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Nearshore Along-Strike Variability: Characteristics, Controlling Factors and Importance for Stratigraphy and Sediment Delivery – A Case Study From the Luanping Basin, Northern China

Abstract

In this study, we utilize field outcrop data to investigate the nearshore along-strike variability in the Lower Cretaceous synrift Xiguayuan Formation in the Luanping basin. Along-strike variability at both facies and stratigraphic scales were recognized. At the facies scale (i) the ratio of conglomerate to sandstone in the Xiyuan fan-delta system is 87.2% and the average thickness of single coarse-grained layers is about 1 m, while the corresponding figures in the Caoying fan-delta system are 52.1% and 0.4 m respectively; (ii) sand-rich gravity flow deposits are developed in front of the Xiyuan fan-delta while mud-rich mass transport deposits (sliding block and slumping debris) are found in front of the Caoying fan-delta. At the stratigraphic scale: (i) in the Xiyuan section a delta clinoform characterized by sand-rich from proximal to distal parts developed in a relatively steep slope setting; (ii) in the Caoying section a delta clinoform characterized by sand-rich proximal to middle parts and mud-rich distal part developed in a gentle slope setting. Sediment grain-size of supply controls the sediment type of gravity flows in front of fan-delta front. Differential basin physiography, subsidence and sediment supply and the interplay between them resulted in significant along-strike variability. In particular, tectonic is the major external control on along-strike variability in rift basins. This study suggests that nearshore along-strike variability is multi-scale and multi-origin and is widely developed. Along-strike variability analysis can give us a more detailed 3D view of rift basin evolution and highlights that rift-climax successions may have a higher hydrocarbon potential.