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Title

Onset, development, and cessation of basal Early Triassic microbialites (BETM) in the Nanpanjiang pull-apart Basin, South China Block

Authors
Bagherpour, Borhan
Bucher, Hugo
Baud, Aymon
Brosse, Morgane
Guodun, Kuang
Published in Gondwana Research. 2017, vol. 44, p. 178-204
Abstract New investigations in the Nanpanjiang Basin indicate that the onset of the iconic microbialites associated with the Paleozoic-Mesozoic boundary was Early Triassic in age. Bathymetry (water agitation, oxygenation, light penetration) and clastic load are shown to have exerted a direct control on the growth of microbialites. Carbonate supersaturation is also required for the deposition of the microbialites. Bathymetric control is further corroborated by the inheritance of the topography of a latest Permian pull-apart basin into Early Triassic times, with a distribution of basal Early Triassic microbialites (BETM) restricted to uplifted blocks and the accumulation of carbonaceous black shales in adjacent troughs. The geographically most extensive Nanpanjiang BETM bloomed on a large NW–SE trending uplifted block exceeding 12,000 km2 (Luolou Platform) centered on northwestern Guangxi. Post-Triassic displacements along the Youjiang Fault obscure the paleogeographic relation of BETMexposed west of this fault. Triassic foraminifers occur in the basalmost BETMepisode,which is locally bracketed by high-energy grainstones made of reworked Permian foraminifers. Therefore, the Permian-Triassic boundary (PTB) is within the unconformity that separates the Late Permian Heshan Fm. from the basal most BETM.Where accommodation space was sufficient, up to five event surfaces are associatedwith the unconformity. Microfacies analysis supports chemical dissolution but did not reveal evidence for subaerial erosion, although intercalated grainstone made of Permian foraminifers indicate reworking. Chemical dissolution and mechanical erosion both conceivably contributed to the genesis of the unconformity. The upward shift fromtabulated to domical microbial build-ups is accompanied by accumulation of coquinoid lenses between domes, which indicate deepening of the Luolou Platform BETM. The main drowning resulting from both regional tectonic subsidence and a global sea-level rise led to the cessation of the BETM that were buried under predominant fine siliciclastics. Any concomitant change in sea water chemistry appears unlikely.
Keywords Permian-Triassic boundaryMicrobialitesBathymetryOxygenAlkalinity
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Research group Sedimentology, Biostratigraphy and Micropaleontology
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BAGHERPOUR, Borhan et al. Onset, development, and cessation of basal Early Triassic microbialites (BETM) in the Nanpanjiang pull-apart Basin, South China Block. In: Gondwana Research, 2017, vol. 44, p. 178-204. https://archive-ouverte.unige.ch/unige:93830

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Deposited on : 2017-04-25

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