The early Paleozoic lithofacie-palaeogeography of Olongbluk micro-massif on the northern margin of Qaidam Basin

Through field geological investigation and elaborate observation of Olongbluk micro-massif on the north margin of Qaidam Basin in combination with the rock, sedimentary structural characteristics and contact relationship of strata in the profiles,the authors studied features of the Early Paleozoic Cambrian to early Ordovician sedimentary and lithofacies palaeogeography in the study area. Olongbluk micro-massif is located in a volcanic island arc back-arc area, and ocean subduction and collision of continental plates control basin-orogen pattern and evolution of sedimentary fillings in North Qaidam. The lower Paleozoic period of Olongbluk micro-massif mainly developed a set of marine carbonatite, with some terrigenous detrital rock in Cambrian bottom and early Ordovician strata. According to lithological association, there are five kinds of sedimentary facies, i.e., evaporated tidal flat facies, restricted platform facies, open platform facies, platform edge facies and flysch sediment. The study area mainly experienced transgression impact in that epoch. In Early Cambrian period, the study area received evaporation tidal flat sediments. In middle and Late Cambrian period, the study area accepted a set of shallow sea carbonatite formation, and the transgression range was enlarged, thus belonging to an area of restricted platform-open platform sediments. In Early Ordovician period, the sedimentary environment shifted into shoal of platform margin facies-foreslope. In the late of Early Ordovician period, the platform migrated southward, and became platform margin or slope under the compensated deep shelf sedimentary environment.