Late Palaeozoic sequence stratigraphy and brachiopod faunas of the Tarim Basin, Northwest China

This thesis deals with the stratigraphy and brachiopod systematic palaeontology of the latest Devonian (Famennian) to Early Permian (Kungurian) sedimentary sequences of the Tarim Basin, NW China. Brachiopod faunas of latest Devonian and Carboniferous age have been published or currently in press in the course of the Ph.D candidature and are herein appendixed, while the Early Permian brachiopod faunas are systematically described in this thesis. The described Early Permian brachiopod faunas include 127 species, of which 29 are new and 12 indeterminate, and six new genera (subgenera) are proposed; Tarimella, Bmntonella, Marginifera (Arenaria), Marginifera (Nesiotia), Baliqliqia and Ustritskia. A new integrated brachiopod biostratigraphical zonation scheme is proposed, for the first time, for the latest Devonian-Early Permian sequences of the entire Tarim Basin on the basis of this study as well as previously published information (including the Candidate's own published papers). The scheme consists of twenty three brachiopod acm biozones, most of which replace previously proposed assemblage or assemblage zones. The age and distribution of these brachiopod zones within the Tarim Basin and their relationships with other important fossil groups are discussed. In terms of regional correlations and biostratigraphical affinities, the Late Devonian to Early Carboniferous brachiopod faunas of the Tarim Basin are closest to those from South China, while the Late Carboniferous faunas demonstrate strong similarities to coeval faunas from the Urals, central Asia, North China and South China. During the Asselian-Sakmarian, strong faunal links between the Tarim Basin and those of the Urals persisted, while at the same time links with central Asia, North China and South China weakened. On the other hand, during the Artinskian-Kungurian times, affinities of the Tarim faunas with the Urals/Russian Platform rapidly reduced, when those with peri-Gondwana (South Thailand, northern Tibet) and South China increased. Thirty lithofacies (or microfacies) types of four facies associations are recognised for the Late Devonian to early Permian sediments. Based on detailed lithostratigraphy, biostratigraphy and facies analysis, 23 third-order sequences belonging to four supcrsequences are identified for the Late Devonian to Early Permian successions, from which sea-level fluctuation curves are reconstructed. The sequence stratigraphical analysis reveals that four major regional regressions, each marking a distinct supersequence boundary, can be recognised; they correspond to the end-Serpukhovian, end-Moscovian, late Artinskian and end-Kungurian times, respectively. The development of these sequences is considered to have been formed and regulated by the interplay of both eustasy and tectonism. Using the system tract of a sequence as the mapping time unit, a succession of 47 palaeogeographical maps have been reconstructed through the Late Devonian to Early Permian. These maps reveal that the Tarim Basin was first immersed by southwest-directed (Recent geographical orientation) transgression in the late Famennian after the Caledonian Orogeny. Since then, the basin had maintained its geometry as a large, southwest-mouthed embayment until the late Moscovian when most areas were the uplifted above sea-level. The basin was flooded again in late Asselian-Artinskian times when a new transgression came from a large epicontinental sea lying to its northwest. Thereafter, marine deposition was restricted to local areas (southwestern and northwestern margins until the late Kungurian, while deposition of continental deposits prevailed and continued through the Middle and late Permian into the Triassic.

Fluctuations of redox conditions across the Permian–Triassic boundary : new evidence from the GSSP section in Meishan of South China

Oceanic anoxia has long been considered as one of the main causes for the end-Permian mass extinction. However, the results obtained by different researchers are rather divergent from different sections, or even on the same section using the same redox proxy. This study aims to examine the causes for some of these divergent results using high-resolution pyrite framboid sampling at the Meishan GSSP section in South China. Detailed microfacies analysis shows that the uppermost Late Permian strata comprises two significantly different sedimentary facies: one characterized by silicious muddy limestone and recognized as representing autochthonous background sediments; the other distinguished by bioclastic grainstone, interpreted to be allochthonous in origin and have been transported from the nearby platform margin. These two different sedimentary facies represent two distinctly different redox conditions. Together with the facies analysis, a statistical analysis of pyrite framboids was carried out to evaluate the redox evolution across the Permian-Triassic boundary. Abundant framboids with average diameters of about 6μm are found in background sediments beneath the extinction boundary, indicating generally anoxic bottom water conditions. But this condition was punctuated by transient intervals of rapid oxygenation interpreted to have been caused by intrusion of intermittent turbidity flows. Our study also showed that anoxic conditions persisted into the immediate aftermath of the mass extinction, thereafter it was quickly followed by a relatively long period of oxic conditions (with rare framboids). However, the redox conditions returned to anoxia (with abundant pyrite framboids averaging about 5μm in diameter), accompanied by a rapid global transgression. The oxygenation manifested near the Permian-Triassic boundary coincides with the negative excursion of carbon isotope. This would imply that, contrary to previous interpretations, this great δ13C negative excursion was probably not caused by the upwelling of anoxic deep ocean waters.

Sedimentology and ichnology of two lower Triassic sections in South China: implications for the biotic recovery following the end-Permian mass extinction

Biotic recovery following the end-Permian mass extinction was investigated using trace fossil and facies analysis of two Lower–Middle Triassic sections in South China. The Susong section (Lower Yangtze Sedimentary Province) comprises a range of carbonate and mudstone facies that record overall shallowing from offshore to intertidal settings. The Tianshengqiao section (Upper Yangtze Sedimentary Province) consists of mixed carbonate and siliciclastic facies deposited in shallow marine to offshore settings. Griesbachian to Dienerian ichnological records in both sections are characterized by low ichnodiversity, low ichnofabric indices (1–2) and low bedding plane bioturbation indices (1–2). Higher ichnofabric indices (3 and 4), corresponding to a dense population of diminutive ichnotaxon, in the Tianshengqiao section suggest opportunistic infaunal biotic activity during the earliest Triassic. Ichnological data from the Susong section show an increase in ichnodiversity during the late Smithian with 11 ichnogenera identified and increased ichnofabric indices of 4–5 and bedding plane bioturbation indices of 3–5. Although complex traces such as Rhizocorallium are present in Spathian-aged strata in this section, low ichnodiversity and ichnofabric indices and diminutive Planolites suggest a decline in recovery. In the Tianshengqiao section, ichnofabric indices are moderate to high (3–5) although only six ichnogenera are present and Planolites burrows are consistently small in Smithian and Spathian strata. Complex traces, such as large Rhizocorallium and Thalassinoides, and large Planolites, did not appear until the Anisian. Ichnological results from both sections record the response of organisms to unfavourable environmental conditions although the Susong section shows earlier recovery during the Smithian prior to latest Smithian–Spathian decline. This decline may have resulted from a resurgence of euxinic to anoxic marine environment in various regions of South China. Ichnological data from the Tianshengqiao section indicate protracted recovery throughout the Early Triassic as previously found elsewhere in South China. Comparison of the South China trace fossil records with global ichnological data show a diachronous pattern of recovery of trace makers and highlights the heterogeneous development of oxic facies on the marked variation in recovery rate.