Types, Textures and Series of the Continental Sequences and Model of Sedimentary Basin-Fill in Rift Basin: A Case Study from Jizhong Basin, China
YANG Ming-hui~1 LIU Chi-yang~2 1 (Basin & Reservoir Research Center, University of Petroleum, Beijing 102249) 2 ( Department of Geology, Northwest University, Xi'an 710069)
The continental sequence deeply affected by natural geographic districts, geological forces and strata types, therefore occurred some kinds of types. There are three kinds of sequence types including alluvial sequence, lacustrine sequence and fluvial sequence that present in Jizhong basin during the syn extensional stage. The alluvial sequence is mainly composed of alluvial fan systems and grows up in the initial stages of the basin development. Here the relief contrast was strong, provenance supply was abundant and climate was arid, so the lacustrine area was narrow in proportion to the alluvial and fluvial area. The lacustrine sequence mainly made up of lacustrine sedimentary systems, and partially fluvial sediments and a little of alluvial interlayer. The stage was a period of rapid growth of the rift basin. The basin base subsidized fast and climate humid, lacustrine area larger, thereby gaining a period for hydrocarbon rock to form. The fluvial sequence began to form in last stage of the basin extend. The geographic contrast gradually was reduced. The deposit was far from the source region,and the area of the fluvial systems was deadly larger than the area of the lacustrine. The authors suggested that a polynomial relation exists between the lake level change and tectonic movement, provenance, climate factor and so on, the former only incarnates the synthetically changes of the latter. The sequence texture of the continental rift basin is under the control of the lake level change, like the sequence texture of the marine basin is controlled by sea level change. Therefore, the continental sequence may be trisection according to the lake level change. The sequences of the continental rift basin controlled by tectonic movement develop in succession, as the alluvial sequence located at the bottom, the lacustrine sequence at the middle and the fluvial sequence at the upper. This serial sequences is a response that sedimentary basin fill to the episodic tectonic movement. During the basin fill, there is clearly a difference among the sedimentary system tract growth of each sequence. In general, the lowstand system tract (LST) of the alluvial sequence is superior to the lacustrine and fluvial sequences. The LST of the lacustrine is absent and/or lack, but the transgressive system tract (TST) and/or highstand system tract (HST) deposited in the lake level rise are best of all development. The LST and/or HST deposited in the lake level fall of the fluvial sequence are the best in the each sequence. The tectonic movement is primary factor to control the continental sequence type, and the lake level change controls the texture of the sequences.