Sediment accumulation on the steep slope zone of the continental down-warped lacustrine basin and dynamics of sequence filling: An example from the Triassic Yanchang Formation in the Ordos Basin,eastern Gansu

The steep slope zone of a continental down-warped lacustrine basin tends to have the features of steep slope,intense tectonic movement,nearer provenance,multiple origins and sharp facies changes.The exploration in recent years has disclosed that all kinds of sandstone bodies associated with the steep slope zone of a down-warped lacustrine basin may be interpreted as most favourable places for oil and gas accumulation due to good conditions of source rocks,reservoir rocks and oil and gas migration.The Ordos Basin was once a typical large-scale continental down-warped lacustrine basin during the Triassic,and the eastern part of Gansu was in the steep slope zone under the influence of the western thrust zone during the Yanchangian of the Triassic.Two models for sediment accumulation are constructed for this steep slope zone: one for the Chang10-Chang7 retrogradational delta and another for the Chang6-Chang1 progradational delta.The accommodation space/sediment supply ratios have changed in different facies zones and periods in response to the base-level fluctuations during the formation and evolution of the basin,giving rise to distinct stratial filling patterns.