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, Hongtao Zhu2,3, James A. MacEachern4
, Wei Zou5, Yinshan Chang5, Jianbin Liu5, Tonglei Zhang5, Qianghu Liu2,3
, Zhiwei Zeng6 and Haijin Wang2,3
Conceptual model of the tectonically controlled washover fan in the Xihu Depression.
Washover fans form during intense storms through barrier breaching and coastal inundation. Despite their importance for understanding coastal response to storms and their potential as stratigraphic traps, ancient washover fans remain poorly documented and underrepresented in subsurface studies, resulting in limited criteria for their recognition. This study investigates the depositional characteristics, controls, and dispersal patterns of a tectonically controlled washover fan succession within the late Eocene Pinghu Formation, Xihu Depression, using 3D seismic, geological and geophysical logs, and petrology. Based on palaeogeography, heavy mineral analysis, seismic‐based provenance analysis, and paleocurrent studies suggest that the Pinghu Formation records a barrier island system. Successions of washover fan deposits are tens of meters thick and comprise stacked 0.5–2.0 m (locally up to 6.6 m) thick, medium‐ to fine‐grained sandstone beds. Individual sandstone beds are generally poorly sorted, normally graded, contain gravel lags, and exhibit parallel stratification. Grain‐size distributions and spatial trends from different wells support a marine‐to‐landward transport process. Petrology shows abundant dolomite crystals and bioclasts. The washover fan deposits are interbedded with thoroughly bioturbated mudstone intervals, which are interpreted as back‐barrier bay deposits. These successions are significantly different from those of river‐dominated deltas and flood‐tidal deltas present in the study area. Washover fan development and preservation are controlled by sea‐level fluctuations, sediment supply, and antithetic faults with associated paleo‐uplifts. The fan dispersal pattern was confined to the syn‐rift period and coincided with rapid sea‐level rise. This study provides criteria for the identification of ancient washover fans and enhances our understanding of their development. Additionally, owing to the succession's significant stratigraphic trap potential, this study is a useful reference for petroleum exploration in the East China Sea Shelf Basin and analogous basins.
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