The Forebulge Migration and Its Influence on Sequence Stratigraphic Architecture of Back-Bulge in Triassic, Tarim Basin

Tang, Wu; Wang, Yingmin; Zhang, Lei

In Triassic, the Tarim Basin become a foreland basin as a result of the strong subduction of paleo-tethys ocean, a series of typical tectonic units (e.g. foredeep, forebulge and back-bulge) formed. Using a broad database comprising of outcrops, well-log and seismic data, this paper qualifies the forebulge migration and assesses its influence on sequence stratigraphic architecture of back-bulge. Our results suggest that the Triassic forebulge experienced three stages of reciprocating migration (stages 1, 2, and 3 roughly corresponding to Early, Middle as well as Late Triassic respectively).

Stage 1: At the beginning, the strata of the back-bulge are characterized by a northward decrease in thickness. When thrusting activities became active, the forebulge migrated northward to Tianshan mountain and then onlapped by back-bulge successions, forming a narrow foredeep. At the end, the study area was characterized by quiet tectonic activities, the forebulge stepped southward and the back-bulge successions were extensively eroded. Meanwhile, foredeep and back-bulge zone were separated by the forebulge, with a series of alluvial fans developed in front of foredeep.

Stage 2: Compared with Stage 1, the general characteristics of forebulge migration were similar. However, the forebugle migrated northward for a larger distance, leading to the back-bulge show a southward decrease in strata thickness. Similarly, back-bulge and foredeep were still two isolated depressions, separating by the forebulge.

Stage 3: The Tianshan mountain actived again, the forebulge stepped towards it and overlapped by back-bugle strata. At the end, the orogenic belts were subjected to erosion and unloading, with the lithosphere rebounded. The forebulge then migrated and uplifted toward to back-bulge again. Thus, the uplift amplitude was relatively small, leading to forebulge migrate under lake level and then sediments cross it. This sedimentary process, in turn, healed back-bulge and foredeep.

Additionally, episodic tectonic activities of forebulge have a profound influence on the sequence stratigraphic architecture. Each of sequence shows an overall fining upward trend, consisting mainly of coarse deltaic and fluvial sandstone or sandy conglomerate in the lower part, and muddy deposits in the upper part. This trend is similar to meandering river, whereas different from the typical sequence stratigraphic architecture, comprising of lower fining upward part and upper coarsening upward part.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013