ANALYSIS OF THE CHIFENG BASINS, INNER MONGOLIA: IMPLICATIONS FOR BASIN MODELING AND CRETACEOUS EXTENSION IN NORTHEAST CHINA AND SOUTHERN MONGOLIA

Cretaceous extension in northeast China and southern portions of Mongolia ranges from high-strain extension, characterized by low-angle detachment faults below supradetachment basins to low-strain extension, characterized by higher-angle normal faults that bound half-graben basins. The Yanshan mountain belt in Inner Mongolia, China, contains a pair of Cretaceous-aged basins near the city of Chifeng that address both of the extensional styles present in this region. The west basin is bounded on the east by a normal fault with dips ranging from 60 to 70 degrees while the east basin is bounded by a low-angle normal fault with dips between 24 and 50 degrees.

The basins near Chifeng are filled with three facies assemblages. Along the fault bounded margin of each basin a cobble to boulder, clast supported, angular to well rounded conglomerate is dominant. These deposits are composed of sediment shed from the footwall of the previously mentioned master faults and volcanic deposits located in the immediate vicinity, and are located proximally to the source. In central portions of each basin, the facies assemblage consists of well sorted, coarse to medium grained, sub-angular to rounded, lenticularly bedded sandstones interbedded with organically rich mudstones, coals, and channelized conglomeratic packages. The third facies assemblage are felsic to mafic volcanic flows and are deposited throughout the basin fill. Paleocurrent indicators, primarily in the form of imbrication, along with clast count data show provenance directly off the structure between the basins.

Proposed models for supradetachment and half-graben basins suggest that each of these basin types have distinctive traits that should be present. Although the Chifeng basins have dissimilar structural geometries their sedimentological characteristics are alike and most closely resemble half-graben models. Thus, we infer that additional variables beyond fault dip angle, such as magnitude and rate of extension are primary controls the geometry and sedimentological characteristics of extensional basins. The data collected in these basins gives further insight to the dynamics of Cretaceous extension within this region while also providing additional information to incorporate into the model for sedimentary basins bounded by low-angle normal faults.