幕式差异沉降运动对断陷湖盆中湖平面和水深变化的影响

EFFECTS OF EPISODIC DIFFERENTIAL SUBSIDENCE ON THE CHANGES OF LAKE LEVEL AND WATER DEPTH IN RIFT LAKE BASINS

湖平面(相对和绝对)和水深是控制湖盆沉积作用的两个基本参数。在断陷湖盆中,由于盆地边界断层的幕式活动,导致盆地缓坡带基底发生差异沉降,相对湖平面、绝对湖平面和水深等参数可出现不统一的变化,主要表现在以下两个方面:a)相对湖平面、绝对湖平面和水深3个参数间出现不统一性变化;b)相对湖平面或水深在湖盆的不同构造位置出现不统一性变化。即在同一沉积时间单元内,湖盆中水深既有增加带也存在减小带,或者相对湖平面既存在上升带也存在下降带;但是绝对湖平面对于同一湖盆来说具有统一性变化。幕式差异沉降运动控制了盆地可容空间的变化,这种非统一性变化是某段时间内新生可容空间量(△Va)与外界水净流入量(△Vw)和沉积物充填量(△Vs)3个参数的综合响应。特别当△Va>△Vw+△Vs时,基底差异沉降运动引起湖盆出现强制性湖退作用,形成层序界面。

Relative and absolute lake level and water depth are two essential parameters controlling lakebasin sedimentation. In a rift lake basin, the differential subsidence of basement in the gentle slope zone of the basin resulted from the episodic movement of boundary faults in the basin can lead to the discordant changes of relative and absolute lake level, water depth and other parameters, that is, the discordant changes of relative lake level, absolute lake level and water depth and those of relative lake level or water depth at different structural location of the lake basin. In the same sedimentary time unit, the water depth of a lake basin had both increasing and decreasing zones, or not only rising but also falling zones existed in relative lake level; but the absolute lake level to the same lake basin had concordant changes. Episodic differential subsidence controlled the changes of accommodating space in a basin. This kind of discordant changes were the comprehensive response of the neogenic volume of accommodating space(△Va), the net incurrent volume of external water(△Vw)and the infilling volume of sediments(△Vs)three parameters during a certain period of time. Especially when △Va>△Vw>△Vs, the differential subsidence of basement resulted in forced regression in a lake basin and formed sequence interfaces.