Micropaleontology and isotope records from mid-Cretaceous organic-carbon-rich lacustrine sediments in Songliao Basin, northeast China

Category Geochemistry
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wan, Xiaoqiao۱; Huang, Qinghua۲
Holding Date 20 September 2008

The Songliao Basin was a big lake during mid-Cretaceous time, when bituminous strata were widespread. The biostratigraphy is well studied, but little has been done on the correlation between lacustrine sediments with standard marine sequence, and on the lacustrine environmental changes. We collected and analyzed ostracods, sporopollen from drill cores of the organic-rich Qingshankou to the overlying organic-poor Yaojia formations, in a pilot feasibility study of stable isotopic chemostratigraphy. In ascending order, ostracodes are recognized as Triangulicypris torsuosus-T. torsuosus var. nota, Cypridea panda-Triangulicypris fusiformis, Limnocypridea inflata-Sunliavia tumida-Triangulicypris symmetrica and Cypridea dekhoinensis-Limnocypridea copiosa assemblages; and sporopollen as Cicatricosisporites-Cyathidites- Pinuspollenites and Balmeisporites-Cyathidites-Classopollis assemblages.
The biotic diversity and abundance decline stepwise toward the boundary between two formations, marking a major extinction event within the lake. Bulk rock and kerogen organic carbon isotope and TOC data have been measured from the sequence. By Rock Eval pyrolysis, the organic matter is III and II2 types, indicating a terrestrial origin. Two positive excursions occur from both lower and top levels of the Qingshankou Formation. At the lower level δ 13C shift from about -30‰ to -26.77‰ with TOC of 20.34%. The C isotope profile of the top shows a positive excursion from values -29.77‰ to -22.9‰ just below the formational boundary, where also shows a biotic crisis. Organic geochemical characters are illustrated by biomarkers of 28,30-bisnorhopane and gammacerane. The Qingshankou interval sampled record the most extensive lacustrine transgressions in the Songliao basin, which occur as black shale intervals over most areas of basin.
The paleohyrdology of the Songliao lacustrine system varied significantly during the mid-Cretaceous, as indicated by shifting deposystems patterns and ostracode paleoecology. It is suggested that the dark mudstones and shales in the Qingshankou Formation are probably the records of lake anoxic periods. They are seemly comparable to that seen in marine sections in mid-Cretaceous. Therefore, a temporally correlation has been proposed. The lower isotopic shift links to OAE 1d and upper one to OAE 2 respectively. The top peak of Qingshankou Formation would appear that the late Cenomanian biotic crisis, hitherto only known from the marine realm, was also seen in lacustrine sections pointing to disturbance of the entire global ecosystem. The elevated burial of organic C in the Songliao Basin at this time points to a further similarity with the world’s oceans. This work was supported by the National Natural Science Foundation of China (Grant No. 40672020), National Basic Research Program of China (No. 2006CB701403)