Stephen A. Royer¹, William E. Galloway²
(1) Shell Offshore Inc, New Orleans, LA
(2) University of Texas at Austin, Austin, TX

Abstract: Late Permian ice-dominated deltas of the Sydney Basin, Australia

From the Early Permian through the Late Triassic, Australia drifted from its Carboniferous position near the South Pole into middle latitudes of the Southern Hemisphere. During Late Permian time (around 250 Ma), the Sydney Basin experienced a seasonally cold climate due to its high-latitude location. Mechanical weathering dominated sediment erosion, and extensive pack ice developed on fluvial, deltaic, and marine waters each year. The icy conditions resulted in significant reduction of fluvial discharge to the deltas and prevented waves from reaching the shore. Consequently, low energy conditions prevailed on the delta systems throughout most of the year. This type of delta is termed ice-dominated. The Wilton and Erin’s Vale Formations comprise Late Permian fluvial, lower delta plain, delta front, pro-delta, and shelfal sedimentary facies.

Late each spring, the ice thaw initiated a series of processes collectively called ‘breakup’. These processes developed facies and morphologies unique to high-latitude deltas. Late Permian deltaic rocks exhibit the abundance of ice-deposited debris expected in polar marine sediments. A high percentage of lithic fragments evidences mechanical weathering. Ice effectively covered and protected the delta from each spring deluge. This created a zone of course-sediment bypass, the sub-ice platform, and a seaward detachment of the prograding delta front from the shoreline. Sub-ice channels, sub-aqueous extensions of distributary channels, crossed the sub-ice platform, delivering sediment to the delta front. Consequently, these polar deltas lack a well-defined distributary mouth-bar facies. Mean grainsize increases from the shoreline, across the sub-ice platform, out to the delta front.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana