46. ORGANIC-MATTER-RICH AND HYPERSILICEOUS APTIAN SEDIMENTS FROM WESTERN MID-PACIFIC MOUNTAINS, DEEP SEA DRILLING PROJECT LEG 62 1 Frederic Mélières, Laboratoire de Géologie Dynamique, Université Pierre et Marie Curie, 75230 Paris Cedex 05, France and Gerard Deroo and Jean-Paul Herbin, Institut Français du Pétrole, 4 Avenue de Bois-Préau, 92502 Rueil-Malmaison, France ABSTRACT A 15-meter sequence of early Aptian organic-matter-rich sediments, cored at Deep Sea Drilling Project Site 463 (western Mid-Pacific Mountains) has been submitted for detailed mineralogical studies (XRD, SEM) and organic- carbon characterization. Although intense diagenesis has obscured the sedimentary record of depositional conditions, the history has been tentatively reconstructed. Through sustained volcanic activity and alteration processes on the ar- chipelago, large amounts of silica were released into the sea water, resulting in a "bloom" of radiolarians. Hard parts settled in large amounts, yielding a hypersiliceous sediment; amorphous silica was diagenetically transformed into chalcedony, opal-CT and clinoptilolite through dissolution and recrystallization. Oxidization of part of the radiolarian soft parts (1) depleted the sea water in dissolved oxygen, allowing the burial of organic matter, and (2) generated carbon dioxide which led to dissolution of most of the calcareous tests. Moderate depositional depth and a high sedimentation rate are though to have prevailed during this episode. An immature stage of evolution is assigned to the studied organic matter, which is of two origins: autochthonous marine material, and allochthonous humic compounds and plant debris. Rhythmic sedimentation characterizes the distribution of the organic matter; each sequence shows (1) an up- ward progressive increase in organic-carbon content, and (2) an upward enrichment in marine organic matter. INTRODUCTION This report is an extended account of the mineralogy and organic geochemistry of early Aptian sediments cored at DSDP Site 463 (Fig. 1), in the western Mid- Pacific Mountains (central subtropical North Pacific). These sediments, very successfully recovered in a com- plete sequence in Cores 70 and 71 (Fig. 2) are character- ized by a high organic-carbon content and the frequent occurrence of volcanogenic constituents. The sediments are cyclic alternations of silicified limestone, ashy limestone, and carbonaceous limestone; colors are generally dark, ranging from greenish gray (silicified limestone) to olive-black (ashy and carbonaceous lime- stone); all this material is fine-grained and frequently laminated in the darker lithologies, whereas the lighter ones show discrete burrowing (see Site 463 report, this volume). The occurrence of organic carbon and volcanogenic constituents together led to detailed investigations of the mineralogy of the sediments and the nature, origin, and history of the organic fraction. For these purposes, 150 samples were selected in Cores 70 and 71 (a 15-meter se- quence). To try to understand the dark facies within the frame of general sedimentation in Aptian time at Site 463, 50 additional samples were taken in Cores 69 and 72. Initial Reports of the Deep Sea Drilling Project, Volume 62. MINERALOGY Analytical Methods Qualitative and quantitative mineralogical analyses were carried out on the bulk sediment by X-ray diffrac- tometry. The analytical procedure, basically operating on non-oriented powder with the use of an internal stan- dard (NaF), is described in detail elsewhere (Mélières, 1974). A resume of the basic steps is given in Mélières (1978). The mineralogical data are graphically represented in Figure 3 (back pocket, this volume), and they are listed in Table 1. Comments on unusual crystallographic data are given in the text. Feldspars Feldspar minerals are represented almost throughout the studied section. They consist of both K-feldspar and plagioclase, but because of their very low concentration (absence to few percent), their exact nature cannot be precisely determined from the routine diffractometry data. Plagioclase occurs only in Cores 70 and 71, where it shows a good correlation with, but remains subordinate to, K-feldspar. Sample 463-72-2, 13-14 cm yielded a minor lithology consisting of a micro-bed of fine-grained, dark sand, in which the K-feldspar content reaches 18%. X-ray dif- fractometry revealed sanidine (K was confirmed through X-ray microprobe). SEM observation shows that the 903
Transcript
46. ORGANIC-MATTER-RICH AND HYPERSILICEOUS APTIAN SEDIMENTS FROM WESTERNMID-PACIFIC MOUNTAINS, DEEP SEA DRILLING PROJECT LEG 621
Frederic Mélières, Laboratoire de Géologie Dynamique, Université Pierre et Marie Curie, 75230 ParisCedex 05, France
andGerard Deroo and Jean-Paul Herbin, Institut Français du Pétrole, 4 Avenue de Bois-Préau,
92502 Rueil-Malmaison, France
ABSTRACT
A 15-meter sequence of early Aptian organic-matter-rich sediments, cored at Deep Sea Drilling Project Site 463(western Mid-Pacific Mountains) has been submitted for detailed mineralogical studies (XRD, SEM) and organic-carbon characterization. Although intense diagenesis has obscured the sedimentary record of depositional conditions,the history has been tentatively reconstructed. Through sustained volcanic activity and alteration processes on the ar-chipelago, large amounts of silica were released into the sea water, resulting in a "bloom" of radiolarians. Hard partssettled in large amounts, yielding a hypersiliceous sediment; amorphous silica was diagenetically transformed intochalcedony, opal-CT and clinoptilolite through dissolution and recrystallization. Oxidization of part of the radiolariansoft parts (1) depleted the sea water in dissolved oxygen, allowing the burial of organic matter, and (2) generated carbondioxide which led to dissolution of most of the calcareous tests. Moderate depositional depth and a high sedimentationrate are though to have prevailed during this episode. An immature stage of evolution is assigned to the studied organicmatter, which is of two origins: autochthonous marine material, and allochthonous humic compounds and plantdebris. Rhythmic sedimentation characterizes the distribution of the organic matter; each sequence shows (1) an up-ward progressive increase in organic-carbon content, and (2) an upward enrichment in marine organic matter.
INTRODUCTION
This report is an extended account of the mineralogyand organic geochemistry of early Aptian sedimentscored at DSDP Site 463 (Fig. 1), in the western Mid-Pacific Mountains (central subtropical North Pacific).These sediments, very successfully recovered in a com-plete sequence in Cores 70 and 71 (Fig. 2) are character-ized by a high organic-carbon content and the frequentoccurrence of volcanogenic constituents. The sedimentsare cyclic alternations of silicified limestone, ashylimestone, and carbonaceous limestone; colors aregenerally dark, ranging from greenish gray (silicifiedlimestone) to olive-black (ashy and carbonaceous lime-stone); all this material is fine-grained and frequentlylaminated in the darker lithologies, whereas the lighterones show discrete burrowing (see Site 463 report, thisvolume).
The occurrence of organic carbon and volcanogenicconstituents together led to detailed investigations of themineralogy of the sediments and the nature, origin, andhistory of the organic fraction. For these purposes, 150samples were selected in Cores 70 and 71 (a 15-meter se-quence). To try to understand the dark facies within theframe of general sedimentation in Aptian time at Site463, 50 additional samples were taken in Cores 69 and72.
Initial Reports of the Deep Sea Drilling Project, Volume 62.
MINERALOGY
Analytical MethodsQualitative and quantitative mineralogical analyses
were carried out on the bulk sediment by X-ray diffrac-tometry. The analytical procedure, basically operatingon non-oriented powder with the use of an internal stan-dard (NaF), is described in detail elsewhere (Mélières,1974). A resume of the basic steps is given in Mélières(1978).
The mineralogical data are graphically represented inFigure 3 (back pocket, this volume), and they are listedin Table 1. Comments on unusual crystallographic dataare given in the text.
FeldsparsFeldspar minerals are represented almost throughout
the studied section. They consist of both K-feldspar andplagioclase, but because of their very low concentration(absence to few percent), their exact nature cannot beprecisely determined from the routine diffractometrydata.
Plagioclase occurs only in Cores 70 and 71, where itshows a good correlation with, but remains subordinateto, K-feldspar.
Sample 463-72-2, 13-14 cm yielded a minor lithologyconsisting of a micro-bed of fine-grained, dark sand, inwhich the K-feldspar content reaches 18%. X-ray dif-fractometry revealed sanidine (K was confirmed throughX-ray microprobe). SEM observation shows that the
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F. MELIERES, G. DEROO, J.-P. HERBIN
10° -
170° 180°
Figure 1. Location of Site 463, and other DSDP sites.
feldspar grains are well rounded, indicating that theyare detrital.
These data suggest that the K-feldspar is terrigenous,probably from on-shore, evolved volcanic material. Thealkalic nature of the feldspar suggests that this volcanicmaterial did not originate from the early archipelagicframework, but more likely from a partly differentiatedmagma. This indicates that at least a part of the westernMid-Pacific Mountains already existed as emergent landmasses in early Aptian time.
Clay Minerals
Clay minerals are represented here exclusively by10-Å phyllosilicate structures, giving X-ray diffractionpatterns characterized in the untreated sample by a dif-fuse "bump" between 10 Å and 17 Å, and in the
glycolated sample by (1) a broad but well-centered 17-Apeak interpreted as smectite and (2) a broad but wellcentered 10-Å peak interpreted as illite.
These 10-Å phyllosilicate structures appear to becharacterized by a complex interlayer population con-sisting of various cations (Ca, K, Na, etc.) yieldingvarious hydration states, and consequently variousbasal spacing, ranging from 10 Å to 17 Å. Therefore,the term "illite" is not to be taken here in its generalsense ("micaceous" clay), but more likely as indicatingthe K-interlayer, saturated, non-expandable fraction ofthe 10-Å phyllosilicate structures.
Smectite and illite occur throughout the studied sec-tion, except in the upper part of Core 69, where they aresporadic. They are present in rather moderate amounts,1 to 2% for illite, and 5% (average) for smectite,
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APTIAN SEDIMENTS
SITE 463
Figure 2. Lithologic column and biostratigraphic zonation of DSDPdrill Site 463.
although the latter may occur in much higher values,reaching 50% of the sediment in the upper part of Sec-tion 1 of Core 71.
SEM and optical microscope observations reveal thatsmectite results from transformation of volcanogenicmaterial (Plates 1 and 2). DTA analysis and systematicstudy of the (060) X-ray-diffraction peak show that thesmectite belongs to the beidellite-nontronite series,displaying an octahedral iron content ranging from 35to 60%. This seems to confirm the relation of the smec-tite with volcanogenic material.
Illite and smectite show good correlation in the varia-tion of their concentrations, except for high smectitevalues. This suggests either that both minerals areoriginally closely related and derived from the samesources (except during the deposition of smectite-richsediments), or that illite resulted from diagenetic trans-formation of part of the smectite during periods of seawater (or interstitial water close to sea water) contactlong enough to allow potassium to be accommodated asan interlayer cation. The latter hypothesis seems morelikely in the light of Core 69 data: within the lower halfof Section 2 and in Section 3, the illite/smectite ratio isthe highest of the entire studied sequence, and theminerals show an excellent correlation. This episode isimmediately followed by normal marine, almost ex-clusively carbonate sedimentation, the deposition rateof which appears to have been much slower than thatexisting during the previous deposition of sediment.
Consequently, the best conditions for diageneticevolution of smectite into illite seem to have been real-ized during that time, because of the possibility of longcontact with interstitial waters close to sea water, whichresulted from the high porosity of the overlying car-bonate beds. Actually, the relationship between illiteand smectite within Core 69 seems to indicate the(early?) diagenetic nature of illite in this case.
In the upper part of Core 69, and in Core 72, smectiteshows good correlation with K-feldspar. This confirmsthe terrigenous nature of smectite, and implies that thismineral was generated on shore through alteration ofvolcanogenic material. (The formation of iron-bearingsmectites through deuteric alteration of the base of ter-restrial lava flows has been recently pointed out [Méli-ères and Person, 1978]). In Core 72, Section 2, the smec-tite peak, indicates an unquestionably terrigenous smec-tite: there is a significant amount of kaolinite, reaching7°7o in a sandy micro-bed which could be interpreted as adistal turbidite.
In smectite-rich sediments of Cores 70 and 71, thereis no correlation between smectite and K-feldspar. Insuch cases the smectite and volcanogenic material arethought to be primarily associated, and the abundancepeaks of smectite are interpreted as direct echoes ofvolcanic events. Such events would have induced highsedimentation rates, preventing the deposited materialfrom long and close contact with sea water. Conse-quently, the smectite would have been unable to partlyconvert to illite. This seems to have occurred, becausethere is no correlation between smectite and illite abun-dance peaks.
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Table 1. Mineralogy of Cores 69 through 72, Site 463.
Clinoptilolite appears to be a diagenetic mineral here,replacing and (or) filling radiolarian tests, as indicatedby SEM and optical microscope investigations (Plates 3and 2). It occurs only in Core 70 and at the top of Core71, and its abundance is closely related to that of smec-tite; its occurrence is restricted to the smectite-richsediments which have been interpreted as direct echoesof volcanic events. Because clinoptilolite has a highersilica content than smectite, it implies the availability offree silica during diagenesis. This silica was suppliedhere in large amounts by dissolution of radiolarian tests(highly soluble amorphous biogenic silica).
Although clinoptilolite does not result here from thedirect transformation of volcanic material (as it is oftenbelieved to happen in marine sediments), this mineralappears to be closely related to volcanic events.
Quartz (Chalcedony)
Quartz is present throughout the studied section; itsX-ray crystallinity2 and the (100)/(101) peak-heightratio show that quartz is not a terrigenous detrital con-stituent, but authigenic chalcedony. Chalcedony is awell-known constituent of cherts (Heath and Moberly,1971; Lancelot, 1973); it has been shown to be derivedfrom highly soluble amorphous biogenic silica (mainlyradiolarian tests) through diagenetic processes. SEMobservations (Plate 4) show that, through fragmenta-tion and dissolution, radiolarian tests released notableamounts of silica and dissolved silica. The biogenicsilica was later converted to chalcedony.
Variations in chalcedony abundance and crystallinitydo not show any significant trend, except in the upperpart of Core 69, where this mineral disappears almostcompletely within the limestone matrix, being restrictedto discrete chert layers.
Opal-CT
The nomenclature used here follows the definition ofJones and Segnit (1971). Opal-CT is present in con-siderable amounts in the studied section; it may con-stitute up to 75% of the sediment.
SEM observations show that it occurs either aslepispheres filling dissolution cavities, or as massive,structureless (at the SEM scale) material constituting thematrix of the sediment—or both. Its diagenetic char-acter, already known from previous studies (Keene,1976), is obvious here, and it is possible to state thatopal-CT crystallized later than clinoptilolite (Plates 3and 4).
Opal-CT occurs in Cores 72, 71, and 70, and at thebase of Core 69. (It should be kept in mind that the in-terval between the base of Core 69 and the top of Core70, as represented in Figure 3, does not necessarilyrepresent reality, because of partial recovery withinCore 69; the base of Core 69 may actually be close to the
The X-ray crystallinity is the difference between the width at half-height of the (101)peak and the instrumental width; the value is given in 1/10° 20 (CuKα) in Table 1 and Figure3.
top of Core 70.) Its abundance, although displayinglarge variations on a decimetric scale, reaches a max-imum throughout Core 70 and in the upper part of Core71; in this silica-rich section, opal-CT constitutes 40%(average) of the sediment. The large variations of abun-dance are clearly controlled by the lithology of the hostsediment. Opal-CT abundance is in remarkable inversecorrelation with smectite abundance. This suggestseither the impossibility of opal-CT crystallization in im-permeable clayey sediments—no mobility of interstitialwater, and/or the presence of foreign cations in largeamounts—or dilution of the "snow fall" of radiolariantests by rapid settling of smectite (volcanogenic mater-ial).
The absence of a significant relation between smectiteand chalcedony (recrystallized radiolarian debris) abun-dance does not seem to favor the second hypothesis, andwe therefore shall tentatively keep the lithological con-trol hypothesis. This hypothesis implies that notablevertical migration, on a decimetric scale, occurred afterdissolution of radolarian tests within the sediment andbefore crystallization of opal-CT. Nevertheless, despitethis vertical migration, resulting in the present distribu-tion of opal-CT, it is possible to characterize the silica-rich sequence (Core 70 and Section 1 of Core 71) by anaverage value of the opal-CT content, about 40%.
The B/A ratio on the diffractograms (height of "trid-ymite" peak at 20.6° 20 CuKα versus height of "cristo-balite" peak at 21.6° 20 CuKα) averages 0.45. This ratiowas defined and tentatively used as a maturity index foropal-CT was defined and tentatively used as a maturityindex for opal-CT from eastern North Atlanticsediments (Mélières, 1978), where the value 0.45 is in-dicative of a burial depth of about 600 meters. Here theburial depth of the sediments (although much older) is610 to 630 meters. This very good agreement betweenthese two locations is encouraging for further investiga-tion.
The presence of opal-CT in considerable amountssuggests that large quantities of biogenic amorphoussilica were deposited and later dissolved before beingconverted to opal-CT. This seems clearly established bythe intense dissolution observed in the radiolarian tests(Plate 4). This indicates that large numbers of radio-larians developed in sea water during the time of deposi-tion of Core 70 and the upper part of Core 71. We knowthat frequent volcanic events occurred during this time;therefore it seems likely that silica was released in seawater through volcanic activity and readily metabolizedby radiolarians (high productivity of the siliceousplankton). Consequently, the sedimentation rate shouldhave been considerable during this episode.
CalciteExcept for scarce siderite, calcite is the only car-
bonate present in the studied section. Besides abun-dance, special attention was paid to accurate measure-ment of calcite crystallographic parameters. X-ray crys-tallinity was measured through slow scanning (0.25°20/min) of the (104) peak; the value given in Table 1 andin Figure 3 is the width at half-height of the (104) peak,
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APTIAN SEDIMENTS
minus the instrumental width; this value is given in1/100° 20 CuKα, the precision being on the order of± 0.005 °20; a perfect crystal has a crystallinity of 0.MgCo3 (mole) content was measured by the angularposition of both the (104) peak and the (108) (116)doublet; absolute precision is on the order of ±0.2%MgCO3. (It should be kept in mind that the diffrac-tometry method does not imply that magnesium is theonly cation responsible for the shift of the (104) calcitepeak, metallic cations smaller than calcium theoreticallybeing able to induce the lattice-contraction effect.Therefore the "MgCO3 content" of calcite determinedhere is rather an "MgCO3 equivalent"; fortunately,there is high probability of magnesium presence.)
These parameters, together with calcite abundance,allow definition of three zones in the studied section:
1) Core 72 and the lower part of Core 71 (from thebase up to about Section 2, 50 cm). Here, calcite is themain constituent of the sediment, averaging 50%. Avery good crystallinity and a very low magnesium con-tent indicate that this calcite is biogenic (calcareous nan-noplankton), and was diagenetically recrystallized.SEM observation (Plate 5) confirms this interpretation.
2) The upper part of Core 71, all of Core 70, and thebase of Core 69 are characterized by low to very lowcalcite contents (in places complete absence), poor cal-cite crystallinities, and notable MgCO3 content. This,together with SEM observation (Plate 5), indicates thatthe calcite is biogenic and underwent considerable dis-solution, but was not recrystallized (or weakly recrys-tallized) during diagenesis. Indeed, dilution of the cal-cite tests "snow fall" by siliceous biogenic remains (nowopal-CT and chalcedony) played a role in the loweringof the calcite content of the sediment, but such dilutioncannot account for the complete disappearance ofcalcite. Therefore, dissolution must have occurred,probably before the settling of the particles, because apost-depositional dissolution would not have eliminatedthe calcium (at least on the episode scale).
The pre-depositional dissolution of calcite is inter-preted as having resulted from an increase of carbondioxide content of sea water, because of the oxidationof large amounts of organic matter released by thesiliceous plankton. Similar processes are thought tohave occurred within Aptian sediments from the easternNorth Atlantic (Mélières, 1979). The absence of calciterecrystallization is attributed to the impossibility ofmigration interstitial water, because of the imperme-ability of the matrix (opal-CT and clay minerals). Thefact that magnesian calcite was preserved suggests thatthe depositional depth was not considerable, probablyon the order of a few hundreds of meters or less.
3) In Core 69, after a transitional zone within thelower part, the calcite content rises rapidly and reachesvery high and constant values (80-90%), except forchert beds and a few small layers of reworked volcano-genie material. Excellent crystallinity and the absence ofmagnesium indicate that the calcite results from intenserecrystallization of calcitic remains, as is seen in SEMobservation. Recrystallization is obviously related to the
high porosity existing after sedimentation because of theabsence of opal-CT, chalcedony, and clay minerals.
Siderite
Siderite occurs sporadically in minor amounts. It ap-pears as perfect rhombohedrons scattered in thin bedswithin the more-clayey lithologies. X-ray diffractionshows that it contains up to 10% CaCO3, and X-raymicroprobe reveals that calcium distribution in the lat-tice is not homogeneous. Siderite is therefore thought tobe diagenetic, iron probably having been supplied byiron-rich smectites.
Pyrite
Pyrite occurs only in Core 70, but in rather constantamounts (a few percent); single peaks exceed 10%. SEMobservation shows that this pyrite is framboidal, andtherefore formed in diagenesis; it results from the activ-ity of sulfate-reducing bacteria. This suggests that thesedimentary medium, at least below the first centimetersbeneath the sea floor, was characterized by reducingconditions. An active bacterial life within the new-bornsediment indicates a high influx of organic matter. Theabundance peaks of pyrite correlate well with those oforganic carbon, and, in the studied silica-rich interval,the occurrence of pyrite matches exactly that of organiccarbon (Fig. 3).
Barite
Barium sulfate occurs in very low amounts (< 1%),but almost continuously, throughout Core 70, and muchmore sporadically in Cores 71 and 72. Because of itsvery low concentrations, barite was not investigated bySEM; therefore, it is difficult to make any positive state-ment about its origin. Nevertheless, because of its cor-relation with the silica-rich interval we tentatively sug-gest that barium could have originated in the organicmaterial,which settled in large amounts. An organicsource for barite in deep-sea barites and the associationof barite with organic carbon in deep-sea sedimentshave been reported often (Dean and Schreiber, 1978).
Halite
Halite does not exist in the sediment as a mineral, butresults from evaporation of the interstitial water whenthe samples dried. Nevertheless, the amount of thishalite in the samples is reported for two reasons: (1)from an analytical point of view this "halite" takes partin the quantitive analysis balance, and (2) whatever theorigin of the interstitial water of the samples (drillingcontamination or genuine), the "halite" content givesan indication of the porosity of the sediment; thedistribution of "halite" in the studied sequence con-firms this assumption.
Amorphous Material
The amorphous material content is estimated by sub-tracting the total amount of crystallized constituentsfrom 100. This actually yields only rough values,especially in clayey lithologies. Amorphous material is
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F. MELIERES, G. DEROO, J.-P. HERBIN
present throughout the studied section; its contentaverages 20% and displays notable variations. In Cores71 and 72, these variations appear in good correlationwith smectite abundance. Because smectite is directlyrelated to volcanogenic material, it seems that amor-phous material proceeds from the same sources andtherefore probably consists mainly of terrigenous alu-mino-silicates. In Cores 70 and 69, the variations do notappear to be related to variations in other constituents;the only notable feature is a slight decrease within thecalcareous material in the upper part of Core 69, sug-gesting that the amorphous material is terrigenous.
Plant debris occurs throughout Core 70. It consists offragile black (carbonized) microscopic ribbons depos-ited within the sediment interbeds; the plants which pro-vided this material were gymnosperms (Plate 6). Theplant debris implies relative proximity of land masses;this confirms the indications of terrigenous K-feldsparand kaolinite (463-72-2, 13-14 cm).
Phosphate grains occur in Core 70 within the layersrichest in organic carbon. They consist of minute brownflakes, probably fish scales (Plate 6). They indicate adepositional environment at times reducing enough topreserve fish remains. This confirms the indications ofthe mineral assemblage.
ORGANIC MATTERFrom 463-72-2, 102 cm to 463-70-1, 4 cm, the sedi-
ment contains 40 to 60% by weight carbonate (5-7%mineral carbon), except for the intermediate interval463-71-1, 123 cm to 463-70-2, 53 cm, where the car-bonate content ranges from 0 to 30% (0-4% mineralcarbon).
Organic Carbon
The vertical distribution of organic carbon is shownin Figures 3 and 4.
Organic-carbon contents of 0.04 to 0.25% character-izes the two extreme carbonate intervals, whereas highercontents ranging from 0.07 to 7% define the intermedi-ate intervals. A detailed examination of the latter per-mits recognition of three main divisions (A, B, and C inFig. 4).
Interval A shows a marked change between 463-70-6, 134 cm and 463-70-6, 123 cm, where subdivisionAl, characterized by a low organic-carbon content(<0.25%), passes to the enriched subdivision A2(0.26-1.00%). The latter is interrupted by two richerlayers (>2%) at 463-70-6, 145 cm to 463-70-6, 35 cmand 463-70-5, 129 cm to 463-70-5, 100 cm.
Interval B has three subdivisions, with progressivelyincreasing organic-carbon contents: Bl, beginning at463-70-4, 78 cm, with a content of 0.40% and less;followed at 463-70-3, 117 cm by B2 (0.29-0.62%); andB3 at 463-70-3, 39 cm (0.51-6.94%).
Interval C has three subdivisions. The boundaries areat 463-70-2, 137 cm and 463-70-2, 91 cm. The increaseobserved for Interval B exists also in Interval C.
Pyrolysis Assay
METHODOLOGYA Rock Eval apparatus was used in pyrolysis. Pyrolysis assay
(Espitalié et al., 1977a, b) of raw samples was a simulation of theelemental analysis method for kerogen (Tissot et al., 1974). Twoparameters, oxygen index (OI) and hydrogen index (HI), allowcharacterization of the organic matter in the same way that the VanKrevelen diagram (H/C versus O/C) can be used for the kerogen con-centrate. Thus, they define the same three types of organic matter,types I, II, and III. Kerogens of type III derive mainly from plantdebris and humic continental matter. Kerogens of types I or II repre-sent aquatic organic material, in which the proportions of planktonic,nektonic, and benthic organisms depend on environmental factors.
Moreover, a correlation between the experimental temperaturereached at the maximum of hydrocarbon production during pyrolysis(S2 peak temperature) and the maturation stages of the kerogen wasestablished. The 400 to 435 °C range corresponds to the immature-kerogen zone, the 435 to 460°C range to the zone of main oil genesis,and beyond 460 °C to the gas or cracking zone.
Maturation of Studied Samples (Table 2)
For the interval 463-71-2, 31 cm and 463-69-3, 14 cm(interval of pyrolysis study), the significant tempera-tures range from 405 to 430°C. This corresponds to theimmature zone, where vitrinite reflectances are less than0.5. At such a low stage of maturation, the observedvariations of pyrolysis indexes are probably moredependent on the organic-matter composition than onmaturation. Some overmatured materials were alsodetected at various places in the interval; they corres-pond to temperatures of 490 °C and more and probablyrepresent high-temperature thermal alteration (volcaniceffects?).
Yield of Hydrocarbons (Table 2, Fig. 4)
If the yields of hydrocarbons (HC) obtained frompyrolysis of kerogen (= hydrogen index related toweight of rock) are considered (Table 2), the verticaldistribution (Fig. 4) is largely comparable to that oforganic carbon, according to the three following classes:(1) 0 to 0.20 mg of HC per gram of rock; (2) 0.21 to 1.50mg/g; and (3) 1.51 to 30.00 mg/g.
The relation between organic carbon and the yield ofpyrolysis HC indicates a relative homogeneous com-position for the analyzed organic matter. However,despite a fair correlation, some discrepancies are de-tected. In subdivision A2 for instance, comparable or-ganic carbon contents in 463-70-6, 22 cm and 463-70-5,47 cm (0.63 and 0.68%) correspond to HC yields of 0.97and 0.67 mg/g, respectively. Likewise, at 463-70-6, 13cm and 463-70-5, 1 cm (organic carbon of 0.53 and0.71%) HC yields are 0.56 and 0.30 mg/gram. Such dif-ferences in pyrolysis yield imply some variations inorganic-matter composition, and a detailed charac-terization of organic matter was therefore undertaken.
Characterization of Organic Matter (Fig. 5)
The samples richest in organic carbon (>3%) revealthe highest hydrogen indexes (>35O) and the lowest
910
LU
oO
gi—oLU
61 5r-
Q.<D
QEo
^ 6 2 0
625
70
71
SMECTITE CARBONATES(vol. % of o 50 100
total rock) ' ' ' ' 'MINERALCARBON
(wt. %)0 6 20 40 60 0 3 6 9 12
- 1<>
LU1-z
ORGANIC CARBON(wt. %) <
>OCLUH
HYDROGEN INDEX(mg HC/g of rock)
ORGANIC-MATTERCOMPOSITION
B
A2
I I > 1.00EHl 0.26 to 100• i 0 to 0.25
(wt. % of total rock)
D 1
r—
t
2 3 A b t
•
*
HI] > 1.50rm 0.21 to 1.50• i <0.20
(mg HC/g of total rock)
1131
1714
M
-Inert material
^^-•-Type lorll (marine!
d type I or II
Type III (terrestrial)
Undifferentiated
Figure 4. Organic-geochemistry profiles, Cores 70 and 71, Site 463.
F. MELIERES, G. DEROO, J.-P. HERBIN
Table 2. Geochemical data, Site 463, Cores 69 through 71.
Type I or II -Φ• Altered type I or II jgf Type I or II mixed with I
# Type III -Φ-Type III mixed with I or II Q) Residual organic matter
-Φ-Residual mixed with type I or II O Undifferentiated organic matter b Burrows
LEG 62, SITE 463
ORG. CARBON CONTENT(wt. %)
o 0.0 to .15
o .16 to .30
O .31 to .50
O .51 to 1.00
O 1.01 to 1.50
O 1.51 to 2.00
O 2.01 to 3.00
Q 3.01 to 4.00
r j ) 4.01 to 5.00
Cj 5.01 to 10.00
b b OO o
100 200 300 500 600 700 800
Oxygen Index
Figure 5. Pyrolysis assays, (hydrogen and oxygen indexes; data related to organic carbon).
913
F. MELIERES, G. DEROO, J.-P. HERBIN
oxygen indexes (<IOO). They contain predominantlyaquatic material of type I or II, and paleontologicalevidence from shipboard studies (Site 463 report, thisvolume) indicates a marine origin for this aquaticmaterial. An oxygen-poor depositional environment isrequired to preserve such material and to account forthe lamination of the deposits.
Marine organic matter also characterizes sampleshaving 1.9 to 2.5% organic carbon, but the lower HI(200-300) can be explained by dilution of the pyrolyz-able material either by inert carbon or by low-hydrogencompounds. More-open environments must have alter-nated with reducing ones during this sedimentation.
Samples with 1.20 to 0.50% organic carbon havesomewhat lower hydrogen indexes, but the variations ofOI identify a population with low OI (60-150) and onewith high OI (150-400). They both derive from marinematerial; however, inert and detrital organic matterdilutes the first, whereas the second should be enrichedwith oxygenated compounds.
In the vicinity of the evolution path of type III (HI100; OI 150-200), a mixed composition of marine type Ior II and of continental humic type III is assumed. Thenin the proper area for immature material derived fromhumic and plant debris of type III (HI 100; OI200-350) a predominance of detrital and inert organicmatter accounts for the lowest HI (<50).
Samples with less than 0.50% organic carbon gener-ally show large oxygen indexes (up to 300) and lowhydrogen indexes. Data are too far off reference pathsto be significant, and the related organic matter cannotbe identified. Hereafter, it is called "undifferentiated."
Samples with the lowest organic-carbon contents(<0.25%) show a nil hydrogen index, while the oxygenindexes are widely variable. Organic matter of thesesamples is considered to be residual material (Tissot etal.f 1979).
Marine organic matter of type I or II is autochtho-nous, and continental organic matter of type III (inertand residual) is allochthonous. The latter derives eitherfrom reworked sedimentary material or from eruptiverocks. Undifferentiated material supposes bad preserva-tion, which can occur both in autochthonous marineand allochthonous continental environments.
Vertical Distribution of Organic-Matter (Fig. 4)
The first marine organic matter appears at 463-71-1,123 cm, just above a carbonate interval at 463-71-1, 127cm, where the organic carbon is derived mainly from in-ert organic matter (Fig. 4). The inert material persists to463-70-6, 134 cm and characterizes subdivision Al.
At 463-70-6, 123 cm, organic-carbon content in-creases, but smectite decreases, and pyrite is present in-stead of siderite. Undifferentiated organic matter ispresent instead of inert matter from 463-70-6, 123 cm to463-70-6, 60 cm. Marine organic material reappears at463-70-6, 95 cm, then characterizes the top of intervalA2a from 463-70-6, 45 cm to 463-70-5, 100 cm.
An abrupt decrease in organic-carbon content isobserved at the base of the overlying interval (A2b). The
levels richest in organic carbon correspond to the largestamounts of smectite; organic matter derives mainlyfrom continental material (type III) mixed with inertmatter. These levels alternate with either undifferen-tiated or marine organic materials. In turn, the upperpart of A2b, from 463-70-5, 1 cm to 463-70-4, 84 cm ischaracterized by continental organic matter alternatingeither with mixed marine and terrestrial or undifferen-tiated organic matter.
Interval B, from 463-70-4, 78 cm to 70-3, 19 cm,comprises a basal zone (Bl) of undifferentiated organicmatter, as at the bottom of A2a and A2b. Then, an alter-nation of marine and continental material defines B2from 463-70-3, 117 cm to 463-70-3, 43 cm, as for A2b.Interval B3, with typical marine material, follows, andis like the upper part of A2a. In Interval B, the highestcontent of smectite (up to 6%) is found at the base ofsubdivision Bl.
Three zones are also present in Interval C (i.e.,463-70-3, 16 cm to 463-70-2, 53 cm). They include azone of undifferentiated organic matter (Cl), a zone ofmixed organic matter (C2) above 463-70-2, 137 cm, anda zone of marine organic matter (C3) above 463-70-2, 91cm.
Thereafter, the major change of carbonate contentobserved from 463-70-2, 46 cm corresponds to anabrupt impoverishment in organic carbon. The corre-sponding organic matter is undifferentiated material,except for two rich levels at 463-70-1, 96 cm and 463-70-1, 4 cm, where more or less altered marine material ispresent.
Organic Matter and Sedimentation
Sediments of the 11.3-meter studied interval containthree horizons of high organic-matter content whichcorrespond to a well-preserved marine organic matterand imply an oxygen-poor depositional environment.Also, influxes of inert organic matter (volcanic origin?)as well as terrestrial matter are defined. Terrestrial mat-ter is commonly mixed with more or less altered marineorganic matter.
The vertical distribution shows rhythmic sedimenta-tion of organic matter, and several sequences can bedefined. Each sequence comprises, from the base to thetop, (1) a lower member, where undifferentiated organicmatter is associated with light-colored sediments; bur-rowing is common and pyrite poorly represented inthese sediments; (2) a middle member, where terrestrialand more or less altered marine materials are mixed oralternate; and (3) an upper member, of wholly marineorganic matter, showing dark, fine parallel laminations.
The analyzed sequence begins with subdivision Al,where inert organic matter is predominant, and thenotable smectite content indicates influx of non-marinematter. An incomplete sequence with the middle mem-ber missing corresponds to A2a, whereas the upper ma-rine member is absent in the next sequence, A2b. Thetwo last intervals, B and C, correspond to complete se-quences with respective thicknesses of 2.08 and 1.13meters. An abrupt change is then observed at top of C:
914
APTIAN SEDIMENTS
the carbonate content increases, and an abrupt decreasein organic matter content indicates the reappearance ofundifferentiated organic matter.
CONCLUSIONSA 15-meter sequence of silica-rich sediments was
deposited during the early Aptian in the western Mid-Pacific Mountains (Site 463). The main feature of themineral assemblage is the fact that the most importantpart of the sediment now consists of diagenetic species.The sedimentary record of depositional conditions istherefore obscured. Nevertheless, it is possible to sketchthe history of the episode as follows.
In the vicinity of emergent land masses, a con-siderable amount of silica from altered volcanicmaterial was released into sea water through sustainedvolcanic activity. This silica was readily metabolized bythe siliceous plankton, the productivity of which in-creased markedly. Consequently, (1) large amounts ofamorphous silica (radiolarian tests) settled; this materialwas later diagenetically transformed into opal-CT,chalcedony, and clinoptilolite; (2) large amounts of car-bon dioxide were generated in the sea-water columnthrough oxidization of the organic matter, resulting indissolution of calcareous planktonic tests before theysettled; (3) a notable amount of organic matter was ableto reach the bottom of the sea and was incorporatedwithin the sediment because of the oxygen-poor deposi-tional environment and high sedimentation rate.
When the volcanic activity ceased, normal calcareousmarine sedimentation restarted, yielding oozes, laterdiagenetically transformed into limestones.
Except for some reworked material, an immaturestep of evolution is assigned to the studied organic mat-ter. The organic matter originated in two sources: anautochthonous source for the marine material, and anallochthonous source for the humic material and plantdebris. The latter probably originated from an adjacentland area. Some residual organic matter was alsodetected; it could have been derived from terrestrialmaterial influenced by volcanic activity.
An oxygen-poor depositional environment was re-quired to preserve the organic matter of marine originand to account for the several organic-rich and lami-nated layers.
Rhythmic sedimentation characterizes the distribu-tion of the organic matter in the studied silica-rich inter-val. This is based on two adjacent sequences in the up-per 3.30 meters of the interval. Each sequence shows anupward progressive increase in organic-carbon content;,it begins with a member of undifferentiated organicmatter, followed by a member of mixed marine and ter-restrial materials, then by a member of abundant, en-
tirely marine organic matter. The underlying 3.50meters are attributed to two incomplete sequences. Thebasal 2 meters of the interval contains predominantly in-ert organic material, as found in the underlying carbon-ate-rich and organic-matter-poor interval.
ACKNOWLEDGMENTS
The authors thank J. Thiede and T. Valuer, Leg 62 Co-ChiefScientists, for entrusting them with this study. The mineralogicalstudies received financial support from CNEXO (grant 78/1951), andF. Mélières warmly acknowledges Mr. Lenoble for his cooperation.DTA analyses were carried out at Université d'Orsay (France) undersupervision of Dr. A. Desprairies. The manuscript benefited from thereviews of Pr. R. Létolle (Université Pierre et Marie Curie, Paris) andDr. L. Montadert (Institut Français du Pétrole).
REFERENCES
Dean, W. E., and Schreiber, B. C , 1978. Authigenic barite, Leg 41Deep Sea Drilling Project. In Lancelot, Y., Seibold, E., et al.,Init.Repts. DSDP, 41: Washington (U.S. Govt. Printing Office),915-931.
Espitalié, J., Laporte, J. L., Madec, M., et al., 1977. Méthode rapidede caractérisation des roches mères, de leur potentiel pétrolier et deleur degré devolution. Rev. Inst. Franc. Pétrole, 32:23-42.
Espitalié', J., Madec, M., Tissot, B., et al., 1977. Source rock charac-terization method for petroleum exploration. Offshore Technol-ogy Conference, Houston, Texas, Paper 2935, pp. 439-444.
Heath, G. R., and Moberly, R., 1971. Cherts from the western Paci-fic, Leg 7, Deep Sea Drilling Project. In Winterer, E. L., Riedel,W. R., et al., Init. Repts. DSDP, 1, Pt. 2: Washington (U.S. Govt.Printing Office), 991-1007.
Jones, J. B., and Segnit, E. R., 1971. The nature of opal. I. Nomen-clature and constituents phases. / . Geol. Soc. Australia, 18:57-68.
Keene, J. B., 1976. The distribution, mineralogy and petrography ofbiogenic and authigenic silica from the Pacific Basin [Ph.D.dissert.]. University of California, San Diego.
Lancelot, Y., 1973. Chert and silica diagenesis in sediments from thecentral Pacific. In Winterer, E. L., Ewing, J. I., et al., Init. Repts.DSDP, 17: Washington (U.S. Govt. Printing Office), 377-405.
Mélières, F., 1974. Recherches sur la dynamique se'dimentaire duGolfe de Cadix (Espagne) [These]. Université Pierre et MarieCurie, Paris.
, 1978. X-ray mineralogy studies, Leg 41 Deep Sea DrillingProject, Eastern North Atlantic Ocean. In Lancelot, Y., Seibold,E., et λ.Jnit. Repts. DSDP, 41: Washington (U.S. Govt. PrintingOffice), 1065-1086.
., 1979. Mineralogy and geochemistry of selected Albiansediments from the Bay of Biscay, Deep Sea Drilling Project Leg48. In Montadert, L., Roberts, D. G., et al., Init. Repts. DSDP,48: Washington (U.S. Govt. Printing Office), 855-875.
Mélières, F., and Person, A., 1978. Genèse de smectites ferrifères paralteration deute'rique de la base de coulees volcaniques du MassifCentral Français. .Rev. Geogr. Phys. Geol. Dyn., 20:389-398.
Tissot, B., Deroo, G., and Herbin, J. P., 1979. Organic matter inCretaceous sediments of the North Atlantic: contribution tosedimentology and paleogeography. In Talwani, M., Hay, W.,and Ryan, W. B. F., (Eds.), Deep Drilling Results in the AtlanticOcean: Continental Margins and Paleoenvironment: Washington(Am. Geophys. Union), pp. 362-374.
Tissot, B., Durand, B., Espitalié, J., et al., 1974. Influence of the na-ture and diagenesis of organic matter in formation of petroleum.Bull. Am. Assoc. Petrol. Geol., 58: 499-506.
915
F. MELIERES, G. DEROO, J.-P. HERBIN
Bui -*"^^E^-•^^ * *"*•
2µm
Plate 1. Photomicrographs.
Figure 1-3. Successive SEM close-ups of a volcanic clast completelytransformed to smectite. XRD and DTA evidence an iron-bearingbeidellite, the structure of which is visible in Figure 3 (flexuouslamellae). This clast is not an aggregate of sedimented clay parti-
cles, but a fragment of altered volcanic material. Sample 463-71-1,32-33 cm.
Figures 4-6. Successive SEM close-ups of a smectitized volcanic clast.The original fluidal structure of the lava fragment is clearly visible.XRD and DTA data as for Figures 1-3. Such clasts, accompaniedby clinoptilolitized radiolarians, constitute the main part of thesediment. See also Plate 2. Sample 463-71-1, 32-33 cm.
916
APTIAN SEDIMENTS
Plate 2. Photomicrographs.
Figures 1-3. SEM (Fig. 1) and optical microscope (Fig. 2, polarizedlight; Fig. 3, crossed nicols) views of smectite-replaced volcanicclasts. Figure 2 suggests a high iron content, and Figure 3 revealsthe crystallized nature (smectite) of the clast. Note the clinop-
tilolitized radiolarians in the sediment. Sample 463-71-1, 32-33cm.
Figures 4-6. Example similar to that in Figures 1-3. Note the crys-talline nature (crossed nicols) of the volcaniclast. Sample 463-71-1,32-33 cm.
917
F. MELIERES, G. DEROO, J.-P. HERBIN
Plate 3. Photomicrographs.
Figure 1. Radiolarian test transformed into clinoptilolite Sample463-71-1, 32-33 cm.
Figure 2. Close-up of Figure 1 (central part), showing the crystallinenature of the radiolarian test.
Figure 3. Radiolarian replaced by and filled with large, euhedral clin-optilolite crystals. Sample 463-70-4, 18-20 cm.
Figure 4. Clinoptilolite crystals partly filling a fragment of radio-larian test. Note the coating of opal-CT on the inner wall of thetest. Sample 463-70-4, 18-20 cm.
Figure 5. Single clinoptilolite crystal within a broken radiolarian test.Sample 463-70-4, 18-20 cm.
Figure 6. Close-up of Figure 5, showing the perfect shape of the clin-optilolite crystal and the coating of opal-CT developed after thegrowth of clinoptilolite. Sample 463-70-4, 18-20 cm.
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APTIAN SEDIMENTS
Plate 4. Photomicrographs.
Figure 1. Intensely corroded radiolarian test, now consisting ofchalcedony. Sample 463-71-2, 142-144 cm.
Figure 2. Close-up of Figure 1, showing the most intensely corrodedpart of the radiolarian framework. The corrosion causes the silice-ous fragments to collapse, forming the matrix of the sediment.Sample 463-71-2, 142-144 cm.
Figure 3. Matrix of chalcedonic sediment consisting exclusively(XRD) of fragments of intensely corroded radiolarian tests. Sam-ple 463-71-2, 142-144 cm. HC1 etched.
Figure 4. Lepispheres of opal-CT in a dissolution cavity. Massiveopal-CT constitutes the matrix of the sediment. Sample 463-70-5,37-38 cm.
Figure 5. Lepispheres of opal-CT in a radiolarian test. Sample463-70-2, 91-92 cm.
Figure 6. Single crystal of clinoptilolite and blades of opal-CT. Sam-ple 463-70-4, 18-20 cm.
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F. MELIERES, G. DEROO, J.-P. HERBIN
Plate 5. Photomicrographs.
Figures 1, 2. Diagenetic euhedral calcite crystals and recrystallizedcalcitic biogenic remains. Sample 463-69-1, 140-142 cm.
Figures 3, 4. Non-recrystallized coccoliths (XRD data) in a clayeymatrix. Sample 463-70-5, 84-85 cm.
Figures 5, 6. Recrystallized limestone, note the abundant euhedralcalcite crystals and rare biogenic remains. Sample 463-71-4, 76-77cm.
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APTIAN SEDIMENTS
Plate 6. Photomicrographs.
Figure 1-3. Successive SEM close-ups of carbonized plant debris. Ra-dial puncticulations on tracheids identify a gymnosperm fragment.Sample 463-70-3, 19-21 cm.
Figures 4, 5. Two different views of a plant fragment. Note the punc-ticulations on the upper edge; note also the flattening throughcompaction. Sample 463-70-5, 105-108 cm.
Figure 6. Organic phosphatic remain (P and Ca from X-raymicroprobe data), crushed through compaction. The lamellarstructure indicates a fragment of a fish scale.
