Paleo-environmental Conditions Of Organic Matter Enrichment In Marine Shale In The Early Cambrian In The Upper And Middle Yangtze Platform

Regarding the mechanism of organic matter enrichment in the Lower Cambrian shale,there is no integrated and through report that has really made a difference.In this study,after completing stratigraphic correlation and sequence stratigraphic framework and analizing sedimentary facies,geochemical indices combined with sedimentary evidence were used to estimate controlling factors on organic matter accumulation.Sedimentology models were also introduced to present organic matter burial during sea transgression and regression.Herein,a variety of geochemical indicators were evaluated to show the variation of redox conditions in vertical and lateral extent.A chemical zonation model of the Yangtze Sea was proposed to present redox heterogeneity.Then an integration of oceanic anoxia and organic matter burial was addressed to show their internal relationship.In the Early Cambrian,significant sea level rise happened in the Yangtze Platform.Two three-order sequences are recognized,where the trangressive marine shale is enriched in organic matter.During the transition from sea transgression to regression,TSTs and HSTs of different lithofacies assemblages were deposited along deepening sequences from shallow shelf to deep water basin.A sedimentology model is put forward to show environmental evolution due to sea level change.Terrestrial influx,primary productivity and reservation are evaluated to discuss controlling factors on organic matter accumulation.Secular variations in Si/Al,Ti/Al and probable biogenic CaCO₃ contents are used to evaluate dilution of terrestrial input to organic matter influx.?¹³Corg and biogenic Barium which may indicating primary productivity are plotted with TOC to show their relationship.While the redox changes are preliminarily indicated by redox-sensitive trace elements and degree of pyritization.In intrashelf basin,organic matter accumulation is decided by both primary productivity and bottom anoxia,while along slope to basin TOC only correlates well with primary productivity.In addition,these environmental factors fluctuate along with sea level changes.Oceanic anoxia is often identified during geologic intervals when organic-rich sedimentary rocks accumulated.Herein we deeply penetrate into redox variations via analysis of Fe speciation and pyritization,Ce anomaly in phosphorous nodules and Mo-U co-variation.Lateral changes of sedimentary?¹⁴N,is also corresponded to redox spatial heterogeneity in different facies,which is further proved by a seawater chemical zonation model.Then euxinia is found to be confined to the semi-restricted intrashelf basin and slope,while suboxic or ferruginous water prevailed in the slope to deep water basin.The evolution of anoxia in bottom water is also constrained by the paleo-environemntal changes.Except for promotion of sea level rise and warm climate in macro-scope,oceanographic conditions and trapping of nutrients also decide the extent and continuity of anoxia.When compared with organic matter accumulation,similar profitable paleo-environmental conditions constraint for the oceanic anoxia.Despite of the similarity with the Lower Silurian Longmaxi shale,we could not agree with those who proposed“preservation decides TOC concentrations”.The accumulation of organic matter in marine shale is tightly linked to paleo-environmental changes.Moreover,the significance of each factor varies largely during different geologic intervals and also in kinds of sedimentary facies.