16. carbonate diagenesis at site 308 koko … · 16. carbonate diagenesis at site 308 koko guyot...

16. CARBONATE DIAGENESIS AT SITE 308 KOKO GUYOT

Albert Matter, Universitat Bern, Bern, Switzerlandand

James V. Gardner, Scripps Institution of Oceanography, La Jolla, California

INTRODUCTIONKöko Guyot is a large seamount located at the

southern end of the Emperor Seamount chain (35°30'N,171°45'E, Figure 1). Leg 32 of the Deep Sea DrillingProject crossed the southern portion of the guyot fromthe southwest, drilled Sites 308 and 309, and proceedednortheastward to complete the crossing.

A detailed study of the morphology and geology ofKöko Guyot is presented by Davies et al. (1972). Thefollowing description is a summary of their findings.The guyot rises to within 270 meters of the sea surfacewith a relatively flat upper surface bounded by steepslopes which descend to depths of 800 to 1200 meters.Two dredge hauls are reported by Davies et al. (1972),one from the western flank of the guyot which returnedbasalt and volcanic breccia (discussed by Clague andDalrymple, 1973) and the other from the southern endof the plateau which collected reef limestones andcobbles of a variety of igneous rock types (Clague andGreenslate, 1972). Davies et al. (1972) calculated thethickness of the limestone cap to be 600 meters, com-parable to that of the other Pacific atolls. Taking intoaccount the present depth of the limestone cap, the op-timum depth of reef growth of 30 meters, and the as-sumed 30 m.y. age of the seamount (taken from Jacksonet al., 1972), Davies et al. (1972) concluded that reefgrowth terminated 5 to 6 m.y. ago. Leg 32 drillingresults indicate that the age of Köko Guyot is at least 50to 51 m.y. old, early Eocene {Discoaster lodoensis Zone).

SEDIMENTS

General Sediment DescriptionSite 308 is located on the southeastern edge of the

plateau, downslope from the coral cap. Only four coreswere recovered from Site 308 before drilling was ter-minated. The cores show a color change, from an oxidiz-ed dark yellowish-brown color at the top (Core 1) tooxygen-depleted dark gray color at the bottom (Cores 2,3, and 4). The X-ray results suggest this change may bedue to increasing pyrite contents with depth. Thephysical character of the cores correlates with the degreeof cementation. Cores 1 and Section 1 of Core 2 are stiffuncemented sediment; Section 2 of Core 2, Core 3, andupper part of Core 4 semilithified; and the lower part ofCore 4 is a well-cemented sandstone. An increase ingrain size with depth is quite noticeable. A sharp contactoccurs at 78 cm in Core 4 between the upper siltstonesection and the lower sandstone (Figure 2). The uppersilty sediments are altered volcanic ash composed ofpalagonite, glass shards, with rare bryozoa (Cheetham,

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170° 170° 160° 150°W

Figure 1. Location ofKoko Guyot.

this volume); ostracodes, benthonic foraminifera (Luter-bacher, this volume); and some weathered carbonatefragments that resemble decomposed mollusk shells.The carbonate content is only 2% to 3%. Coarse sand-sized well-rounded volcanic rock fragments, benthonicforaminifera, bryozoa, ostracodes, coralline algae, andcalcite cement all increase with depth until in Core 4 thesediment becomes a well-cemented biogenous volcanicsandstone. The only apparent internal structure is in thecoarse grains which show a rough horizontal alignmentand occasionally imbrication.

A biostratigraphic unconformity, based on nanno-fossils, occurs in Core 1 at approximately 70 cm depthseparating Quaternary from lower Eocene sediments(see Biostratigraphic Summary, Site 308, this volume).However, this unconformity is not apparent from thelithology or color of the sediments.

Microfacies and PetrographyThe early Eocene section drilled on Köko Guyot

shows three different microfacies (Figure 3):1) A dark yellowish-brown, soft clayey silt was

recovered in the top core. This silt consists mainly of

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A. MATTER, J. V. GARDNER

Figure 2. Site 308, Core 4, Section 1, sharp lithologic breakat 78 cm separating the lower sandstone from the uppersiltstone.

palagonite, light volcanic glass, montmorillonite, partlyaltered potassium feldspar and Plagioclase, and lesseramounts of clinoptilolite, augite, and pyrite. Charac-teristically, this volcanic clayey silt is very low in car-bonate (2% to 3%). Rare nannofossils, benthonic foram-inifera, and bryozoa are seen in thin section and smearslides.

Clinoptilolite occurs as 2-25 micron long single andtwinned crystals which are moderately corroded. Theaugite crystals (30-80 microns in size) are fresh but showcockscomb-like terminations indicating considerable in-trastratal dissolution of this relatively unstable heavymineral.

308

T.D.68.5 m

Lithology

CLAYEY SILT (altered volcanic ash) consistingof montmorillonite volcanic glass, feldsparsand rare benthic foraminifera, bryozoa andnannofossils.

BIOGENOUS VOLCANIC SILT with layers of finesand. Volcanic rock particles and sameshallow water fossils as in lower unit areembedded in pseudosparitic groundmass.

BIOGENOUS VOLCANIC SANDSTONE containingabundant well-rounded altered volcanic rockparticles, ooids fragments of shallow waterfossils (benthic foraminifera, codiacean andcoralline algae, bryozoa, corals etc.)cemented by fibrous and bladed calcite.

β Fossil iferousoOoids• Silt• Sand

Figure 3. Sequence of major lithologies drilled at Site 308,Köko Guyot.

This sediment probably represents an altered volcanicash (see also lithologic description, Site 308 chapter).

2) A biogenous volcanic silt of medium dark graycolor occurs in Cores 2 and 3 (Figure 3). Texturally, thesediment is mainly a clayey silt with occasional faintlaminations, bioturbate structures, and rare interbeddedlayers of fine sand. The consolidation increases fromstiff in Core 2 to semilithified in Core 3 due to pro-gressive cementation and recrystallization of the carbon-ate particles and the calcareous groundmass. The car-bonate content of bulk samples ranges from 43% to 63%.

This microfacies is characterized by abundant angularto rounded, sand- to silt-sized, highly altered volcanicrock and glass particles which are embedded togetherwith skeletal fragments in a pseudosparitic groundmass(Figure 4). The major skeletal elements are, in order ofdecreasing abundance: benthonic foraminifera (per-forate, porcelaneous, and arenaceous), bryozoa, and os-tracodes. Planktonic foraminifera, echinoderm remains,and fragments of pelecypods and coralline algae arerare.

A few ovoid to slightly irregularly shaped ooids wereobserved ranging in size from 20 to 30 microns. Theyshow a brown ferruginous stain and pyrite inclusionsconcentrated in the outermost part of the cortex. Theseooids are micritic, have a faint concentric structure, andshow pronounced thin radial fibers. The core of theooids is either a volcanic rock fragment or a bioclast.The cortex measures between 0.5 and 0.6 of the ooidradius.

Pyrite and ilmenite are conspicuous as minor con-stituents (see Zemmels and Cook, this volume). Fram-boidal pyrite occurs mainly in chambers of foraminiferaand other fossils. Where pyrite is found in the ground-mass and in grains of volcanic rocks, it occurs domi-nantly as tiny single cubes and aggregates of cubes.

3) Biogenous volcanic sandstone. This microfacies isrestricted to the lowest section sampled (Core 4) at Site308 (Figure 3). It consists mainly of well-rounded, sand-sized volcanic rock particles and different types ofskeletal fragments that are cemented by fibrous and

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CARBONATE DIAGENESIS AT KOKO GUYOT

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500µm

Figure 4. Biogenous volcanic silt consisting of angularto rounded epiclastic volcanics, palagonite, and bio-clasts embedded in a pseudosparitic groundmass.Thin section Sample 308-2, CC.

sparry calcite (Figures 5 and 6). Although the abun-dance of the different constituents varies between thinsections taken at different levels, all the studied samplesfrom Core 4 belong to a single characteristic micro-facies.

The volcanic rock particles are flat, oval-shaped andoccasionally imbricated (Figure 5). Commonly theyhave a superficial oolitic coating, 10 to 40 microns thick,or a thin (10 microns) crust of authigenic montmoril-lonite (Plate 3, Figures 3 and 6). The oolitic coating isgenerally absent on the rounded corners of the grains,indicating abrasion of the coating during movement ofthe grains.

The most abundant skeletal particles are benthonicforaminifera (perforate, porcelaneous, and arena-ceous), bryozoa, and plates of Halimeda, a calcareousgreen alga. Ostracodes, lamellibranchs, gastropods,echinoderms, solitary corals, serpulidae, coralline algae,and some dasycladacean algae all occur but vary fromcommon to rare in abundance. Both disarticulatedbranches and nodules (up to 1 cm in size) of corallinealgae were observed. The serpulids were attached tohard substrates such as pelecypod shells, plates of cal-careous green algae and bryozoan colonies (Figure 6).Miliolids are the most common porcelaneous foram-inifera.

Ooid abundance ranges from 10% to 20% in somesamples. Ooids occur as large single grains 0.15 to 0.25

500µm

Figure 5. Biogenous volcanic sandstone as in Figure 6, but with dominant epiclastic volcanics, common ooids, grape-stones, and other intraclasts and few bioclasts. Many of the volcanic particles have a superficial oolitic coating.Notice their excellent roundness, flat shape and imbrication. Thin section Sample 308-4-1,126 cm.

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500µm

Figure 6. Well-washed and moderately sorted biogenous volcanic sandstone consisting mainlyof well-rounded epiclastic volcanics and bioclasts cemented by low-Mg sparry calcite. Hali-meda plates, benthonic foraminifera, and bryozoa are the dominant skeletal remains. Noticeserpulids growing on and encrusting bioclasts. Thin section Sample 308-4-1, 68 cm.

mm in diameter as well as in grapestone lumps wherethey are associated with volcanic rock fragments andbioclasts. Intraclasts other than grapestones are alsopresent in small amounts. The nuclei of the ooids areeither altered volcanic rock particles or bioclasts, mostfrequently coralline algal fragments and tests of foram-inifera. The radius of the ooid cortex generally equalsthat of the nucleus (Figure 5).

Ooids seen in thin section display alternate clear andyellow-brownish micritic layers. The clear layers consistof radially oriented neomorphous calcite crystals,whereas the darker layers are made up of minute grains("nano-grains" of Loreau and Purser, 1973) whenstudied in the electron microscope (Plate 1, Figure 4).An identical relationship between micro- and ultra-structure of ooids has been described by Loreau andPurser (1973).

Pyrite is present in clusters of framboids in chambersof fossils, but it also occurs as tiny crystals in micritizedtests.

The sediments within this microfacies are well sortedand only rarely contain patches of micritic groundmass.However, interbedded with microfacies 3, some layers ofmicrofacies 2 were observed.

The carbonate content of the bulk sediment variesfrom 60% to 70%. This rock type, as well as the previousone, would be classified as a limestone on a chemicalbasis. However, taking into account the cement whichcontributes about 10% to 30% of the carbonate, thesediments described above are best classified as

biogenous volcanic sandstone and biogenous volcanicsilt.

Carbonate MineralogyThe mineralogical composition of the calcareous skel-

etons, matrix, and cement was investigated with the aidof X-ray diffraction and electron microprobe analysis,combined with conventional thin-section examination.

Because of the different primary mineralogy of therock-forming skeletal-carbonate constituents, thesediments at Site 308 originally consisted of a mixture ofstrontium aragonite, (corals, Halimeda, ooids); highmagnesian calcite (coralline algae, miliolids); and lowmagnesian calcite (planktonic foraminifera, nanno-fossils).

However, all the grains which originally consisted ofeither of the metastable phases (aragonite and magne-sian calcite) have been converted to stable low magne-sian calcite with less than 4 mole % MgCCh (bulkanalysis).

Thin sections were stained for aragonite and high-Mgcalcite but no relicts of these minerals were discovered.Strontium, although not measurable quantitatively, wasobserved in the micritic parts of ooids, suggesting thepresence of relicts of aragonite microcrystals.

DIAGENESISThe flood of scientific articles on shallow-water car-

bonates published during the past 20 years has greatlyincreased our understanding of how accumulations of

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metastable aragonitic and Mg-calcitic particles becomestabilized and lithified. Stabilization and lithificationtake place most rapidly when the sediments are exposedto the vadose and meteoric realm. However, during thelast few years increasing evidence for submarine dia-genesis and cementation has been cited (Bathurst, 1971;Milliman, 1974).

Because the alteration of carbonate grains in shallow-water sediments has been widely studied, we will onlybriefly describe the diagenetic features of some Mg-calcitic and aragonitic grains. Then we will describe ingreater detail the ultrastructure and fabrics of the car-bonate cement and discuss their origin.

Alteration of Carbonate GrainsThe alteration of carbonate grains at Site 308 is best

studied in the coarse-grained biogenous volcanic sand-stone which contains abundant grains composedoriginally of metastable aragonite or Mg-calcite. The be-havior of grains during diagenesis, and thus the ultimatestate of preservation of different kinds of skeletal andnonskeletal carbonate grains, depends largely on theirinitial mineralogy.

The most abundant strontium-rich aragonitic par-ticles are plates of the calcareous green alga Halimedaand ooids (Figures 5 and 6). The flakes of living Hali-meda consist of intertwined and branching filaments orutricles which are more or less calcified by fusedaragonite needles (Milliman, 1974). Infilling of the utri-cles by secondary aragonite needles (Glover and Pray,1971; Marszalek, 1971) or high-Mg calcite or with bothminerals (Winland, 1971) starts soon after burial,rendering greater strength to the algal plates. Milliman(1974) also reports disc-like secondary aragonite in theutricles.

In our Eocene material from Site 308, the calcifiedfilaments have been dissolved to various degrees and ina patchy manner. The aragonite of the remainingfilaments, their secondary carbonate infillings, as well asthe micrite envelope, were subsequently replaced bymicrocrystalline low-Mg calcite. The intraparticle voidspace (primary and secondary) is partly filled withfibrous and bladed calcite cement (Plate 1, Figure 1).Progressive replacement of micrite by calcite spar resultsin a finely crystalline blocky calcite fabric leaving notrace of the primary skeletal structure in thin section. Asshown by Schneidermann et al. (1972) the micritic walland the void-filling spar may become replaced byneomorphic spar (Plate 1, Figure 2) which may containrelict aragonite needles. In the advanced state of replace-ment, the algal origin of the particle is recognized onlyby the micrite envelope (Plate 1, Figure 2). Finally, themicrite envelope also may be replaced by coarser calcitecrystals, resulting in a continuous mosaic of intra-particle and interparticle calcite cement. These resultsare similar to the observations made by Tebbutt (1967)on Halimeda plates of Pleistocene age.

Ooids which initially consisted of strontium-rich ara-gonite (see Carbonate Mineralogy section above) dis-play yet another pattern of replacement by low-Mg cal-cite. As mentioned previously, the ooids cortices consistof alternating clear lamellae of radially oriented calcite

and yellowish-brown micritic laminae made up ofminute grains (Plate 1, Figure 3 and 4). Similar ooidswere described by Hesse (1973) from the Ita Matai Sea-mount. The width of the clear lamellae and thus the sizeof the subequant calcite crystals range between 1 to 10microns. Thin streaks of microcrystalline grains oftenseparate individual clear cyrstals within a clear lamina(Plate 1, Figures 3 and 4).

Some ooids consist mainly of thicker clear calcitelayers alternating with thin micritic layers, whereasothers, such as the ooid shown on Plate 1, Figure 5, havemainly a thick yellowish-brown micritic cortex with onlyfew clear layers. The micritic layers are composed oflow-Mg calcite and possibly some relict aragonite grainsand a cryptocrystalline clay mineral (possibly of themontmorillonite series) as well as diffuse iron-oxide(limonite?). The presence of the yellow clay mineral andthe brown-red iron oxide renders a yellow-brown colorto the micritic laminae.

The yellow-brown laminated zone of micrite seen inthe ooid cortex shown in Plate 1, Figure 5 is much lowerin calcium than the clear layers (Plate 1, Figure 6). Also,the micritic parts contain high concentrations of Si, Al,Mg, and Fe, all of which are present in much lower con-centrations or missing in the clear laminae. Optical in-vestigation of stained thin sections suggests that Si, Al,and also most of the Mg are located in clay minerals andFe in limonite(?). The chemical composition of thecalcite in the clear laminae is identical with that of theintergranular cement.

It appears that the layers of micrite correspond withthe organic-rich layers of nano grains described in recentooids by Loreau and Purser (1973). According to theseauthors, the layers of nano-grains alternate with clearlayers of either radially or tangentially oriented elon-gated aragonite crystals.

Although ooid diagenesis has attracted much scien-tific effort since Eardley's (1938) classical paper (seeBathurst, 1971 for review; Hesse, 1973; Kahle, 1974;Sandberg, in press), a brief account on the diageneticfeatures of ooids encountered at Site 308 is justified be-cause of their unusual content of clay minerals.

We observed in a few ooids primary clear layers whichwere partly dissolved and had a lining of first-generationmicron-sized calcite (?) crystals. This suggests that layersof primary clear aragonite (Loreau and Purser, 1973)were dissolved and infilled by radially oriented low-Mgcalcite crystals on a piecemeal basis, thereby avoidingcollapse of the cavity. This mechanism was first envis-aged by Shearman et al. (1970). These features havebeen documented with excellent SEM photomicro-graphs by Fabricius and Klingele (1970).

The clay minerals associated with minute carbonategrains not only occur in the concentric laminae ofmicrite, but also in the irregular, 1 to 5 micron wide "pil-lars" which separate the crystals of secondary calciteconstituting the clear layers mentioned above (Plate 1,Figures 3 and 4).

The mode and time of formation of the clay mineralsare not certain. They might either be of accretional ordiagenetic origin. In the first case they would have beentrapped during periods of ooid stability by the

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mucilaginous matter which coated the ooid surface. Thediagenetic hypothesis assumes that the clays formed inprimary or secondary pores generated by decay oforganic matter and by dissolution of minute aragonitenano-grains. It appears that the "pillars" originated bygrowth of authigenic clay minerals in algal borings. Byanalogy, a diagenetic origin also is favored for the clayminerals in lamellae of the micritic ooids. Furthermore,authigenic montmorillonite predating deposition of cal-cite cement commonly coats volcanic grains in thestudied samples. Therefore, the microenvironment wasapparently very favorable to the formation of this claymineral at times of very early diagenesis. The texturalrelationships indicate that the clay minerals and micro-crystalline carbonate grains predate growth of secon-dary calcite in the clear laminae.

The diagenetic alteration, and thus the preservation,is different for the most common Mg-calcite particles,namely coralline algae and miliolid foraminifera. In allof the specimens studied the magnesium is gone (lessthan 4 mole % MgCO3).

The skeletal microstructure of altered coralline algaeis almost always well preserved in ancient sediments.Land (1967) suggested that magnesium is selectivelyremoved and replaced by calcium through incongruentdissolution. Although this process is not understood indetail, it apparently does not involve a void stage, andthus would explain the perfect preservation of corallinealgae.

Unlike coralline algae, miliolid foraminifera, whichhave an amber color and are translucent and crypto-crystalline under the microscope, are frequently re-placed by microcrystalline low-Mg calcite. Usuallymiliolid tests first become spotted with micrite grainswhich increase in number until finally the entire test ismicritized (Plate 2, Figures 1 and 2). High-Mg calciteechinoderm fragments altered to low-Mg calcite showthe same sequence of replacement features (see, also,Crickmay, 1945).

Our data are insufficient to determine whether this al-teration takes place by dissolution-reprecipitation be-neath a mucilaginous envelope (Kendall and Skipwith,1969) or by boring and subsequent precipitation of tinycarbonate crystals in the excavated tunnels as describedby Bathurst (1966) and Alexandersson (1972). Neitherof these mechanisms provides a sufficient explanationfor the different state of preservation of the corallinealgae and miliolids.

Moreover, it is not entirely clear why the mineralogictransformation from aragonite to calcite generally is tex-turally destructive. However, it does involve a dissolu-tion-reprecipitation mechanism with an intermediatevoid stage as mentioned above. Purdy (1968) ques-tioned the validity of "incongruent dissolution" as amechanism for mimicry of biogenic fabrics and pointedout that perhaps the problem of textural preservation ordestruction is only a matter of the scale of the samedissolution-reprecipitation process. Dissolution andreprecipitation on the ultramicroscopic scale wouldappear texturally nondestructive in the light microscope.The textures would be destroyed only where largercavities are formed.

However, the most recent results on neomorphic re-placement of aragonitic skeletons by calcite (Schneider-mann et al., 1972; Sandberg, in press) do not supportPurdy's hypothesis. Although the neomorphic processappears to operate along thin-film fronts without forma-tion of visible voids, it is texturally destructive and re-sults in a coarse neomorphic calcite fabric which may ormay not contain relic structures.

Habit and Fabrics of Calcite CementsThe ultrastructure and the fabrics of carbonate

cements are best developed in the biogenous volcanicsandstone because of its high initial porosity (at least40%) and the large size of the pores. Cement lines or fillsinterparticle and intraparticle voids. Two kinds of ce-ments are found: palisade cement consisting of fibrouscrystals and blocky cement made up of blade-like crys-tals. No needle-like cement occurs in these sediments.Where both cements occur together, palisade cementalways forms the first crust on the grains. Where thecrystals rise from a polycrystalline substrate, which pro-vides a high nucleation rate (such as oolitic coatings,porcelaneous foraminifera, coralline algae, or micriteenvelopes), they are subparallel, radiate very regularly,and increase in length towards the center of the pore(Plate 2, Figure 5 and Plate 3, Figure 4). The correlationbetween regularity of crystal orientation and crystaldensity was pointed out by Schroeder (1972).

The length of the crystals is highly variable and rangesfrom less than 10 microns up to 150 microns. Very small(<IO microns) fibrous crystals, which are rooted on thesediment grains and which themselves serve as a sub-strate for larger fibrous crystals, are faintly recognizablein thin sections. It is possible that the latter are com-posite crystals of several tiny fibrous crystals which havebeen overgrown by larger ones. In linings with highcrystal density, the fibrous crystals have an almost con-stant width which decreases only slightly towards the tip(Plate 2, Figure 6). The ratio of length to width variesbetween 3 and 10. Commonly, the crystals terminatewith two sets of rhombic faces which are rotated 60°against each other (Plate 3, Figure 1 and Schroeder,1972, fig. 5d). Steep rhombic faces form the long sides ofthe crystals. The terminal part of the long sides of thecrystals often shows curved faces (Plate 3, Figure 1).

Substrates on which cement would have a slownucleation rate, such as volcanic grains coated with athin layer of authigenic clay (Plate 3, Figure 3), are linedwith subequant crystals about 5 microns in length.These abut against bladed to blocky calcite spar. If thesubequant crystals are missing, bladed or blocky calciteis in direct contact with the detrital grains (Plate 3,Figure 3).

The true nature of the bladed crystals, which grewrooted on fibrous or subequant crystals, is best seen insamples only partially cemented. As shown in Plate 3,Figures 2 and 4, the bladed crystals are steep-sided com-posite crystals which consist, at their base, of severalsmaller crystals. The composite crystals reach about 350microns in length and decrease stepwise in widthtowards the tip, where they almost always terminatewith crystal faces and never show the depressions char-

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acteristic of the submarine aragonite cement describedby Schroeder (1972). Because the bladed crystalsnucleate rather irregularly and are wider spaced than thefibrous crystals, the growth direction of the bladedforms is less controlled and therefore often oblique tothe substrate.

If the crystal blades arrive from different directions,they interfere with each other in an intergranular poreand display a bulky fabric (Plate 3, Figures 4 and 5). Asthey continued to grow, they fill the pore completely(Plate 3, Figure 6). Such pore fillings show, in thin sec-tion, all the characteristics of blocky calcite spar(Bathurst, 1971, p. 417). In this case the sparry calcitemosaic is caused by growth of blade-like calcite crystals,whereas in deep-sea chalk the same fabric results fromgrowth of equant calcite rhombohedrons (Matter, 1974,PI. 9, fig. 6).

Microprobe profiles run across fibrous crusts and intoblocky calcite cement revealed that the concentrationsof MgCθ3 decreased from about 3 mole % MgCCh closeto the substrate to 1.6 mole % MgCCh in the blockycalcite. Traces of iron were also observed, but no stron-tium was detected.

Syntaxial overgrowths on crinoid fragments are com-mon in Core 4 (Evamy and Shearman, 1965; 1969). Inthe same sample some crinoid fragments may have asyntaxial overgrowth, and others may have a crust offibrous calcite cement whose crystals are not in latticecontinuity with the host. Apparently, growth of thelatter calcite crystals is favored on crinoid fragmentswhich have an oolitic coating or a micrite rim.

In the porous biogenous volcanic sandstone twobasically different kinds of cement fabrics were ob-served. The cement shown in Plate 2, Figure 5 ischaracterized by subparallel fibrous calcite formingthick coatings on the grains and relatively few bladedcomposite crystals. The bladed crystals occupy, as se-cond generation cement, the interior of some pores. Asutured contact is seen in thin sections (Plate 2, Figure 3)where fibrous crusts meet and fill entire pores. The samekind of cement fabric has been described by Cullis(1904) from Funafuti (see Bathurst, 1971, fig. 269). Ofcourse, changes in the relative amounts of fibrous tobladed crystals vary the aspect of this basic fabric (Plate2, Figure 4). Such changes are present within the rangeof a single sample.

The second type of cement fabric observed in Core 4is shown on Plate 2, Figures 1 and 2. A thin layer offirst-generation subequant-micritic crystals is followedby last-generation bladed-composite crystals whichocclude most of the intergranular pore space. The blad-ed crystals form a sparry mosaic which shows some ofthe criteria, such as straight boundaries and enfacialjunctions, which, are diagnostic for void-filling sparBathurst (1971). It does not show, however, the increaseof crystal size towards the center of the pore.

CONCLUSIONS

The abundance of shallow-water benthonic foram-inifera, calcareous green algae, coralline algae, and

ooids in Core 4 indicates that the biogenous volcanicsandstone accumulated in a water depth of less than 4 to6 meters. Furthermore, the coarse-grained, well-rounded and moderately sorted nature of the sedimentand the absence of a micritic matrix suggest mechanicalabrasion and winnowing in a high-energy environment,such as the surf zone or lower foreshore. The sharp con-tact between the sandstone and the siltstone representsthe change from a high-energy environment to a low-energy environment, possibly as the surface subsidedbelow wavebase.

Many of the carbonate grains in the studied sedimentshave been micritized to various degrees by boring algaeand fungi while still at or close to the sea floor. Manyborings have been infilled by early diagenetic mont-morillonite instead by micrite. This clay mineral alsoforms thin fringes around volcanic grains. Neoforma-tion of montmorillonite thus predates deposition of thecarbonate cements.

The Eocene sediments from Köko Guyot havereached the diagenetic stage V of Land et al. (1967), i.e.,Mg-calcite has lost its magnesium, most of the ara-gonitic particles have been dissolved, and much of theprimary and secondary pore space has been cemented bywhat is now low-Mg calcite.

Generally bladed calcite, frequently forming a blockyfabric, succeeds either fibrous calcite fringes or thincrusts of subequant micritic crystals. Two main ques-tions arise as to their origin: (a) are the crystals in theiroriginal mineralogical and chemical state? and (b) inwhich diagenetic environment did they grow?

Aragonite cement in shallow-water sediments com-monly occurs as fibrous or needle-like crystals. Thefibrous habit is not indicative for aragonite only becausefibrous high-Mg calcite rim cement also has been de-scribed (Taylor and Illing, 1969; Schroeder, 1973; andothers). However, from the absence of orthorhombicsymmetry and absence of even traces of strontium, ara-gonite can be precluded as a precursor of the fibrouslow-Mg calcite cement found at Site 308 (A. Stalder,personal communication).

Lindholm (1972) suggests that the amount of mag-nesium present in what is now low-Mg calcite might givea clue to its original mineralogy (low- vs high-Mg cal-cite). The relatively high magnesium content (approx. 3mole % MgCθ3) together with the regular outline andfabric of the fibrous crystals may indicate that they arean ancient analog of the submarine high-Mg calcitepalisade cement described by Schroeder (1972; 1973).Because the MgCθ3 content is only slightly lower in thebladed cement (approx. 1.5 mole %) and because nohiatus is observed between the two cement generations,the bladed composite crystals might have been depositedas high-Mg calcite as well. However, lacking any safecriterion or set of criteria which would allow a deter-mination of the kind of primary calcite in our Eocenesamples, the possibility that it has grown as low-Mg cal-cite cannot be ruled out with certainty.

In the absence of any features diagnostic of the vadoseenvironment, it is concluded that the carbonate cementsare of submarine origin.

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ACKNOWLEDGMENTSG. Burri, A. Mahé (Lausanne), and H. Walter generously

carried out analyses with the microprobe and H. Ischi helpedwith the evaluation of the data. F. Zweili kindly operated theSEM and gave valuable technical advice. U. Ernst and U.Furrer prepared the photomicrographs, and Miss S. Sahlityped the manuscript.

F. Allemann and R. Herb helped with the identification offossils, and A. Stalder gave advice on crystal morphology.R.N. Ginsburg and J.D. Milliman reviewed the manuscript.We thank all these individuals for their help and constructivecriticism.

REFERENCESAlexandersson, T., 1972. Micritization of carbonate particles:

Processes of precipitation and dissolution in modernshallow-marine sediments: Geol. Inst. University of Upp-sala Bull., new ser., v. 3, p. 201-236.

Bathurst, R.G.C., 1964. The replacement of aragonite bycalcite in the molluscan shell wall. In Imbrie, J. and Newell,N.D. (Eds.), Approaches to paleoecology: New York(Wiley), p. 357-376.

, 1966. Boring algae, micrite envelopes and lithifica-tion of molluscan biosparites: Geol. J., v. 5, p. 15-32.

., 1971. Carbonate sediments and their diagenesis:Amsterdam (Elsevier).

Clague, D.A. and Greenslate, J., 1972. Alkali volcanic suitefrom the Emperor Seamounts: Geol. Soc. Am., Abstracts,v. 4, p. 136.

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PLATE 1Diagenesis of aragonitic particles

Figure 1 Photomicrograph of Halimeda which now consistsof low-Mg calcite. Utricles which had been infilledby ?high-Mg calcite (Winland 1971) and micriteenvelope are preserved whereas the aragoniticthallus has been dissolved. The secondary in-traparticle void space is being infilled by smallfibrous acicular calcite crystals which are rootedon the utricles. Note similar habit of intra- and in-terparticle calcite cement. Thin section. Sample308-4-1, 126 cm.

Figure 2 Halimeda plate replaced by neomorphic calcite. Inthe microscope brownish-stained neomorphic sparis seen where filaments have been replacedwhereas clear spar marks former intrabiotic voids.Shape of Halimeda is retained by micrite envelopewhich also shows relicts of borings.Thin section. Sample 308-4-1, 58 cm.

Figures 3,4 Scanning electron photomicrograph of stronglyetched ooid showing alternation of clear calcitelaminae (deeper etching) and micritic laminae.Under higher magnification (Figure 4) the micriticlaminae as well as the micritic "pillars" separatingclear calcite crystals are seen to consist of nano-grains which are mainly montmorillonite and low-Mg calcite. Clear calcite has been etched awaycompletely in Figure 4.Sample 308-4, CC.

Figure 5 Electron micrograph of ooid. Light laminae aremicritic, darker ones are clear calcite. Inner part ofcortical complex consists mainly of micritic layers.Note thick layer of clear calcite beneath outermostmicritic lamina. Black line indicates where micro-probe measurements have been made.Sample 308-4-1, 126 cm.

Figure 6 Beam scan image showing distribution of calciumin ooid of Figure 5. Nucleus which is a volcanicrock particle appears black. Micrite layers arerecognized by their low concentration and clearcalcite by high concentration.Sample 308-4-1, 126 cm.

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Figures 1,2 Cryptocrystalline miliolids, partly micritized inFigure 1 and completely micritized in Figure 2.Test shown in Figure 2 displays bores and a partlycollapsed micrite wall. Collapse predates deposi-tion of first-generation stubby calcite crystalswhich also grow on fracture surfaces. Noticecoarse sparry calcite cement which immediatelyfollows a thin layer of stubby first-generationcrystals. Thin sections. Sample 308-4-1, 68 cm.

Figure 3 Fibrous palisade cement filling entire in-tergranular pore space. A sutured contact is seenwhere rim cements meet. Thin section. Sample308-4-1, 126 cm.

Figure 4 Palisade rim cement followed by pore filling blad-ed crystals which show the typical blocky fabric ofsparry calcite. Sample 308-4, CC.

Figures 5, 6 Fibrous low-Mg calcite rim cement in biogenousvolcanic sandstone. In side view and under highermagnification the slender shape of the crystals isseen which taper only slightly towards the endbecause of very steep rhombohedral faces (Figure6). SEM photomicrographs. Sample 308-4-1, 126cm.

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PLATE 2


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PLATE 3Scanning electron photomicrographs of low-Mg crystals

cementing biogenous volcanic sandstone.

Figure 1 Fibrous calcite with steep rhombohedral as well ascurved faces on sides of crystals. The crystals oftenterminate with two sets of rhombohedral facesrotated against each other by 60°. Some of thecrystals are composite ones. Sample 308-4-1, 126cm.

Figure 2 Short fibrous crystals rooted on foraminiferal wallare overgrown by large-bladed composite crystals.Sample 308-4-1, 126 cm.

Figure 3 Surfaces of volcanic grains coated with box-workauthigenic montmorillonite. Only few fibrouscrystals grow on the montmorillonite substrateand large pore-filling second-generation bladedcomposite calcite may therefore be in indirect con-tact with detrital grain. No centripetal increase ofcrystal size is observed (Bathurst's rule not ful-filled). Sample 308-4, CC.

Figures 4-6 Subparallel radiating fibrous rim cement over-grown by bladed composite crystals. Figures 4 to 6show progressive infilling of pore by second-gen-eration bladed calcite. Notice step-like thinning ofthese crystals in Figure 5. Progressive growth ofbladed crystals results in bulky texture (Figure 5)where single crystals are still recognizable and to asparry mosaic where bladed crystal habit is nomore recognizable. Sample 308-4, CC.

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16. carbonate diagenesis at site 308 koko … · 16. carbonate diagenesis at site 308 koko guyot...

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