse, blocky calcite cements. In the latter case, the limitsof calcite crystals may crosscut both the initial aragonitic structuresand the bladed to fibrous cements that fill the intragranular pores(Plate 4C). The stable carbon and oxygen isotope compositions of therudist shells are plotted in Figure 7.

The stable carbon and oxygen compositions of well-preservedaragonitic rudists (60‰-100‰ aragonite) range, respectively, from4.53%o to 6.35%o PDB (average 5.08‰ PDB) for δ13C and from-1.32‰ to -0.94‰ PDB (average -1.115‰ PDB) for δ 1 8θ. Thesevalues are similar to the stable isotopic composition of aragoniticrudists such as Antillocaprina and Plagioptychus (Al-Aasm andVeizer, 1986). As the diagenetic changes are very slight, these valuesprobably reflect the isotopic values of initial seawater.


Isotopic values obtained in inclusion-rich calcites exhibiting relicsof aragonitic skeletal structures range, respectively, from +2.05%e to+2.78%o PDB (average +2.46%o PDB) for δ13C and from -l.Ol‰ to-0.07‰ PDB (average -0.484%o PDB) for δ 1 8 θ. These values arevery close to those obtained on similar neomorphic calcites by Al-Aasm and Veizer (1986). With respect to values measured on arago-nitic shells or fragments of shells, only a slight shift occurs in δ 1 8θ,whereas the decrease in δ'3C is sharper. This suggests that the δ l 8 0and the temperature of diagenetic water did not vary much from thatof the coeval seawater during an early stage of diagenesis, implyingthe wet replacement of aragonitic components.

Isotopic values obtained in inclusion-free calcites exhibiting fewor no relics of aragonitic skeletal structures range, respectively, from+1.15‰ to +1.3‰ PDB (average +1.21‰ PDB) for δ13C and from-7.64‰ to -5.95%o PDB (average -7.06%o PDB) for δ 1 8θ. The iso-topic shift, especially in δ13C confirms solution-reprecipitation pro-cesses, implying a greater alteration at a later diagenetic stage. Thearagonite-calcite replacements may have occurred in waters that weredepleted in 18O compared to depositional waters, possibly in meteoricwaters, as noted by Sandberg and Hudson (1983) and Al Aasm andVeizer (1986), for the calcitization of aragonitic bivalve shells. Thesetransformations were only noted at the top of the carbonate sequenceat Site 874.

Dolomitization

A few dolomitic beds were reported in lagoonal facies from Sub-unit IIB (interval 144-874B-2R-2,47-55 cm); these correspond to thefaint dolomitization of gastropod-foraminifer wackestone. The lackof any dolomitization in the overlying and underlying intervals sug-gests that early dolomitization processes should be related to tempo-rary and local emergence.

Dissolution Processes and Cavity Fillings

Dissolution may affect formerly aragonitic shells (e.g., gastro-pods, caprinid rudists, corals) and encasing sediments, thus creatingboth moldic pores and solution cavities, which are either still open orfilled by cements and/or internal sediments. Relative timing of theinvolved dissolution processes can be determined precisely with re-spect to the ages of sediment fillings.

Yellowish, reddish, or rusty brown internal sediment displaying ageopetal pattern may fill partly moldic pores; it consists of micritewith scarce, smooth-shelled ostracodes and very small benthic fora-minifers. This implies dissolution of rudist and gastropod shells,followed by sediment infilling of the moldic pores. The same type ofinternal micritic sediments may partly fill irregular solution cavities,up to 1 cm in size, which occur especially in lagoonal deposits andunderlying "reef" facies. In this case, the internal micritic sedimentspostdate successive generations of banded isopachous crusts of radi-axial and fascicular-optic calcite cements. This implies a dissolutionduring an early stage of diagenesis. The remaining moldic porosity isusually occluded by scalenohedral and blocky calcite cements. Earlymeteoric diagenetic processes are indicated also by the postdating ofequant, blocky, calcite cements by fibrous and bladed calcites inlagoonal beds.

Late fillings of centimeter-sized solution cavities in the platformcarbonates consist of white to orange-brown micritic internal sedi-ments corresponding to mudstone and wackestone rich in pelagicorganisms associated with micritized and phosphatized fragments ofbenthic foraminifers and rudists (radiolitids). The biostratigraphic datashow that the filling seems to be an episodic process. Planktonicforaminifers and calcareous nannofossils constitute distinctive assem-blages, ranging in age from the middle Maastrichtian (interval 144-874B-3R-1, 114-118 cm, and between intervals 144-877A-1R-1, 3-24cm, and -3R-1,92-97 cm), to the middle Eocene (interval 144-877A-1R-2, 0-37 cm), through the late Paleocene and the early Eocene

(intervals 144-874B-1R-1, 47-50 cm, and -3R-1, 114-118 cm, andintervals 144-877A-1R-1, 3-24 cm, to -3R-1, 92-97 cm) (ShipboardScientific Party, 1993a, 1993b). This implies that the drowning ofthe carbonate platform occurred during the Maastrichtian and thatpelagic sedimentation lasted until the middle Eocene, even if no earlyPaleocene sediments were recovered. Phosphate and manganese grainsand phosphate staining occur within these pelagic sediments. Thedeepest occurrence of pelagic sediments within platform carbonates isrecorded in interval 144-874B-6R-1, 0-4 cm, about 50 m beneath thetop of the carbonate sequence. The mechanism involved in the forma-tion of these cavities cannot be deciphered because of the uncertaintiesconcerning their morphologies, widths, and relationships with encas-ing shallow-water carbonates. These cavities could correspond eitherto narrow fractures affecting the carbonate cap or to larger cavitiespossibly related to karstification processes.

POST-DROWNING FACIES

The platform limestone sequence is topped by a complex series ofphosphate-manganese dense black crusts (Plate ID), up to 3 cm thick,sealing well-cemented, foraminifer-rudist grainstone and manganese-phosphate-coated limestone conglomerate. The upper few centimetersof these limestones are recrystallized and red stained by phosphate,which may coat skeletal grains. Manganese dendrites and threads growinward, suggesting a secondary mobilization caused by the good solu-bility of the Fe-Mn oxides. The contact surface between the platformcarbonates and the manganese-phosphate crust is scalloped and dis-plays multiple generations of borings, ranging in size from 20 to 2 mm.The crust includes multiple generations of stacked digitate and lami-nated accretions embedding clasts both of foraminifer rudist grain-stone, 2-3 mm in size, and late Paleocene pelagic limestone. The crustsgrew probably during periods of low or no sedimentation and havebeen mechanically reworked several times. The production of clastsincluded in the crusts is seemingly related to the intense boring activityand to erosion and redeposition, probably because of bottom-wateractivity, during Eocene time. Pores between the manganese accretionsare filled with several generations of pelagic sediments rich in plank-tonic foraminifers and nannofossils, late Paleocene to middle Eocenein age, implying a deep-marine depositional environment (i.e., tens tohundreds of meters) and very slow accretion rates. These sediments aresimilar to those infilling the irregularly shaped solution cavities withinshallow-water carbonates.

As noted by Grötsch and Flügel (1992), the geologic setting andtime relations leading to phosphatization processes recorded at the topof the guyots are clearer than for phosphate deposits formed in regionssubjected to coastal upwelling. Because the amount of phosphate in theCentral Pacific photic zone is almost nil, the upwelling of deep wateror exposure to it is the only available source for phosphate (Grötschand Flügel, 1992). A combination of winnowing and physical andbiological erosion, together with the continuous process of mineraliza-tion and dissolution, results in concentrations of phosphate on thehardgrounds (Baturin, 1971; Jarvis, 1980; Föllmi, 1989; Grötsch andFlügel, 1992). The microbial role, especially sulfato-reducing bacteria(Williams, 1984), in phosphogenesis has been reported in similarseamount phosphorites of the Central Pacific by Rao and Burnett(1990) and on Cretaceous pelagic highs from the Helvetic Domain(Delamette, 1988). Phosphatization may have started after drowningwhen the guyot entered the oxygen-minimum zone (see Burnett et al.,1980, for Albian counterparts), thus favoring microbial "blooms."Furthermore, paleoceanographic events that promote productivity andintensification and expansion of the oxygen-minimum zone shouldproduce crusts with increased amounts of Mn and manganophile ele-ments (Hein et al., 1992). Another interpretation could rely on theintensification of upwelling, because Wodejebato Guyot was probablylocated in the area of the central equatorial upwelling at the end of thecarbonate platform development.

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ATOLL GENESIS AND DEPOSITIONAL HISTORY

Flooding of the Volcanic Substrate

The depositional history of the inner perimeter ridge of Wodeje-bato Guyot began with the end of volcanism before or during Cam-panian time (78-83 Ma; Pringle et al., this volume). As other mid-oceanic platforms, this guyot initially consisted of a broad shieldvolcano. The dusky red to gray clay and claystone, which form thebulk of Unit III at Sites 874 and 877, are a subaerial weathering profileof the basalt as shown by the preservation of vesicular texture, includ-ing laths of plagioclase and vesicles filled by white patches of zeolitesin these flows. The contact between the basalt and the clay is grada-tional and the degree of alteration decreases downward. The durationof subaerial exposure required for the formation of such a 15-m-thickweathering profile is consistent with the radiometric ages obtained onlavas and with the biostratigraphic data obtained from the overlyingmarine deposits. With subsidence and coeval erosion, the summit ofthe shield volcano was truncated, creating topographic depressionsfilled with clay-rich sediments produced by pedogenetic processes.At this time, a clearly defined lowstand systems tract (LST) of slopefans and a prograding slope wedge were lacking on these platformsbecause the flanks of the volcano were sufficiently steep to bypasssediment directly to the adjacent deep ocean basin.

The initial overlying marine deposits, dated late Campanian(Premoli Silva, Haggerty, Rack, et al., 1993), correspond to blackorganic-rich clay containing marine and continental material (e.g.,calcareous nannofossils and preserved kerogen-type woody material)deposited in a quiet, reducing, shallow-marine environment (seeBuchardt et al., this volume). The basal limestone recovered fromabove the clay consists of a grainstone that includes fauna and flora(larger foraminifers, coral fragments, and red algae) typical of a shal-low-marine environment with an open circulation. The partial preser-vation of the weathering profile and overlying marine clays, and thelack of any clay reworking at the base of the carbonate sequence ofUnit II, are consistent with a slow rise in sea level and rapid burial.

Development of the Carbonate Platform

The development of the carbonate platform was clearly governedboth by the growth potential of the carbonate systems and by relativesea-level fluctuations, corresponding to a balance between thermalsubsidence rates averaging 25 m/m.y. (see Detrick and Crough, 1978for thermally rejuvenated lithosphere) and eustatic sea-level changes.

A two-stage transgression initiated the carbonate platform on agently inclined volcanic island shallowing toward a central substratehigh. This transgression is clearly correlated with a two-step develop-ment of the carbonate platform. The Holocene transgressive model ofa "barrier-island complex" moving landward over a sheltered "estu-ary" has some similarities to the landward movement of an algal-coral-grainstone shoal over an organic-rich nearshore marsh under-lying the base of the carbonate sequence occurring at Sites 874 and877. The overall vertical evolution of platform carbonates corre-sponds to an upward-shallowing sequence, as shown by the relaysdetected by means of correspondence analyses conducted on skeletalcomponents (Figs. 5-6; see also Hennebert and Lees, 1991).

The environmental gradient is obvious in Sites 874 and 877 wherean arch effect, the so-called "Guttman effect," appears (lower part ofFig. 5). Evolution of biological components fits with a relay effect,especially at Sites 874 and 877. These relays might be characterizedby a progressive and continuous shift of the organisms in time andspace, related to hydrodynamism and confinement.

Some of the major successions at Site 874 consist of a series of 6-m-thick, upward-shallowing sequences. The 6-m-thick sequences at Site874 are similar in general aspect and thickness to the cycles observedon other guyots drilled during Leg 143 (P. Cooper, pers. comm., 1993).These parasequences were interpreted as 100,000-yr, Milankovitch-driven, sea-level fluctuations (P. Cooper, pers. comm., 1993).

Sequence types are closely related to sea-level fluctuations thatinduced variations of accommodation with subsequent shift fromrhythmic sequences when the ridge was near sea level (i.e., during thelate part of highstand systems tracts [HST] or at the base of transgres-sive systems tracts [TST]), to cyclic sequences when sea level washigh and accommodation was at a maximum.

The first phase of carbonate platform development corresponds tothe accumulation of skeletal sand piles edging a still-emerged vol-canic relief, near the shelf margin (Fig. 8). They accumulated in ashallow-marine depositional environment with a sedimentary regimedominated by currents. The initial buildups correspond to the earlystages of a transgressive systems tract (TST); their formation prob-ably occurred during a time when sea level was low or when the car-bonate factory kept pace with a slow rise in sea level.

The overlying sequence corresponds to a series of stacked retro-gradational sand shoal units with reflectors truncating against thevolcanic substrate reflector (Fig. 8). This backstepping onto the gentleslope of the volcanic rocks implies that the carbonate factory failed tokeep up with a rapid rise in sea level and spread landward. Thisimplies that these sand shoal units still belong to a TST. A maximumflooding surface is tentatively assigned at about 152.5 mbsf, wherethe maximum peak of natural gamma-ray intensity is interpreted asthe development of maximum algal encrustation (Fig. 3). An upwardshallowing in depositional depth is suggested, but no well-definedupward-shallowing cycles could be identified.

The stabilization of sea level induced the progradation of sand shoalunits toward the shelf margin, characterizing the beginning of an HST(Fig. 8C-D). However, short-term backstepping or aggradation andprogradation may have been induced by minor sea-level changes (seeArnaud Vanneau et al., this volume). At this time, the central volcanichigh was still emerged and exposed to weathering processes.

The bulk of sand shoal units seemingly corresponds to an LSTdeposited near the edge of the summit plateau after a seaward migra-tion of the carbonate factory; at this time, carbonate production wasminimal. The partial sediment starvation of this debris apron mayhave caused enrichment of iron oxides and the reddish coloration ofthe grainstone. The occurrence of weathering processes was not sup-ported by microfacies (no vadose-type cement), or by isotopes (novadose signature), or by chemistry (no thorium or silica/aluminumincrease suggesting soil clays).

A subsequent rise in sea level is responsible for backsteppingtoward the center of the plateau of the overlying carbonate sequence,including first sand shoal units and then early cemented rudist-coralgalframeworks facing the open sea, with a grainstone shoal accumulationon its lagoonward edge. This sequence corresponds to the second stepof carbonate platform development (Fig. 8) and was deposited in ashallow-marine environment. Coevally, the central volcanic high wasflooded and covered by sand shoal facies similar to those reported atthe base of the carbonate sequence from the inner perimeter ridge(Arnaud Vanneau et al., this volume). This flooding induced changesin the dominant stratal pattern of the TST and the HST: from retrogra-dational and progradational to aggradational. As sea level stabilized,the rudist coralgal units began to aggrade and then prograde. At thisstage of development, the relief of the inner perimeter ridge wasaccentuated, probably on the order of 36 m above the coeval lagoonalsediments, and the carbonate platform was therefore forming an atoll-like structure (Arnaud Vanneau et al., 1993). The more pronouncedrelief of the perimeter ridges on the northeastern flank of the guyotsuggests that prevailing wind and wave activity occurred in this zone.

The continued seaward progradation of reef units induced the shiftof depositional setting and the lagoonal deposits invaded the innerperimeter ridge, forming the upper part of the HST. The seawardmigration of lagoonal deposits resulted in the complete filling-up of theback-reef area and the leveling of the carbonate platform topography.The prolonged sea-level stand, or the lowering of sea level, may haveinduced local and temporary subaerial exposures by means of thedissolution of lagoonal beds and, possibly, dolomitization processes.

286


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Figure 8. Correlations between reflectors occurring in the carbonate sequence and successive systems tracts, from the initiation of the carbonate platform (A) tothe final drowning (F).

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G.F. CAMOIN ET AL.

However, no meteoric cementation has been observed in the recov-ered intervals. The weak downward penetration of dolomitizing flu-ids suggests a limited fall in seawater and water table levels, implyingjuvenile karstification processes. Such a short emergence near the topof a carbonate sequence is a common feature reported on Type Aguyots (see Winterer and Metzler, 1984; McNutt et al., 1990) andother platforms (Erlich et al., 1990). The duration of emergence isusually estimated to be on the order of 100 k.y., which is consistentwith the data reported from Wodejebato Guyot.

The next transgression caused backstepping of the carbonate fac-tory toward the lagoon, forming a TST. This rapid transgression andlagoonward shift of depositional environments initially caused theaccumulation of wave-reworked grainstone from the coral-algal belt,followed by temporary establishment of rudist-coralgal frameworksonto the backstepped grainstone. The lack of any lateral zonation inthese organic frameworks should be related to the short duration ofthis "reef" unit. The sequence, involving a barrier-reef growth aftershort-term emergence, is similar to those reported on Type A guyots(see Winterer and Metzler, 1984; McNutt et al., 1990) and other plat-forms (Erlich et al., 1990).

Demise and Drowning of the Carbonate Platform

The solution cavities filled by pelagic sediments that occur as deepas 50 m in the platform carbonates may have been formed by theenlargement of fractures in a deep-marine environment or, more prob-ably, by karstification as indicated by the timing and the nature of thecavity system. The thorough penetration of platform carbonates bythese vertical cavities indicates exposure of the top of the formation.The creation of such deep cavities during the juvenile karstificationrecorded at the top of lagoonal beds is unlikely because of its shortness.

The morphology of the drowned atoll (i.e., hummocky upper sur-face of platform carbonates) recorded on seismic lines suggests thatkarstification processes were in motion before the final drowning. Fur-thermore, there seems to be no mechanism for marine erosion to haveproduced the trough separating the inner perimeter ridge from the outerone (Enos et al., this volume). If subaerial exposure has produced thetrough, a minimum lowering of 90 m is required, as shown by the reliefbetween the top of the inner ridge and the floor of the trough; thistrough would therefore correspond to a large doline parallel to adjacentslope (Enos et al., this volume). However, the only isotopic evidenceof the occurrence of meteoric waters relies on the stable carbon andoxygen values of some neomorphosed rudists at the top of the innerridge (see Fig. 7). All the cements analyzed in the upper "reef" unit dis-play a marine isotopic signature (see Fig. 7). The formation of 50-m-deep cavity systems in the platform carbonates and the topography ofthe upper surface of platform carbonates through karstification pro-cesses would imply an emergence of about 1 m.y. (see Montaggioniand Camoin, in press, for Miocene atoll counterparts). During such anemergence, karstification processes should have removed partly the topof the carbonate sequence at Wodejebato Guyot, based on an averagedissolution rate of 35 m/m.y. (Lincoln and Schlanger, 1987). This couldexplain the lack of pre-drowning lagoonal beds, which could be relatedto a regression in the late stage of carbonate platform development.

The occurrence of Maastrichtian pelagic sediments as cavity fill-ings implies a rapid sea-level rise of a few tens of meters duringMaastrichtian time, just after exposure. The association of planktonicforaminifers with Gansserina wiedenmayeri (see Premoli Silva etal., this volume) confines the age of the formation and the infilling ofcavities to a time span ranging from the G. gansseri Zone to the lowerpart of the A. mayaorensis Zone (Fig. 9). The data suggest a short-term regressive-transgressive cycle with a high amplitude in the lateMaastrichtian (G. gansseri Zone) that is consistent with sea-levelchanges with an amplitude of 100 m at the top of the SupercycleUZA-4 (Haq et al., 1987) (Fig. 9). This sequence of events, whichimplies a strong fall in sea level, leading to karstification before aneven more rapid rise of greater amplitude, and ultimately causing

drowning, is similar to the process found in the Early Cretaceous atollreefs described by Grötsch and Flügel (1992).

The complete shutdown of platform carbonate production duringre-flooding cannot be explained only by the rise of sea level becausesubsidence rates and long-term sea-level changes are at least 1 orderof magnitude lower than the potential growth rates of carbonate plat-forms (Schlager, 1981). We assume that benthic growth was reduced,or even hampered, by the deterioration of environmental conditions,which appear as a cause of abrupt platform drowning (Schlager,1981). Furthermore, because of their size, small platforms are moresusceptible to drowning events that can place them out of the photiczone and effectively shut down the shallow-water "carbonate fac-tory" (see Dominguez et al., 1988, for Holocene counterparts).

Strong but short perturbations of climatic equilibrium may explainrapid sea-level changes and environmental deterioration that couldhave removed the possibility of reef colonization and caused thecomplete drowning of the previous carbonate platform. The involvedregression-transgression cycle with an amplitude on the order of 100m (Fig. 9) during greenhouse time suggests a short-term coolingevent during the late Maastrichtian (see Camoin et al., 1993). Thistime corresponds to a transitional period between two modes of oce-anic circulation (e.g., equatorial to polar-dominated thermohaline cir-culation), inducing a sharp decrease in organic productivity (Camoinet al., 1993). In particular, a sharp decrease in sea-surface tempera-tures to about 21°C in the Pacific Ocean has been considered byBarrera et al. (1987). This time of moderately cool conditions beganin the Maastrichtian and is thought to have continued, with fluctua-tions, into the late Paleocene when considerable warming occurred inbottom/polar water masses (see Camoin et al., 1993). A drastic col-lapse of carbonate platforms occurs in the latest part of the Maas-trichtian throughout the Tethyan realm, mostly related to a regressivephase seemingly coinciding with the marked global eustatic fall re-corded before the KTb (Camoin et al., 1993).

Post-drowning Evolution

Following the demise of the carbonate platform, subsidence (pos-sibly combined with eustatic sea-level changes) rapidly carried theplatform carbonates in a basinal setting below the aragonite saturationdepth, resulting in the dissolution of aragonite and the subsequentin-place precipitation of calcite cements. Deep-marine waters thatwere responsible for these processes probably continuously invadedthe carbonate cap of the guyot during its subsidence, especiallythrough the very permeable sand facies constituting the bulk of thecarbonate sequence. As dissolution processes in deep waters are muchslower than in the meteoric environment, it seems likely that theylasted throughout the entire Cenozoic.

The first post-drowning sediments are late Paleocene in age. Con-sidering an average rate of subsidence of 25 m/m.y. (Derrick andCrough, 1978), the guyot was probably between 250 and 300 m deepin the late Paleocene and entered the oxygen-minimum zone when thephosphatization started and, with the intensification and expansion ofthe oxygen-minimum zone, increased amounts of Mn and mangano-phile elements produced a manganese crust. These data are also con-sistent with the present-day aragonite saturation depth, which is 300-400 m in the modern Pacific Ocean (Li et al., 1969; Scholle et al., 1983).

CONCLUSIONS

The shallow-water carbonate sequence of Wodejebato Guyot doc-uments the submergence of a volcanic island, followed by the devel-opment and the demise of a Campanian-Maastrichtian carbonateplatform or atoll-like structure.

The depositional history of the inner perimeter ridge of Wodeje-bato Guyot began with the end of volcanism, either before or duringCampanian time, and the subsequent subaerial exposure that createda 15-m-thick weathering profile.


250 200 150 100 50 0M

MAJOR EVENTSMAYARONENSIS

DrowningFinal emergence(Karstification)

MAASTRICH-TIAN 4 Emergence

(juvenilekarstification)

G. TRICARINATA

Developement of thecarbonate platform

Flooding of thevolcanic substrate

Figure 9. Correlations between Wodejebato Guyot depositional history (major events) and eustatic curve, supercycle, and systems tracts of Haq et al. (1987).

The initial marine deposits correspond to organic-rich clay depos-ited in a quiet, reducing, shallow-marine environment.

A two-stage transgression initiated the carbonate platform, lead-ing to the accumulation of skeletal sand piles near the shelf margin,followed by the development of early cemented coralgal "reef" unitsthat constituted barrier-like features. At that time, the carbonate plat-form was forming an atoll-like structure. The prolonged sea-levelstand or the lowering of sea level may have induced local and tempo-rary subaerial exposures in the last stages of platform development.

The morphology of the drowned atoll (hummocky upper surfaceof platform carbonates), the occurrence of a trough separating theinner perimeter ridge from the outer one, and the thorough penetra-tion of platform carbonates by deep solution cavities suggest karstifi-cation processes before the final drowning of the carbonate platformduring Maastrichtian time. The occurrence of Maastrichtian pelagicsediments as cavity fillings implies a rapid sea-level rise of a few tensof meters during late Maastrichtian time (G. gansseri Zone), just afterexposure. The complete shutdown of platform carbonate productionduring re-flooding was probably related to the combination of sea-level rise and deterioration of environmental conditions caused bystrong but short climatic perturbations that characterize late Maas-trichtian time.

Following the demise of the carbonate platform, subsidence possi-bly combined with eustatic sea-level changes to rapidly carry theplatform carbonates in a basinal setting below the aragonite saturationdepth, resulting in the dissolution of aragonite and the subsequent in-place precipitation of calcite cements, possibly throughout the Ceno-zoi

14. ANATOMY AND EVOLUTION OF THE INNER ......sediments correspond to a phosphate-manganese crust...

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