Journal of Conference Abstracts

A Carbonate Crust in the Permian-Triassic Boundary Interval in Sichuan, China: Structure, Setting and Distribution

Stephen Kershaw (Stephen.Kershaw@Brunel.ac.uk)¹, Li Guo (lg203@hermes.cam.ac.uk)² & Jiasong Fan³

¹ Department of Geography and Earth Sciences, Brunel University, Uxbridge, Middlesex, UB8 3PH, United Kingdom

² Department of Earth Sciences, Cambridge University, Downing Street, Cambridge, CB2 3EQ, United Kingdom

³ Institute of Geology, Chinese Academy of Sciences, Deshengmenwai, Qijiahuozi,, Beijing 100029, China

A recrystallised carbonate crust with digitate architecture, capping reef complexes of the Permian-Triassic boundary interval in Sichuan, China, was recently interpreted as either a microbialite or an inorganic cement growth (1). New, better-preserved material displays a partially altered radial fabric not seen before. Interpretations of this fabric include: inorganic cement growth similar to cavity fills in the Permian reef cores; microbial (or microbially-mediated) cement, similar to Proterozoic precipitated stromatolites; and partly altered skeletal frame of a microtubule-bearing Solenopora-like red algae. Digitate architecture of the c.1 m-thick crust, in which the branches mostly exhibit pronounced lobate margins, is more like an organic than an inorganic nature. Unfortunately, all published examples of the crust show diagenetic alteration, presently precluding unequivocal identification of its nature. Its abrupt appearance and disappearance in the boundary interval appears to be a widespread feature of that level in northeast Sichuan Province over a distance of at least 240 km, and represents a major regional event. Evidence so far shows that the crust mostly formed on reef complexes, extending for only short distances beyond their edges, and wedges out down reef flanks. In one site the crust formed on c.30 m of fine-grained dolomite overlying the reef complex suggesting that: (a) topography, not the reef facies, is the main control on crust location; and (b) processes which led to crust formation were not responsible for extinction of the reef systems. The sum of evidence so far indicates a sea-level rise in the interval.

Kershaw S, Zhang T & Lan G, Palaeogeography, Palaeoclimatology, Palaeoecology, 144, (1998).

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