DEPOSITIONAL FEATURES AND DEPOSITIONAL MODEL OF ANCIENT LACUSTINE GRAVITY FLOW DEPOSITONAL SYSTEM

Similar to marine depositional settings, deep-lacustrine gravity flow depositional systems comprise of two kinds of gravity flows deposits attributable to sandy debris flows (sandy debrites) and turbidity currents (turbidites). The sandy debrites are clean, massive, fine-graded sandstones without size segregation, grading and Bouma-sequences, indicating rapid halt deposition. The sandy debrites are also characterized by floating and tearing mudstone clasts at the bottom. These poorly sorted mudstone clasts show various orientations, either floating parallel to bedding plane or tearing shape with random fabric. The sandy debrites have an abrupt contact with underlying deep-lake mudstone; a boxed-shapes GR log curve and minor mud content (<5%). The turbidites are mostly fine, silty to muddy sandstones, and display a variety of sedimentary structures, including graded beds, parallel beds, ripple cross-lamination, that are indicative of partial Bouma-sequences, and suggest that the turbidites were deposited layer-by-layer. Their GR log curves display normal cycle features.

The sandy debrites and turbidites were deposited in a deep-lacustrine, non-channelized sedimentary system that was dominated by sandy debris flows associated with a sand-rich delta front. The sandy debris flows originated in the steeply inclined delta front where sediment failure was triggered mainly by earthquakes. The sandy debris flows are high density laminar flows that easily deposit their sediment load on relatively flat gradient slopes. During the progress, the sandy debris flows are diluted by water and clays, and transform into turbidity currents. Therefore, the resulting turbidites have high mud content and usually become deposited far away from the sediment source.