TOPOGRAPHIC CONTROLS ON SEDIMENTATION DURING EARLY LAKE EXPANSION AT THE STEEP MARGIN OF DONGYING DEPRESSION, EASTERN CHINA

Two contrasting depositional systems and basin-filling processes had occurred during early lake expansion at the steep margin of Dongying Depression, Eastern China: fluvial and lacustrine deposition in an incised valley, and lacustrine sandstone and shale deposition in a mountain-front trough. Well log and 3-D seismic data were used to delineate the location, spatial geometry, and sedimentological and stratigraphic characteristics of the two systems. In the first case, the incised valley is oriented NE-SW along dip direction of the border fault and has an erosional base, which was formed by fluvial incision during subaerial exposure and is defined by truncational seismic termination patterns. The valley is about 5 km long, widens from 1 km at the head to 2.5 km at the mouth, and bifurcate into two branches at the lower reach. It has a concave-upward longitudinal profile, gradually steep at the head and gentle toward basin, and an asymmetrical cross-sectional profile. Valley fills contain lateral accretionary and cross-valley sheets composed of sandstone and shale with no clear grain size trend. The maximum thickness is ~250 m in the downstream. They are interpreted as fluvial channel-levee systems, progradational fans, and profundal mud. In the second case, lacustrine sediments filled a gradually expanding lake developed above the sequence boundary during early lake expansion. The lake occupied an elongate topographic low, which is oriented E-W, parallel to the border fault and mountain front. The initial transgressive lake fill is 2-30 m thick, consisting of sandstone and shale. Onlap patterns indicate gradual lake expansion in all directions. The initial lake is oval shaped, maximum 10.5 km long and 4 km wide. It is a hanging lake perched on the steep margin and, eventually, merged with the main lake to the south during maximum expansion. In summary, two styles of transgressive sedimentation, both occurred above a sequence boundary during early stage of lake expansion, demonstrate significant topographic control on paleoenvironmental and paleogeographic evolution at the steep margin of a rift lake basin.