PRELIMINARY RESEARCH INTO LACUSTRINE STORM-CURRENT DEPOSITS: AN EXAMPLE FROM THE WESTERN DONGPU DEPRESSION

Bohai Bay basin was apparently influenced by the oceanic climate due to the pacific Ocean' s progressing to China' s continent in Eogenc.Dongpu Depression in the basin had a subtropical climate, and was not far from the Pacific.Thus the powerful storm from the Pacific used to attack the area.At the same time, Dongpu Depression was a large open lake with gentle toporgraphy and the developed delta and shore beaches, which helped form storm flow and its deposits. Storm deposit or tcmpcstitc in the study area can be identified by: (1) abrupt top and sole contact, (2) wave-produced beddings, (3) low maturity in composition and relatively high naturity in texture, (4) biological Fodinichia and Fugichnia, (5) Bouma-like succession, and (6) abundant resedimented intrabasinal components. As a complete system, storm-flow dcpositional system consists of provenance facies, storm facies and turbidite one.Provenance lacies rclers to the beach and/ or delta supplying the storm now with suth-cicnt sandy source.The storm sedimentation facies, located between fair-weather (littoral lake) wave base and that of storm, can be subdivided into two units, proximal subfacies characterized by Sa, Sb and Sc in Bouma-like sequence and distal subfacies with Sabc and Sdc and lower sand / shale ratio.Storm waves were not available below the wave base of storm, where the unloading of storm-underflow formed deep-water storm-generated turbidite.It seems that the storm deposition followed the following ru es: (1) tempestite developed in the middle of the lake-regressive sequence during the evolution of the basm; (2) tempestite is intcrbcdcd with non-tempestite and takes a smaller proportion in strata records; and (3) storm deposit is surrounded by acltas and/ or beaches onshore and by turbidites (flood-formed) offshore. The discovery of ancient storm deposit is of great significance.Oscillation ripples common in pestite have been used to evaluate paleobathymetry quantitatively.As a result, the fair-weather wave base is about 5m, and the storm one around 15m.And what is more important is that tempestite in the area acts as type of reservior. It is further concluded that tempestite might be widely spread in the near-sea lake basins of coastal lake basins, for example, the Eogenc of Bohai Bay basin.