The Analysis on the Evolution of Depositional Sequence in the Mid-Depth Formation under the Control of the Medium-Long Base-Level Change in Huanghekou Depression

The time and space evolution of sequence architecture and depositional systems has attracted much attention in the geological field. Meanwhile, due to the recent excellence, the Oligocene lacustrine rift basin takes a key position during geological research in Huanghekou depression of Bohai Bay Basin in east China. However, the systematic researches of the depositional sequence of Dongying formation in Huanghekou depression appear little, thus this study aims that using many seismic lines, complemented by well logs and cores to thoroughly analyze the time and space allocation of depositional sequence of Huanghekou depression and its major controlling factor, base-level change, in a typical rift lacustrine basin. The Oligocene Dongying formation in this article will be divided into three composite sequences on the basis of unconformities on basin margins and correlative conformities in the basin center. Every sequence is composed of a regional depositional cycle from transgression with an onlapping lacustrine expanding systems tract (EST) to regression with a prograding highstand systems tract (HST). And five sequences can be further identified by subordinate discontinuous boundaries in Huanghekou depression. The evolution of depositional systems is associated with base-level change within sequences. In Lower and Middle Dongying formation (ESTSQ3, HSTSQ3, ESTSQ2), depositional systems are dominated by braid river delta and sandy beach-bars. During the late period of Dongying formation (HSTSQ2, SQ1), depositional systems are characterized by meandering river delta or braid river delta deposits. And base-level change is one of major controlling factors, of which the shape and scale control the development of the depositional sequences. Base-level changes, via affecting the A/S, directly control the depositional states and determine the unique depositional sequences structure: in the northern steep slope, multiple meandering river deltas systems exist. In the middle of the depression, the large areas of beach bars can be found. And the depositional system is continuously developed with the large-scale braided river delta in the southern gentle slope.