793 Geologische Rundschau 78/3 [ 793-806 ] Stuttgart 1989

Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments from the southern Black Sea margin By M. G. WIESNER,H. K. WONG and E. T. DEGENS,Hamburg*) With 4 figures and 2 tables

Zusammenfassung Die organische Substanz in den Sedimenten der Oberkreide und des Terti~irsder siidlichen Schwarzmeerregion ist dem terrestrischen bis marin-terrestrischen Bereich organischer Fazies zuzuordnen. Innerhalb dieses Bereiches weisen die stratigraphischen Abschnitte unterschiedliche organische Faziestypen auf, die auf unterschiedliche, die Akkumulation und den Erhaltungsgrad der organischen Substanz kontrollierende Prozesse zuriickzufiihren sind. W~ihrend des Obercampan-Maastrichtiums und des Pal~ioz~ins (fore-arc Becken) wurde organisches Material des Schelf/-hanges in den tieferen oxischen Beckenbereichen resedimentiert. Die rasche Zufuhr und Ablagerung flihrte zu einem gegeniiber den autochthonen Sedimenten h6heren Erhaltungsgrad an lipidreichen, terrestrischen Komponenten (Sporinit, Cutinit, Resinit). Die Zunahme an organischem Kohlenstoff mit steigendem Silt-/Tonanteil bei insgesamt niedrigen Kohlenstoffkonzentrationen in den Resedimenten l~if~tvermuten, dat~ die Akkumulation organischer Substanz in den Liefergebieten durch terrigene Zufuhr bestimmt wurde und die Akknmulationsbedingungen ungiinstig waren. Fiir das Eoz~in ist ein erh6hter Eintrag an marinem organischem Material zu verzeichnen, der mit der zunehmenden Verflachung des Ablagerungsraumes (fore-arc Becken) und einer Reduzierung im Sauerstoffgehalt des Bodenwassers (abnehmende Bioturbation) erkl~rt wird. Fiir das Mioz~in und Plioz~n (back-arc Becken) ist die organische Fraktion der Ablagerungen des Beckenrandes rein terrestrisch und besteht zum gr6f~ten Tell aus Inertinit und wieder aufgearbeitetem terrigenem Liptinit, die oxidative Verh~iltnisse anzeigen. Die Dominanz von Inertodetrinit im Mioz~in und Semifusinit im Plioz~in indiziert eine Anderung im Liefergebiet oder ein h/Sheres Energieniveau beim Transport bzw. im Ablagerungsraum der mioz~nen Randsedimente. Im Beckeninneren ist ein erh6hter Anteil an mariner organischer Substanz festzustellen, der auf Sauerstoffverarmung oder anoxische VerhSlmisse im Bodenwasser zuriickgefiihrt wird. Die Mineralassoziationen in den Sedimenten weisen auf vollst~ndige Sulfatreduktion und nachfolgende Methanogenese hin, die sich auch mit den Kohlenwasserstoffverteilungen nachvollziehen 15f~t. Periodisch oxische Bedingungen *) Author's address: Dr. M. G. WIESNHL Prof. Dr. H. K. "~,VONGand Prof. Dr. E. T. DEGENS,Geologisch-Pal~iontologisches Institut and Museum, Universit~it Hamburg, Bundesstrage 55, D-2000 Hamburg 13, Federal Republic of Germany.

fiihren zu einer Reduzierung der marin-liptinitischen Komponente. Im Beckeninneren dominiert jedoch auch die terrigene Fraktion (Hnminit/Vitrinit, Inertinit), was auf kontinuierliche Zufuhr vom Beckenrand schliegen l~igt. Die Sedimente der Oberkreide bis Plioz5n sind thermisch unreif (Rm < 0.5%, Tma~< 435 ~

Abstract The organic matter in the Late Cretaceous and Tertiary sediments from the southern Black Sea margin is assigned to the terrestrial-marine/terrestrial range of organic facies. Within this range, the stratigraphic section yields different organic facies types in response to different accumulation and preservation controlling processes. During the Late Companian-Maastrichtian, organic material from the shelf and slope was re-deposited in the deeper oxic parts of the basin. Rapid transport and sedimentation resulted in a higher degree of preservation of lipid-rich, terrestrial components (sporinite, cutinite, resinite) in comparison to the autochthonous sediments. The increase in organic carbon with increasing silt/clay content together with low carbon concentrations in the allochthonous sediments suggest that the accumulation of organic matter in the source areas was controlled by terrigenous influx and that the accumulation conditions were not favorable. In the Eocene (fore-arc basin), the higher content of marine organic matter can be explained by progressive shallowing of the environment and by reduced oxygen content in the bottom waters (reduced bioturbation). In the Miocene and Pliocene (back-arc basin), the organic fraction of the sediments from the basin margin is purely terrestrial and consists mostly of inertinite and reworked terrigenous liptinite indicating oxidative conditions. The dominance of inertodetrinite in the Miocene and of semifusinite in the Pliocene point to a change in the source area or to a higher energy transport or deposition conditions for the Miocene marginal sediments. In the basin interior, the higher content of marine organic matter is due to an oxygen deficiency or anoxic conditions in the bottom waters. Mineral associations indicate complete sulfate reduction and consequent methanogenesis. This is also implied in the hydrocarbon distributions. Periodic oxic conditions lead to a decrease in the marine liptinitic component. In the basin interior, however, the terrigenous fraction is still dominant, implying a continuous influx from the basin margins. The Late Cretaceous to Pliocene sediments are thermally immature (Rm < 0.5%, Tmax< 435 ~

794

M . G . WIESNER et al. R6sumd

La mati6re organique contenue dans les s6diments du Cr& tac~ sup&ieur et du Tertiaire de la pattie sud de la MeT Noire est ~ rapporter au domaine de facies organique terrestre marin-terrestre. La s&ie stratigraphique pr&ente, dans les limites de ce domaine, divers types de facies organiques qui traduisent les divers processus qui r6gissent 1'accumulation et la pr&ervation. Au cours du Campanian sup&ieur-Maastrichtien, des mat&iaux organiques provenant du shelf et du talus continental ont &~ red@os& dans les parties oxyg~n&s plus profondes du bassin. La rapidit6 du transport et de la s6dimentation a entrain& la pr6servation de composants terrestres riches en lipides (sporonite, cutinite, r&inite), dans une mesure plus 61ev& que dans les s6diments autochtones. L'augmentation de la teneur en carbone organique corr61ative ~ celle de la fraction fine (boue et silt), de m~me que la faible concentration en carbone des s6diments allochtones indique que, dans la r6gion-source, l'accumulation de mati~res organiques &air r6gie par un afflux terrigEne et que les conditions d'accumulation n'&aient pas favorables. A l'Eoc~ne (bassin d'avant-arc) le contenu en mati~re organique marine est plus ~lev6, ce qui s'explique par la diminution progressive de la profondeur et par la r~duction de la teneur en oxyg~ne des eaux du fond (bioturbation r6duite). Au Mio&ne et au Plio&ne (basin d'arri~re-arc), la fraction organique des s6diments de la bordure du bassin est purement terrestre et consiste principalement en inertinite et en liptinite terrig~ne remani6e, ce qui indique des conditions oxydantes. La predominance d'inertod&rinite au Mio&ne et de semifusinite au Plio&ne indique soit une source diff& rente, soit un transport ou un d@6t dans les conditions de plus haute 6nergie des s6diments mio&nes marginaux. Vers l'int&ieur du bassin, le contenu plus dlevd en mari~re organiqne marine est dfi ~ une d~ficience en oxyg~ne ou ~tdes conditions anoxiques dans les eaux de fond. Les associations rain& rales indiquent une r6duction complete des sulfates et en cons6quence une m&hanogen~se, ce qui ressort &galement de la distribution des hydrocarbures. Des conditions oxydantes p&iodiques provoquent une diminution du composant liptinitique marin. Dans l'int6rieur du bassin, toutefois, la fraction terrig~ne res~e dominante, ce qui implique un afflux continu depuis les marges du bassin. Les s~diments d'age cr&ac6 sup&ieur fi plioc~ne sont the> miquement immatures (Rm < 0,5%; Tmax < 435~ Kpa,lvxoe co~ep~arme OrpaHn~ec~oe Beu4eCTSO BhI;~e.~e~Hoe ~Ia ce~MeftTOB ~ow,Jaoro peruoHa q e p ~ o r o Mop~l, Bo3pacT KOTOpblX OTnOC~T K 13epx~teMy Meay ~ TpeWgqttOMy ~epno~Iy, cneJIyeT npIlq~ICn~ITb K MaTepI~KOBOMy lI MapHHO-MaTepHKOBOMy c~almlO. B aTOM pervloHe cTpaTnrpaqb~r OT~eJlbI OTHOC~ITC~IK paaJIrlqHMM T~InaM opraFtwqecKoro qbatlv6t, HTO CBSt3blBatOT C paan~qn~tMn B Hpoi~eccax, yHpaBn~eMbIXHaKOnneHIIeM oprauuqecKoro 8eiIIecTga u CTelIenb~o eoxpaHHOCTg epo. B nep~o)~ 13epxHero KaMnaHa-MaacTpi~xTa MB naaeot~eHe opraHHqecKgft MaTep~aa CKnOga menbqba nepeoTnoaoxnc~ ~ 6once rny6oK~e, HO ~ce >Ke OK11Cn~Ott~t~ec~

oTpe3KI4 6acceftHa. BblCTpt,Ift npHItOC MaTep~a~a a 6oasmaa CKOpOCTBOTJIOXeHIaN ero n p ~ s e a n K 6onbmeft cre~eHtt coxpamfocTH wepprarenH~,ix KOMnOHelrrOB, 60raT*,Ix OI~Inn~aMn (CTOpMHHT, KyTHHHT, pesmiVlT), no cpasnemno c TaKOShlMUaBTOXTOHHblXceamMeI~TO~. u aln~enne KOanqecTBa opralmqecKoro yraepo~a c nOB~IILIeHHeM KOalHqeCTBa CHnTa H FJIHHI/ICTOIIKOMIIOHetvrt~I npn 06It~eft nHaKO~IKOHI~eHTpaK11Hyrnepo~a 13nepeoTao~KeHHB[X ceRttMeHTax pa3pemaeT Ilpe~IlonaraT~, qTO HaKOnaeH~e opranH~eCKOft cy6cTattR~m B O6JIaCTH CHOCa 3a13nceno OT IlpHHoca Ty~a Tepp~reHHoro MaTepmla, H ~ITO yCgIOBIIa Rn~ ero 11aKonneItnH 6bla~t He 6narorrpH~THBIMH. Hepno~ ao~ella xapaKTepnayeTc~ noB~1meHnblM npuaOCOM opralmqecKoro MaTepnaaa MopcKoro n p o a c x o x genl,I~, qTO O6~bUCliStlOT 1303pacT51IOUlIaM oS~aeaeuneM o6aaCTH OTnOX(eI~n~(force-arc Basin) n yMem,mem~eM coalep~I~annn *mcnopo~la B rrpnalonnoft BOale (nonmKatomanca cMena 6uotlenoaoB). BO Bpe~a~ MgotteHa n nnlm~eHa (back-arc Basin) oprawaqecKa~ d o p a ~ a a OTamacem~fi ~ta Kpam 6acceftlla as~seTcz qnCTO MaTepnKoBOft n COCTOnTB 6om, mencTBe c~OeM na n~epT11nuTa n nepepa6oTallnoro Tepp~rengoro JIIelHTIlHHTa, HTO yKaabIBaeT Ha OKl,Ic,naTeJIbftble yCAIOB!a~t epe~3,~. FOeI1OJICT130rmeTpo;ReTpm~rra B MFlol~ettel,I CeMl/IdOy3l,lHHTa 13 nngot~eae roBopl,tT O cMeue O6JIaCT11 cuoca, m m o 6oaee BLICOKOM 3HepFeTHqeCKOM ypoBHe I~p*t TpaHcHOpTe, UaU g peruollax OT.IIO)KeHYdSI KpaeeNx ce~HMeHTOB MHoIIeHa. B i~ewrpe 6acceftHa OTMeRaeTc~t 31~IaqHTeJIbFlaYlnprlMeCb opraHwtecKoro MaTepHana MopcKoro IlpOncxox~elm~, KOTOpyIO CB~IaBIBaIOT C o6eJIHeHIIeM KI,IcnopoJIOM nplt~IOHIfOft13O~BI,110"Ill;laxce c 6ecKncnopo~ttbIMll ycnoBtIStM11B Heft. AccolmaKg11 MHaepaaoB B ceRHMeHTaX yKaaMBatOT Ha iio~Ittoe BOCCTaHOBnHHecep~I nocne~Iy~Olllee o6paaoBam~e MeTaHa, qTO IIOJITBepZ~JIaeTC~ *I paclIpeRenem~eM yram30~lopo~IoB. IlepHolI~necK11 no~tBa~fmmI~eest OKHCnlITenbFlble ycaomoI IIp11BORaT K yMeHsmeag~o amm~VlOft KOMnOUeI~TbIMopcKoro itpogcxox~e~mm B ~xenTpe 6accefiua npeo6na~aeT Bce me ~opa~IN MaTep!,IKOBOFO rlpoHcxoTgqReI-Imt(ryMHH11T H BHTpHFI~T, 14HepTmfHT), ~ITO FO13Op~T o HellpeKpaII~aionleMc:a npm~oce c MaTepnKo13oro ~pax. Ceav~MeHT~,I BepxHero ~eaa ~lo nnno~tena a13a~mTC~ "rep~nqecKri He 3pe:I~iMrI: (Rm < 0,5 % ; T ~ < 435 ~ C).

Introduction T h e g e o c h e m i s t r y of organic matter in postTortonian abyssal plain a n d slope sediments of t h e Black Sea has been studied by various investigators using a large n u m b e r of core samples f r o m R / V A T L A N T I S II cruise 49 (stations 1430 to 1487) (DEG~NS & ROSS, 1974) and D S D P Leg 42B (sites 379 to 381) (ROSS, NEPP,OCHNOV et al., 1978). A recent synthesis o f t h e results (DEGENS et al., 1986) d e m o n s t r a t e d that t h e q u a n t i t y a n d provenance o f t h e organic matter varied considerably in geological t i m e and geographic location, as well as w i t h variations in t h e depositional e n v i r o n m e n t recorded in lithological

Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments changes within the sedimentary sequence. All of the primary organic matter is thermally immature as is reflected in low vitrinite reflectance (Rm) values and in low temperatures of maximum pyrolysis yield. Nevertheless, it was recognized that for a given stratigraphic unit, the maturation level increases from the Black Sea margin to the basin centre as expected. However, significant differences in rank gradient within the sequences that could provide clues to changes in the rate of tectonic subsidence were not obvious (DEGENS et at., 1986). This finding has very important implications because, based on evidence of subaerial exposure and probable erosion in sediments on the slope and of very shallow water conditions on the basin apron, it has been suggested that the entire Black Sea was very shallow until as late as the Cromerian (0.7 m.y. ago) when it subsided by about 2000 m to its present abyssal depth (STOFFERS et al., 1978; SCHRADER, 1978; DEGENS et al., 1978). It could be argued though that in a young, rapidly subsiding basin, the equilibrium temperature as well as the equilibrium degree of coalification at any given depth may not have yet become established. On the other hand, the rank indicators used may not be appropriate for the overall low stage of coalification encountered (0.19% < Rm < 0.32%). An alternative to the subsJdence hypothesis is the assumption that the Black Sea has not undergone any large scale subsidence in the Quaternary and that the sedimentary characteristics described above result from a much lowered sea (or lake) level in a basin having a bottom morphology similar to that of today (HsO, 1978). The purpose of this study is to extend the data set for a reconstruction of the pre-Pleistocene history of the Black Sea margin by an organic facies characterization of Late Cretaceous and Tertiary sediments from Lhe shelf province between Istanbul and Sinop (Fig. 1; core sites not released). Organic carbon content, Rock-Eval pyrolysis yield, maceral composition and geolipid distribution are used as the most important parameters. A comparison is made between stratigraphically equivalent sediments from the shelf and from the slope and basin apron (DSDP sites 380 and 381). The results are discussed in terms of the origin and depositional environment of the organic matter, and its diagenetic alteration and maturation are delineated in a broader geological context by a comparison with pre-Late Cretaceous lithologies recovered from the coastal areas of northern Anatolia (core sites not released).

Experimental Methods Total organic carbon (TOC) was determined using a Carlo Erba 1104 C H N analyser after treatment of

795

the sediment samples with hydrochloric acid to remove carbonate carbon. The extractable lipids of the sediments were obtained by ultrasonic extraction (toluene/methanol 4:1) and subsequently desulfurized with activated copper. Thin-layer chromatography afforded several lipid fractions, of which only the nalkanes and acyclic isoprenoid alkanes are considered here. Gas chromatography (GC) was performed on a HewIett-Packard 5710A gas chromatograph equipped with a 25 m x 0.25 mm SE30 fused silica capillary column and programmed from 80-300 ~ at 4 ~ with hydrogen as carrier gas. Individual compounds were identified by comparing their GC retention times to those of an authentic standard injected under the same conditions. They were then quantified with respect to the internal standard. Rock-EvaI pyrolysis was carried out according to the method described by ESPITALII~ et al. (1984). Hydrogen index (HI) values representing yields of hydrocarbon-type compounds upon pyrolysis, normalized to organic carbon, were used as a bulk measure of the type of organic matter. Kerogen microscopy was performed on both whole rock samples and separated organic matter, The separation procedure included treatment with HC1 and HF followed by centrifugation in ZnBrz. The maceral types were analysed using both transmitted and normal or fluorescence reflected light. For a description of maceral types, the reader is referred to STACH et al. (1982) and GORMLY & MUKHOPADHYAY(1983). The maturation level of the sediments was assessed by huminite/vitrinite reflectance (Rm at 546 nm in oil, measured on particles larger than i0/zm) and by the temperature of maximum pyrolysis yield ( T ~ ) . Cretaceous and Tertiary Tectonic Development Starting with the earliest Upper Cretaceous, northward subduction of the Neo-Tethys became widespread (Fig. 1A; DERCOURT et al., 1986; SENGd3R & YILMAZ, 1981). The coastal and shelf areas of the Istanbul-Sinop segment (which belonged to the socalled Rhodope-Pontide Fragment) was north of the volcanic axis and was hence in a fore-arc position. Thick, Turonian-Cenomenian flysch sequences intercalated with volcano-clastics were deposited. During the Maastrichtian and the Paleocene, these flysch basins shallowed considerably. The Black Sea opened as a back-arc basin of this island arc (DERCOURT et al., 1986; LETOUZEY et al., 1977; SENG()R & YILMAZ, 1981). Nappe emplacement with concomitant subsidence e n m a s s e took place along the southern borders of the Rhodope-Pontide Fragment. By the middle Eocene, subduction of the Neo-

796

M.G. WIESNERet al.

tii

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Fig. 1. Paleogeography of the Turkey-Black Sea region during the Late Cretaceous-Paleocene(A), Early-Middle Eocene (B), Late Eocene-Early Miocene (C), and Middle Miocene-Pliocene (D). After SENGOt~& YILMAZ(1981).

Tethys along the Istanbul-Sinop segment was complete. Magmatism and pyroclastic and volcanogenic flysch deposition accompanied folding of the Pontides which reached its maximum during the Upper Eocene (Fig. 1, B and C). By middle Miocene to Pliocene time, squeezing of the Anatolide-Taurides by the converging Eurasian and African plates have led to the

development of the North Anatolian Transform (Fig. 1D; MCKENZIE, 1972; SENGOR & KIDD, 1979). Sedimentary facies Late Cretaceous and Paleogene In onshore northern Anatolia and along the southern Black Sea shelf, the Late Campanian-

Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments Maastrichtian stage which marks the shallowing forearc basin is represented by a series of calcareous flysch. This series consists of two basic sediment types: a thick allochthonous calciturbidite made up mainly of graded calcarenites and slump clasts containing shallow water fossils; and a thin autochthonous unit of marlstones which are intensely bioturbated with an ,excellent preservation of trace fossils. This limestone flysch extends to the end of the Paleocene with little or no perceptible break, but the fine and coarsegrained terrigenous siliciclastic components increase continuously in abundance. These deposits, particularly in the Late Cretaceous section, are intimately mixed with andesitic island arc volcanics in the form of tufts, pillows and tuffaceous sandstones (see also KAZMIN et al., 1986). Olistostromes are widespread (BRINKMANN, 1976). The overlying Eocene comprises an alternation of calcarenaceous beds (mostly at the base), sandstones containing pebbles and siltstones. It cannot be rated as true flysch since it has been deposited in a shallow marine environment (RIGASSI, 1971). Sediments definitely of an Oligocene age are missing ,an the shelf, since during this time, folding of the Pontides has just encompassed this area. The Middle or (?)Late Eocene sections are unconformably capped by Middle-Late Miocene or Pliocene strata.

797

Miocene to Early Pliocene in age containing intraclasts, dolomitized crusts, algal mats and oolites which suggest a supratidal evaporitic regime with subaerial exposure (STOFFERS & MIdLLER, 1978). Towards the basin apron, these strata grade into dolomitic siltstones deposited under shallow water, brackish-marine conditions. They are topped by brecciated sediments, which are coeval to the upper section of the supratidal facies of the slope, thus implying a progressive lowering of the sea (lake) level of the Black Sea (STOFFERS& MULLER,1978). On the shelf, this shoaling event probably coincides with the occurrence of the oolitic dolomites, but here indicators for subaerial exposure have not been found. The dolomites, therefore, might have been deposited in a somewhat disconnected, possibly lagoonal environment. The brecciated strata on the slope and basin apron are covered by an alternating succession of diatomaceous clay and seekreide. At the base of this succession, brackish-marine conditions prevailed in response to marine invasion of the Black Sea in the Early Pliocene (MURATOVet al., 1978). Higher in the succession, however, salinity decreased and the basin was gradually freshened (isolated) until the end of the Pliocene (SCHRADER, 1978).

Organic Facies Neogene On the shelf, the Middle to Late Miocene is represented by compound lithologies although the elastic phase dominates. On the shelf, the basal beds are conglomeratic and are succeeded by a series of variegated siltstones, sandstones and marls of a fluviodeltaic or shallow, brackish-marine origin. Occasionally, the sediments grade into lignite seams with the gastropod Patomides of coastal and brackish water affinities. The Late Miocene to Pliocene transition is characterized by a complex sequence of pebbly layers, marls, silts and in particular oolitic dolomites, dolomitic limestone fragments and beds with aragonitic shell debris indicating very shallow water conditions as well as enhanced salinities. The transition zone is followed by a succession of coastal sands, gravels and silts which extends to the Late Pliocene. Beyond the shelf edge, the oldest sediment recovered so far is Late Miocene in age. It consists of black siltstones that have been assigned to a freshwater environment on the basis of diatoms (SCHRADER,1978). However, this fossil assemblage may not be significant, since downhole displacement cannot be ruled out (SCHRADER, 1978). On the slope, the siltstones are overlain by brecciated dolostones of Latest

Late

Companian-Maastrichtian and Paleogene

In the autochthonous rock units of the Late Campanian-Maastrichtian and Paleocene series, the total organic carbon content (TOC) varies in concentration from less than 0.1% in the limestones to between 0.3 and 0.9% in both the marlstones and sikstones (Tab. 1). The latter two sediment types have been subjected to organic geochemical analyses and Table 1 shows that they are similar to each other in organic matter composition. The hydrogen index (HI) values lie between 90 and 190 mg HC/g TOC and plot within the range for kerogen type III (Fig. 2), thus implying a predominance of terrigenous organic constituents (efTISSOT & WELTE,1984, for a definition of kerogen types). Visual examination under the microscope revealed that most of the organic matter consists of huminite/vitrinite (ligno-cellulose components of higher plants) associated with inertinite (residual or oxidized ligno-cellulose components) and microbially altered amorphous humic and marine organic material (Tab. 1). The n-alkane distributions (Fig. 2) are dominated by the homologues C21 to C32 which yield odd over even preference index (CPI) values of over 2 (Tab. 2). This is indicative of higher

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grey silty to sandy marlstone

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[~ab. 1. Organic Facies in Late Cretaceous and Tertiary Sediments from the Southern Black Sea Margin

auminite/vitrinite; terrigenous fiptinite; amorphous humic matter; inertinite; phytoclasts

~uminite/vitrinite; inertinJte; amorphous marine organic ~aatter; amorphous humic matter

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huminite/vitrlnite; inertinite; amorphous humic matter; amorphous marine organic matter

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htmainite/vltrinite; amorphous marine organic matter, inertinite, phyto-/zooclasts

inertinite, bnmlnite/vitrinite; recycled/oxidized terrigenous liptinite

zuminite/vitrimte; amorphous ~aarine organic matter,spores, nertinite, phyto-/zooclasts

inertinite, huminite/vitrinite; recycled/oxidized terrigenous liptinite

Dominant Macerals d

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Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments

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Fig. 2. Hydrogen index-Tma~diagram and n-afkane distributions of the Late Cretaceous (L. Campanian-Maastrichtian), Paleocene and Miocene sediments from the Black Sea shelf and coastal areas. Dotted lines denote relative weight percent of pristane and phytane. Sample numbers (in circles) are the same as in Table 2.

been mineralized and, as a result, comparatively up to 45% of the total macerals. Thus, rehydrogen-poor terrigenous organic matter prevails in sedimentation played an important role in improving the sediment since it is more resistant to oxic decom- the preservation of land-derived lipid compounds by position (less susceptible to bacterial degradation; TIS- reducing the effect of long-term transport through SOT et al., 1979; WAPLES, 1983; BERNER,1984). The the oxic water masses and by ensuring rapid burial. scarcity of authigenic pyrite in both the marlstones The overall low TOC levels in the allochthonous and siltstones indicates that only limited amounts of rock units imply that conditions for the accumulareadily metabolizable organic matter were available tion of organic matter in the source areas (on the shelf for bacterial sulfate reduction (BERNER,1984). A limi- or upper slope of the fore-arc basin) cannot be rated tation by an insufficient supply of reactive detrital as favorable. The fact that in the calciturbidite seciron is a less likely explanation because the sediments tions, TOC increases with an increase in the contents contain abundant fine-grained chlorite. of clay and silt suggests that changes in the accumulaIn the allochthonous series, TOC concentrations tion of organic matter in the source area probably reare generally very low (< 0.1%). An exception are tho- flect changes in terrigenous influx from adjacent land. se rock sections which contain an appreciable admixture of fine-grained terrigenous siliciclastics and this is Eocene in particular the case in slump clasts consisting of clayey limestone (0.4-0.9%; Tab. 1). The HI values The TOC contents of the Eocene calcarenites, silty of these slump clasts are slightly higher than those in sandstones and siltstones cover ranges of less than the autochthonous sediments (less than 25% relative 0.2%, 0.1 to 0.6% and 0.5 to 1.3% respectively. In difference on the average; Tab. 1). Examination under comparison to the autochthonous marlstones and the microscope confirms that this is a result of the siltstones of the older sections, the Eocene sihstones presence of well-preserved terrigenous liptinites (spo- yield somewhat higher HI values of between 140 and finite, resinite and cutinite; Tab. 1) which comprise 250 mg H C / g TOC (Tab. 1). Terrigenous organic

800

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alkanes around C17 (Tab, 2; Fig. 2) which are usually regarded as indicators of planktonic (as well as bacterial) inputs (SIMONEIT,1978; FARRIMOND et al., 1986). The terrigenous signal in the region n - C 2 1 t o n-C32 is also present (Fig. 2). The gradual increase in TOC contents from the calcarenites to the siltstones is accompanied by a % decrease in bioturbation and vice versa. This suggests ~> an autochthonous deposition of the sediments in a progressively more anoxic regime; sedimentary struc~.~ tures indicative of slumping have not been observed in the core sections studied. Completely anoxic conditions in the bottom waters, however, are unlikely to have developed since (1) the siltstones usually contain "go a iow diversity epibenthic fauna (gastropods) and (2) the Pr/Ph ratio still lies above unity (1.3, see Tab. 2), . though it is lower than in the pre-Eocene samples, a fact possibly attributable to oxygen-depletion (DIDYK + et al., 1978). "~ The enhanced degree of preservation of marine organic matter in the siltstones may thus be related to reduced bottom water oxygenation and may have been further improved by the shallow water condi8 tions prevailing in the Eocene (see section on sedimentary facies). The burial of more highly reactive -~..==~ organic matter, in turn, promoted bacterial sulfate ~ reduction as is reflected in the larger amounts of early ~a.~ diagenetic pyrite compared to the pre-Eocene :~ siltstones and marlstones (up to 60% relative dif~ ference).

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matter is predominant (huminite/vitrinite, amorphous organic matter), but the kerogens contain appreciable quantities of phyto- and zooclasts which have been partially converted into amorphous material by bacteria (Tab. 1). An elevated contribution of marine liptinitic matter is consistent with the higher relative abundance of short chain length n-

and

Pliocene

Within most of the Middle/Late Miocene and Pliocene siliciclastic sections recovered from the Black Sea shelf, TOC contents are found to be less than about 0.2%. Higher values are measured for the Late Miocene variegated siltstones and carbonaceous silty sandstones (0.3 to 1.4%) as well as for the Early Pliocene silts (0.4 to 0.7%, Tab. 1). The HI values of the variegated siltstones and silty sandstones range from 10 to 100 mg HC/g TOC (Fig. 3 and Tab. 1) indicating a mainly residual and oxidized type of organic matter. Kerogen microscopy showed that inertinite together with huminite/vitrinite are the major macerals in both the Late Miocene and Early Pliocene samples (Tab. 1). Recycled vitrinites have also been noted. The terrigenous liptinites present (sporinite, cutinite) may account for up to 20% of the total maceraIs, but most of these particles appear to be recycled because the spore colour is largely brown (as opposed to the greenish-yellow colour of the few primary spores recognized).

Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments The n-alkane distributions consist almost entirely of the long chain wax alkanes of terrestrial origin as expected, but the odd-numbered homologues, are less pronounced than in the Late Cretaceous and Paleogene samples (Tab. 2, Figs. 2 and 3). A thermal maturation effect can be excluded because the Neogene sediments are still in the early stages of diagenesis (see section following). Hence, with respect to the inertinitic nature of the organic matter, the lower CPI values can be explained by a contribution of recycled (more mature) components to this nalkane range. The dominance of terrigenous recycled particulate organic matter provides evidence for oxidative depositional conditions along the margins of the back-arc basin during the Neogene. Of particular interest is the fact that the inertinitic fraction of the Late Miocene sediments consists largely of highly reflecting inertodetrinites (Rm > 2.0%) whereas in the Pliocene sediments, semifusinites with reflectivities mostly around 1.5% Rm prevail. This may point to significant changes in the source area or, since inertodetrinite is chemically very resistant (STACH et al., 1982), to higher ,energy transport and deposition conditions in the Late Miocene. The organic facies changes towards the interior of the back-arc basin. The Late Miocene siltstones and dolostones from the Black Sea slope (site 381) have TOC contents of 1.0 to 2.4%. Measurements on the total reduced sulfur content of the siltstones were carried out in order that the possible freshwater depositional conditions for these sediments as suggested by SCt-IRADEP, (1978) may be better evaluated. For the TOC range of 1.5 to 2.4%, the total reduced sulfur varies between 0.4 and 0.8%, yielding a TOC to total reduced sulfur ratio of 3.2 on the average. This value implies sufficient availability of dissolved sulfate during pyrite formation and thus at least brackish water conditions (BERNER & RAISWELL, 1984). Hence, as was stated by SCHRADER (1978), the freshwater fauna present in the sediments may be due to downhole displacement. The HI values of the siltstones and dolostones range broadly from 170 to 360 mg HC/g TOC (Tab. and Fig. 3). The kerogens consist of huminite/vitrinite (commonly the main maceral) associated with amorphous marine organic matter, inertinite and F,hyto-/zooclasts. In some of the samples, inertinite exeeds 30% of the total macerals. The (brackish marine) Pliocene clays obtained from the slope and the basin apron (site 380) yield TOC values between 0.8 and 2.0% and an HI of 280-460 mg/g TOC. Again huminite/vitrinite is the main maceral in co-occurrence with amorphous marine organic matter, terrigenous liptinite (spores), inertinites

801

and phyto-/zooclasts. The inertinites generally account for less than 20% of the macerals. Thus, the basinal sediments are distinguishable from their coastal counterparts by the presence of marine liptinitic components. This stands in agreement with the higher relative abundance of n-alkanes in the range of Cls to C20 (Tab. 2 and Fig. 3). Furthermore, the CPI values are above 2.5 (Tab. 2) and this may reflect the lower relative proportion of recycled terrigenous organic matter in the kerogens. Other distinct features are that phytane is the most pronounced component among the short chain n-alkanes and that the Pr/Ph ratios are below unity (Tab. 2 and Fig. 3). In addition, the pyrite found in the basinal sediments (up to 15% by volume) is generally associated with siderite of early diagenetic origin as evidenced by relict structures clearly showing replacement of the sediment matrix (see also ROSS, NEPROCHNOVet al., 1978, for a description of sediment structures). This mineral association is indicative of complete sulfate reduction and consequent methanogenesis (BERNER, 1981). Since methanogenic bacteria generate phytane during aerobic fermentation of organic matter (RIsATTI, 1984) and since such fermentation can occur under anoxic conditions deep in the sediments or at the water/sediment boundary, the low Pr/Ph ratios may record bacterial fermentation rather than anoxic depositional conditions. Nevertheless, based on the fact that the basinal sediments investigated exhibit a distinct lamination and that epibenthic (trace) fossils are scarce or lacking, an oxygen deficiency in the bottom waters during deposition is likely. The alternating succession of the basinal clays with seekreide in the Pliocene (see section on sedimentary facies) has been explained by an oscillating pynocline with oxic conditions during seekreide deposition (DEGENS et al., 1978). The seekreide beds are generally characterized by lower TOC levels compared to the clays and, according to the data of KENDRICK et al. (1978), the relative abundance of liptinitic material is also lower, while huminite/vitrinite and inertinite prevail. Thus, high oxygenation corresponds to a low degree of preservation of organic matter. This underscores the importance of bottom water oxicity as a controlling factor for organic matter accumulation in the basinal parts of the back-arc basin in the Late Miocene and Early Pliocene. The persistance of terrigenous organic material in these areas points to a continuous influx from the coastal zones.

Diagenesis and Maturation The huminite/vitrinite reflectance values measured for Pliocene and Late Miocene samples from the shelf

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803

Tab. 2). Hence, all of the pre-Late Cretaceous stratigraphic units are thermally mature and, in particular, the Early Cretaceous to Early Carboniferous strata plot within the zone of petroleum formation (435 to 465 ~ Tmax; see Fig. 4 and ESPITALIEet aL, 1984). The positions of the Early Cretaceous, Early Jurassic and Permo-Triassic samples in the HI-Tm~x diagram (Fig. 4) indicate a mainly terrestrial provenance of their organic matter. However, for the older sediments, a characterization is not possible because of their advanced stage of maturation. For the Silurian samples, terrigenous inputs can be precluded apriori because of the lack of higher plants in the Early Paleozoic. The n-alkane distributions imply distinct compositional changes with increasing rank: (1) There is a shift from a heavy-end biased to a front-end biased n-alkane distribution as reflected in an increase in the ratio of low to high molecular weight n-alkanes from 0.3 in the Early Cretaceous to 2.7 in the Middle Devonian (Tab. 1, Fig. 4); (2) The CPI values decrease from 1.51 to 1.03 and (3) The Pr/Ph ratio increases from a value of 5.5 (0.56% Rm) to 7.0 (0.68% Rm) and then decreases drastically to 0.90 in the Silurian sample (Tab. 2). Very similar compositional trends have been reported for coals and source-bed type shales bearing significant amounts of terrestrial organic matter. They have been explained by (1) thermal catalytic cracking of carbon/carbon bonds, (2) n-alkane generating reactions, or at higher rank levels alkane-cleavage reactions, and (3) asynchronous formation of pristane from kerogen during coalification (BROOKS et al., 1969; ALBRECHT et al., 1976; DURAND et al., 1977; RADKE et al., 1980). Thus, it might be interpreted that the organic matter initially delivered to the Early Carboniferous and Middle Devonian strata was mainly terrigenous. The Silurian sample also fits into the above trend because it is the most mature in the sequence and at such a rank, coals would yield comparable values (RADKE et al., 1980). The (TOC-normalized) n-alkane contents lie roughly between 4 and 8 mg/g in the Early Cretaceous to Permo-Triassic samples, and decrease rapidly to 0.1 mg/g in the Silurian sample. The highest value is measured in the Early Jurassic claystone which has a reflectivity of 0.68% Rm (Tab. 2). However, since the CPI of this sample is still higher than that for the Permo-Triassic (Tab. 2), the n-alkane maximum may reflect differences in the initial composition of the organic matter rather than the peak of hydrocarbon generation. Among the samples studied, the Early Carboniferous shale appears to be a promising source rock in view of its high TOC, high n-atkane content,

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Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments low CPI and its still appropriate maturity level (1.17% Rm). More detailed investigations are necessary to evaluate the hydrocarbon potential of this older series.

Conclusions (1) The organic facies in the Late Cretaceous and Tertiary sediments from the southern Black Sea margin varies significantly, but a generally strong influence of terrigenous organic matter can be recognized throughout the stratigraphic units. (2) During the Late Cretaceous and Paleocene (forearc basin), organic matter preservation was improved by rapid burial and by downslope transport, so that oxygenation, bacterial degradation and decomposition eli route were inhibited. Conditions for organic matter accumulation in the source areas were not favorable; the accumulation of the organic matter was probably governed by terrigenous influx. In the

805

Eocene, shallowing of the fore-arc basin and reduced bottom water oxygenation allowed much of the organic matter to be preserved. (3) During the Miocene and Pliocene (back-arc basin), oxygen-deficient conditions in the basin interior yielded the highest preservation potential for marine organic matter in the Tertiary. Highly oxidative depositional environments prevailed along the coastal zones of this basin and reworked organic material was dominant. There was a continuous transport of terrigenous organic matter from the basin margin to the centre. (4) All of the primary organic matter in the Late Cretaceous and Tertiary sediments is thermally immature. The substratum of these units along the southern Black Sea coast is mature, with the Early Carboniferous to Early Cretaceous shales in the principal zone of hydrocarbon formation. The Early Carboniferous yields the most promising source beds.

References ALBRECHT, P., VANDENBROUCKE,M. & MADENGUE, M. DERCOURT,J., ZONENSHAIN,L. P., Rrcou, L. E., KASMrN,V. G., LE PICHON, X., KNIPPER,A. L., GRANDJACQUET,C., (1976): Geochemical studies on the organic matter from St3ORTSHmOV, I. M., GEXSSANT, J., LEPVRIER, C., the Duala Basin (Cameroon) - I. Evolution of the extracPECHERSKY,D. H., BOULIN,J., SIBUET,J.-C., SAVOSTIN, table organic matter and the formation of petroleum. L. A., SOROKHTIN,O., WESTPHAL,M., BAZHENOV,M. Geochim. Cosmochim. Acta, 40, 791-799. BERNER, R. A. (1981): Authigenic mineral formation L., LAUER, J. R & BIJu-DuVAL, B. (1986): Geological evolution of the Tethys belt from the Atlantic to the resulting from organic matter composition in modern Pamirs since the Lias. - Tectonophys., 123, 241-315. sediments. - Fortschr. Miner., 59, 117-135. DIDYK, B. M., SIMONEIT,B. R. T., BRASSEL,S. C. &; EGLIN- (1984): Sedimentary pyrite formation: an update. TON, G. (1978): Organic geochemical indicators of Geochim. Cosmochim. Acta, 48, 605-615. paleoenvironmental conditions of sedimentation. -- & RAISWELL,R. (1984): C/S method for distinguishing Nature, 272, 216-222. freshwater from marine sedimentary rocks. - Geology, DUe,AND, B., NICAISE,G., ROUCACHE,J., VENDENBROUCKE, 12, 365-368. M. ~; HAGEMANN, H. W. (1977): Etude g6ochimique BRINKMANN,R. (1976): Geology of Turkey. - Enke, Stuttd'une s&ie de charbons. - In: Campos, R. & Goni, J. gart, 158 pp. (Editors): Advances in Organic Geochemistry 1975. ]3ROOKS,J. D., GOULD,K. & SMITH,J. W. (1969): Isoprenoid Enadimsa, 601-631. hydrocarbons in coal and petroleum. - Nature, 277, 284-287. EGL]NTON, G. & HAMILTON,R. J. (1963): The distribution of alkanes. - In: Swain, T. (Editor): Chemical Plant TaxDEGENS, E. T. & ROSS, D. A. (1974): The Black Sea onomy, Academic Press, I87-208. Geology, Chemistry, and Biology. - Amer. Assoc. Petroleum Geol. Memoir, 20, AAPG, Tulsa, Oklahoma, ESPlTALIg, J., MARQUIS, E & BAP,SONY, I. (1984): Geochemical logging. - In: Voorhees, K. J. (Editor): 633 pp. Proc. 5th Intern. Symposium Analyt. Pyrolysis. Butter--, STOFFERS,P., GOLUBIC,S. & DICKMAN,M. D. (1978): worth, 276-304. Varve chronology: estimated rates of sedimentation in the Black Sea deep basin. - In: Ross, 17).A., Neprochnov, Y. FARRIMOND, P., EGLINTON,G. & BRASSELL,S. C. (1986): P. et al. (Editors): Initial Reports of the Deep Sea Drilling Geolipids of black shales and claystones in Cretaceous Project. U.S. Govt. Printing Office, Washington, D.C., and Jurassic sediment sequences from the North Ameri42, Pt. 2, 499-508. can Basin. - In: Sommerhayes, C. R & Shackleton, N. --, WONG, H. K. & WIESN~R,M. G. (1986): The Black Sea J. (Editors): North Atlantic Paleoceanography. Geol. Soc. region: sedimentary facies, tectonics and oil potential. Lond. Spec. Publ., 21, 347-360. In: Degens, E. T., Meyers, P. A. & Brassell, S. C. (Editors): GORMLY,J. & MUKHOPADHYAY,1). K. (1983): Hydrocarbon Biogeochemistry of Black Shales. Mitt. Geol:Pal~iont. potential of kerogen types by pyrolysis-gas chromatograInst. Univ. Hamburg, SCOPE/UNEP Sonderband, 60, phy. - In: Bjoroy, M. et al. (Editors): Advances in Orga127-149. nic Geochemistry 198i. Wiley, Chichester, 597-606.

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HsO, K. J. (1978): Stratigraphy of the lacustrine sedimentation in the Black Sea. - In: Ross, D. A., Neprochnov, Y. P. et al. (Editors): Initial Reports of the Deep Sea Drilling Project, U.S. Govt. Printing Office, Washington, D.C., 42, Pt. 2, 209-524. KAZMIN, V. G., SBORTSHIKO,I. M., RICOU, L.-E., ZONENSHAIN, L. R, BOULIN,J. 8~;KNIPPER,A. L. (1986): Volcanic belts as markers of the Mesozoic-Cenozoic active margin of Eurasia. - Tectonophys., 123, 123-152. KENDRICK, J. W., HOOD, A. & CASTAND,J. R. (1978): Petroleum-generating potential of sediments from the Eastern Mediterranean and Black Seas. - In: Ross, D. A., Neprochnov, Y. P. et al. (Editors): Initial Reports of the Deep Sea Drilling Project. U.S. Govt. Printing Office, Washington, D.C., 42, Pt. 2, 729-735. LETOUZEY,J., BIJU-DLWAL,B., DORKEL,A., GONNARD,R., KR1SCHEV,K., MONTADERT,L. & SUNGURLU,O. (1977): The Black Sea: a marginal basin. Geophysical and geological data. - In: Biju-Duval, B. & Montadert, L. (Editors): Structural History of the Mediterranean Basins. Editions Technip, Paris, 363-376. McKENzE, D. P. (1972): Active tectonics of the Mediterranean region. Geophys. J. R. Astron. Soc., 30, 109-185. MURATOV,M. V., NEPROCHNOV,Y. R, Ross, D. A. & TRIMONIS, E, S.: Basic features of the Black Sea Late Cenozoic history based on results of Deep Sea Drilling Leg 42B. - In: Ross, D. A., Neprochnov, Y. R et al. (Editors): Initial Reports of the Deep Sea Drilling Project. U.S. Govt. Printing Office, Washington, D.C., 42, Pt. 2, 1141-1148. RADKE, M., SCHAEFER,R. G. & LEYTHAEUSER,D. (1980): Composition of soluble organic matter in coals: relation to rank and liptinite fluorescence. - Geochim. Cosmochim. Acta, 44, 1787-1800. RIGASSI, D. (1971): Petroleum geology of Turkey. - In: Campell, A. S. (Editor): Geology and History of Turkey. Tripoli, 453-482. RISATTI,J. B., ROWLAND,S. J., YON, D. & MAXWELL,J. R. (1984): Stereochemical studies of acyclic isoprenoids XII. Lipids of methanogenic bacteria and possible contribution to sediments. Org. Geochem., 6, 93-104. Ross, D. A., N~PROCHNOVet al. (Editors) (1978): Initial Reports of the Deep Sea Drilling Project. U.S. Govt. Printing Office, Washington, D.C., 42, Pt. 2, 1244 pp.

SCHP,ADI~R,H.-J. (1978): Quaternary through Neogene history of the Black Sea, deduced from paleoecology of diatoms, silicoflagellates, ebridians, and chrysomonads. In: Ross, D. A., Neprochnov, Y. P, et al. (Editors): Initial Reports of the Deep Sea Drilling Project. U.S. Govt. Printing Office, Washington, D.C., 42, Pt. 2, 789-901. SENO6R, A. M. C. & KIDD, W. S. E (1979): Post-collisional tectonics of the Turkish-Iranian Plateau and a comparison with Tibet. - Tectonophys., 55, 361-376. -- & YmMaZ,Y. (1981): Tethyan evolution of Turkey: a plate tectonic approach. - Tectonophys., 75, 181-241, SIMONEIT,B. R. T. (1978): The organic chemistry of marine sediments. - In: Riley, J. P. & Chester, R. (Editors): Chemical Oceanography, Academic Press, London, 7, 233-311. STACH, E., MACKOWSKY,M.-TH., TEICHMOLLER,M., TAYLOR, G. M., CHANDI