Organic matter in Australian Cambrian oil shales and other Iower Palaeozoic shales

Abstract

Reflected light microscopy, transmission electron microscopy (TEM), elemental analysis, carbon isotopic compositions and gas chromatography—mass spectrometry (GC—MS) have been combined in the study of the organic matter from Cambrian oil shales in the Georgina Basin. These have shown two main kinds of organic matter to be present. The first kind consists of autochthonous well-preserved filamentous structures of inferred cyanobacterial origin with occasional remains of possible methanogenic bacteria. The latter point to limited microbial degradation of the substrate namely, the inferred cyanobacterial mats represented by the filamentous structures. The second kind is a significant humic fraction and a rare third kind of organic matter possibly represents the remains of planktonic algae. A close resemblance to organic matter from the Thorntonia Limestone sequence of Middle Cambrian age is apparent from TEM observations. The oil shales may have been deposited in a shallow marine environment where oxygenated waters were separated from reducing sediments by a cyanobacterial mat. The humic acids probably represent organic matter which was partially oxidised before it settled on the mat and was ultimately incorporated into the sediment.