Petrography and Geochemistry of Organic Matter in Triassic and Cretaceous Deep-Sea Sediments from the Wombat and Exmouth Plateaus and nearby Abyssal Plains off Northwest Australia

Abstract

Triassic (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. Although deposited in a nearshore deltaic environment, little liptinitic material was preserved. The dominant vitrinites and inertinites are hydrogen-lean, and the small quantities of extractable bitumen contain w-alkanes and bacterial hopanoid 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 have an organic matter composition similar to that in the Wombat Plateau sediments with the exception of a smaller particle 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 addition to n-alkanes and hopanoid hydrocarbons.Thin, black shale layers at the Cenomanian/Turonian boundary, although present at several sites (Sites 762 and 763 on the Exmouth Plateau, Site 765 in the Argo Abyssal Plain, and Site 766 on the continental margin of the Gascoyne Abyssal Plain), are particularly enriched in organic matter only at Site 763 (up to 26%). These organic-matter-rich layers contain mainly bituminite of probable fecal-pellet origin. Considering the high organic carbon content, the moderate hydrogen indices of 350-450 milligrams of hydrocarbon-type material per gram of C org , the maceral composition, and the low sedimentation rates in the middle Cretaceous, we suggest that these black 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 primary bioproductivity.

Pb–Nd isotopic constraints on sedimentary input into the Lesser Antilles arc system

“…However, if black shales contaminated the Lesser Antilles lavas during their ascent through the Caribbean crust, the origin of the north-south Pb isotopic changes would still remain unclear because the black shales seem to be deposited all over the Caribbean plate. More generally, the presence of Cretaceous black shale units has been reported in other ocean basins (Sliter, 1989;Rullkötter et al, 1992) but none exists in any of the geochemically well characterized sites drilled in front of the main subduction zones (see Langmuir, 1998 andPlank et al, 2007 for further details). In the present situation, it is difficult to evaluate if similar effects can be found in other island arcs.…”

Geochemistry of oceanic anoxic events

“…In Tibet, TOC values of Cenomanian–Turonian boundary strata are generally <2 wt % [ Wang et al , 2001]. Continuing further east and entering the southern hemisphere, on the Exmouth Plateau off western Australia, black shales of this age reappear with TOC values locally rising to >25 wt % [ Rullkötter et al , 1992; Pancost et al , 2004]. In the Pacific Ocean, coeval organic‐rich sediments are recorded from oceanic plateaus (e.g., Shatsky Rise: TOC of ∼9 wt %; Hess Rise; the sediment burned when heated) as well as areas with greater paleodepth, such as the Mariana Basin, where TOC rises to ∼10 wt %.…”

Further evidence for the development of photic-zone euxinic conditions during Mesozoic oceanic anoxic events

“…Previous analyses of similar samples revealed TOC contents exceeding 25% (Rullko ¨tter et al 1992), whereas stratigraphically equivalent sediments at Site 762, 84 km to the north, do not exceed 2% (Haq et al 1990). Organic matter, consisting of structurally degraded bituminite, is inferred to be of marine origin, has hydrogen indices of c. 450 mg hydrocarbon g À1 C org , and is thermally immature (Rullko ¨tter et al 1992).…”