The Stratigraphic Division And Facies Analysis On Early Carboniferous Yong Zhu Group In Luocang Area, Tibet

The study area located in Gangdise-Tengchong formation, which belongs toa part of Longuel-Nanmulin stratigraphic division. The geotectonic position isin the suture zone of longgeer-Jomda faulted uplift belt and the Gangdise magmatic arc belt. The geotectonic position of the suture zone belongs to the secondary structure unit of the Gangdise block. This paper analyzes the lithologic association, fossil assemblages and sedimentary structural characteristics of YongZhu group in comprehensive method. The Yong Zhu group is divided into four lithologic section from bottom to up: the first section of Yong Zhu group(C1y1), black and very thin slake with gray thin sandstone (brown after weathering), intercalary strata of the middle sandstone; the second section of Yong Zhugroup (C1y2), grey and thin to middle sandstone and siltstone, intercalary strata black thin slate or yellow brown pebbly coarse-grained sandstone; the third section of Yong Zhu group (C1y3), black thin slate, dark grey thin siltstone andfine grain quartz sandstone, the three make up a series of basic rhythm a single rhythm is0.2to1meter in thickness, and the clastic grain get greater grained from bottom to top. Band structures is visible in siliceous slate; the fourth section of Yong Zhu group (C1y4), yellow and gray thick quartz sandstone and siltstone. Sandstone and grey black thin slate form a series of rhythmic layer. The layer is20to40meter in thickness, and the grain is greater grainedupward. The slate section contains a few sandstone layers, and sandstone section contains a few slate layers.The formation age of Yong Zhu group is preliminarily determined Early Carboniferous according to the fossil Agishanoceras sp.picked up in field during fieldwork and the brachiopod assemblages zone formed bv predecessors in the north of the study area Yong Zhu group.This paperestablishes sedimentary facies types that reflect the characteristics of sedimentary system through the comprehensive analysis of rock characteristics, sedimentary and paleontological data observing in the profile on the field and the usingof the principles of sediment logy. The subfacies of Yong Zhu group in Xizan g Luocang area can be divided into three types. The three subfacies types includes inner shelf facies, outer continental shelf facies and nearby strand facies.This paper the spatial distribution characteristics of the petrographic facies.Thewriter analyzed the composition of Yong Zhu group sandstone and make pointdiagram in Dickinson’s way. The preliminary analysis shows that the tectonic background of the Yong Zhu group sandstone’s provenance is orogenic belt in recycling period. The result of main and trace elements expose the types of Yong Zhu group sandstone mainly include lithic sandstone, feldspathic sandstone and feldspathic quartz sandstone. The writer proposed that Yong Zhu group belongs to the edge of the Gangdise terrane according to the diagrams of main and trace elements and REE of Yunzhug formation sandstone. During this period,Gangdise terrane was in a rift of the passive continental margin, and was ina primary stage when passive continental margin was transforming into activecontinental.