Research On Sedimentary Facies And Sedimentary Evolution Of Cambrian In Zhuozi Mountain Region

This paper is a section of an oil exploration project, Research on Strata and Lithofacies Palaeogeography of Lower Paleozoic, the Southwest Part of Ordos Basin, from Sinopec. The study area of this paper belongs to Zhuozi Mountain region and its neighborhood, west margin of Ordos Basin. On the basis of lots of previous data, geologic reconnaissance and actual measurements, this paper detailed studies the characteristics of petrology, strata, sedimentary structures, paleontology, distribution and evolvement of sedimentary facies of Cambrian.In study area, the sediments are carbonates firstly, and clasolite secondly. Clastic rocks with carbonate interbeds are developed in Lower Cambrian Series. From Hulusitai Formation, Middle Cambrian Series, abundant brecciola appear, which are different from typical shallow-water wormkalk. By study of petrographic thin section, lithological association and sedimentary structures, the paper believes that this kind of brecciola belongs to debris-flow deposition.The paper believes that the area developed four sedimentary systems in Cambrian, marine depositional system, platform depositional system, platform margin depositional system and fore-platform margin depositional system. The main types of sedimentary facies are offshore shoal, tidal flat, restricted platform, open platform, platform margin shallow shoal and fore-platform margin slope. On the basis of previous studies of carbonatite depositional model, that of Zhuozi Mountain in Cambrian is founded. Depositional models and evolution patterns of Early Cambrian and Middle-Late Cambrian are expounded separately. Then, composite columnar sections of wells and profilesand lithofacies paleogeographic maps are made.According to lithology, paleontology and sedimentary structures, the paper thinks that, from middle-upper Hulusitai Formation, Meer Valley has already located in deep slope depositional environment, which is different from previous opinion that Cambria system in this place was in shallow water environment.Overall, Meer Valley area is lack of early Early Cambrian strata. From Taosigou Formation to Abuqiehai Formation, Zhuozi Mountain area went through three depositional environments, coastal deposition, platform deposition and slope deposition. Gravity flow late stage of Xuzhuang period in Middle Cambrian shows the change from shallow-water platform deposition to deep-water slope deposition, and the latter lasted the whole Middle-Late Cambrian.