The Study On Mordern Sedimentary System In The Qinghai Lake

Qinghai Lake forming in the Miocene is the largest faulted brackish lake in China, which contains various types terrigenous clastic sediments, providing an ideal site to compare with ancient terrestrial rift lacustrine basin in East China. Different researchers conducted comprehensive study via multi-disciplinary methods between 1960 and 1970, and however, the modern sedimentary researchers commonly focus on single facies instead of systemic understanding of sedimentary systems of Qinghai Lake and its surrounding areas. Based on observation of outcrop and exploratory trench and bathymetry, remote sensing image and meteorological data, the combined research of the modern sedimentary systems in Qinghai Lake are conducted assisted by petrographic microscope and scanning electron microscopy observation, mechanical analysis and geochemical test.The theory of “source-to-sink” system is brought out in this study. Based on sedimentary facies analysis in single sites, six linear“source-to-sink” sedimentary systems and lake sedimentary system are well built. We analyzed the plane characteristics of modern sedimentary systems in the Qinghai Lake, and then established the new planar graph of sedimentary systems in the Qinghai Lake.The following new knowledge on sedimentary facie types and formation mechanism has been established.(1) The Ⅲ channel lake embankment between Qinghai Lake and Erhai Lake is controlled by lake level declining, wave and coastal current.(2) The formation of lake beachrock is related to microbes.(3) Foreslope exists in Buha River delta.(4) The so-called Shaliu River fan-delta is actually composed of alluvial apron, fan-delta and meandering stream delta.(5) Barrier island-lagoon-delta, hom-shaped estuary and barrier island-watter channel-lagoon are found in the Harergai River and Ganzi River estuary.The formation and distribution of the modern sedimentary system is not only constrained by “source-to-sink” system, but also related to direction and power of wind. The Qinghai Lake modern sedimentary systems are the product of the systematic combination of wind(wind field), provenance and lake(lake basin/basin evolution). The control factors conclude provenance lithology, topography, drainage, vegetational spacies, plant coverage, wind, wave, coastal current and lake level fluctuation. Finally, the two sedimentary systems are established, the strong source-leeward system and the week source-windward system. The strong source-leeward system is composed of alluvial fan, fan dela, river, delta et al. The week source-windward system is composed of sand dune, barrier, lagoon, hom-shaped estuary et al. Establishment of two sedimentary systems are key to predicting distribution of sandbodies in the ancient terrestrial rift lacustrine basin. The fine-grained sediments in the semideep-deep lake could be potential source rocks. Delta front and fan delat front in the strong source-leeward system and barrier and sand dune in the week source-windward system are potential reservoir of high quality.