A Study Of Early Triassic Vermicular Limestone And Its Genesis In Lower Yangtze Region

The vermicular limestone is widespread in the Early Triassic of South China.It is a kind of mudstone and composed of dark "vermicuform bodies" and tint "matrix".The vermicuform bodies can be continuously laminar,banded,irregularly granular or ellipsoidal,and they have clear bounderline with matrix.The vermicuform bodies are about 0.1cm-1cm wide and varied in lengh. The calcite grains of vermicuform bodies under the scope are crassitude(25-60μm)with better degree of crystal and purer than the counterpart of matrix.The components of vermicuform bodies are mainly calcite(＞97%)and little quartz under X-diffraction,while the components of matrix are complex,which are mainly calcite(85-90%)and remaining insoluble hydrosialite,including illite,smectite and their admixture,quartz,little chlorite and so on.Under the microscope the vermicuform bodies and matrix are gradual contact or comparted by stylolite.The vein of matrix often converged in each side of lenticular vermicuform bodies.The suture joints are developed. The main authigenic non-carbonate minerals in vermicular limestone are pyrites and sulfates. There are very few macro-fossils can be observed but some 1-2mm depth ichnofossils Planolites. In thin section,fossils are rarely seen and only some eurytrophic forms occur,such as ostracods, bivalves,foraminifers,small brachiopods and echinoderms.The algal macula,algal lines,algal lamina are common.Fine pyrite grains spreaded in the vermicular limestone.The vermicular limestone is less restricted by environment.It could form in open platform, half-restricted platform,slope at platform edge and other marine environments.The condition includes intertidal zone,shallow sea and subtidal zone,has relatively higher salinity and are quiet.So far,there have been a lot of studies on the genesis of vermicular limestone in various parts, but it is still under disputation.Various hypothesis have been proposed to interpret its genesis, including chemical coacervation,bioturbation and mechanical origin.In the Early Triassic,the Lower Yangtze that was between North China block and Jiangnan submarine uplift developed a suit of epicontinental carbonate sediments.The Helongshan and Nanlinghu Formation from 6 sections in South central Anhui and South Jiangsu developed integral vermicular limestone sediment sequence,which has the variation:(1)Wide and straight dark gray vermicuform stripe and narrow tint matrix stripe alternation.The vermicuform bodies which became curve upwards have strong resistibility to weathering and are laminar in three-dimensions. (2)Wave-shaped stripe of the vermicuform bodies alternated with matrix,brokenly upwards.(3) The vermicuform bodies are laminar-like and discontinuously distribute in the matrix.The cross sections are lentiform,like-tadpole and so on.Millimeter-width and spread out stratiformly.They become clutter upwards.(4)The vermicuform bodies are irregularly granular,ellipsoidal,and disorder in the matrix.(5)Upwards,the immethodical vermicuform bodies disappeared,which are superseded by homogenous mudstone.In ascending order,the wide and straight vermicuform bodies→wavy vermicuform bodies stripe→like-tadpole vermicuform bodies stripe→cluttered vermicuform bodies→homogenous mudstone succession appeared againThe vermicular limestone can be divided into three types that are laminar vermicular limestone,banding vermicular limestone(subdivided into trace/boring and laminar-like styles), and contaminated vermicular limestone.These deposits start to become diverse in the early Triassic with special chemical conditions of the ocean and then undergo the synsedimentary lithification and different extent effect of macro life-form and animalcules.And then come into being different kinds of vermicular limestone.Lack of vertical bioturbation make the lamella limestone preserved and the latter quickly benthic cementation result in the formation of vermicular limestone.Macro life-form together with the animalcule played important part during the formation of vermicular limestone,and these two are as one falls,the other rises.Meanwhile, the chemical conditions of the seawater played key effect.The spreading algal macula in vermicular limestone become lighter under the fluorescence microscope,illustrating that there were high content of the organic matter in the algal macula and the algal and its life behavior had given contribution to the formation of the vermicular limestone.There existed a close relationship between the formation of vermicular limestone and the special chemical conditions of the sea water.Poor oxygen,negative shift of carbon-oxygen-sulfur isotope,increased proportion of H₂S and CO₂,increased alkalinity and salinity of the sea water, and the aragonite ocean etc had formed main features of the oceanic environment in the early Triassic.The vermicular limestone developed at the latter period of anoxic events and the release of poisonous gas events,when the values of carbon isotope returned to higher levels.Subsequently, the environment improved gradually and the biota became active.As a result,the vermicular limestone could be considered as the indicator of the improvement of environment and biotic recovery.As an important "anachronistic facies" of Phanerozoic,vermicular limestone,identical to other abnormal sediments that are the results of global anoxic/dysoxic condition following the end-Permian mass extinction,indicates the oceanic condition returning to the early aragonite ocean interval of Phanerozoic.These anachronistic facies,appeared in special interval,could be considered as the mark of an extreme environment relating to the global anoxic/dysoxic events.