Stratigraphic Evolution of the Barrow Group (Northern Carnarvon Basin, North West Shelf, Australia): Controls on the Architecture of a Shelf-Margin During a Syn-Rift to Post-Rift Transition

Abstract

The Barrow Group constitutes a unique example of a regressive system developed continuously during a syn-rift to post-rift transition. The Lower Barrow Group (LBG) was developed concomitantly to the last phase of rifting in the Northern Carnarvon Basin, before the final break-up during the Valanginian, marking the onset of deposition of the Upper Barrow Group (UBG). The study is based on the interpretation of a regional grid of 2D seismic and ~55,000 km2 of 3D seismic, integrated with ~100 exploration wells. Regional seismic unconformities, approximating the base and the top of each dinoflagellate zone from the latest Tithonian to Valanginian (P. iehiense to S. areolata) were identified. For each seismic sequence, a quantitative analysis of clinoform geometries and shelf-edge trajectory has been conducted, along with a mapping of the different depositional domains (i.e. topset, foreset, bottomset). The results show that the LBG was deposited in three depocentres (Investigator, Exmouth and Barrow) separated by paleo-structural highs. In the first stages (P. iehiense and K. wisemaniae sequences), the system displayed an overall progradational pattern; whereas the onset of aggradation and / or retrogradation is observed regionally during the following C. delicata sequence, interpreted as an increase in the rate of accommodation creation due to a regional pulse of subsidence. Initiated during the D. lobispinosum sequence, a progressive shift of sediment supply occurred towards the east, which continued during the B. reticulatum and E. torynum sequences. The S. areolata sequence marks the onset of break-up resulting in uplift and erosion of the LBG and its resedimentation as a prograding UBG restricted in the east of the study area. These results highlight the strong partitioning of subsidence, sediment supply and accumulation rates across the three regional depocentres, as a result of late syn-rift and break-up geodynamics. They provide new insights on the sensitivity of regressive margins to varying allochtonous controls during an active period of rifting. The study also offers new constraints on the regional changes in shelf-margin architecture of the Barrow Group through time and space, and its relationships with the sand delivery to the slope and basin areas. In addition, the results show that a detailed analysis of the shelf-margin architecture in an active extensive setting (i.e. syn-rift) can help to refine the rift timing before the final break-up.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017