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Winterer, E.L., Sager, W.W., Firth, J.V., and Sinton, J.M. (Eds.), 1995Proceedings of the O cean Drilling Program, Scientific Results,Vol. 143

30. SEDIMENT FACIES AND ENVIRONMENTS OF DEPOSITION ON CRETACEOUS PACIFICCARBONATE PLATFORM S: AN OVERVIEW O F DREDGED ROCKSFROM WESTERN PACIFIC GUYOTS1Robert J. van Waasbergen2

ABSTRACTMany years of dredging of Cretaceous guyots in the western Pacific Ocean hav e shown the widespread o ccurrence of drownedcarbonate platforms that were active in the Early to middle Cretaceous. Through petrographic analysis of available dredgedlimestone samples from these guyots, eight limestone lithofacies are distinguished, of which the first three are found in greatabundance or in dredges from more than one guyot. The eight lithofacies are used to form a comp osite image of the depositionalenvironments on the C retaceous Pacific carbonate platforms. The most ab undant facies (Facies 1) is a coarse bioclastic grainstonefound in the forereef environment. Facies 2 co mprisesmudstonein which small bioclasts are rare to abundant. This facies is typicalof the platform-interior ( lagoon ) environment. Facies 3 comprises pack stones and wa ckestones of peloids and coated grains,and is attributed to deposition in environments of moderate energy dominated by tidal currents.A number of lithofacies were recognized in only a few samples, or only in samples from a single guyot, and are therefor termed minor facie;. Facies 4 comprises muddy sponge-algal bafflestone deposits probably associated with shallow, platform-interior

bioherms. Facies 5 comprises oolite grainstones and is attributed to high-energy platform-margin environments. Facies 6 is a mixedcarbonate/siliciclastic deposit and may be assoc iated with an episode of renewed volcanic activity on one of the guyo ts (Allison)in the Mid-Pacific Mountains. Facies 7 is a mixed shallow-water and pelagic sediment, attributed to deposition in the middle-slopeenvironment, seaward from the coarse forereef sands. Facies 8 comprises muddy sediment with a high species diversity. It isattributed to deposition in a near-marginal open-lagoon environment.The m ost striking aspect of the recovered lithofacies is the strong contrast in depositional energy presented by the platform-margin and platform-interior facies. The absence of evidence of reef-framework structures at the platform margins suggests thatwave and current energy in the open ocean either was not very great in the Cretaceous Pacific Ocean, or was efficiently dampedby lack of depositionalrelief.Early lithification of platform margin sediment by diagenesis may have helped prevent rapid erosionof the platform margin deposits during the buildup of the platforms.INTRODUCTION

Western Pacific guyots (flat-topped seamounts) are the sites ofnumerous carbonate platforms that formed during the middle Creta-ceous. Many of these platforms, the summits of which were at sealevel roughly during the middle C retaceous, have been the focus of anumber of studies (e.g., Menard, 1964; Winterer and Metzler, 1984;Winterer etal.,19 93; van Waasbergen and Winterer, 1993). Most ofthese studies concentrated on the geophysical aspects of the guyottops and their relationship to the tectonic and volcanic history of thewestern Pacific seafloor. T he expeditions that gathered the nec essarygeophysical data in many places also collected rock samples of theplatform carbonates. During 1992,several of the Cretaceous c arbon-ate platforms were drilled during Legs 143 and 144 of the OceanDrilling Program, which provided a great amount of new materialfrom a few platforms.

This study examines the sedim entological aspects of the platformsbased on the petrographic analyses of limestone samples that weredredged from many different guyot tops in the western Pacific Ocea n.Many of the samples used have not previously been described from asedimentological perspective. The objectives were to identify thevarious sedimentary e nvironments that were present on the platforms,to examine the Oceanographic conditions in the Cretaceous PacificOcean, and to arrive at a composite image of a typical CretaceousPacific shallow-marine carbonate platform.

Winterer, E.L., Sager, W.W., Firth, J.V., and Sinton, J.M. (Eds.), 1995.Proc. OD P,Sci. Results, 143: College Station, TX (Ocean Drilling Program).2Department of Geosciences, University of Tulsa, Tulsa, OK 74104, U.S.A.

PREVIOUS WORKDredged carbonate samples from the W estern Pacific Ocean havebeen studied and described by Hamilton(1956),Heezen et al.(1973),Ladd et al. (1974), and by Grtsch (1991). The work of Hamilton

(1956) was of groundbreaking importance: prior to the discovery ofCretaceous limestones during the Scripps Institution of Oceanography(SIO) Mid-Pacific Expedition of1950,the guyots were thought to beislands ofPrecambrianage (He ss, 1946) that had sunk below the seasurface by the weight and water-displacement effects of sedimentsaccumulated on the seafloor over eons. Ham ilton(1956)described therecovered shallow-marine carbonates only as coquina. . . , cementedby calcium carbonate, fragments of . . . reef-coral (quoted fromHamilton, 1956), and as fragments of individual fossil types (corals,stromatoporoids, gastropods, etc.).Heezen et al.(1973)described shallow-marine limestones dredgedduring Leg 5 of the1971SIO Aries expedition. Their analyses focusedmainly on the phosphatization process, which affected much of thelimestones dredged from these guyots, and on thebiostratigraphicagesof the recoveredmicrofossilsand m egafossils. Little distinction among

lithofacies types (lumped as bioclastic calcarenites andrudistidlime-stones [Heezen etal.,1973])was made.Ladd et al. (1974) described shallow-marine limestone dredgedfrom Darwin Guyot during the 1968 SIO Styx expedition (Leg 7).Their analyses focused on constraining the ages of the fossil m aterial,in particular the tests of planktonic foraminifers found in ma nganesecrusts, which they determined to be Albian toTuronian in age, but probably Cenom anian (Ladd etal.,1974). The assemblage of fossilgastropods was determined to belong to anintertidal to near-reefenvironment, based on comparisons to gastropod forms found onmodern carbonate platforms.Grtsch (1991)presented a somewhat more in-depth analysis ofsome limestone and phosphorite samples recovered during the SIO

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10

140 150* 160 170' 180 170 160* 150Figure 1.Bathymetry of the western Pacific Ocean, based on DBDB-5 5-min gridded bathymetry. Th e contour interval is 1000 m. Guyots surveyed duringRoundaboutLeg 10, are indicated as blacktriangles. 1: Takuyo-Daini.2:Takuyo Daisan.3 : Winterer .4: CharlieJohnson .5: Stout . 6: Thomas Washington .7:Isakov.8:Makarov.9: MIT. 10: Scripps . 11: Lam ont . 12: Pot . 13: Vibelius . 14: Wilde . 15: WoodsHole . 16:Darwin. 17: Heezen .18 :Resolution.19: Caprina .2 0: Jacqueline .2 1 :Allison.Roundabout expedition (Leg 10) and divided the material into pre-drowning , drown ing, and post-d rowning facies. These subdivi-sions were based largely on the observation of planktonic foramin-ifers in the shallow-marine rocks, morphologic interpretations ofdredge locations, the presence of fossil faunal assem blages associatedwith deposition in deeper water, or during periods of transgression,and the interpretation of diagenetic features with respect to the historyof the movement of the platform sum mits relative to sea level. Manyof the rocks assigned by Grtsch (1991) to a drowning faciesprobably are not. Grtsch(1991)used the lack of meteoric diage neticfeatures as an argument that the materials had been deposited after theexposure events inferred from morphologic evidence. However, mostof the dredged materials came from locations on the outer slopes ofthe guyot tops, below the level to which the summits had becomeexposed. Grtsch assumed (based on correlations to events on theDinaric Platform in Yugoslavia) that the inferred drop in sea leveloccurred during the late Albian and, therefore, assigned rocks bea ringlate-Albian-agemicrofossils to a facies that had been deposited aftersea level returned to norm al. I will argue that most, if not all, of theshallow-water facies dredged from the guyot tops were depositedbefore the events leading to summit exposure took place.

METHOD S AND LIMITATIONSLimestone rock samples for this study were obtained from theflanks and tops of the guyots by application of the SIO marine rockdredge. The samples were collected during the Styx, A ries, andRound-about expeditions. Most of the dredges of Aries-5 andRoundabout-10(Fig. 1) that recovered shallow-marine lim estone were taken up slopeon the upper 500 m of the guyots (Table 1).Some were taken acrossthe flat tops, and a rare few targeted specific bathymetric features (e.g.,the top of the outer rim on Resolution Guyot (RNDB10 D65); theinsides ofkarstholes on MIT Guyot,RNDB10D56).Much of the material in the dredges consists of slabs of partially tocompletely phosphatized pelagic and shallow-water limestone, com-monly encrusted on both sides with marine manganese-oxyhydroxidedeposits up to 25 cm thick (Ladd etal.,1974). Samples of unphospha-

tized limestone were separated for further study from the mass ofphosphorite and manganese, along with whole and fragmented m ega-fossils (mostly mollusks), many of which were identified in paleon-tological studies. Thin sections were prepared of slabbed limestonesamples and were studied with a standard petrographic microscopewith camera-attachment. Based on observations of grain types, bio-clastassemblages, rock-textures, and diagenetic features, three differ-ent major andfive minor lithologic facies were identified. These arethought to represent different environments of deposition on the Cre-taceous platforms. M ajor facies are those recovered in many dredges,either on multiple guyots or in large amounts in a single dredge. Minorfacies a re those seen in only a few rock samples from one guyot.

SUMMARY OF CARBONATE LITHOFACIESThis section briefly introduces the main petrographic aspects ofthe eight lithofacies. More detailed observations and descriptions ofeach facies are given in the Appendix. Each facies summary is accom-panied by a cartoon that shows the typical microscopic features ofthe sediments. Facies 1, 2, and 3 were dredged from more than oneguyot and were generally seen in sufficient abundance in the dredgedrocks to be termed major facies. Facies 4 through 8 were found ononly one guyot and are considered minor facies.

Facies 1 (Coarse Bioclastic G rainstone and Packstone)This facies consists of abundant skeletal debris of mollusks, withfragmentsofechinoderms, red- andblue-greenalgae that form a coarsesandstone torudstone loosely cemented by fringing, and bladed andsyntaxial calcite cement (Fig. 2). The mollusk grains are generallyreduced to micrite envelopes, although some can be identified asfragments ofcaprinidandradiolitidrudists.Algae include solenopo-raceanandsquamaracian(red) algae as well as blue-greenCayeuxia.Echinodermdebris is mostly fragments of cideroid e chinoids. Litho-clasts of previously cemented material of similar composition arecomm on. Most of the m aterial has grainstone textures, but some inter-granular, pelletal mud occurs, which is in m any places phosphatized.

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Table 1. Dredging results from Thomas Washingtoncruise Roundabout 10, November through December, 1988.

Contents5 kg of altered basalt and volcaniclastic rubble1 kg pumice750 kg phosphorite breccia, coarse bioclastic grainstone, wholemdistshells150 kg layered phosphorite, bioclastic grainstone andrudstonewithmolluskandechinoid debris250 kg phosphatized pelagic chalk, loose fragments of gastropod and mdistshells,coarse bioclastic grainstone, one small basalt pebble30 kg altered basalt, volcaniclastic debris25 kg altered basalt, volcaniclastic debris35 kg phosphorite breccia, pelagic chalk, lagoonal mudstone,nerinid gastropod shells.40 kg breccia of altered basalt in phophatized pelagic matrixPhosphatized pelagic limestone and manganese oxide2.5 kg phosphorite and manganese, two pieces of silicified ooliteAltered basalt andvolcaniclastics45 kg phosphatized pelagic chalk, rounded basalt pebbles, manganese oxide, fragmentsofrudists5 kg bioclastic grainstone, packstone and wackestone w ith abundant coral debris45 kg fossiliferous limestone breccia with mdist megafossils, manganese oxide crusts50 kg altered basalt and hyaloclastite, and 100 kg golfball-sized manganese nodules200 kg manganese-encmsted mdistpavements, fragments of coralline limestone,phosphoriteAltered basalt and volcaniclastics, phosphatized pelagic limestoneManganese oxide, phosphorite breccia, gastropod packstone350 kg mangane se-encmsted phosphorite breccia, volcaniclastic debris0.2 kg limestone, sandstone with volcanic and carbonate debris, manganese oxideLost dredge20 kg altered basalt, hyolclastite, phosphorite breccia, coarse bioclastic grainstone100 kg pelagic chalk, manganese-encmsted ph osphorite, basalt75 kg lagoonal bioclastic packstone, pelagic chalk, m anganese nodules

edge49505152

53

5455565758596061

626364656667686970717273

Location (degrees)Start

32-19.9N148-24.8 E31-59.28N148-17.41 E31-58.4N148-19.9 E31-53.3N151-12.1E31-30.0N151-12.1E31-33.9N151-03.8E27-25.6 N152-02.4E27-13.6 N151-44.0E23-43.2N159-16.5 E21-16.2N162-43.6E21-13.4N162-42.6E21-08.6N162-50.3 E21-06.8N

166-27.8E21-59.25N171-39.21E21-11.6N173^0.6 E21-06.6N173-40.3E21-08.5N174-23.4E19-11.36N176-41.47E18-37.2N179-33.3 W18-27.3 N179-16.6 W18-19.9N179-27.6 W18-20.1N179-24.7 W18-10.5N179-43.5W18-22.3 N179-27.2 W

End32-50.5 N148-23.9 E31-59.6N148-15.6E31-57.3 N148-18.7 E31-53.5N148-13.5 E31-30.6N151-10.1E31-34.1 N151-04.7E27-24.2 N152-01.2E27-11.7N151-44.8E23-13.5N159-16.5 E21-17.0N162^3.8 E21-14.1 N162^3.6E21-10.2N162-50.8 E21-06.8N

166-28.3E21-59.4 N171-38.5 E21-10.4N173^2.3E21-06.8N173^t2.3E21-09.1E174-24.4E19-11.89N176-42.50E18-37.8 N179-32.9 W18-28.6 N179-16.4 W18-19.7N179-28.3 W18-21.2N179-24.7 W18^0.7N179-11.6W18-21.9 N179-26.2 W

Depthrange(m)234429961646178017002087175023751655237523663747253732002537320013321420187319501935262022413390184013102500289012652200186225001337134626813100163519241435177321252529260629222567333719372000

SiteWinterer Guyot Charlie Johnson Guyot Charlie Johnson Guyot Charlie Johnson Guyot

Isakov GuyotIsakov GuyotMIT GuyotMIT Guyot Scripps Guyot Vibelius Guyot Vibelius Guyot Wilde Guyot Woods Hole Guyot

Darwin Guyot Heezen Guyot Heezen GuyotResolution Guyot, south marginalong top of reef-rim Jacqueline GuyotAllison GuyotAllison GuyotAllison GuyotAllison GuyotAllison GuyotAllison Guyot

The mud postdates formation of early fringing spar cement in virtu-ally all samples, but appears to have inhibited the formation of syn-taxial cement on echinoid g rains. Little to no evidence of com pactionis present: even the fragile micrite husks of mollusk shell fragmentsremain intact.Coarse bio clastic shell debris was drilled in the interior of Allisonand Resolution guyots during Leg 143 (Sites 865 and 866), wherethey are thought to represent either brief transgressive events over theplatform or storm washovers from the rim (Shipboard ScientificParty, 1993a, 1993b). This facies appears to be far more common indredges from the more northerly Japanese guyots than in dredgesfrom guyots in the Mid-Pacific Mountains (Table 2).

Facies 2 (Mudstone, Fossiliferous M udstone,and Wackestone)Well-lithified lime mud with variable fossil content dominatesthis facies (Fig. 3), which was found in dredged samples from MITand Allison guyots (Table 2). The micrite matrix is a mixture ofpatches of dark, granular mud in a more common tan, finer-grainedpelletal mud. Most particles are bioclasts, among which foraminifersare most common. These include benthic agglutinated forms, such as

Cuneolina, textularia, and miliolids. Sand-sized sponge spicules in-clude both massive and hollow forms. Ostracode tests and fragmentsof green algae are common in samples from Allison Guyot. Molluskdebris consists of very fine, abraded fragments of bivalves and recrys-tallized fragments of thin-shelled gastropods, but more important arelarge, often whole, gastropod shells of the type Nerinea.In most samples, bioclasts of a single type occur in clusters orpatches, rather than randomly distributed. This, and the uneven distri-bution of the different types of matrix, suggests that the sediments w ereoriginally deposited in discrete laminae dominated by a singlematrix-and fossil-type, which became partially mixed by bioturbation. Mostsmaller bioclasts are dissolved, leaving uncemented molds. The gas-tropod shells have been replaced w ith clear,equant,anddrusy-mosaiccalcite cement, indicating that the mud m atrix wasrecrystallizedpriorto dissolution of the shells.

Facies 3 (Packstone and G rainstone of Peloids, Ooids,and Coated Grains)The m ost characteristic aspects of this facies are the predom inanceof coated grains, the variability in abundance and types of bioclasts,and the presence of diagenetically altered rock fragments (Fig. 4). It

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Algae

Finecalcite cementCoarse calcite cement

Benthic foraminifers

Planktonic foraminifersOstracodeSpongespicule

Sponge structure

Volcanic fragments

Peloids pellets

Ooids aggregate grains

LegendFigure 2. Cartoon representation of Facies 1.Most of the mollusk grains are shown as micrite envelopes or spar-filled molds surrounded with bladed spar cement.The width of the figure represents approximately 4 m m.was found on platforms in the Japanese and in several of the Mid-Pacific Mountains guyots (Table 2). The matrix consists of fine mudthat can be clotted and slightly cemented with microspar, to densepelletal mud that is somewhat phosphatized in many places. Grainsinclude bioclasts of near-reef (mollusks, red algae, and coral) toplatform-interior (foraminifers and green algae) origins. Most abun-

dant, however, are a variety of coated grains, including ooids, aggre-gate grains, and peloids. These suggest thorough reworking, coating,and micritizationof sediments was com mon. M any of the bioclastsare also thickly coated with mud. Rock fragments include abundantmud chips, but also pieces of diagenetically advanced limestone, inwhich spar-filled veins and stylolites occur.

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Figure 3. Cartoon representation of Facies 2 : Scale and legend as in Figure 2.Table 2. Facies distribution of dredged limestone samples.

Facies1

23

45678

Found on Charlie Johnson GuyotIsakov GuyotThomas W ashington Guyot Winterer GuyotResolution Guyo t Woods Hole Guyot Jacqueline GuyotTakuyo-DaisanGuyotMIT GuyotAllison Guyot Charlie Johnson GuyotAllison Guyot Jacqueline GuyotCape Johnson GuyotAllison Guyot Vibelius GuyotAllison GuyotAllison GuyotAllison Guyot

Location(degrees)32.0 N, 148.4 E31.5N ,151.2E32.0 N, 149.3 E32.5 N, 148.3 E21.3N ,174.3E21.1 N, 166.5 E19.3N ,176.6E34.2 N, 144.3 E27.3N ,151.8E18.5 N, 179.5 W32.0 N, 148.4 E18.5N ,179.5 W19.3 N, 176.6 E17.2 N, 177.2 W18.5 N, 179.5 W21.2N ,162.8E18.5 N, 179.5 W18.5 N, 179.5 W18.5N ,179.5W

DredgelocationUpper slopeUpper slopeUpper slopeUpper slopeMarginal rimUpper slopeUpper slopeUpper slopeCenter of platformSlump scar in lagoon sedimentsUpper slope, platform marginSlump scar in lagoon sedimentsUpper slopeUpper slopeSlump scar in lagoon sedimentsEdge of erosional remnant on summitUpper slope, marginal platformSlopeFurrow in summit platform

All the samples have packstone textures with little evidence ofsorting. Only one sample from Allison Guyot contains fining-upwardlaminae of pellets and coated grains. The matrix material is well-lithified, and m any of the bioclasts are dissolved, leaving uncem entedor partially cemented molds.Facies 4 (Bafflestone of Sponge and Algal Structureswith a Peloidal and Bioclastic Matrix)This facies, found on Allison Guyot (Table 2), consists of a matrixof variable composition between m illimeter- tocentimeter-sizedstruc-

tures of spong es, encrusting algae, and coral (Fig. 5). The matrix is aloose, clotted micrite that, in some places, is nearly absent as a resultof dissolution of larger bioclasts. Unevenly distributed in this matrixare grains, including bioclasts (foraminifers, mollusk fragments, redalgae, and echinoids) and micritic grains and lithoclasts. Large (sev-eral millimeters), poorly preserved fragments ofrudistsare common.Red algae form elongate grains and, in many places, encrust molluskand sponge debris. The micritic particles appear to have a number oforigins, but most common are angular mud chips and fecal pellets.Sponge/algal structures form bulbous masses up to 2 cm in sizewith irregular walls of dense micrite and finely crystalline spar. The

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Figure 4. Cartoon representation of Facies 3:Scale and legend as in Figure 2.

Figure 5. Cartoon representation of Facies 4: Scale and legend as in Figure 2.

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centersof thestructures contain im printsofnow-dissolved corallitesvisiblein hand specimen. Between thestructuresis the mudmatrixwith a variable and irregular distribution of small bioclasts, rudistshells,andmicriticparticles. Where dissolutionofsponge structuresand bioclastshas left vugs, micritic particles occur thatarelooselyconnected with m icritic cement, indicating possible vado se diagenesis.Fades (Silicified Oolites)

Oolite sandstonewasdredged from V ibeliusGuyot (Table2). Itconsistsofsand-and coarse sand-sized, tangential, laminated ooidscementedby fringing bladed sparand coarse, blocky spar cements(Fig.6).Silicificationhasreplaced virtuallyall theoriginal c arbonatematerial with fine silica thatisrecrystallizingtosmall euhedral quartzcrystals. Onlya fewconcentric layersofmicritearepreservedin theooids,aswellassomeof thebladed, fringing sparry calcite cementcrystals.Noevidenceofinterstitialmud isfound,and theooids forma packed, grain-supported texture with some fused, interlocking ooidgrains, possiblytheresultofpressure solution.Facies 6 (Mixed Carbonate and Siliciclastic Sand)

Shallow-water-derived carbonate clastsare mixed together withvolcanic rock fragments andmineral grains (Fig. 7). Thedominantcarbonate fraction consists ofwell-rounded, thoroughly micritizedbioclastsofalgae, bivalve fragments andechinodermdebris,aswellas micritic intraclasts andpeloids.Thevolcanic fraction includesmafic lithoclasts, twinned and untwinned feldspars, volcanic glass,and mica.Thesandiswell sortedandwell compacted,andcementedwith clayey alteration products. Thereis noevidenceforlamination,beddingorgrading.Facies7(PackstoneofPeloidswith Planktonic Foraminifers)

This facies, foundonAllison G uyot, consists mo stlyofsand-andfine sand-sized micritic particles in aslightly recrystallized micritematrix (Fig. 8).Scattered throughout are abundant planktonic fora-minifers that formalate Albian assemblage.Asecond assem blageofplanktonic microfossils occursinsmall cracksandmolds. TheseareLate CretaceoustoCenozoicin age.Particles include abundant mi-critic peloids, micritized benthic foraminifers andpoorly preservedmollusk fragments including radiolitid rudists. Many bioclasts weredissolved leaving uncemented molds. Echinoid grainsarecommon,in some places overgrown with syntaxial cement rinds.Thematrixishomogeneous and very loosely packed, leaving much intergranularporosity. Minor cementationhasoccurred,in theformofclear equantspar, that fills some m oldsaswellasmostoftheintraparticularpores.Facies8(PackstoneandWackestoneofBioclastic DebrisinaLime-mud Matrix)

This facies, dredged from the northwest sideof Allison Guyot(Table2),consistsoffine limemudwith abundantanddiversebio-clasts (Fig. 9). Dark granularand tan lime mud form irregularlydistributed patches with sharp, irregular shaped boundaries betweenthem.Athird, pelletal-gran ular mudpartially fills molds, vugsandcracks. Particles include large, whole gastropod shellsandsand-tosilt-sized fragmentsofbivalves, algae, foraminifers, andostracodes.Mostof thebioclasts o ccurin the tanmatrix, along with smallmud-stone lithoclasts.Themolluskandrare echinoid fragm ents tendto bepoorly preserved and abraded, whereas theforaminifers andgreenalgaearemuch better preserved.The large gastropods were completely dissolved andlargelyre-placed with coarse, clear spar cement. Smaller bioclasts includinggreen algaeandmollusk fragments w ere dissolvedaswell, leaving

small moldsandsolution-enhanced vugs thatarepartially filled withpelletalmud andclear spar.DISCUSSION ANDCO N CL U SI O N S

The samples dredged from the Pacific guyots represent onlyafragmented viewof carbonate depositionon the mid-Pacific Creta-ceous platforms. Nevertheless, each facies described here representsthe resultsof auniqueset ofcircumstances thatcan beinterpretedinterms of sedimentary environments. How these environmentsaredistributedon the platforms cannot be easily known from sucharelatively random and sparse sampling. Comparisonof thePacificfaciesto those described from Cretaceous carbonate platforms else-where mayallow for a better understandingof thedistributionofenvironments,and thedevelopment of a composite mode l of aCretaceous Pacific mid-oceanic carbonate platform.Onemodel thatrelates thedistribution of facies anddepositional environments isshownas acartooninFigure10.The closest related platforms, bothinspaceandtime,arefoundinMexico and around theGulf of Mexico in Texas (Bebout,1974;Cooganeta l , 1972;Freeman-Lynde,1983; Scott, 1990).Anespe-cially good analogis theVallesSanLuis Potosi Platform (VSP) inwest-central M exico. Thisis arudist-reef-bounded carbonate plat-form surroundedon allsidesby deep marine basins that formedataboutthesame timeasmanyof thePacific platforms du ring Albianto early Cenomanian time. Another seriesofreefsandplatformmar-gins collectively formstheStuart City Trend (SCT) platform margininthesubsurfaceofTexas, whichis roughly parallelto thepresent-day coast lineof theGulf of Mexico (Scott, 1990).Avarietyofdepositional environments was identified on these platforms fromlithofaciesandfossil comm unities, someofwhich bear strong resem-blancetothose foundon thePacific p latforms.Facies 1,thecoarse bioclastic g rainstoneandpackstone,issimilartoonefoundin the forereef slope environmentof the SCT,wherecoarse sandsoffragments ofcorals, caprinid,andradiolitid rudistsand whole rudist skeletons occur. Unlikethe western Pacific plat-forms,theforereef depositsof the SCTcontain significant amoun tsof coral.Theabsenceofcoral fragmentsinFacies1mightbe anindi-cation that these sediments representanenvironment more interiorofthe platform margin thanthe SCTdeposits,but thedredge locationsin which th is facieswasrecovered (upper foreslopesofthe platform)suggest otherwise. Likewise,on the VSP,such a rudist grainstonefaciesisfoundin thefore-slope environment, whereitforms stronglydipping beds thatcan lie at angles as steepas 43. Similar steepupper-slope angles were seeninmanyofthe Japanese guyots, wherethis facieswasfoundingreat abundance.On theVSP, similar debrisis also partofthe reef-core environment, whereitfills space betweenrudistbiohermsalongtheplatform margin.Facies2consists primarilyofmudstone w ith small bioclasts, suchasmiliolidforaminifers, algae, sponge spicules,andostracodes. Sucha lithofaciesandfossil asso ciationiscommonin theopen-lagoon partsofthe SCT,whereit isassociated with mollusks, echinoids,and redalgae.On the VSP,mostof thelagoon facies consistofm iliolidandpeloid sands, with sparse mudstone present onlyin themost interior,distal portionsof thelagoon, where miliolid micrite (Scott,1990)

formsone of thelithofacie s.Itappears thatthelagoon-derived faciesfromthewestern Pacific platforms representamore restricted, quiet-water environment than wascommon on the Mexican andTexanplatforms.Thenerineidgastropods commoninFacies2 arefoundininterior portionsofthe SCTaswell, generally associated w ith bivalvesand redalgae, ostracodesandserpulids(worms),anassociation thatisinterpreted asrepresenting a stable environment withamuddysub-strate,in thebackreeflagoon portionsofthe platform.Facies3, thepeloid/ooid/coated-grain assemblage,iswell repre-sentedin theSCT, whereitforms chann el-filling sandsandshoal areasin environments marginalto thelagoon.On theVSP,peloid-miliolid

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Figure 6. Cartoon representation of Facies5 :Scale and legend as in Figure 2.


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~ mLEGEND

777? .upper slope debris (Facies 1)sparsefossiliferous mudstone(Facies 2)peloidalpackstone(Facies 3)

L (cj Gastropods

Caprinid rudists

Green algae

sponge-algal bioherm (Facies 4) @oolite bar (Facies 5) \mixedcarbonate/siliclasticsands (Facies 6)transitionalplatform/pelagicmaterial (Facies 7)

OoidsSponge spicules

coral-rudistbiohermFigure 10. Composite model of facies distribution on a Cretaceous Pacific carbonate platform.

packstones form a commonbackreeffacies, where they are associatedwith reef-derived skeletal debris and intraclasts, and form upward-shallowing sequences (Enos, 1983; Minero, 1983). On these plat-forms, this facies represents open-marine,backreef/lagoon environ-ments that were relatively energetic and dominated by tidal currents(Enos, 1983). The same facies on the Pacific platforms shows a morevariable contribution of reef-derived components relative to lagoon-derived components, suggesting that similar, moderately energeticenvironments were present in various places across the platform.No obvious analog is described for Facies 4 in either the VSP orthe SCT platforms. Shallowbiohermsare described in the reef coreand marginal backreef environments where coral, red algae, andsponges are common (Scott, 1990). In the samples from Allison

Guyot, the muddy matrix that fills the space between sponge/algalstructures consists of fine mud w ith miliolid and orbitolinid foram in-ifers, and closely resembles the dominant backreef facies on the VSPplatform. Th is suggests that this facies occupied a similar zone, some-what inward from the margin, but with fewer contributions from themargin than would be expected in a part of the platform where coraland red algae thrive. The presence of vadose textural features andreworked intraclasts as part of the matrix indicates this was probablya very shallow-water environment that was emergent at times.Facies 5, the oolites, is a common h igh- to moderate-energy faciesassociated with prograding platform margins (Scott, 1990; Halley etal., 1983). Shoals just interior to the platform margin become domi-nated by tidal currents that winnow particles to develop peloid sands

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17940 WFigure 11. Bathymetry of Allison Guyot from multibeam data. The Roundabout Leg 10 dredge locations for rocks used in this study are indicated by heavy lines.Also shown is the ODP drill site (865).and, subsequently, ooid san ds. These can be swept into tidal bars andeven islands and may represent the shallowest parts of the platform.They typically separate the marginal ( re ef and foreslope) from theplatform-interior ( backreef' and lagoonal ) environments. On theSCT, ooid-bank facies occur in the reef-flat portions of the RodessaFormation, which is associated with the SCT platform margin belt.On the VSP, ooid shoals are not comm on.The mixed carbonate siliciclastic sediments of Facies 6 are prob-ably specific to a volcanic-island setting, and no analog for this faciesoccurs in the Mexican or Texan platforms. Textural features suggestthat the sand was rapidly transported and deposited, with the carbon-ate components showing m ore evidence of reworking and abrasionthan the siliciclastic grains. The location of the dredge site (Table 2and Fig. 11), on the upper slope of Allison Guyot, suggests this maybe a beach or shelf-edge sand. The paucity of platform-margin-derived components (such asmdistfragments) suggests that the car-bonate sands may have been transported by a stream or channelthrough the interior portion of the platform and quickly deposited onthe upper slope without incorporating other grains.

Facies 7 is a mixture of platform-derived andmicritizedgrains andpelagic compo nents and strongly resembles facies associated with boththe VSP and SCT platforms. On the VSP platform, facies downslopefrom the fore-slope deposits include mdist fragments and pelagic

microfossils in a micrite matrix. On the SCT platform, wackestoneswith small bioclasts of mollusks, echinoids, and peloids, and withabundant planktonic microfossils are attributed to a forereef basinalfacies.This suggests that Facies 7 represents the deepest of allplat-form-derived facies and occurs below the coarse bioclastic debrisapron, transitional to basinal pelagic facies, in a few tens to hundredsof meters water depth.Facies 8 is heterogeneous with large whole gastropods in a mudmatrix that contains fragments of green and blue-green algae, aggluti-nated foram inifers, echinoderms,ostracodes, and peloids. Such a di-verse assemblage indicates an open-marine environm ent with a stable,muddy b ottom, such as is found in the near-marginal backreef env iron-ments on the V SP (Scott, 1990).The site of the dredge in which thisfacies was recovered, just interior of the outer rim of Allison Guyot(Table 2 and Fig. 11), is consistent with such an interpretation.A composite model for the distribution of facies and depositionalenvironments, as interpreted from dredged samples, is shown in Fig-ure 10. As this is based on samples from several sites, it is only anapproximation of what may have been present on any individualplatform in the Cretaceous Pacific. Several aspects stand out. Noneof the dredged material contained anything resembling a platform-margin, reef-framework facies. Such a facies is characteristic forpresent-day coral-algal atolls in the Pacific. Furthermore, S cott (1990)

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described the VSP platform as clearly reef-bounded, with in-situlenticular bodies ofcoral-rudistbuildups acting as frameworks alongthe edges of the platform. A second interesting aspect is the strongcontrast in energy of deposition between the outer-platform/upper-slope facies and the platform-interior facies. The latter is composedprimarily of fine-grained m udstone. In both the VSP and SCT plat-forms, the platform-interior environments are dominated by sandypeloid/foraminifer packstone and grainstone and show a clear gradi-ent of depositional energy over many kilometers into the platforminterior. Especially on the VSP, true mudstone is relatively rare. Onthe Pacific platforms, energy of deposition dropped from high on themargins and uppermost slopes to very low immediately interior to themargin, so that most of the platform interior was exceeding ly m uddy.Drilling on Allison and Resolution guyots during Leg 143 showed agreat abundance of sediments that matched Facies 2 (Shipboard Sci-entific Party, 1993a, 1993b) in the upper parts of the sections recov-ered on these sites. This implies that, although there are no signs ofreef structures or other firm barriers, damping of wave energy at themargins w as efficient. Very little coral was found in the dredge m ate-rial. One explanation for this is that the Pacific was truly pacific anddid not subject the mid-oceanic platforms to particularly energeticconditions. During the middle Cretaceous, the platforms w ere thoughtto be located in the tropical Pacific Ocean (Sager, 1992). Platformssuch as the VSP were clearly subjected to storms and other con ditionsof strong energy, the effects of which are noticeable in the sedimen-tary facies for many kilometers into the platforms. There is no reasonto assume that the open ocean was exempt from such conditions.Another possibility is that the platform margin efficiently dampenedwave energy by having little depositional relief (Enos, 1983). Inabsence of other forces, the mid-oceanic tidal range may have al-lowed the bank tops to build near (1-2 m) sea level, and in manyplaces, intertidal conditions may have prevailed, preventing waveenergy from reaching very far beyond the margins. Sediments at themargins, which contained a large percentage ofaragonitein the formof mollusk shells, were subject to rapid lithification during earlydiagenesis, which stabilized the platform margins and prevented sub-stantial wave erosion during the upward growth of the platforms.Textural evidence of early cementation of the mollusk grainstoneswould support this possibility.In summary, from petrographic analyses of dredged limestone sam-ples, western Pacific Cretaceous carbonate platforms developed intoshallow, exceedingly muddy banks. These were held together, not byreef structures, but by rapid lithification of sediment during earlydiagenesis. The platform-interior environments were protected fromstorms and ocean w aves by the damping effects of shallow water.

A C K N O W L E D G M E N T SThe author gratefully ackn owledges I. Premoli Silva and B. Sliterfor their patient help with identification of microfossils, and E.L.Winterer for overall guidance and support. Some of the figures weremade u sing software developed by P. Wessel and W.H.F. Smith. Thework was partly supported by the JOI-USSAC post-cruise sciencesupport program and by NSF Grant 8717079-O CE.

REFERENCES*Aguayo, J.E.C., 1976. Sedimentary environments and diagenesis of El AbraLimestone at its type locality, eastern Mexico.AAPGBull. 60:644.Bathurst, R.G.C., 1965. Boring algae, micrite envelopes and lithification ofmolluscan biosparites.Geol.J. 5:15-32.

Abbreviations for names of organizations and publications in ODP reference lists followthe style given inC hemical Abstracts Service Source Index (published by AmericanChemical Society).

Bebout, D.G., 1974. Lower Cretaceous Stuart City shelf margin of SouthTexas: its depositional and diagenetic environments and their relationshipto porosity.Trans. Gulf Coast Assoc. G eol. Soc, 24:138-159.Carannante,G.,Esteban, M., Milliman, J.D., and Simone, L., 1988. Carbonatelithofacies as paleolatitude indicators: problems and limitations.Sediment.Geol, 60:333-346.Coogan, A.H., Bebout, D.G., and Maggio, C , 1972. Depositional environ-ments and geologic history of Golden Lane and Po za Rica Trend, Mexico:an alternative view.AAPGBull 56:1419-1447.Enos,P., 1983. Shelf environment.InScholle, P.A., Bebout, D.G., and M oore,CH. (E ds . ) , Carbonate Depositional Environments. AAPG Mem. ,33:267-295.Flgel, E., 1982.Microfacies Analysis of Limestones: New York (Springer-Verlag).Freeman-Lynde, R.P., 1983. Cretaceous and Tertiary samples dredged fromthe Florida Escarpment, eastern Gulf of Mexico.Trans. Gulf Coast Assoc.Geol. Soc, 33:91-99.Grtsch,J.,1991.Die Evolution vonKarbonatplatformen des offenen Ozeansin der mittleren Kreide (NW-Jugoslawien, NW-Pazifik, NW-Griechen-land): Mglichkeiten zur Rekonstruktion von Meeresspiegelvarenderun-gen verschiedener Groszenordnung [Ph.Ddissert.].Inst. Paleontol. Univ.Erlangen, Neurenberg, Germany.Halley, R.B., Harris, P.M., and Hine, A.C., 1983. Bank margin environment.In Scholle, P.A., Bebout, D.G., and M oore,C H . (Eds.),Carbonate Depo-sitional Environments.AAPG Mem., 33:463-506.Hamilton, E.L., 1956. Sunken islands of the Mid-Pacific Mountains.Mem.Geol. Soc.Am., 64 .Heezen, B.C., Matthews, J.L., Catalano,R.,Natland,J.,Coogan, A., Tharp,M., and Rawson, M., 1973. Western Pacific guyots. In Heezen, B.C.,MacGregor, I.D., et al., Init Repts. DSDP,20: Washington (U.S. Govt.Printing Office), 653-723.Hess,H.H., 1946. Drowned ancient islands of the Pacific basin.Am. J. Sci.,244:772-791.Ladd, H.S., Newman, WA ., andSohi,N.F., 1974. Darwin Guyot, the Pacific'soldest atoll.Proc. 2nd. Int. Coral Reef Symp.,2:513-522.Menard, H.W., 1964. Marine Geology of the Pacific: New York (McGraw-Hill).Minero, C.J., 1983. Sedimentation and diagenesis along open and island-pro-tected windward carbonate platform margins of the Cretaceous El AbraFormation, Mexico.Sediment.Geol. 71:261-288.Sager, W.W., 1992. Seamount age estimates from Paleomagnetism and theirimplications for the history of volcanism on the Pacific Plate. InKeating,B.H., and Bolton, B.(Eds.),Geology and Offshore Mineral Resources ofthe Central Pacific Basin. Circum.-Pac.Counc. Energy Miner.Resour.,

Earth Sci.Ser.,14:21-37.Sager, W.W., Winterer, EX., Firth,J.V.,etal., 1993.Proc. ODP, Init. Repts.,143:College Station, TX (Ocean Drilling Program).Sartorio, D. , and Venturini, S., 1988. Southern Tethys Biofacies: Milan(AGIP).Schlanger, S.O., 1964. Petrology of the limestones of Guam .Geol. Surv.Prof.Pap. U.S.,403-D:1-52.Scott, R.W., 1990. Models and stratigraphy of M id-Cretaceous reef commu-nities, Gulf of Mexico. Soc. Sediment.Geol. Concepts Sedimentol. Pa-leontol. 2.Shipboard Scientific Party, 1993a. Site 865.In Sager, W.W., Winterer, E.L.,Firth, J.V., etal.,Proc.ODP Init. Repts., 143: College Station, TX (OceanDrilling P rogram), 111-180., 1993b. Site 866.InSager, W.W., Winterer, E.L., Firth, J.V., etal.,Proc. ODP, Init. Repts., 143: College Station, TX (Ocean Drilling Pro-gram), 181-271.van Waasbergen, R.J., 1993. Western Pacific g uyots: summit geomorphology,sedimentology and structure of Cretaceous drowned carbonate platforms[Ph.D. dissert.].Univ. of California San Diego, La Jolla, CA .van Waasbergen, R.J., and Winterer, E.L., 1993. Summit geomorphology ofWestern Pacific guyots. In Pringle, M.S., Sager, W.W., Sliter, W.V., andStein, S.(Eds.),The Mesozoic Pacific: Geology, Tectonics, andVolcanism.Geophys.Monogr.,Am. Geophys. Union, 77:335-366.Wilson, J.L., 1975.Carbonate Fa cies in Geologic History:New York (Sprin-ger-Verlag).Winterer, E.L., and Metzler, C.V., 1984. Origin and subsidence of guyots inthe Mid-Pacific Mountains.J. Geophys.Res., 89:9969-9979.Winterer, E.L., Natland, J.H., van W aasbergen, R.J., Duncan, R.A., McNutt,M.K., Wolfe, C.J., Premoli Silva,I.,Sager, W.W., and Sliter, W.V., 1993.Cretaceous guyots in the Northwest Pacific: an overview of their geology

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SEDIMENTFACIESOF CRETACEOUS PACIFICand geophysics.InPringle, M.S., Sager, W.W., Sliter, W.V., and Stein, S.(Eds.), The Mesozoic Pacific: G eology, Tectonics, and Volcanism.Geo-phys.Monogr.,Am. Geophys. Union, 77:307-334.

Date of initial receipt: 1 December 1993Date of acceptance: 21 June 1994Ms 143SR-242

APPENDIXDetailed Descriptions of Carbonate Facies

This appendix contains detailed observations and descriptions of eachfacies,g enerally organized by particles, matrix com position, texture, diagen-esis,and an interpretation of the environment of deposition. The dredge loca-tions for samples collected during the Roundabout Expedition (Leg 10) aretabulated in Table 2. Dredge locations for the Aries Expedition (Leg 5) aregiven in Heezen et al. (1973).Facies 1 (Coarse Bioclastic Grainstone and Packstone)

This facies is characterized by abundant coarse skeletal debris of mollusks(predominantly rudists, but including abundant gastropods and common neriticbivalves such asInoceramus ,red algae, echinoids, and, to lesser extent, benthicforaminifers and limestone rock fragments, which are loosely to completelycemented together by fringing, bladed and syntaxial calcite cement. It is by farthe most common nonphosphatized type of shallow water and shallow water-derived limestone dredged from the western Pacific guyots.

ParticlesGrains are typically angular, partially micritized bioclasts, which are verypoorly preserved, although in most samples mollusk microstructures werepreserved in certain grains. Mollusk fragments (80 ) include fragments ofthick-shelled rudistbivalves, mainly from massively walled caprinids. Somefragments show the two-layer structure of radiolitids. The outer layer of these,originally aragonite, has been dissolved away except for a thin micritized rind.The thicker inner layer of low-magnesium calcite is preserved w ith recogniz-able lamellar and crossed-lamellarmicrostructure. Fragments ofInoceramuscan be recognized by the typical prismatic crystal structure of the shell.Fragments of various types of algae occur(10 )among which a massive,often poorly preserved form of red algae is the most common. This form is

characterized by a microstructure of fine radial, nonbifurcating tubules ap-proximately 50 in diameter, that are generally preserved as cast structures:the tubules are filled with micrite, whereas the surrounding skeletal materialof the calcareous algae has been replaced by finely crystalline spar (Fig.A l ) .Clasts of this algal material can be up to several cm in diameter, but aregenerally sand- to coarse sand-sized.Other forms of algae include Cayeuxia,a blue-green algae that is charac-terized by closely packed, bifurcating micritic tubules, and Polystrata alba(Fig. A2), a member of the red algae group Squamaraceae. Numerous sand-sized micritic particles with poorly defined internal structures may likewise bealgal in origin, but are too poorly preserved to be identified as such withcertainty. Some of the grains that appear to be micritic in thin section may infact be sections through a micritic rind at the edges of grains. They occurcommonly as smaller grains in pockets between larger, more readily identifi-able skeletal fragments.Fragments ofechinoderms are common (5%), generally in the form ofequant and prismatic angular grains, which beha ve optically as single crystalsof calcite. They most likely originated from echinoids (sea urchins), becausethis facies occurs commonly in association with well-preserved thick club-shaped spines of cideroid urchins.Other constituents (5%) include benthic and planktonic foraminifers, bryo-zoa, limestone rock fragments, and very rare coral fragments. Many samplescontain comm on lithoclasts of w ackestone and packstone. These grains can bedistinguished from ordinary grains by the occurrence of broken fossil-fragmentsand discontinuous cement-rinds at the grain-boundaries. Coated grains are rare,except in samples from Thom as Washington Guyot. The micritic outer wallsof most grains are clearly produced bymicritizationof the shell walls, ratherthan by coating with mud. This may be interpreted as indicative of depositionunder high-energy condition of short duration or temporary nature, such as achannel- or storm-deposit.

0 5 mmFigure A l . Photomicrograph of skeletal grainstone (Facies 1). The large,rounded grain in upper left part of the photograph is a fragment of solenoporanred algae. The other grains are fragments of mollusks and echinoids, allpartially cemented by equant spar cement (calcite). Sample RNDB-D53-1.

0 5 mmFigure A2. Photomicrograph of skeletal grainstone (Facies 1).The elongategrain (center) is a fragment of the squamaracian red algae Polystrata alba.Other grains include poorly preserved fragments of mollusks and echinoderm s,and rounded micritic grains (peloids). Sample A5-31-A12.

TextureGrainstone and packstone textures predominate. In samples with a muddymatrix (packstones) the matrix is commonly partially to completely ph ospha-tized. In the most phosphatized samples, only echinoid fragments remain ascalcite particles; other particles have been reduced to molds in a finelycrystalline phosphatic matrix.In nonphosphatized and partially phosphatized samples of this facies,grains are fine sand to gravel in size, poorly sorted, and with no noticeablegrading or bedding. Im bricate textures, which indicate deposition in a strongcurrent, occur in samples from Resolution Guyot (RNDB-D65). Little evi-dence for compaction can be seen. Few examples were seen of collapsedmolds, which typically result from compaction after early partial dissolutionof grains. In packstones, the muddy matrix consists of micrite of variabledensity that fills allinterparticularspace, as well as somemoldicporosity. Inmany samples, the interstitial micrite appears in the form of small pellets that

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0 5 mmFigure A3 . Photomicrograph of skeletal grainstone (Facies 1).Grains includepoorly preserved bioclasts of mollusks, algae and echinoderms, loosely ce-mented with equant and bladed spar cement. Echinoid fragments are charac-terized by syntaxial diagenetic overgrowths. Mollusk fragments remain onlyas micritic envelopes. Sample RNDB-D53-1.commonly fill both mollusk molds and intergranular space, but that postdatethe earliest generation of cement.

BiotaMost of the grains appear to be fragments of rudists, both caprinids andradiolitids. Other common skeletal fragments include gastropods, solenopo-racean (red) algae, and coralline (red) algae. Less common constituents arebenthic foraminifers, rare planktonic foraminifers (not identifiable), greenalgae (Woods Hole Guyo t), and red algae belonging to the group Squamaracia,possibly Polystrata alba,as well asLithocodiumcf.aggregatum.Some samples of packstone were studied by Scanning Electron Micro-scope (SEM) to search for nannofossils in the muddy matrix, but none werefound (T. Bralower, pers. comm., 1990).

DiagenesisThe grains were rapidly cemented with fine-bladed spar cement duringdiagenesis, which preceded the dissolution of mollusk grains. Most of themollusk fragments are recognizable only as micritic envelopes, partially tocompletely filled with bladed and blocky spar cement (Fig. A3). These micriticenvelopes originate from the destruction of the outermost part of the originalshell fragment by microscopic organisms (Bathurst, 1965). Micritic calcitecement precipitates in these holes. This process occurs during deposition,while the grain is still at the sediment surface. T he micrite enve lope is generallymore resistant to dissolution than is the original shell, so that ultimately onlythe envelope rem ains, after the rest of the shell dissolves. Following dissolutionof the shell, the molds and the remaining pore space were partially filled withcoarse blocky spar cement. In some places, interstitial mud infiltrated therocks,but this generally postdated early cem entation.In samples with little to no interstitial mud, echinoid fragments have

characteristic syntaxial overgrowths: the grains are enlarged with opticallycontinuous calcite cement. In packstones, the echinoid debris commonlyoccurs without cement, and may even be partially micritized.Syntaxial ce-ments appear to have formed preferentially on nonmicritized parts of theechinoid grains.Interpretations

Material of peri-reefal origin, including large whole mdist specimens,predominates, whereas lagoon-derived clasts (mudstones, sponge-debris, high-spired gastropods) are notably absent. Coral-debris is also very rare, possiblyowing to the low preservation potential of thearagoniticcoral skeletons. Size

0 5 m mFigure A4. Photomicrograph of sparse fossiliferous mudstone (Facies 2). Twoforms of matrix mud are irregularly distributed throughout: a dominant,light-colored micrite, and a darker, more granular micrite. Sample RNDB-D56-15.

ranges, lack of sorting, and occurrence of imbricate structures suggest depositionunder conditions of high energy, possibly in strong currents. The lack of coatingof grains, grain-angularity and co mmon occurrence of postdepo sitional infiltra-tion of lime mud suggest that the high-energy conditions we re intermittent or ofshort duration.The texture and constituents of this facies closely resemble those found atthe very top and outer slope of the platform-margin deposits of the Tamabralimestone formation in west-central Mexico (Aguayo, 1976). There, coarsebioclastic grainstones and packstones form the final transgressive layer mark-ing the end of deposition on the shallow platform, and commo nly occur as talusblocks on the upper slope of the platform (Enos, 1983).Facies 2 (Mudstone, Fossiliferous Mudstone and Wackestone)

This facies is characterized by a predom inance of a lime-mud matrix, inwhich bioclasts of benthic foraminifers, algae, sponge spicules, and ostracodetests occur in varying abundan ces, and large nerineid gastropods are comm on.

MatrixThe matrix consists of well-lithified lime mud of very fine grain size(micrite). Variability in color and granularity is common within single thinsections: darker patches of micrite 0.5 m m-5 mm in size occur within the moreabundant lighter micrite, with irregular boundaries. In hand specimens the dark matrix is actually whiter than the surrounding tan (light) matrix Thedarker micrite appears to be slightly more granular (Fig. A4). It consists ofsmall clots 20m in diameter that contain submicrometer-size specks ofopaque m aterial. In reflected light these are metallic yellow and m ay be pyrite.The micrite crystals themselves are0.1-0.4m in size. These two types ofmatrix will be referred to as tan (the lighter, homogen eous, more abundantmatrix) and granu lar (the darker matrix associated with pyrite).The tan micrite also has a clotted fabric, with individual clots typically

50-75 m in diameter and far less well developed than in the granular micrite.In some samples, small(50-100)opaque dendritic structures occur that areprobably manganese-oxide replacements of the original limestone. The granu-lar patches yield slightly lighter 813C values than do the tan micrite (vanWaasbergen, 1993). This may indicate a slightly higher content of organic-derived carbon in the calcite in the granular micrite. The apparent associationbetween granular micrite and the occurrence of pyrite may indicate that thegranular patches represent originally more organic-rich parts of the sediment.The highly irregularly shaped boundaries between the patches of granularmicrite and the surrounding tan micrite suggest that this distribution is theresult of incomplete mixing of originally layered sediment, most likely bybioturbation beforelithificationof the sediment. Evidence ofbioturbationca n

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be seen in rare recognizable tubular burrows and in sheltered areas such as theinsides of gastropods, where the m icrite occurs as ovoid fecal pellets 0.5 to 1mm in diameter.Particles

Particles in this facies consist exclusively of bioclasts, recrystallizedbioclasts, and molds of bioclastic material. For the purpose of understandingthe original sediment com position, the molds will be treated as particles as faras they can be identified. Bioclasts and molds of foraminifers, ostracodes,mollusks, spicules, algae, and pellets occur in variable amounts and relativeabundances. Bioclasts range in size from 0.1 mm to 2 mm. The origin offragments smaller than0.1mm cannot be determined.Foraminifers are predominantly small (up to 0.5 mm) agglutinated formsincluding biserial Cuneolina, Textularia, rare planispiral forms, and somemiliolids. Only a few planktonic foraminifers have been reported (Grtsch,1991) and were not observed in these sam ples. The benthic foraminiferal testsare generally composed of recrystallized micrite identical in composition tothat of the matrix (Fig. A5). Foraminiferal cham bers are filled with fine equantspar. Most of the tests appear to be com plete, rather than fragmented. Mo st ofthe small round molds that are common in most samples can probably beattributed to dissolved foraminiferal tests. Miliolid tests are more comm onlypreserved as a slightly darker, denser micrite, and contrast well with thesurrounding matrix. The abundance of foraminifers varies between samples.Cuneolina-typesappear to be more comm on in the samples from M IT Guyot,whereas those from Allison Guyot contain more miliolids.Ostracode tests are common in the samples from Allison Guyot, less so inthose from MIT Guyot. The tests are up to 0.5 mm long, a few tens ofmicrometers thick, and most commonly found in disarticulated form. Theyretain their characteristic prismatic microstructure, indicated by a sweepingextinction pattern under crossed nichols.Two types of algae w ere observed in this facies: a very poorly preservedmassive-branching form similar toCayeuxia,and fragments of Dasycladacea(green algae). Algal structures are commonly filled with clear equant spar,whereas the w all structures are micritic, or completely absent. In the latter case,they remain unfilled by cement or sediment.Fragments of bivalves occur in most samples, and range in size from finesand to coarse sand. These fragments are typically very poorly preserved andshow signs of abrasion by transport and by biological processes: most grainshave irregular, notched and serrated edges, and aremicritizedand bored. Theoriginalshell-wallstructure is preserved as ghostly micritic bands in otherwiseclear crystalline calcite. Many of the bivalve fragments have a two-layerstructure: a thin layer on the convex, outer side of the slightly curved fragmentsconsists of clear prismatic spar, whereas the thicker inside layer consists ofcoarse spar in which an original crossed-lamellargrowth structure is visible.Gastropod remains are present in two main forms: small, thin-shelled,low-spired forms and large, thick-shelled, ornamented, high-spired forms.Debris from the thin-shelled forms occurs as recrystallized shells and shellfragments and as mud- or spar-filled molds, all of which are from broken-updebris, rather than from whole shells. The large, thick-shellednerineidgastropodshells also are not preserved, but replaced with blocky spar cement. The originalshell wall was dissolved, and fringing anddrusy-mosaicspar is in its place. Som esamples lack this secondary cem ent, leaving only empty molds in which lithifiedmud that had filled the shell at time of deposition remains as a loose cast. OnMIT Guyot such loose casts from the insides of nerineid gastropods were veryabundant. The chambers of the gastropod shells are generally filled with pelletalmud identical to that surrounding the shells. Most shell walls are micritized,leaving a thin micritic rind between the matrix and the mold-filling spar cement.This indicates that the large nerineid shells were abandoned, the outer shell wallsmicritized, and the shell cavities filled with mud. The m ud became lithified, anddissolution removed the original aragonitic shell wall, leaving a mold thatbecame, in some places, filled with clear fringing and equant spar cement.Lithification of the surrounding and interior mud preceded dissolution, or thespace left by the dissolved shell-wall would have been filled with mud.Many sam ples contain thin spicules up to 2 mm in length. These generallyare recrystallized to clear spar cement (Fig. A6). The spicules are straight,tapered in longitudinal section and circular in transverse section. Some of thethicker spicules appear to have micritic cores, indicating they were originallyhollow. These are likely sponge spicules.Round and oval fecal pellets,0.25-0.5 mm in diameter, are common inuncompacted burrows. They consist of micrite and small particles, identical inappearance to the surrounding m atrix. Where occurring in burrows, the pelletsare cemented by clear crystalline spar, indicating that burrowing occurred infairlywell-lithified sediment, and that burrows did not refill with mud.

0 5 mmFigure A5 . Photomicrograph of sparse fossiliferous mudstone (Facies 2). Tanmicrite matrix contains agglutinated benthic foraminifers and sponge spicules.The larger, cone shaped foraminifer is Cuneolina sp., a miliolid. SampleRNDB-D56-29-R1.

0 5 mmFigure A6. Photomicrograph of fossiliferous m udstone (Facies 2). Homogene-ous tan matrix with abundant, calcareous spicules. Sample RNDB-D56-28.

Small equant crystalline fragments of miscellaneous unidentifiable debrisare common in most samples of this facies. These m ay represent bits of bioclaststoo small to be identified. No ooids, intraclasts, or lithoclasts were recognized.Texture

The texture of this facies varies between sparse fossiliferous micrite( l % - 1 0 % particles) and sparse biomicrite (more than 10% particles, butmud-supp orted). The original primary porosity was probably hig h, in the formofintercrystalline microporosity of the unlithified m ud, but has been reducedupon lithification and com paction. Such m ud-porosity can be more than 35%(Flgel,1982). Other primary porosity included the intragranular spaces, suchas foraminiferal spaces and the perforations of algal fragments. Rare articu-lated ostracodes remain unfilled. The current porosity is 15%-40% (deter-mined by visual estimates of hand specimens as well as thin sections) andlargely moldic. Some molds appear to have been slightly enlarged by dissolu-tion, whereas others are partially to completely occluded with spar cem ent.

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Sorting, bedding, or lamination is not evident in samples cut for thinsections. Patchily distributed dark-granular mud may indicate some originalbedding that was disturbed by bioturbation. Incomplete m ixing by bioturbationalso resulted in patchy distribution of the various types of bioclasts. Hence,bioclasts of any particular type now tend to occur concentrated in 1 -3mm areasof a thin section. Sample RNDB10-D56-28 (MIT Guyot) contains curiouselongate, subparallel or radially oriented filamentous strings of calcite thatresemble calcified rootlets.Many of the samples also contain fractures that appear to cross both typesof matrix and cement-filled molds, indicating the fractures formed at a latestage in the diagenetic history. Most of the fractures are filled w ith clear, equantspar, although some are wholly or partially filled with mud and small mudclasts (Fig. A7). Parts of molds and bioclasts appear to b e slightly offset acrossthe fractures, indicating these are extensional cracks, rather than openingscaused by solution, although they may have been enlarged by solution.

Diagenetic HistoryThe diagenetic events and processes that have affected the rocks of thisfacies appear to include the following:1.Lithification of bioturbated mud occurred at an early stage, with verylittle compaction. Tubular structures and patchy matrix inhomogeneities re-main unflattened, leaving a firm, micritic texture.2. Dissolution of bioclasts occurred after lithification, because all molds

appear to beundeformed,and none appear to have collapsed. This process mayhave coincided with nondestructive recrystallization of some original shellmaterials, including originally calcitic parts of bivalve fragments and the thinshell walls of some gastropods, ostracodes, and of sponge-spicules, some ofwhich may have been originally silicious.3. Some of the moldic porosity was subsequently filled with clear sparcement, which grew inward from fringing bladed spar into adrusy mosaiccement texture. Other open pores such asintragranular spaces (foraminiferalchamb ers, pores in fragments of algae)filledwith fine clear equant spar cem ent.The mudstone matrix is rather porous, and only some of the larger open poreswere occluded with cement. Finally, some of the samples display fractures thatwere subsequently filled with granular mud and clear equant spar cement.Interpretations

The predominance of mud-supported textures indicates a low-energy envi-ronment of deposition, such as a platform-lagoon or bay. The assemblage ofbiota is typical of that of Cretaceous restricted shallow-water marine environ-ment (Enos, 1983). Somewhat bioturbated mudstones and wackestones with alow species diversity indicate a subtidal setting in a lagoon protected from wavesand currents. Some evidence of episodic changes in depositional environmentoccurs in these rocks, but the textural effects of these changes have becomeobscured by bioturbation: laminae of organic-rich, granular mudstone h ave beenmixed into the more comm on lime-mud, w hereas apparent clustering of particlesinto patches dominated by single fossil types may likewise indicate episodes ofrather restricted conditions, during which species diversity was very low. Theenvironment seems to have varied between rather restricted and more open-marine conditions, the latter indicated by increased bioturbation, occurrence ofnormal-marine indicator organisms (green algae, sponges [Flgel, 1982]),anddebris transported from the shelf margin, including abraded mollusk-shellparticles and fragments of coral. Water depth was probably no more then a fewmeters: deep enough to be subtidal.

When comparing samples from Allison and MIT guyots, those fromAllison appear to be consistently richer in green algae, ostracodes, and miliolidforaminifers, whereas those from MIT Guyot contain more biserial foramin-ifers and spon ge spicules. These differences co uld be attributable to differencesin paleolatitude of the two sites(Carrananteetal., 1988). MIT Guyot is to thenorth of Allison, and appears to have a slightly more temperate fossilassemblage. The differences could also be due to slight difference in depthof deposition, where the assemblage from A llison Guyot represents the shal-lower environment.Facies 3 (Packstone and Grainstone of Peloids,Ooids, and Coated Grains)

This facies is characterized by the predominance of peloids and coatedgrains in grainstone or packstone. In addition, a variety of bioclasts occurs,

including mollusk and echinoid debris, red and green algae, and benthic fora-minifers. Grains of reworked, previously lithified limestone are common also.Matrix

Most samples from this facies contain a muddy, micritic matrix that iscommonly partially to completely phosphatized (Heezen et al., 1973). Thismatrix is heterogeneous: patches of dense, dark, relatively mold-free, matrixoccur in more common light, clotted m atrix w ith interstitial microspar cement(Fig. A8). The denser matrix tends to contain smaller particles. The samplefrom Allison Guyot (RNDBD71-14;Fig. A9) has interstitial mud concentratedin the finer-grained horizons.

ParticlesThe abundance and types of bioclasts among the coated grains and peloidsappear to cover a range from near-reef (many mollusk shells, fragments of redalgae, and coral) to distal-lagoon (rare bioclasts of foraminifers and fragmentsof green algae). The most common particle types in this facies are coated grainsand micritic peloids and pellets. Coated grains include ooids, superficial ooids,and aggregate grains. The ooids are partially micritized which has obscuredtheir original concentric structure. Superficial ooids are fine-sand-sized claststhat consist of one or more concentric micritic laminations around molluskfragments or foraminiferal tests. The cores of most superficial ooids have beendissolved, leaving mo lds, or recrystallized to fine-crystalline equant spar (Fig.A8).In some grains, concentric layers of micrite coat more than one particle,forming aggregate grains.Peloids and pellets are very abundant in this facies. Peloids are micritic,rounded grains that can have a variety of origins. They range in size from0.1mm to 2.0 mm. Extensive micritization of algal fragments, foraminifers, andooids suggests that many peloids may have originated in this way. Others maybe small redeposited clasts of mudstone (Fig. A8). Pellets are round and ovalmicritic particles that have a narrow grain-size range of 0.01 to 0.1 m m. Theyconsist of dark homogeneous micrite and lack any suggestions of preexistingmicrostructures. Their consistent shape and size suggest they are fecal pellets,produced by small detritus-feeders, such as many types of crustaceans, soft-bodied invertebrates, and fish (Flgel, 1982).Bioclasts include fragments of bivalves, gastropods, red and green algae,echinoderm fragments, foraminifers, and coral. The bioclasts are angular andsubangular, except where rounded by thick coats of micrite. They range in sizefrom 0.01 mm to 0.5 mm. M ost bioclasts, especially the bivalve fragments, arevery poorly preserved. M ost are extensively micritized and coated (Fig. A10).Much of the original bioclast material is dissolved, leaving molds. Somemollusk fragments retain part of their original wall-structure.Mollusks include fragments ofInoceramus, rudists (probably radiolitids)and gastropods. Fragments of algae are common, including green and redalgae. Fragments of solenoporan red algae were observed at Jacqueline Guy ot(Sample A5-D9-7). Rare fragments of echinoderms occur in all samples andare generally coated with micrite and extensively bored and m icritized.Benthic foraminifers, mostly unidentifiable agglutinated uniserial, biserial,and planispiral forms, occur in most samples. In the partially phosphatizedsamples with heterogeneous matrices, the foraminifers occur preferentially inthe denser, finer-grained parts of the matrix. Chambers are filled with finemicrospar cement. A specimen of Favusella washitensis, a planktonic fora-minifer that ranges in age from lower Albian to lower Cenomanian (Sartorioand Venturini, 1988) was found on Cape Johnson Guyot (Sample A5-D5-1).Other bioclasts include rare fragments of bryozoans and co ral.Samples from Charlie Johnson and Cape Johnson guyots contain abundantfragments ofprelithifiedlimestone. These rock fragments consist of mudstoneand pelletal packstone and grainstone. The mudstone fragments can be distin-

guished from peloids by their more angular appearance and by the abrupttermination of small veinlets and grains at the boundary of the fragments. Thepelletal packstone and grainstone fragments are similar to the rest of thematerial in which they were reworked. They distinguish themselves by beingfar more diagenetically altered than the surrounding m aterial. Most of the clastshave micritized edges. Some have spar-filled veins that cut through the entiregrain and terminate sharply atthe grain boundary. The rock fragments rangein size from 0.5 m m to several mm .Texture

All samples in this facies have a grain-supported texture, with varyingamounts of micrite between the grains. Samples from Charlie Johnson andCape Johnson guyots are the muddiest. Most of the matrix in those samples

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0 5 mmFigure A7. Photomicrograph of sparse fossiliferous mudstone (Facies 2).Small, irregular fractures in the mudstone have been filled with clear, equantspar cement (calcite). The small fossil in the center is a gastropod fragment.The straight, light-colore d line near the right edge of the photograph is a scratchon the thin-section. Sample RNDB-D56-29-R1.

0 5 mmFigure A9. Photomicrograph of peloidal packstone (Facies 3). This sampleexhibits fine-scale fining-upward laminae of pellets, coated grains and ooids.Transitions from fine to coarse are sharp, whereas transitions from coarse tofine are gradual. Sample RNDB-D71-14.

0 5 mmFigure A8. Photomicrograph of peloidal packstone (Facies 3). The grainsconsist of micritic pellets, peloids and partially micritized and coated fossilfragments. The bottom half of the photograph is dominated by dense, darkmatrix, whereas the upper half consists of a lighter, clotted matrix with abroader range of particle-size, interstitial microspar cement. SampleA5-D5-1.appears to have been affected by phosphatization. Little evidence of compac-tion can be seen, except in some of the rock fragments: grains are touchingonly tangentially and at extremities. No evidence of sorting or grading occurs,except on Allison Guyot (Sample RNDB D71-14; Fig. A9). This sampleconsists of 0.2- to 0.5-mm-thick graded laminae of pellets and coated grains.Although the orientation of the sample is not known, the gradual transitionfrom coarse tofine,and the sharp boundary between fine and coarse intervalssuggest these are fining-upward laminae. The textural inhomogeneity of themuddy samples may have been caused by bioturbation.

Porosity in most sam ples is high, mostly in the form of molds of bioclasts.Little primary porosity remains: the lightly compacted, rather granular micritehas been completely cemented with microspar. On Jacqueline Guyot (SampleA5-D9-7), grains are cemented by a very fine microspar cement, which maybe neomorphic after micrite. Because many grains are surrounded by fineradial-fibrous cement, this micrite would have been a later mud that infiltratedthe already partially cemented sediment.

0 5 mmFigure A10. Photomicrograph of peloidal packstone (Facies 3). The largemollusk fragment has been extensively micritized, as indicated by the micritictendrils that intrude into the particle. Subsequent co ating of the fragment withlime mud has trapped sm all particles and formed a thick layer of micrite aroundthe grain. Sample RNDB-D51-33.

DiagenesisThe postdepositional history of these samples varied, but began with lithi-fication of micrite matrix and the formation of microspar cement in the less-dense micrite as well as in intraparticular pores such as the chambers offoraminifers and the space inside the structures of algal fragments. This wasfollowed by dissolution of bioclasts, especially of mollusk fragments, and thepartial occlusion of some molds by bladed and clear coarse spar cement. Laterdiagenetic events include the partial phosphatization of some of the matrix,which does not appear to have affected particles; replacement of limestone bydendritic mang anese o xide; and the partial filling of open pores and mo lds withacryptocrystallinebrown material, probably phosphorite and manganese oxide.Stylolites were observed in rock fragments at Charlie Johnson Guyot(Sample RNDB-D51-18; Fig.Al1). The stylolites form low-amplitude, angu-lar peaks in muddy parts of the fragments, and appear to terminate smallveinlets, indicating their formation after formation of veins. Cementation of

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0 5 mmFigure A l l . Photomicrograph of peloidal packstone (Facies 3). Part of awackestone rock fragment has a distinct stylolitic structure (bright jagged line)against which small, spar-filled fissures terminate. The edges of the grain arebeyond the boundary of the figure. Sample R NDB-D 51-18.

0 5 mmFigure A12. Photomicrograph of sponge-algal bafflestone (Facies 4). Notesheltered part of matrix between large sponge-algal structures. Micritic in-traclasts are loosely bound with micrite. Bright areas are void space. SampleRNDB-D71-10.

pore space, including molds, in the rock fragments is generally complete,although some molds of m ollusk shells are not completely occluded, and arelined with coarse, bladed, spar cement.Interpretations

The coated grains indicate the original environment of deposition was inmoderately agitated shallow water. Extensive micritization and abrasion ofmost grains indicate they were moved around in well-lighted water prior tofinal deposition. Grains of reworked, previously lithified limestone may indi-cate the presence nearby of exposed parts of the platform where this sedimentwas exposed by erosion. The presence of ooids, superficial ooids, and aggre-gate grains likewise suggests a periodically well-agitated environment, suchas a marginal bank-top or shelf. Bioclasts include particles derived fromnear-reef environments, such as radiolitid rudists and other thick-shelledbivalves, and red algae. Others, such as agglutinated benthic foraminifers andgreen algae are more commonly associated with a lagoonal environment.Samples of this facies from the different seamounts appear to representslightly different parts of the bank-top environment: samples from CharlieJohnson Guyot contain the most reef-derived material (mollusk deb ris, echi-noids,and even coral), whereas those from Allison Guyot contain the least reefmaterial and the most elements associated with a lagoonal environment(micritized green algae and foraminifers). In between these two extremes arethe sample from Cape Johnson Guyot (which has fairly abundant molluskfragments, but also foraminifers and green algae) and the sample fromJacqueline Guyot (in which foraminifers are more abundant then molluskfragments). The Facies 3 environment was less sheltered than that of themudstones of Facies 2, but was inside the platform margin, as indicated by themixed occurrence of lagoonal and platform-margin derived bioclasts. Thisenvironment corresponds m ost closely to Facies Belt 7 of Wilson (1975):anopen platform lagoon, interior of the margin, but well agitated and with opencirculation to the ocean.The occurrence of diagenetically mature limestone rock fragments indi-cates the presence of elevated (exposed) terrain on Charlie Johnson and CapeJohnson guyots at the time of deposition of these sedim ents. The presence ofstylolites in these fragments indicates that they were eroded from previouslyburied limestone. The depth of burial required formicrostylolitesto form maybe only a few tens of meters (Schlanger, 1964), but usually requires a fewhundred meters of overburden to develop. The microstylolites in these samp lesappear to be completely nontexture controlled, indicating that the sedimentswere well lithified and diagenetically mature when the stylolites formed. Thepresence of exposed, previously buried limestone may support the hypothesisthat the guyots were uplifted and their limestone summits exposed as much as180 m some time in late Albian-early Cenomanian time. Such uplift andexposure is suggested on the basis of geophysical and morphological evidence(van Waasbergen and Winterer, 1993; Winterer etal., 1993).

Minor FaciesThe following rock types were identified only in minor amounts and insingle dredges. Most of them were found on Allison G uyot, which, with sixsuccessful dredge-attempts (Fig.11),is one ofthemost widely-sampled of theCretaceous Pacific guyots.

Facies 4 (Bafflestone of Large Sponge and Algae Structureswith a Peloidal and Bioclastic Muddy Matrix)This facies is characterized by packstone and wackestone of fine sand- tosilt-sized micritic particles and bioclasts of foraminifers, coral, mollusks, andechinod erms, as well as wackestone rock fragments in a porous matrix in whichlarge structures of encrusting algae and massive calcareous sponges occur.

MatrixThe matrix consists of chalky, loosely packed micrite light gray in color,which appears to form small, rounded clots about 0.02 mm in size. Some of thematrix wasrecrystallizedto veryfinespar. Patches of micrite are fairly homo-geneo us, but areas between the large algal and sponge structures differ from oneanother in density and particle abundance, which suggests the matrix may be asecondary infilling of the space between the sponge and algal structures.In sheltered parts of the matrix, and in partially filled molds of bioclasts,the matrix consists of a very loose, open structure offine-sand-sized subangu-lar micritic clasts that are loosely held by micritic cement that often formsbridging structures connecting individual grains across a void space (Fig. A12).This fabric closely resembles meniscus cement fabrics commonly associatedwith precipitation in the vadose zone (Flgel, 1982).

ParticlesParticles include common bioclasts of foraminifers, mollusks, red algaeand echinoids, and abundant micritic particles, including peloids, intraclastsand wackestone rock fragments. Among the foraminifers, the most commonforms are micritic agglutinated biserial forms (textularids), miliolids, andfragments of Orbitolina. In addition, many samples contain specimens ofthick-shelled hy aline forms (pos sibly rotalids) and rare plankton ic foraminifersincluding Planomalina buxtorfi(I. Premoli Silva, pers. comm., 1990), whichhas a narrow biostratigraphic range within the late Albian (Sarterio andVenturini, 1988). Chambers are commonly filled with fine equant spar cement,or can be unfilled.Mollusk fragments include fragments of Inoceramus, gastropods, andradiolitid rudists that can be up to several mm in size. Mollusk fragmentscommonly have micritized outer shell walls, whereas the rest of the shell

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SEDIMENT FACIES OF CRETACEOUS PACIFICmaterial may be entirely dissolved, recrystallized, or partially preserved.Where dissolved shell material has left large molds, the pore space is com-monly filled by loosely bound micritic particles and small bioclasts in whatappears to be a vadose meniscus cement fabric.Fragments of red algae are rare to common as angular, elongate bioclastswith a fine-mesh microstructure. The algal fragments are partially micritized,but some of their fine microstructure remains visible.Rare echinoid fragments form poorly preserved, partially micritized andabraded fragments, silt- to fine-sand-sized, that can be identified in thinsections by their single-crystal extinction. The co res of many of these particleswere recrystallized to an equantpolycrystalline texture.Micritic particles in the wackestone matrix include fine-sand- to sand-sizedpeloids, intraclasts, and wackestone rock fragments (Fig. A12). The roundedpeloids are difficult to distinguish within the identical surrounding m ud, exceptin the porous parts of the matrix. Intraclasts are angular and subangular clastsof mudstone, with clearly distinguishable grain boundaries, but otherwiseidentical to the common wackestone matrix. Rock fragments include angularclasts in which small spar-filled molds can be observed to terminate againstgrain boundaries. Whereas the intraclasts may be derived by early in-situredeposition of slightly lithified mud, the rock fragments appear to be diage-netically more advanced, because spar-filled molds are virtually nonexistentin these sediments. Other micritic particles occur within the irregular sponge-structures (Fig.A13).These silt-sized, round, dark-micriticparticles may besmall fecal pellets.

Massive Sponge, Coral, and Algal StructuresBulbous and irregular-shaped, com monly hollowed structures up to 2 cmin size occur throughout these rocks. In hand samples, these are often observedas light gray, porous limestone patches and irregular cavities lined with muddycasts of corallites, indicating some of them are molds of coral knobs. Some ofthem appear to have small stems that give them the appearance of fragmentsof cauliflower. In thin section, the sponge structures (Fig. A13) consist ofirregular walls that enclose chambers and channels0.1to 1 mm in size and arecoated withfinefringing and equant spar cement. The space enclosed by thesewalls is commonly filled with wackestone matrix or pelletal grainstone,whereas the walls themselves are filled with dense micrite (Fig.A13)or remainvoid-space partially occluded with equant spar. The outer edges of the struc-tures are commonly coated by encrusting coralline algae. Individual algae canbe traced over several millimeters in a single thin section. The hollow aspectof these structures can most likely be attributed to dissolution of the frame-work-material, w hich has left large cavities, which in some places are linedwith imprints of corallites.

TextureThe bulk of these sediments consist of porous brown-gray w ackestone withsand-sized micritic particles and abraded bioclasts. Individual patches ofwackestone are fairly homog eneous and show no evidence of sorting, gradingor lamination. Within these patches are uncemented cavities, mostly molds oflarger bioclasts, in which a very loose meshwork of micritic grains heldtogether by small amounts of m icritic cement occurs. The p atches of wacke-stone occur between large, irregular shaped sponge, coral and algal frameworkbodies, which acted as stabilizing structures. No evidence of compactionwas recognized.Porosity throughout is very high. Primary porosity includesinterparticularpore space in the loose wackestone, which is only partially occluded withcrystalline cement. Primary growth-framework pore space in coral/sponge/algal structures is largely filled with mud. Secondary porosity includes largeand small molds of bioclasts, generally without any crystalline cement. Som emolds may have been enhanced by solution. The largest cavities are thosewhere coral and sponge structures have been dissolved away. Within thepreserved sponge-structures, original wall-material has commonly been re-moved, leaving open, spar-lined channel-like pores among the infilling pel-sparite and wackestone matrix.

DiagenesisSyndepositional alteration includes boring and micritization of bioclasts.Most fine-sand and smaller sized particles have been thoroughly micritized.Early postdepositional diagenesis includes the infilling of coralline and spongestructures with mud and pellets. Following lithification of the sediment,including the precipitation of fine-crystalline equant spar in the loose pelletalsilt parts of the matrix, unstable minerals dissolved, leaving largely unfilled

0.5 mmFigure A13. Photomicrograph of sponge-algal bafflestone (Facies 4). Thesponge structure has cav ities that are partially filled with mud. The wa lls of theoriginal sponge body are micritized and lined with fine, fibrous spar cement(calcite). Sample RNDB-D71-13A.open m olds. This may hav e occurred at the same time as recrystallization oftheechinoderm fragments.Very fine-crystalline, marine, fringing-spar cement lines many of the openmolds. These became subsequently partially filled with micritic particlescemented by what appears to be meniscus m icrite cement. This suggests thatthese rocks have undergone a brief episode of vadose diagenesis, likely veryearly in the diagenetic history, after initial lithification, and perhaps contem-poraneous w ith the dissolution of unstable mineral grains.

InterpretationsThe centimeter-size coral, sponge, and algal structures may be part of asmall bioherm. Trapped b etween these structures are large radiolitid rudists,in peloidal mud containing abundant lagoonal organisms such as micriticbenthic foraminifers and algae. Bioclasts associated with platform margin andfore-slope deposits (mollusks and echinoids) occur rarely and are generallyabraded and micritized, indicating the source for such particles may berelatively far away. These elements suggest deposition occurred on smallmounds inside the platform margin, which may have become emergent asindicated by the apparently vadose secondary diagenetic textures. Other indi-cators of emergent terrain include locally derived lithoclasts (pieces of barelyfirm mud at time of deposition) and diagenetically advanced w ackestone rockfragments. The relatively common occurrence of planktonic foraminifers inthe matrix indicates an open circulation with the ocean.

Facies 5 (SilicifiedOolites)Samples that belong to this facies w ere dredged from the edge of a smallplateau about 5 km in diameter that stands about 100 m above the main summitplateau on V ibelius Guyot. They consist of poorly preserved, wholly siliceousoolite grainstone. Sample RNDB D59-1 is a solid, nonporous siliceous rock,and in sample RNDB D59-2 all the ooids have been removed, leaving

oomoldic porosity in a fine-grained siliceous matrix.Grains

Ooids and oomolds up to 2 mm in diameter show (in Sample RNDB D59-1)relict cortical structures, indicating these were tangential-laminated rather thanradial ooids (Fig. A14). No nuclei remain in either sample. The concentricstructures occur as brown, discontinuous laminae of micrite. Between the relictcarbonate laminae, the ooids consist of chert or microcrystalline quartz thatforms small subrounded optically continuous patches and clusters up to 0.1mm in diameter. The chert patches are observed to enclose the carbonatecrystals as poikilitic cement. In some ooids, clotted micrite occurs betweenquartz patches (Fig. A14). Near the outer boundary of the ooids, there typically

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0 5 m mFigureA14. Photomicrograph ofsilicifiedooid grainstone (Facies 5). Originalconcentric layering in the ooids is preserved as traces ofmicrocrystallinecalcite.The rest is completely replaced with fine-grained silica. Euhedral quartz formssmall equant crystals within the replaced ooids. Sample RNDB-D59-1.occurs a10-m-widelamina of small equant qu artz crystals that lies just insid ea 20-m-thick lamina of fine micritic calcite that defines the outer edge of theoriginal ooid.

MatrixThe ooids are cemented by isopachous calcite cement, which grades intochert only a few tens of microns into the material between the grains. The chertcement replaces the original two-generation calcite ce

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