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Depositional Environment and Stable Carbon Isotope Chemostratigraphy of the Early Cretaceous Xiagou Formation in Changma Basin, Gansu Province, China

Suarez, Marina B.1; Gonzalez, Luis A.1; Ludvigson, Gregory A.2; You, Hailu 3
1 Department of Geology, The University of Kansas, Lawrence, KS.
2 Kansas Geological Survey, Lawrence, KS.
3 Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China.

Sedimentary basins in northwest China contain terrestrial Mesozoic rocks that have produced significant amounts of fossil fuel resources. In addition, important fossil discoveries are improving our knowledge of the evolution of many modern groups of taxa during the Cretaceous, such as birds. The Xiagou Formation occurs in outcrops along the Altyn Tagh fault system in the northwestern edge of the Hexi Corridor. Much of the strata in this region, however, lack from significant means to constrain ages. This study utilizes sedimentary bulk organic δ13C chemostratigraphy and total organic carbon to help constrain ages of the fluvial-lacustrine Xiagou Formation, as well as carbonate carbon and oxygen stable isotopes to provide data on lake conditions. Bulk organic δ13C chemostratigraphy has proven useful in correlation of Jurassic and Cretaceous terrestrial and marine sequences because perturbations in the global carbon cycle by oceanic anoxic events also affected terrestrial carbon reservoirs. Two sections were collected systematically at 1 meter intervals. δ13C values range from -31.83‰ (VPDB) to -20.92‰ (VPDB), with an overall increasing trend from the base of one of the sections. Values for the second section range from -33.18‰ (VPDB) to -20.66‰ (VPDB), and have a similar increasing trend. These isotopic profiles can be correlated with each other as well as with the marine carbon isotope excursion known as the “Selli Event” which is associated with the Early Aptian Ocean Anoxic Event 1a.

Carbonate stable isotope geochemistry of carbon and oxygen are characterized by both variable carbon and oxygen values. Oxygen values range from -7.67‰ to 1.23‰ (VPDB), and carbon values range from 5.98‰ to 11.13‰ (VPDB). Some horizons have positive linear covariant trends in δ18O vs. δ13C space, indicating that the lake experienced evaporative losses to the atmosphere, with resulting isotopic enrichment. The δ13C enrichment, particularly those in excess of 10 ‰ are attributed to methanogenesis via CO2 reduction, indicating that the lake experienced episodic anoxia. Interestingly, total organic carbon values of the lacustrine shales range from 0.47% to 15.80%. The rapid sedimentation and lack of oxygen in the sediments were favorable conditions in the superb preservation of vertebrate and invertebrate fauna, including the primitive bird Gansus yumenensis.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009