Types, Distributions And Origins Of Early Triassic Anachronistic Facies In Chongqing Area

In the Chongqing area of Upper Yangtze there widely sedimented variousabnormalities carbonate rocks and tectonics-" Anachronistic ", these carbonatesediments were widely distributed in the marine environment before Ordovician, butafter the great development of Ordovician metazoan, they were found only in someextreme and unusual environments, not seen in normal circumstances. When theywere widespreadly found in the environment of early Triassic normal shallow again,considered it was a dislocation on time or environment, so we called it "non-normal"deposition or " anachronistic ", the main types of anachronistic in Chongqing areainclude Flat-pebble conglomerates, Vermicular limestone, Subtidal wrinkle structures,Microbialite, Ribbon limestone and so forth.The vermicular limestone in Chongqing area is widely sedimented in theformations of Feixianguan and Jialingjiang, and the vermicular limestone consists ofdark vermicuform bodies and tint matrix, among which the vermicuform bodies are invaried shapes. The main composition of the vermicuform bodies is calcite and itscontent is above95%. The matrix has more clay content while its calcite still reach85%-90%. There are few metazoan fossils can be found in vermicular limestone,while pyrite and algae are observed with the microscope. According to the shape, thevermicular limestone is divided into three types, i.e. laminar-like, spot-like andmessy-like vermicular limestone. The formation of vermicular limestone is related tocyanobacteria or algae activities, and the storms play a very important role in theprocess of its formation. Disturbance of metazoan (e.g. brachiopods, gastropods andbivalves) will inhibit the development of the vermicular limestone, thus thevermicular limestone are both biological and mechanical. The formation of vermicularlimestone is due to such extremly grim conditions of the Early Triassic Tethys oceanas high seawater temperature, high content of CO2, low content of O2, stagnant anoxiaand frequent storms.Flat-pebble conglomerates are mainly sedimented in the Feixianguan formationof Chongqing area. The flat-pebbles are often exist with oolitics and bioclastics, andare composed of debris and dark matrix. It is often interbedded with shale, inclusionswhich has a rounded, and the lack of fossils gravel, plaster matrix may contain smallamounts of biological crumbs. And its cause maybe relates with the storm.Microbialite is found in many sections’ Lower Triassic just like Beibei Baimiaozi section in Chongqing area, related with a kind of microbial community and in itsformation had not affected by biological infestation. Its main types includestromatolites and microbial mound which may reach2meters high, microbialitecontain fossils of high abundance and low diversity micro-gastropods, bivalves andostracods and other euryhaline microbial composition. The high abundance organismshould be the one who likes eating cyanobacteria.Ribbon limestone is often found in the Feixianguan formation first LowerTriassic, interbedded by limestone and mudstone or siltstones, observed almost nolimestone fossils or biological disturbances, maybe is a product of chemicalprecipitates.Thin limestone is usually found in Feixianguan formation and Jialingjiangformation, often centimeter layer thickness, poor biological bioturbation in thevertical direction, however is rich remains in the horizontal direction, a mobile orfixed levels of benthic life can adapt relatively well in lower Triassic underseaenvironment.The anachronistic in the Early Triassic appeared again illustrates the depositionalenvironment was special, its special paleoenvironmental conditions may have certainsimilarities with the ancient Precambrian and Cambrian,means the ancient ocean isstagnant, anoxic and hypoxic marine environment of this stagnation is also extendedto the shallow water areas. And because of the volcanic eruption and the same periodof dramatic ocean bottom to promote water-soluble toxic greenhouse gas CO、CO2、H2S、CH4and others large released, and then once again intensified the scale andspeed of the mass extinction.After the P-T mass extinction, the anachronistic in theLower Triassic on the one hand is related to the extinct, on the other hand is relatedwith the long recovery of the biological (about five million years).