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

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 topographic inheritance of a latest Permian pull-apart basin into Early Triassic times, with a distribution of basal Early Triassic microbialites (BETM) restricted to uplifted blocks. This control is also reflected by 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 BETM exposed west of this fault. Triassic foraminifers occur in the basal most BETM episode, 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 associated with 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 from tabulated to domical microbial build-ups is accompanied by accumulation of coquinoid lenses between domes, which indicates 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.

New data from Oman indicate benthic high biomass productivity coupled with low taxonomic diversity in the aftermath of the Permian–Triassic Boundary mass extinction

“…The occurrence of crinoid or bivalve assemblages thriving on a well-oxygenated offshore seamount during the Griesbachian in Oman stands in sharp contrast with the interpretation of a widespread anoxia as a cause for a delayed biotic recovery (Hallam 1991;Isozaki 1997;Algeo et al 2008Algeo et al , 2010Brennecka et al 2011;Schoepfer et al 2012;Elrick et al 2017). The presence of Griesbachian well-oxygenated conditions has been inferred not only in the Tethyan offshore seamounts (Krystyn et al 2003; this study) but also on the Northern Indian Margin (Hermann et al 2011;Schneebeli-Hermann et al 2013), in the north-east Greenland rift (Sanson-Barrera et al 2015) and in shallow water microbialite of Guangxi, South China (Kaim et al 2010;Forel et al 2013;Hautmann et al 2015;Bagherpour et al 2017). In the Nanpanjiang pullapart basin, anoxia has been shown to be restricted to grabens while microbialite flourished in oxic conditions on intervening horsts ).…”

“…; Bagherpour et al . ). In the Nanpanjiang pull‐apart basin, anoxia has been shown to be restricted to grabens while microbialite flourished in oxic conditions on intervening horsts (Bagherpour et al .…”

“…In the Nanpanjiang pull‐apart basin, anoxia has been shown to be restricted to grabens while microbialite flourished in oxic conditions on intervening horsts (Bagherpour et al . ). On the Northern Indian Margin as well as on microcontinents transiting northward across the Tethys (e.g.…”

“…), where water masses were trapped in silled basins (Bagherpour et al . ), where the siliciclastic load of the water column was high, where the flux of terrestrial organic matter was high, and where salinity may have been variable. The ‘disastrous’ view of the recovery could reflect the extremely unbalanced palaeogeographical distribution of studied sections between continental shelves and offshore records.…”

Controls on the formation of microbially induced sedimentary structures and biotic recovery in the Lower Triassic of Arctic Canada

“…The post-mass extinction world of the Early Triassic is often considered to represent an unusual time in Earth history when marine recovery from the Permo-Triassic mass extinction (PTME) was delayed by the prevalence of marine anoxia (Hallam, 1991;Twitchett and Wignall, 1996;Woods et al, 2007;Wignall et al, 2010Wignall et al, , 2016Grasby et al, 2013Grasby et al, , 2016Pietsch et al, 2014), and anachronistic facies better known from Cambrian and earlier times were well developed; especially carbonate microbialites (e.g., Bagherpour et al, 2017) and microbially induced sedimentary structures (MISS) in sandstones (Wignall and Twitchett, 1999;Pruss et al, 2004Pruss et al, , 2005Baud et al, 2007;Noffke, 2010). In recent years a broad range of sandstone bedding features have been related to the presence of microbial mats at the time of deposition.…”