High-resolution sequence stratigraphy in continental lacustrine basin: A case of Eocene Shahejie formation in the Dongying Depression, Bohai Bay Basin

Abstract

Approximate 500 m deltaic-lacustrine successions were formed within 2 Ma in the Liangjialou area of the Dongying Depression in the Bohai Bay Basin (eastern China). However, few studies on sequence stratigraphy were previously conducted in the east of Liangjialou. In the present study, we integrate high-resolution three-dimensional (3D) seismic data with well-logging data to study the sequence stratigraphy and the depositional system.Our study shows that: (1) Four systems tracts have been identified in the study interval, they are HST, FSST, LST and TST. The HST has an ascending shelf-edge trajectory and aggradation to progradation stacking pattern. FSST has a descending shelf-edge trajectory and progradation to degradation stacking pattern. LST has a low-angle ascending shelf-edge trajectory and progradation to aggradation stacking pattern. TST has a backstepping shelf-edge trajectory and retrogradation stacking pattern. (2) Five high-resolution sequences (equal to 4th-order sequence), HRS1∼HRS5, were further subdivided within the 3rd-order LST. In addition, the depositional systems of HRS1∼HRS5 are significantly variable in 3D space. The delta and deep-water deposits in the north are larger than those in the south. (3) The formation of the sequence is controlled by both tectonics and dry-wet climatic cycles. The tectonic subsidence and high sediment supply in overall wet climate play a major role in the formation of the 3rd-order sequence. The higher frequency dry-wet climatic cycles dominate the formation of internal systems tracts. We further divide the climate of study interval into three stages that are excellent correlated to the specific systems tracts: Stage 1 (evaporation < precipitation) corresponds to HST; Stage 2 (evaporation > precipitation) corresponds to FSST; Stage 3 (evaporation < precipitation) corresponds to LST. (4) The basin morphology and source location mainly control the lateral variability of the depositional system of LST. Low paleotopography, narrow shelf width and steep slope dip in the north cause a large accommodation, and promote the development of the deltaic and deep-water deposits. In contrast, a high relief, wide shelf width and gentle slope cause only minor accommodation in the south, and limit the development of the deltaic and deep-water deposits.