裂陷盆地重力流沉积对基准面变化的响应--以乌里雅斯太南洼腾一下亚段为例

在重力流沉积区,沉积物粒度旋回难以反映基准面的变化情况,但重力流流态与类型对地形变化具有良好的响应关系。利用上述关系对重力流沉积区基准面变化的研究方法进行了探讨。研究区相对古地貌恢复结果表明,乌里雅斯太南洼腾一下亚段沉积时期控坡断层的活动性在各层序内存在明显差异,由于这种差异性导致凹陷在地形上经历了由缓至陡的变化过程,相应地,可容纳空间经历了缓慢增长、加速增长两个时期。钻井取芯、三维地震反演等资料显示,在地形与可容纳空间的变化过程中,重力流类型也随之发生变化,沉积物沉积位置亦发生迁移。因此推断重力流流态特征与沉积位置对地形以及可容纳空间变化存在一定的响应关系:断裂活动性强,可容纳空间增长速率高,地形陡,以浊流或砂质碎屑流沉积为主,沉积物主要沉积在断坡下方;断裂活动性弱,可容纳空间增长速率低,地形缓,以强黏结性碎屑流沉积为主,沉积物沉积范围广。综上认为利用重力流流态特征与类型转换可以判断地形变化趋势,进而推断基准面的变化情况。

Responses of Gravity Flow Deposits to Base-Level Variation in Rift Basin Using a case Study of Lower Teng1 Formation in South Wuliyasitai Sag

In gravity depositional area although the sediment grain-size cycle cannot reflect the base-level changes, the flow condition characteristic and types of gravity flow have an apparent response to the landform. Based on this response, our study discusses how to investigate the base level variation in gravity depositional area. A difference in activity rate of the synsedimentary faults that made slope breaks of Teng 1 Formation, southern Wuliyast sag, is shown within different medium cycles through relative paleogeomorphic restoring and analysis of strata thickness. Because of the difference, the landform of the study area underwent a flat to steep process with the accommodation rate changing from slow to fast. Within the process, the flow condition characteristic and deposition location of gravity flow show a regular changes through core observation and 3-D seismic inversion: turbidity current developed and deposited on the bottom part of slope break, as well as weak cohesive debris, during the activity rate of the synsedimentary faults was fast, which led to a steep landform and high creation rate of accommodation; while in the case of low activity rate resulting in flat landform and low creation rate of accommodation, debris developed and deposited wildly on the flat slope. Consequently, our research suggests that the landform variation can be confirmed by the changes of flow regime and taypes of gravity deposition, then the scenario of base level variation can be investigated.