The Late Triassic Karst In Northwest Sichuan Basin,China:Characteristics And Research Significance

The Carnian Climate Event,one of significant global climatic events during Triassic,was identified in Guizhou,Yunnan province and northwestern Sichuan Province.Sedimentary sequences of Carnian in Qingyangou,Guanyinya and Jushui sections reveals the lithological change from biolithite limestone to black shale(siltstone).Someone thought this lithological change is an evidence of deepening of the ocean during Carnian.We found a karst surface in the field sections.The discovery of ancient Karst at the top of the sponge reef limestone in the northwest of Sichuan Province shows that the strata have been exposed during the Carnian.And the water became more and more deep after the exposure.In order to ascertain the sedimentary characteristics in this kasrt and explore its relationship between the Carnian Humid Event and the uplift of Longmen Mountain,the research observed and sampled outcrops and statistically analyzed thin sections,also included the identification of fossils,collected foreign research data and methods of karst and Carnian Humid Event.Piebald structures and honeycomb dissolved pores are obvious in the karst outcrops,and karst breccia,filling material(e.g.,micritic-sparry calcite and terrigenous quartz grains)in the dissolved pores and micritization granules are discovered with microscope.According to the microscopic characteristics and the lithology,the eogenetic karst is determined to be a single layer,overlied yellow and black shale containing plenty marine fossils.The identification of ammonoids in the shales shows that the karst is an important isochronous surface regarded as the boundary of the upper and lower Carnian.The strata above the karst are different between the Hanwang sections and Jushui section,i.e.,the overlying rocks above the karst in the Qingyangou section,Hanwang are black silty shale and bioclastic limestone(e.g.lamellated sponge limestone and crinoidal limestone)while in Guanyinya section,Hanwang,the overlying rocks above the karst are shale and moundy sponge limestone.However,karst caves are developed at the top of sponge boulders in the Jushui section.The formation of Karst is usually related to the exposure of strata and the scour of rainfall and other fresh water.Comprehensive research implies that the formation time of this karst is close to the occurrence of the Longmen Mountain uplift and the Carnian Pluvial Event,it is thus inferred that the karst was caused by the joint effect of uplift of Longmen Mountain and Carnian paleoclimatic event.