Thomas, Stephanie G.¹, Christopher R. Fielding¹, Tracy D. Frank¹ 
(1) University of Nebraska-Lincoln, Lincoln, NE

ABSTRACT: Sedimentology of the Wandrawandian Siltstone in the Southern Sydney Basin, NSW, Australia: Record of a Middle Permian Glacial Period?

The middle Permian Wandrawandian Siltstone forms a distinctive marine mudrock-rich interval within the succession of the southern Sydney Basin, New South Wales, Australia. The unit, < 120 m thick, was deposited during a major marine transgression that can be correlated over 2000 km from south to north and is enclosed by sandstone-prone formations of shallow marine to coastal origin. The Wandrawandian Siltstone comprises mainly siltstones and fine sandstone rich in invertebrate fossils. The unit is generally heavily bioturbated containing a trace fossil assemblage that may be referred to as the Cruziana ichnofacies. Outsized lonestones, interpreted as ice-rafted debris, are abundant throughout the unit and reach up to 1.5 m in diameter. A few horizons contain “cannonball” concretions, some of which surround glendonites or lonestones. Six distinct horizons preserve concentrations glendonite psuedomorphs after ikaite (a metastable, hydrated CaCO₃ mineral that forms in the presence of water close to freezing temperature). Some previous interpretations have therefore suggested that the unit records a temporary return to glacial conditions in the middle Permian of the Sydney Basin. In addition to the siltstones, deposited in quiet, offshore shelf environments, the Wandrawandian Siltstone contains storm-emplaced sandstone beds, an interval characterized by numerous slump sheets, and an incised channel fill. The slump sheets, along with sand-injection structures and load casts, indicate instability of the submarine sediment surface and are related to fluid pressure change during lowstand. With the incised channel also offering evidence for lowstand, a temporary return to glacial conditions is probable during the middle Permian.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.