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Sedimentary Characteristics of Hyperpycnites of the Lower Cretaceous Xiguayuan Formation in Luanping Basin, Northeast China: Implication for Lithological Reservoir Exploration in Ramp Margin of Rift Lacustrine Basins


Relatively dense underflows generated from river flooding termed hyperpycnal flows are common in modern oceanic and lake environments. Although most of the observed ancient extrabasinal turbidite systems develop in deep water settings, research of hyperpycnal deposits in shallower water settings needs further research. The Luanping Basin is a typical Cretaceous rift basin in Northeast China. The outcrops studied in the Luanping Basin provide a good analog for the sedimentary facies’ distribution of widely developed rifted basins formed simultaneously. Thus, the outcrops provide a good opportunity to conduct a detailed study of shallow water hyperpycnal flow deposits in a continental rift basin. Flood-generated hyperpycnal flow deposits were identified in the Lower Cretaceous Xiguayuan Formation of the Luanping Basin. The diagnostic characteristics of the shallow water hyperpycnites were recognized based on the description and interpretation of the eleven lithofacies in the bedload (Facies B) and suspended-load deposits (Facies S). According to the vertical association of the different facies, three facies associations were identified from the hyperpycnal flow deposits in the ramp margin of the rift lacustrine basin. Sedimentary elements of shallow water hyperpycnites can be interpreted from the facies associations. That is, channel deposits (facies association A) distributed in the proximal region, channel-lobe deposits (facies association B) in the intermediate region, and the lobe zone (facies association C) distributed in the distal part. A depositional model was proposed for the Cretaceous Xiguayuan Formation in the Luanping Basin that reflects the sedimentological characteristics and depositional processes of the shallow water hyperpycnal flows. The depositional model consists of a proximal channel zone, intermediate channel-lobe, and distal lobe zone. The sedimentary process of the hyperpycnites indicates a cycle of velocity increases and decreases of the long-lived delta-fed hyperpycnal flows in a shallow lacustrine environment. Overall, this study suggests a possibility of the existence of ancient hyperpycnal flow deposits and provides a new perspective for understanding a sedimentary model of gravity flow in rift lacustrine basins. Furthermore, the identification of hyperpycnal flow deposits sheds new light on future exploration and development of lithologic reservoirs in rift lacustrine basins.