The Pre-Permian history of the North Perth and South Carnarvon Basins, Western Australia

Abstract

The Lower Palaeozoic history of the Perth and Southern Carnarvon Basins is enigmatic and is often based on the occurrence of “undifferentiated Palaeozoic red sandstones” encountered at the base of a number of wells. These are often assumed to be equivalent to the Tumblagooda Sandstone, an Ordovician aged terrestrial deposit which outcrops in a fault bounded slither between the Perth and South Carnarvon Basins. Onshore seismic data is generally of poor quality at this level, making it difficult to assess the extent and tectonic significance of this unit, or the relationship between the lithologies described in the wells and the outcrop. Seismic data from the Houtman sub-basin, in the northern, offshore part of the Perth Basin shows very clear evidence of extensional fault activity that pre-dates an unconformity at the base of a well-established sequence of Permian-Triassic aged sediments. The only well penetration of this pre-Permian sequence again refers to “undifferentiated Palaeozoic red sandstones”. It is thus possible that this data provides clear evidence of Lower Palaeozoic extension on what is now the western margin of Australia. However, given the almost complete absence of stratigraphic control, these sequences could equally as well correlate with Devonian and Carboniferous sequences that are present in the Southern Carnarvon Basin, immediately to the east. It is only by careful seismic-stratigraphic correlation that these two alternative interpretations can be evaluated. Whatever the exact age of this deformation, it is clear that it established the fundamental structural grain that was repeatedly reactivated during Mesozoic and Cenozoic events. The Pre-Permian faults mimic the strike of younger structures, and several show spectacular examples of repeated reactivation as both normal and reverse faults. Understanding this early episode of basin formation will help to establish the early tectonic history of both the Perth Basin and Southern Carnarvon Basin – two important basins that still have considerable exploration potential.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015