Sources and preservation of organic matter in the Cretaceous Toolebuc Formation, eastern Australia

Abstract

The organic matter-rich Toolebuc Formation of eastern Australia was deposited in a Lower Cretaceous epicontinental sea. Parameters from biological marker studies indicate that the organic matter is immature to marginally mature for hydrocarbon generation. The occurrence of abundant coccoliths and the distribution of alkane biomarkers suggest that the organic matter (Type II) is largely of planktonic origin and only in the southeastern part of the depositional area can a terrestrial influence be discerned. Variations in kerogen composition can be attributed to the extent of the oxidation of the source materials and the degree of incorporation of sulphur. The atomic H/C ratios ( c. 1.1) are remarkably constant for most of the Toolebuc Formation. Atomic O/C ratios vary from 0.1 and 0.4 and can be related both to depth and paleogeographic position. Kerogen sulphur contents range up to 7%, and the highest values occur in the most carbonate-rich sediments. Total sulphur (inorganic + organic) to carbon ratios in the sediments vary from 1 to <0.2 and are a function of paleogeographic position and lithology. Most of the sulphur in the sediments is in the form of pyrite, but the proportion of sulphur in organic form increases as the total sulphur content decreases. The evidence for oxidation of the organic matter and incorporation of sulphur into it during deposition suggests that bituminite, which is the dominant organic maceral in the Toolebuc Formation, was formed from an organic gel derived by decay of predominantly algal material. These data support a modified gyttja model (Kauffman, 1981) for the deposition of organic matter in the Toolebuc Formation.