Stratigraphic architecture, depositional systems and controls on sediment filling in Nanber sub-basin, Tamtsag basin, Mongolia

Abstract

The Nanber sub-basin is a secondary and minor tectonic unit in the centre of the Tamtsag Basin, Mongolia. The structural and sedimentary evolution of the depositional systems were significantly affected by active faults at the basin boundaries. In this paper, full coverage of the study area is achieved using high-quality 3D seismic and comprehensive drilling, test data, and analysis, while the stratigraphic architecture is finely characterized in an isochronal framework. The study reveals that the infilling of the lacustrine basin in the Early Cretaceous can be divided into three different evolutionary stages: syn-rift, rift climax, and post-rift. These evolutionary stages have different sequences and sedimentary systems: (1) the syn-rift sequence stage (SQ1+SQ2), which is characterised as “narrow basin-shallow water”, developed primarily from fan-delta sedimentary systems; (2) the rift climax sequence stage (SQ3), which is characterized as “wide basin-deep water”, developed primarily from fan-delta and nearshore subaqueous fan sedimentary systems; and (3) the post-rift sequence stage (SQ4), which is characterized as “wide basin-shallow water”, developed primarily from braided river delta sedimentary systems. The varying tectonic evolutionary stages formed different types of stratigraphic structures and sedimentary fillings. Simultaneously, differential tectonic settlement and fault block rotation controlled the different palaeogeomorphology backgrounds in the basin. The fault-scarp formed by rapid subsidence of the boundary faults, which predominantly dictated the formation and development of fan-deltas. The fault-terrace formed by differential settlement and controlled the formation and development of fan-deltas and subaqueous fans, while the secondary fault-break in the basin largely controlled the formation and development of sub-lacustrine fans. During the post-rifting period, most of the boundary faults that had been active, such as controlling traps, basically ceased, and formed a braided river delta depositional system characterised by widely developed sedimentation along the long-axis of the basin. Thus, under conditions featuring plenty of sediment and weak tectonic activity, the key factor controlling sequence stratigraphy and sedimentary system filling patterns within the lake basin was the abundant source supply, not tectonic activity.