Sequence architecture of a Miocene marginal shelf sea influenced by tectonic activity: A case study of Eastern Yinggehai Basin, South China Sea

Abstract

While sequence stratigraphic frameworks of passive continental margins have been extensively studied, their counterparts in semi-enclosed marginal shelf seas remain poorly understood. This study investigates the sequence architecture and depositional patterns of a marginal shelf sea system through integrated analysis of 3D seismic, well-log, and core data from the Miocene succession in the Yinggehai Basin, South China Sea. The Miocene strata were subdivided into seven third-order sequences whose architectures and sedimentary associations were governed by the interplay between slope-break geometries, sea-level fluctuations, and sediment supply. Three distinct shelf sequence types (A1, A2, and B) were recognized based on spatial configurations of slope breaks and relative sea-level positions. Type A1 sequence (Huangliu Formation and Mei1 Member) consists of multiple slope breaks at basin margins and in the interior basin, with sea-level falls exceeding the elevation of interior slope breaks. The sequence preserves complete systems tracts, including early lowstand (gravity flow-dominated), late lowstand, transgressive, and highstand tracts. In contrast, Type A2 sequence (Mei2 Member), though retaining multiple slope breaks, lacks early lowstand systems tracts due to insufficient sea-level fall to expose interior slope breaks. Type B sequence (Sanya Formation) features solely basin-margin fault-controlled slope breaks and shares systems tract compositions with Type A2. Sedimentary processes vary markedly among sequence types. Type A1 sequence hosts multi-phase, large-scale gravity flow deposits encompassing submarine fans, slope-perpendicular channels, and slope-parallel channels, with maximum development during early lowstand conditions. Type A2 sequence is characterized by braided river deltas interfingering with shelf mudstones, accompanied by limited gravity flow activity. Type B sequence comprises shallow marine facies and small-scale braided deltas. Hydrocarbon implications emerge from the preferential occurrence of reservoir-quality gravity flow sands in Type A1 lowstand systems tracts, while regional seals and source rocks are associated with shelf mudstones in Type A2 and B sequences. The sequence evolution is primarily controlled by syn-depositional tectonics (fault reactivation, post-rift subsidence) and semi-enclosed paleogeomorphology, secondarily by sediment influx and eustasy. This framework advances predictive models for shelf-scale sequence development in restricted marine settings and provides critical insights for global hydrocarbon exploration in analogous basins.