von Rad, U., Haq, B. U., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 12217. PETROGRAPHY AND GEOCHEMISTRY OF ORGANIC MATTER IN TRIASSIC ANDCRETACEOUS DEEP-SEA SEDIMENTS FROM THE WOMBAT AND EXMOUTH PLATEAUS ANDNEARBY ABYSSAL PLAINS OFF NORTHWEST AUSTRALIA1Jürgen Rullkötter,2 Ralf Littke,2 Matthias Radke,2 Ulrich Disko,2 Brian Horsfield,2 and Jürgen Thurow3ABSTRACTTriassic (Carnian-Rhaetian) continental margin sediments from the Wombat Plateau off northwest Australia(Sites 759, 760, 761, and 764) contain mainly detrital organic matter of terrestrial higher plant origin. Althoughdeposited in a nearshore deltaic environment, little liptinitic material was preserved. The dominant vitrinites andinertinites are hydrogen-lean, and the small quantities of extractable bitumen contain w-alkanes and bacterialhopanoid hydrocarbons as the most dominant single gas-chromatography-amenable compounds.Lower Cretaceous sediments on the central Exmouth Plateau (Sites 762 and 763) farther south in general havean organic matter composition similar to that in the Wombat Plateau sediments with the exception of a smallerparticle size of vitrinites and inertinites, indicating more distal transport and probably deposition in deeper water.Nevertheless, organic matter preservation is slightly better than in the Triassic sediments. Long-chain fatty acids,as well as aliphatic ketones and alcohols, are common constituents in the Lower Cretaceous sediments in additionto n-alkanes and hopanoid hydrocarbons.Thin, black shale layers at the Cenomanian/Turonian boundary, although present at several sites (Sites 762 and763 on the Exmouth Plateau, Site 765 in the Argo Abyssal Plain, and Site 766 on the continental margin of theGascoyne Abyssal Plain), are particularly enriched in organic matter only at Site 763 (up to 26%). Theseorganic-matter-rich layers contain mainly bituminite of probable fecal-pellet origin. Considering the high organiccarbon content, the moderate hydrogen indices of 350-450 milligrams of hydrocarbon-type material per gram ofCorg, the maceral composition, and the low sedimentation rates in the middle Cretaceous, we suggest that theseblack shales were accumulated in an area of oxygen-depleted bottom-water mass (oceanwide reduced circulation?)underlying an oxygen-rich water column (in which most of the primary biomass other than fecal pellets is destroyed)and a zone of relatively high bioproductivity. Differences in organic matter accumulation at the Cenomanian/Turonian boundary at different sites off northwest Australia are ascribed to regional variations in primarybioproductivity.INTRODUCTIONDuring Ocean Drilling Program (ODP) Legs 122 and 123 aseries of holes was drilled to study the Mesozoic sedimentaryhistory on the continental margin off northwest Australia (vonRad et al., 1989). For this purpose, two transects were drilled onthe Wombat Plateau (Sites 759-761 and 764) and on the Ex-mouth Plateau down to the margin of the Gascoyne AbyssalPlain (Sites 762-763 and 766). These holes were complementedby drilling at Site 765 in the Argo Abyssal Plain (Fig. 1). Triassicearly rift sediments, unconformably overlain by Cretaceouspost-rift sediments, were recovered, whereas Jurassic sedimentswitnessing a major late-rift block faulting event were missing(Shipboard Scientific Party, 1990b). This situation is roughlysimilar to that on the Galicia passive continental margin offPortugal where drilling during Leg 103 recovered only pre- andpost-rift deposits (Boillot, Winterer, Meyer, et al., 1987).The Northwest Shelf of Australia, comprising four majorsedimentary basins (Carnarvon, offshore Canning, Browse, andBonaparte, from south to north, respectively), is one of thiscontinents important petroleum provinces (Purcell and Purcell,1988a) with estimated undiscovered reserves of 1500 millionbarrels of oil and 20 trillion cubic feet of gas (Purcell and Purcell,1 von Rad, U., Haq, B. U., et al., 1992. Proc. ODP, Sci. Results, 122:College Station, TX (Ocean Drilling Program).2 Institut fur Erdöl und Organische Geochemie, Forschungszentrum JülichGmbH (KFA), Postfach 19 13, D-5170 Jülich 1, Federal Republic of Germany.3 Institut für Geologie, Ruhruniversitàt Bochum, D-4630 Bochum, FederalRepublic of Germany.1988b). In contrast, exploratory drilling on the Exmouth Plateau,which forms a distal part of the Northwest Shelf of Australia,discovered only noncommercial dry gas accumulations, includ-ing the Scarborough Gas Field, and many of the commercialwells were abandoned dry (Barber, 1988). A minor petroleumgeneration potential was ascribed to some of the sediments onthe Exmouth Plateau (e.g., Exon and Willcox, 1978,1980; Cooket al., 1985; Barber, 1988), although lack of sufficiently deepburial apparently prevented its realization (Barber, 1988).It was our objective to characterize the distribution andcomposition of organic matter in deep-sea sediments fartheroffshore but stratigraphically equivalent to those studied earlier.Special emphasis was placed on the possible occurrence ofCretaceous black shales, particularly those at the Cenomanian/Turonian boundary which are well documented in many otherareas of the workTs oceans (e.g., Herbin et al. 1986; Arthur etal., 1987). Furthermore, we were interested in the effects of thesupply of terrigenous organic matter on the bulk preservation oforganic matter in continental margin sediments. Our organicpetrographic and molecular organic geochemical analyses ofdeep-sea sediments from the Exmouth Plateau are complemen-tary to those of Meyers and Snowdon (this volume) and Snow-don and Meyers (this volume).SAMPLES AND EXPERIMENTAL METHODSSamplesA total of 57 samples was collected for this study aboardthe JOIDES Resolution at eight sites during Legs 122 and 123(Fig. 1) with a bias toward sediments that gave the impression317J. RULLKOTTER ET AL.15°S17°19°21'Rankinl" J. fc Nortt‰kiπ IP JLL•^ Goodwyn Ius 1^ 0 Dampler ISaturn \ yj/p P North Tryal Rocks ISultanAUSTRALIA109°E111'113°115C117°119CFigure 1. Location of drilling sites on the northwest Australian continental margin during Legs 122 (solid circles) and 123 (open circles) (fromShipboard Scientific Party, 1990b). Also shown are locations of commercial wells (standard industry symbols). Locations of Sites 762 and 763,although shown slightly offset, actually are identical to those of commercial wells Eendracht I and Vinck I, respectively.of having high organic carbon contents