Sedimentary Environment And Cold Rift Evolution Of Miboshan Formation In Helan Mountain

Helan mountain is situated in the northern part of South-North trending structural zone which lies between different tectonic realm of east and west china. It is focused on obvious geomorphic character, special tectonic pattern, "multicycle" tectonic activity. This area is a long term active region, its important constituted section of sedimentary tectonic evolution is Ordovician. According to the systematic research of petroface, microelements, rare earth elements and sedimentary environment, based on full reorganization existence information and in predecessor research results, we discuss its sedimentary type of Ordovician, we obtain several preliminary understanding as follows:1. The sedimentary combination is half deep water-deep aqueous phase from early Ordovician late stage to middle Ordovician at Helan mountain, its main lithology is clastic rock, argillite, carbonatite, The rock composition is complex and petrographical facies change sharply.2. Miboshan formation is composed by these sedimentary facies such as debris-flow sedimentary mud flow turbidite sedimentary and so on, the gravity flows is developed, it belongs to Continental slope region submarine fan deposition. The sedimentary microfacies includes breccia microfacies counter grain of foreword lamination rock microfacies strong affection calcirudite with the deposition transformation microfacies, these microfacies compositions has the character of continent edge microfacies composition.3. The element geochemistry research natures shows that the sedimentary interface was mainly reduction, partial reductional environment was strongly; the sedimentary rate was fast, and it had the character of continent pitch gravity flow fast accumulation.4. The synthesis considered the regional structure background of Miboshan formation in Helan Mountain, its activity epicontinental and the resource of the arc nature possibly comed from the south northern Qilian area.5. The sedimentary structure of Helan Mountain area changed obviously in laest Early Ordovician.The south area fast extendly and grown gravity-current carbonate; Because the areal structure activity intensifies, the subsidence scope enlarges and terrigenous clastics gravity current flowed frequently; Afterwards, the structure activity tends to steadily, the water level changed deeply, belong to deep basin. To latest Middle Ordovician, the sea water gradually changed shallowly, were littoral-neritic deposit.6. In latest Early Ordovician, We think that Helan Mountain area that lied in the northern Qilian back-arc basin where near to the continent possibly affected by the back-arc expansion and the relative motive of the Alashan massif and sino-korean massif, and it restarted to expansion subside, and formed passive rifited basin—Helan cold rift. In latest Middle Ordovician, affected by Qilian ocean closed, this cold rift vanished because of the reformation.