Silicified brachiopod and bivalve fossils from the mid-Permian (Kungurian−Roadian) Wandrawandian Siltstone of the southern Sydney Basin, southeastern Australia: implications for taphonomy and silicification

The occurrence of silicified brachiopod and bivalve fossils in carbonate rocks is a common feature of the fossil record, but they have rarely been reported from siliciclastic facies. In this paper, 125 fully silicified brachiopod and bivalve internal moulds and composite casts are documented for the first time from the mid-Permian (Kungurian−Roadian) Wandrawandian Siltstone in the southern Sydney Basin, southeastern Australia. Although most of the silicified fossils could not be definitively named at species level due to lack of sufficient external characteristics, nine distinct species have been identified and briefly described. Though all are preserved either as siliceous internal moulds or silicified composite casts, the latter consisting of both a siliceous internal mould as well as a silicified shell, three different types of preservation mode have been recognized. Type A preservation is predominant among the specimens studied and is characterized by fully siliceous internal moulds without incorporating any shell material. This mode of silicification is interpreted to have taken place by void-filling silica within the articulated shells of the brachiopods, and probably within the taphonomic active zone in the very early stage of diagenesis. Type B mode of preservation is the least represented among the studied material and characterized by the partial composite casts of one rhynchonellide and one terebratulide species, consisting of both fully siliceous internal moulds and silicified parts of the shell that have preserved evidence of their original shell microstructures, including the cross-laminated and fibrous secondary layer and punctae. The formation of these siliceous moulds is interpreted to likely have taken place during early diagenesis within the articulated shell cavities of the brachiopods, and involved the process of simultaneous dissolution of the shell and precipitation of silica. Type C mode of preservation is restricted to two bivalve species. It is characterized by the excellent preservation of both fully siliceous internal moulds and also the silicification of originally aragonitic shells. Based on these interpretations, a new taphonomic model is proposed to account for the formation of fully siliceous internal moulds of brachiopods and that of silicified composite casts of bivalves. <i>G. R. Shi [[e-mail]], Sangmin Lee* [[e-mail]], Brian G. Jones [[e-mail]] and Paul F. Carr [[e-mail]] Environmental Futures, School of Science, University of Wollongong, Northfields Avenue, New South Wales 2522 Australia; Bruce Runnegar [[e-mail]] Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA.</i>