Coevality of the sea-level fall and main mass extinction in the Permian-Triassic transition in Xiushui, Jiangxi Province, southern China

A continuous Permian-Triassic boundary （PTB） section has been found and studied for the first time in Xiushui, Jiangxi Province, South China. Evidence for a large sealevel fall has been found in the horizon of 0.8 m below the PTB, from the beginning of Hindeodus changxingensis zone （correlatable to Hindeodus typicalis Zone of the Meishan section）. Sedimentary record indicates that the sea level kept at Iowstand, or occasionally rose slowly during the whole Hindeodus parvus zone, except another substantial sea-level fall in early H. parvus zone. It began a quick rise from the beginning of Isarcicella staeschei zone, kept rising for the whole/, staeschei zone, and probably caused the stagnation of sea water. The first severe change in the biota, marked by the sudden disappearance of all steno- tropic organisms such as fusulinids and dasycladacians, happened at the same time as the first sea-level fall, and is regarded as the first and main episode of the end-Permian mass extinction in this area. A microbe-dominated biota followed the first extinction, and spanned the late H. changxingensis zone and the whole H. parvus zone. All the microbes and some other eurytropic organisms including gastropods and ostracods disappeared at the end of the H. parvus zone, and the following biota in the/. staeschei zone is very simple. The coevality of the main sea-level fall and the main extinction episode might be causal： both of them might be caused by a drastic climatic cooling.

Growth characteristics and sedimentary mode of Permian reefs,Lengwu,Tonglu,Zhejiang Province,southern China

Organic reefs are favourabte accumulation spaces for hydrocarbons and various mineral resources. A complete Permian organic reef profile about 44 m thick with distribution area no more than 1 km2 is exposed near Lengwu, Tongtu, Zhejiang Province. Examination of outcrops and thin sections revealed that the main reef-building organisms are catcisponges, with inozoans as dominant type. Five types of rocks have been recognized in the reef, and they are catcisponge framestone, catcisponge bafflestone, bindstone, rudstone and bioctastic wackestone. The profile was constructed in three reef-building stages. The thickness of the second stage is largest, followed by the first stage and the third stage is smallest. Each stage started with a frame-stone or bafflestone, ended as the reef grew near the sea-level, and died because of influx of terrigenous sediments. The development of the Lengwu reef is controlled by biological factors and sea-level changes. Based on the study a reef sedimentary model is established.

Microfabric features of microbial carbonates:experimental and natural evidence of mold holes and crusts

Results of our study based on examination of induced precipitation of carbonate by a cyanobacterium, Lyngbya in the laboratory, and the analyses of microphotographs of both modern and ancient microbial carbonates,demonstrated the importance of recognition of mold holes and carbonate crusts in understanding microbial carbonates. In the experiment, only cyanobacteria Lyngbya can induce precipitation of carbonate, forming scattered grains on the surface of Lyngbya filaments. Carbonate crusts enclosing the old parts of the filaments were formed through aggregation of these scatter grains while mold holes were formed after decay of the filaments. Mainly based on the experiment, six different ways of microbial carbonate formation were recognized:(1) trapping without mold holes,(2) trapping with mold holes,(3) particle-forming induced-precipitation of carbonate,(4) discrete crustforming induced-precipitation of carbonate,(5) induced precipitation, forming tangled crusts that build a porous construction, and(6) induced precipitation, forming a dense construction. And mold holes and crusts can form in ways(4),(5), and(6). Examination of both modern microbial carbonates from the Shark Bay of Australia, Highborne Cay of Bahamas and the atoll of Kiritimati and the microbialites from the Cambrian dolostone sequence in Tarim,Xinjiang, China all demonstrated the limitation of recognizing only mesofabric features and importance of examining microfabric features for understanding of the genesis of the microbial carbonates and their proper classification. The shape, size and arrangement of the mold holes, crusts, and the features of the minerals filling in pores between the crusts, which are referred as the microfabric features here, are keys to better understand the formation and environments of both modern and ancient microbial carbonates.

Halysis Høeg,1932—An ancestral tabulate coral from the Ordos Basin,North China

The problematic calcareous microfossil Halysis is abundant in the Middle Ordovician Darriwilian Stage of the western edge of the Ordos Basin,North China.The rich and well-preserved specimens of Halysis in this area facilitate detailed studies for its skeletal construction and tube microstructure.Halysis differs from calcified cyanobacteria and calcareous red and green algae in morphology,skeletal construction and microstructure,as well as reproduction mode.Halysis typically consists of multiple juxtaposed parallel tubes arranged in sheets(‘multiple-tube'type)or is just composed of one tube(‘single-tube'type).In‘multiple-tube'Halysis,tube fission by bifurcation results from the insertion of a microcrystalline wall at the center of a mother tube.This study demonstrates for the first time that the tube walls of Halysis have a laminofibrous(fibronormal)microstructure,composed of fibrous calcite perpendicular to wall surface,and recognizes the‘single-tube'type Halysis composed of one tube;in addition,for the first time,this study finds out that‘multiple-tube'Halysis develops buddings from the conjunction of two tubes and‘single-tube'Halysis shows wide-angle Y-shaped branchings.Based on these findings,this study further compares Halysis with tabulate corals.Halysis appears stratigraphically earlier than Catenipora and Aulopora,and has a smaller tube size.‘Multiple-tube'Halysis resembles Catenipora and‘single-tube'Halysis resembles Aulopora in skeletal construction and microstructure,and in their tube walls of laminofibrous microstructure composed of fibrous calcite perpendicular to the tube wall surface.Catenipora and Halysis are both characterized by the absence of septal spines.The similarities suggest that Halysis may be the ancestor of Catenipora-like and Aulopora-like tabulate corals.

Macro-and microfeatures of Early Cambrian dolomitic microbialites from Tarim Basin, China

The fabrics of microbialites preserved in limestones are generally better than in dolostones. What are the fabrics of the microbialites preserved in heavily dolomitized dolostones? This paper presents an example of a strongly dolomitized Cambrian microbialite profile. The Xiaoerblak Formation(Cambrian Series 2 Stage 3 and lower Stage 4)of the Sugaitblak section in Aksu, Xinjiang Uygur Autonomous Region, China is mainly composed of microbial dolostones. Due to strong alteration by diagenesis, their features, formation and environments have not been fully understood. Here, based on detailed observation on outcrops and thin sections, we show that this formation comprises four kinds of microbialites: laminite, thrombolite, thrombolitic laminite, and Renalcis framestone, in five intervals(Interval I to Interval V). We identified three main types of microbialite fabrics, i.e., clotted fabric, laminated fabric and skeletal fabric, and established a high-resolution vertical evolution sequence of the microbialites. The clotted fabric and the laminated fabric were further divided into subtypes. We found that the original fabrics were mainly affected by dolomitization, recrystallization and dissolution, and the alteration degree of the microbialite fabric is stronger in the lower part of this formation. The laminated fabric has the strongest resistance to diagenesis,followed by the clotted fabric. Based on studies of different rock types and sedimentary structures, we concluded that the sedimentary environment of Xiaoerblak Formation consists of three settings: a) Intervals I to III formed in restricted tidal flat environments, b) Interval IV and the lower part of Interval V in restricted deep subtidal environments, and c) upper part of Interval V in shallowing-up open subtidal environments.