Upper Devonian reefs and microbialite at Maoying, South China-implications for paleoenvironmental changes

摘要

Reefal carbonate deposits in South China preserve a record of community structure and environmental change across the Frasnian-Famennian boundary and through the Late Devonian, detailing a regional response to one of the largest mass extinction events in Earth history. The Upper Devonian (Frasnian to Famennian) limestone outcrops in the Maoying area of northern Ziyun County, South China, record a carbonate platform depositional setting with some patch reef development. The Upper Devonian carbonate deposits in Maoying area are distinctive from those in correlative sections in Dushan County with rare argillaceous limestone and dolomitic limestone as well as less dolostone. Exposures to the south in the Guilin area of northeastern Guangxi Province are similar to those seen in the study area. Upper Devonian carbonates in the Maoying area are divided into two lithologic groups, reef limestones or dolostone beds, composed of five lithologically distinctive intervals. The Upper Devonian strata have a cumulative thickness of 530 m. The Upper Devonian reefs in the Maoying area were developed on a shell pavement of brachiopod- and bivalve-shell shoal packstones and evolved into patch reefs in the interior of the carbonate platform. Patch-reef development was limited to early Frasnian time. Reef building organisms are dominated by massive and tabular stromatoporoids, Amphipora sp., Stachyodes sp., and coral Thamnopora sp., while brachiopods and algae accumulated between stromatoporoid skeletal frameworks. Three evolutionary stages of the Frasnian patch reefs, including an initial development stage, maturity stage and demise stage, can be recognized according to sedimentary structures and fossil assemblage characteristics. The Frasnian strata overlying the patch reefs represent a well-bedded back-reef depositional setting and are mainly composed of Amphipora floatstone and packstone containing a few bulbous stromatoporoids and small fasciculate corals. Famennian strata are characterized by laminated limestone and fenestrate limestone that are interbedded with infrequent mudstone and argillaceous limestone horizons. A prominent microbialite bed developed in the laminated limestone of the Famennian carbonate platform consists of stromatolite and thrombolite framestone. This bed can be correlated, in horizon and age, with microbial reefs and mounds in the Famennian offshore carbonate platform in the Huanan Epeiric Sea (such as Famennian microbial reefs and mounds in Guilin). This represents a biogenic carbonate formation characterized by microbial fabrics that accumulated in a nearshore carbonate platform setting after the global Frasnian/Famennian biotic extinction event, which promoted the growth of microbialites during the latest Devonian in South China. (C) 2016 Elsevier B.V. All rights reserved.