断陷湖盆陆相层序中体系域四分性探讨──泌阳断陷下第三系核桃园组为例

DISCUSSION ON SYSTEM TRACT FOURDIVISION FOR A TERRIGENOUS SEQ UENCE STRATIGRAPHY IN FAULTDEPRESSED LACUSTRINE BASIN

断陷湖盆中一个完整陆相层序的体系域具四分性，即低水位体系域（ＬＳＴ）、水进体系域（ＴＳＴ）、高水位体系域（ＨＳＴ）及水退体系域（ＲＳＴ），该四分体系域具显著的几何构型特征，它们分别由低位加积小层序组、退积小层序组、高位加积小层序组及进积小层序组构成。陆相层序四分体系域是一个沉积基准面旋回所引起的可容空间旋回性变化的沉积响应。对于断陷湖盆的一个完整陆相层序，其体系域划分原则为：湖相厚层泥岩代表高水位体系域，它是水退体系域和水进体系域的分界；初始湖泛面之上为水进体系域，其下为低水位体系域。断陷湖盆中一个完整的陆相层序自下而上依次由下列层序单元构成；层序界面（ＳＢ）→无湖岸上超的低水位体系域→湖岸显著上超的水进体系域→由沉积速率缓慢的垂向加积型湖相泥质岩构成的高水位体系域→湖岸显著退却的水退体系域→层序界面（ＳＢ）。

A complete terrigenous sequence in faultdepreesed lacustrine basin can be divi ded into four system tract: Lowstand System Tract (LST), Trangressive System Tra ct (TST), Highstand System Tract (HST) and Regressive System Tract (RST), which has typical geometrical configuration and respectively consists of lowstandagg radation regression, highstandaggradation and progradation. The fourdivided system tracts in a complete terrigeous sequence were sedimentational response to accommodation change resulting from a sedimentary baselevel cyclicity. The prin ciple to divide system tracts for a complete terrigeous sequence in faultdepre ssed lacustrine basin is that the lacustrine thicklayer mudstone represents HS T and boundary between RST and TST, and that the above first flooding surface (f .f.s) is TST and below f.f.s is LST. From bottom to top, a complete terrigeous s equence in faultdepressed lacustrine basin consists of the following sequence units in turn: Sequence Boundary (SB)→Lowstand System Tract (no coastonlap)→ Trangressive System Tract (distinct coastonlap)→Highstand System Tract being composed aggradational lacustrine mudstone with low sedimentation rate→Regressi ve System Tract (distinct coastdropback)→Sequence Boundary (SB).