Sublacustrine gravity-induced deposits: The diversity of external geometries and origins

Abstract

Our current knowledge of deep-water depositional systems is mainly built of extensive achievements obtained from marine basins. Although lacustrine basins are important oil & gas productive areas of the world, gravity-induced deposits in these basins are poorly understood in the literature when compared with those documented in marine basins. Based on new insights from 3D seismic data, this paper attempts to investigate the effect of the interaction between gravity-induced flows and lake-floor topography on the resulting external geometry of gravity-induced deposits in the Cretaceous Qingshankou and Nenjiang formations of the Songliao Basin (SLB).Eight different shapes of gravity-induced deposits were identified in the SLB, and which were in turn grouped into four distinct categories according to external shape, internal architecture and spatial assembly: (1) Channel-fan complexes; (2) Fan-form complexes; (3) Strip-form complexes; and (4) Pond-form complexes. Furthermore, fan-form complexes can be subdivided into four subcategories (isolated fan with compressional ridges, isolated fan without compressional ridges, mother-son fans and stacked fans). Strip-form complexes can be subdivided into two subcategories, including strip-form controlled by gully system and strip-form controlled by growth faults.Sediment transport mechanism (density flows or mass transport flows) and lake-floor topography together influence the pathways, runout distances, discharge places and internal characteristics of gravity induced deposits, which could ultimately influence external geometries of gravity-induced deposits in the SLB. Consequently, sublacustrine gravity-induced deposits does not exhibit classic fan-like geometry. Frequent and powerful hyperpycnal flows associated with flood events are fairly common, and develop channel-distal fan complexes. MTDs exhibit various external geometries, which are significantly affected by growth faults, slope gullies, gradient, and basin-floor irregularities. The new depositional model built from the SLB is vital to understanding the distribution of gravity-flow deposits in a sublacustrine slope.