Abstract

Sediments enriched in continental organic matter evidently accumulated on the northern margin of Gondwana during the Late Triassic. Dark colored Upper Triassic siltstones were recovered at four sites on the Wombat Plateau offshore of northwest Australia by Ocean Drilling Program Leg 122. These rocks typically contain between 1-3% organic carbon and reach as high as 13% by weight. They were deposited in a Carnian-Norian fluviodeltaic-lagoonal sequence which gradually deepened to a Rhaetian reef complex. The organic matter throughout this transgressive sequence is predominantly type III (land plant) kerogen, based on low Rock-Eval hydrogen index values, C/N ratios between 8 and 45, δ¹³C values between -24 and -26%‰, and dominance of long chainlength n-alkanes. The thermal maturity of organic matter is below the level necessary for significant hydrocarbon generation, yet these sediments contain adequate organic matter to generate and expel commercial amounts of gaseous hydrocarbons if subjected to higher thermal stress than found on the Wombat Plateau. Two other organic carbon-rich siltstones which are chronostratigraphically equivalent to the Wombat Plateau rocks are the Mungaroo Formation in the Carnarvon Basin of northwest Australia and the Thini Formation in central Nepal. Unlike material from the Wombat Plateau, organic matter in parts of the Mungaroo Formation and in the Thini Formation is thermally mature. The three settings comprised a continuous Late Triassic depositional system on the northern margin of Gondwana which was broken up during Jurassic rifting and drifting.