Thomas, Stephanie G.1, Christopher R. Fielding1, Tracy D.
(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.