Abstract: Triassic Burial of Organic Matter on the Northern Margin of Pangea and Consequences of Subsequent Continental Breakup

Philip A. Meyers, Elizabeth A. Kowalski

Dark-colored, Upper Triassic siltstones were recovered at four sites on the Wombat Plateau offshore of northwest Australia by Ocean Drilling Program Leg 122. The rocks are enriched in organic carbon, typically containing between 1-3% and reaching as high as 13% by weight. They were deposited during Carnian-to-Norian time on the northern Pangean margin as a fluviodeltaic-lagoonal sequence which gradually deepened to a reef complex during the Rhaetian. 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, ^dgr¹³C values between -23 and .27^pmil, and dominance of long chain length n-alkanes. Two other organic-carbon-rich siltstones which are chronostra igraphically and paleogeographically equivalent to the Wombat Plateau rocks are the Mungaroo Formation in the Carnarvon Basin of northwest Australia and the Thini Formation in the Himalaya Mountains of central Nepal. The three settings comprised a continuous Late Triassic depositional system on the northern Pangean margin which became separated and eventually dispersed during Jurassic rifting and drifting. Different post-rifting histories created different amounts of thermal maturity in the organic matter in these related deposits. The thermal maturity of organic matter in the Wombat Plateau sections is below the level necessary for significant hydrocarbon generation, whereas organic matter in parts of the Mungaroo Formation is thermally mature, and it is overmature in the Thini Formatio .

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994