The well-known Permian Changhsingian calcisponge reef located at Panlongdong section,Xuanhan county,northeastern Sichuan Basin has attracted wide attention.Due to severe dolomitization and poor quality of the fossils,the P-T boundary in this section is difficult to determine.This study,for the first time,recognized six communities in the Upper Permian Changhsingian Changxing Formation through the Lower Triassic Induan Feixianguan Formation of the Panlongdong section.They are Bryozoan-Archaeolithoporella-calcisponge Community,Calcareous green algae-foraminifer-crinoid Community,Microgastropod-foraminifer Community,Ostracod-microgastropod-cystic microbe Community,Ostracod-small brachiopod Community,and Non-calcified cyanobacteria Community.By using community replacement and palaeoenvironmental analysis,for the first time,we set the P-T boundary of the Panlongdong section at the middle of the calcimicrobialite containing cystic microbes.The community replacement sequence in the Panlongdong section is similar to that in other contemporaneous sections in reef areas of South China,indicating universal palaeoenvironmental changes during the Permian-Triassic transition.The results show that:(1)Changhsingian calcisponge reefs in South China generally vanished before the mass extinction and may be related to the large regression in the Late Permian.(2)The Calcareous green algae-foraminifer-crinoid Community replaced reef community and continued till the mass extinction.The mass extinction was probably related to the global sea-level drop.(3)The first aftermath community was dominated by specialized microgastropods,followed by the microbes,and then by the specialized microgastropods and small brachiopods.The succession of the three communities reflected the change in environmental conditions from dysoxic to anoxic and again to dysoxic.(4)In the Early Triassic,the relict community in shallow sea had very low diversity and low abundance,and was dominated by crinoids,gastropods,and bivalves.
The second member of the Triassic Xujiahe Formation in the Sanhuiba area in Huaying Mountain is an important section of sedimentary delta facies in the Chongqing Tianfu Geological Survey and Training Base. In spite of some initial efforts on its lithology and lithofacies, the existing research is yet to be completed and suffers from contradictory conclusions and inadequate evidence. Starting from the sedimentary microfaeies types and cyclic features, we analyzed the vertical sequence of a sedimentary delta model and discovered that this section is a constructional delta deposit. Eight incomplete and asymmetric regressive cycles are marked off, given that frequent erosion surfaces appeared in the section. Based on the analysis of structural characteristics of every monocycle, we summarized the system of cycles, tracts and sequence characteristics of the overall section. In view of this systematic series of cycles, we discussed the features of the syndepositional tectonic movements in the area and conclude that, due to frequent erosion, migration and diversion of a sub-channel branch was brought about by oscillating movements of the crust. In a comparison with neighboring sections, we explored the cause of erosion and the relations between erosion and cycles. This study greatly strengthens existing research and theories, makes an important and comprehensive contribution to geological surveys and training in the area and provides essential suggestions for coal exploration, oil and gas evaluation and in general to exploration and development in the target stratum.
An interval of limestone,which generally has a dendroid appearance on outcrops,has been found in several Permian-Triassic boundary(PTB) sections in South China,and interpreted to be microbialites.Previous research has examined the paleontology and sedimentology of the unit,however,little attention has been devoted to its diagenesis.This paper discusses the diagenesis of the microbialites in the PTB section at Laolongdong,Chongqing,South China.Examination of multiple outcrops reveals that the structure in this kind of rock is not always dendroid;in many cases it is irregular in shape,and seldom shows the "up-branching" form.In addition,the speckled interval has also been found beneath the dendroid interval.In outcrop,both the speckled and dendroid rocks consist of the darkercolored areas and the lighter-colored areas.Examination of thin sections reveals that the darker-colored areas on outcrops are actually lighter-colored in thin sections,consist of larger crystals,and are more transparent.The lighter-colored areas in thin sections generally consist of large blocky calcite containing scattered small dirty calcite rhombs and irregular residual limestones,which are the same as the lime mudstones between the lighter-colored areas.It is inferred that the dendroid and speckled rocks have experienced the following diagenetic processes:(1) exposure of the carbonate sediments to the subaerial environment because of a sea level drop;(2) dolomitization caused by the downward migration of fluids formed the dendroid and speckled dolomitic patches;(3) dedolomitization of the dolomitic patches formed dendroid and speckled patches of calcite;(4) dissolution occurred in the interstices between relic dolomite crystals formed spongy pores;and(5) filling of the spongy pores by large blocky calcite.Therefore,even though microbes played a critical role in the formation of these microbialites,diagenesis contributed greatly to the formation of the speckled and dendroid pattern of the microbialites.
