THICK TRANSGRESSIVE FAN SYSTEMS AT STEEP MARGIN, DONGYING DEPRESSION, EASTERN CHINA – CONTRADICTION TO THE “NORM”

Transgressive fan systems are normally poorly developed. However, fans composed of mass transport complexes and turbidites at the steep margin of Dongying Depression, eastern China, developed during lake expansion are extraordinarily thick. Cores and well-logs from 21 wells and 3-D seismic data covering 27x17 km² were used to characterize fan systems in a stratigraphic sequence, consisting of early expansion (EE), late expansion and early contraction (LEEC), and late contraction (LC) systems tracts (STs). Marginal fan systems are variably developed among STs, prograde or retrograde, and interfinger with basinal deposits. Fan systems of EEST are 3-22 times thicker than those in other STs and cover ~70% of the margin. EEST is 149-315 m thick, composed of conglomerate and sandstone successions showing fining or coarsening-upward trends within an overall fining-upward trend. Individual fans are wedge- and mound-shaped in dip and strike sections, respectively, and progressively fine and thin basinward. They migrate landward, showing an overall retrogradational pattern. Fan systems are rare in LEEC and LC STs; instead, evaporite-shale systems dominate. The fans are only 14-52 m thick, consist of interbedded sandstone and shale without a prominent grain size trend, and cover only 5-20% of the margin. The great thickness and abundance of fan systems in the EEST may have been caused by factors and processes controlling the sediment supply and accommodation space ratio. It was slightly less than one at the margin during EE. Sediment supply was copious because of increasing precipitation, which increased slope instability and inflow runoff, resulting in large sediment yield and river influx. However, the large supply still could not keep pace with the increasing accommodation space, resulting in an overall retrogradational pattern. In contrast, the ratio was negative during LEEC and LC stages. Sediment supply was limited due to increasingly arid condition, which caused reduction in sediment yield and runoff. The increasingly arid condition led to lake contraction and reduced accommodation space. As a result, fan systems were poorly developed and replaced by evaporitic and muddy deposits. In summary, thick transgressive fan systems may actually be the norm at steep margin of half grabens.