Geological Controls of Source Rock Geochemistry Through Relative Sea Level; Triassic, Barents Sea

Abstract

Lower to Middle Triassic mudrock cores from the Svalis Dome (Dia-Structure); (Barents Sea, Norway) demonstrate the control of depositional conditions on the physical and chemical properties of mudrocks deposited in offshore and shelfal environments. Five cores (112 m) of Smithian to Late Anisian age were analyzed in detail to establish their environment of deposition. The details of the depositional environment were revealed by changes in lithology, bedding geometry, sedimentary structures, and trace-fossil assemblages, as well as bulk and molecular geochemical parameters. The redox condition at the sediment/water interface in these offshore and shelfal environments is likely to have been dysoxic. Inorganic and bulk-organic geochemical indicators (major and minor elements, trace metals, natural radioelements; organic carbon, HI, OI) provided additional parameters to define the facies within each depositional environment. Ni/Ni + V and Al2O3/TOC ratios are both good indirect measures of the quality of the organic matter; each facies has a characteristic range of these ratios. Saturate and aromatic hydrocarbons show an early mature to immature molecular distribution. Estimates of maturity from biomarkers suggest a vitrinite reflectance value of 0.4%Ro, whereas measured vitrinite reflectance values are in the range of 0.3 to 0.5%Ro. Pristane/phytane ratios support the interpreted dysoxic to anoxic conditions near the sediment-water interface and within the sedimentary column. Triterpanes are present in near-equal or greater quantities than steranes, with hopane/sterane ratios of 1.1 to 3. Sterane distributions show a predominance of C29 regular steranes and appreciable quantities of C30 desmethylsteranes, indicating mostly marine-algal organic matter in the kerogen. This study demonstrates the integration of molecular geochemistry with sequence stratigraphy. Steranes and triterpanes show predictable variations within each systems tract. C30 desmethylsteranes show a quantitative increase in absolute concentration that correlate with the second-order rise in sea level during the Lower to Middle Triassic.