Aragonite in Permian Reefs

Abstract

PRESERVATION of aragonite in fossils decreases the further we go back in the geologic record. In the Palaeozoic, aragonitic remains have been found only in clayey–bituminous deposits which have largely inhibited diagenetic alterations of unstable mineral phases (for example, Buck-horn Asphalt and Kendrick Shale, both Upper Carboniferous of USA¹, and Upper Oil Shale Group, Lower Carboniferous of Scotland²). Palaeontological information preserved in these facies is restricted, however, to those organic remains which lived in the equivalent basinal environments or were transported into them. Original mineralogy and microstructures of ancient reef organisms can generally only be deduced from analogies with equivalent younger counterparts. The oldest known reef-organisms with preserved aragonitic microstructures were found in the Upper Triassic (Lower Carnian) of the Southern Alps³. Here, early burial of patch reefs and reef detritus by clayey–tuffaceous sediments and porosity diminution of the carbonates by subsequent cementation have probably inhibited transformation of aragonitic micro-structures into calcite⁴. In this report we describe investigations of micrestructures and diagenesis in calcareous sponges which have revealed aragonitic primary structures and cements in material from the Upper Permian of Djebel Tebaga, Southern Tunisia.