Genetic types and characteristic research on deep-water gravity flows of Yingcheng Formation in Longfengshan subsag, Changling Depression

Key words:  tectonic-paleogeomorphology  gravity flow type  sedimentary facies  Yingcheng Formation  Longfengshan subsag

Abstract:

The gravity flow in the Yingcheng Formation of the Longfengshan subsag was studied in this research. Based on the differences on the sediment composition structure, transportation-sedimentation mode and fluid rheological characteristics, we made full use of core, well logging, seismic explanation and other data, and carried out a study on the lithfacies types, lithofacies association characteristic and genetic mechanisms of gravity flow. The results are as follows. During the sedimentation of the Yingcheng Formation, the Longfengshan subsag can be divided into three tectonic-paleogeomorphology units, which are southwestern fault-slope zone, southeastern steep slope zone and northern depression belt. The paleogeomorphology steepens gradually from west to east. Faulted troughs formed by synsedimentary faulting are the main sand transporting channel. There are five gravity flow types developed in the Longfengshan subsag, including mud-only turbidite, low density turbidite, high density turbidite, glutenite debris flow and muddy debris flow. According to characteristics in triggering mechanisms and sedimentary facies sequences, the lithofacies association can be divided into two types, the gravity flow caused by slumping and the gravity flow caused by flooding. The whole flood process can be divided into three stage, including water-rising stage (low density turbidity flow-debris flow), flood peak stage (debris flow-high density turbidity flow), and water-recession stage (low density turbidity flow-muddy flow). The slumping process can be divided into four stages, including sliding, sandy debris flow, high density turbidity flow, and low density turbidity flow and muddy flow. The sedimentary characteristics of different tectonic-paleogeomorphology in gravity flow have been established. In the southwestern fault-slope zone, affected by flooding, terrigenous debris are transported along the faulted troughs into lacustrine and fan delta developed. Because of the development of slope break belt, the sublacustrine fan is developed by slumping of the fan delta front in the northern depression belt. In the southeastern steep slope zone, influenced by tectonic activities, debris slump into semi-deep lake and deep lake and form nearshore subaqueous fan.