Print this pageF. E. Burns1, E. Hooper2
(1) University Western Australia, Perth, Australia
(2) Apache Energy Ltd, Perth, Australia
ABSTRACT: Omission Colonisation Surfaces within the Lower Cretaceous Mardie Greensand, Northern Carnarvon Basin, NW Shelf, Australia
The Mardie Greensand is a regionally extensive, glauconite-rich, transgressive unit associated with the flooding of the Barrow Group delta during the Early Cretaceous. In the Northern Carnarvon Basin, the reservoir in the study interval comprises dominantly inner shelf deposits. Due to the intensity of bioturbation, an ichnofabric approach to depositional modelling and stratal surface recognition is adopted.
The trace fossils identified in core are dominated by Teichichnus, Palaeophycus, Skolithos, Ophiomorpha irregulaire, Diplocraterion habichi, Thalassinoides, spindle-shaped Asterosoma and Chondrites, with indeterminate mottling due to pervasive siderite cementation. Primary sedimentary structures include wavy lamination and low-angle HCS. Deposition is interpreted to have occurred within a low energy, gently dipping, lower shoreface and inner shelf setting, between fairweather and storm wave base. Where bioturbation is abundant, coarse grains are dispersed throughout the greensand, producing a 'floating grain' texture attributed to softground burrowing communities. However, concentrations of coarse-filled (up to granule size) Thalassinoides and Skolithos are also evident, representing periods of omission associated with both transgressive ravinement surfaces and downshift surfaces. The coarser-grained components are derived from both storm-generated lag deposits and more laterally-extensive, shoreface erosion surfaces associated with fluctuations in relative sea level. Further basinward, within the inner and outer shelf successions, flooding surfaces are marked by concentrations of Skolithos, Diplocraterion habichi and sand-filled Thalassinoides.
Recognition of omission colonisation surfaces marked by firmground Thalassinoides and Skolithos, and characterisation of their lateral variability, is key to the precise identification and correlation of stratal surfaces of sequence-stratigraphic significance.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado