Early Mesozoic Paleogeographic Reconstruction and Hydrocarbon Implications for the Southern Ordos Basin, China, Based from U-Pb Detrital Zircon Geochronology

Xiangyang Xie¹, Paul L. Heller², and Kevin R. Chamberlain^2
1University of Texas at Austin, Institute for Geophysics, Austin, TX
²Geology and Geophysics, University of Wyoming, Laramie, WY

The discovery of the giant Xifeng Oilfield in the southwestern part of the Ordos Basin, China, has renewed interest in the paleogeographic evolution of this intracontinental basin. U-Pb detrital zircon (dZ) dating provides a tool for determining source area evolution as well as reconstructing clastic transport routes into the basin. Previous studies suggest that the southern Ordos Basin primarily received sediment from two bounding deformation belts — the Qinling orogenic belt (QB) to the south and the Western Liupanshan thrust belt (WLB) to the west. Detrital zircon analysis from three sites in the Yanchang Fm (Triassic age) demonstrate the interaction between these southern and western source areas. A total of 333 grains were analyzed using LA-ICP-MS. Three major age populations — 240-490 Ma, 1.8-2.0 Ga, and 2.2-2.8 Ga — with a few minor age groups characterize the dZ age spectra. The two oldest groups match ages of basement rocks that underlie the entire region and are interpreted to represent mostly recycled sediments. However, for the younger age group, even though a few minor age peaks can be correlated with the QOB, majority of those younger dZ ages have no known sources in either the QOB or the WLTB. Instead, suitable sources are only found in the Qaidam-Qilian terrane, over 500 km to the west-northwest of the WLTB. The abundance of sediment from this far western source area suggests that the WLTB had not yet formed at the time of deposition of the Yanchang Fm. Thus the western basin boundary likely laid much farther west than the present basin margin. If true, Mesozoic basins to the west, now separate from the Ordos Basin (Liupanshan and Dingxi basins) may have once been parts of a much larger basin. Therefore, these basins may contain as yet unexplored hydrocarbon reservoirs similar to, and once connected to, those found in the Xifeng Oilfield.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas