THICK COARSE-GRAINED TRANSGRESSIVE SILICICLASTIC DEPOSITS IN LACUSTRINE SEQUENCES

Coarse-grained shorezone transgressive deposits, as an integral part of a T-R cycle, are commonly perceived as thin and insignificant in comparison to regressive deposits. Three ancient lacustrine examples show the contrary. First, 21 consecutive sequences in Upper Permian Wutonggou low-order cycle in Zaobishan section, Bogda Mountains, NW China, are 4-100 m thick, and were deposited in a half graben in Turpan-Junggar intracontinental rift basin. Coarse deposits in transgressive systems tracts (TSTs) are 1-30 m thick and represent 4-60% of total thickness of each sequence; 18 of the 21 TSTs (86%) represent 10-60% of the total sequence thickness; 11 TSTs (52%) 25-60%. The TSTs consist of persistent well x-stratified, well-sorted conglomerate and very coarse-fine arenites. They overlie deltaic, fluvial, or paleosol deposits with an erosional base and underlie prodeltaic shale with a sharp contact, contain multiple m-scale upward-coarsening beach-littoral cycles, and form an overall upward-fining trend, indicating deepening and retrogradation. Second, transgressive sandstones in Upper Triassic Zhangjiatan Shale sequence on the ramp margin of Ordos foreland basin, N-C China, are 10-65 m thick in 72 wells, and represent 15-70% of total thickness of the sequence and 25-70% in 80% of the wells. The TST is composed of upward-coarsening wave-dominated deltaic cycles and have an overall fining-upward pattern. Finally, transgressive fan-delta and deltaic deposits in a sequence in Eocene Shahejie Fm. in Dongying half graben, eastern China, are 100-300 m thick and represent 40-92% of the total thickness of the sequence. The TST is composed of fining or coarsening-upward conglomerate and sandstone successions within an overall fining-upward trend, and was deposited at the steep margin during lake expansion. Thick transgressive shore-zone coarse siliciclastics in the examples occur as different systems on gentle or steep margins, and suggest the sediment supply-accommodation space ratio is close to one. Sediment supply was copious and almost kept pace with increasing accommodation space and is the determining factor. In addition, the steep topography on steep margins promotes mass deposits and stalls shoreline transgression. Thick transgressive deposits may be more common than expected in the geologic records.