GA28 Guizhou

LANGBAI site, Great Bank of Guizhou; PTB microbialite

This page authored by Steve Kershaw

This area was made famous by the work of Dan Lehrmann and colleagues. Below are some photos of the Langbai site on the Great Bank of Guizhou, where PTB microbialite encrusts an irregular erosion surface. There is great interest in this surface because some authors consider it as a submarine dissolution surface caused by ocean acidification associated with the end-Permian extinction event. Others drew attention to the presence of vadose fabrics in the foraminiferal grainstone (light-coloured area below the erosion surface).

General view of karst landscape in the Great Bank of Guizhou. The Langbai site is a few metres behind and to left of the position of the photographer in this photograph.

View of outcrop. The base of the microbialite is at the bottom of this slope.

Close-up view of the erosion surface; below it is pre-extinction Changhsingian limestones of latest Permian age; the darker grey rock is the calcimicrobial framework. Sample R20 is shown on the left.

Detail of the erosion surface; the microbialite frame encrusts directly onto it. The microbialite has micrite infilling the frame.

Polished vertical section sample showing two erosion surfaces, the upper one of which is the latest Permian surface in this area. The microbialite (black) is a framestone of renalcid-group calcimicrobes. Curiously there is no grading the the sediment grainsizes above the erosion surface. White grains below the erosion surface are foraminifera, forming grainstone in which there are vadose cements (described by Collin et al., 2009 in Sedimentology). There is no direct evidence of sea-floor dissolution, and we await proof that ocean acidification, proposed for the end-Permian event, can actually be demonstrated.

Three photographs below show field appearance of a small microbial dome, one of the microbial domes described by Dan Lehrmann as part of the post-earliest Triassic microbialites. Note the vertical fabric. Such domes do not occur in the northern South China Block localities described elsewhere in this atlas.

Photo: Steve Kershaw