Tectono-Sequence Stratigraphy of Lower Cretaceous in Tamtsag Basin, Mongolia: Sequence Architecture, Depositional Systems, and Controls on Sediment Infill

Zhou, Yong; Ji, Youliang

Tectonics is very important to the depositional record preserved in nonmarine sedimentary basins. The episodic syndepositional faulting and the related paleogeomorphology controll both the formation of sequence boundaries and the stratal stacking patterns. In this study, the sequence architectures, depositional systems and controls on sediment infill of the Lower Cretaceous in Tanan Depression, Tamtsag Basin in Mongolia were investigated using seismic profiles, cores and well logs.

Based on the identification of unconformities of different hierarchies, three second-order sequences and four third-order sequences are identified in the Lower Cretaceous lacustrine rift-basin successions. According to the subsidence rate and the intensity of tectonic activity, three types of lacustrine sequences, consisting of distinctive depositional systems, were distinguished: (1) simple half-graben sequences developed during the initial rifting stage; (2) tectonic-rollover sequences developed in response to rapid and differential tectonic subsidence during the climax -rifting stage; and (3) depression sequences formed during the late-rifting stage.

The sequence development is mainly controlled by tectonics and sediment supply. Due to the differential tectonic subsidence rate and sediment supply, the accommodation/sediment supply ratio (A/S) varies greatly in both different tectonic positions and stages of rifting, resulting in a wide variety of stratal stacking patterns.

Through episodic rifting and differential subsidence, various types of transfer zones and structural slope-break zones were formed, both of which play significant roles in formation and distribution of different types of sequences and depositional systems within. Transfer zones controlled the locations of sedimentary provenances, entry points of sedimentary material into the basin, and the resultant development of depositional systems. The structural slope-break zones of Tanan Depressoion during the climax rifting stage of K1n would be subdivided into four types from the steep slope to the gentle slope, they are: fault scarp zones, fault terrace zones, intrabasinal fault break zones and gentle slope zones.

Furthermore, three fault array patterns were identified according to their distribution on the structure map, which are "comb-like", "fork-like and "parallel". The structural slope-break zones mainly influence the distribution of depositional systems and sand bodies, the sand bodies are mainly accumulated at the lower part of tectonic slope-break zones, and the rift-interior sediment dispersal directions are consistent with the strike of the slope-break zone.

Areas where the structural slope-break zone overlapped with transfer zones are sites for major drainage systems and the optimum locations of fan deltas and sublacustrine fans. The sand bodies deposited here are favorable targets for the exploration of litho-stratigraphic traps in Tanan Depression.

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