Using Basement Exposures to Constrain the Structural Evolution of the Southeastern Georgina Basin, Northern Territory, Australia

Abstract

The Georgina Basin of central Australia is a remnant of the ~2 × 106 km² Neoproterozoic intraplate Centralian Superbasin, and a proven hydrocarbon producing province. The southern Georgina Basin contains >1.5 km thick Neoproterozoic and <2.2 km thick Cambrian-Devonian stratigraphic sections. Reconnaissance exploration in the southeastern Georgina Basin suggests that source rock presence and preserved reservoir geometry were likely controlled by the basin's structural evolution, as opposed to eustatic influences. Major NW-SE structural lineaments in the basin align with faults exposed in the adjacent basement. These faults have been interpreted as normal faults formed during Neoproterozoic NE-SW extension that were reactivated by contraction during the Devonian Alice Springs Orogeny. One reactivation model invokes coeval N-S and NE-SW dip-slip inversion with one NW-SE directed sinistral fault. However, existing models are limited by extremely sparse seismic reflection and well data within the basin, and because they do not use constraints from kinematically-equivalent basement exposures nor have they assessed possible oblique fault motion. New geological mapping in the adjacent basement raises the possibility of multiple hydrocarbon systems and trap types in the southeastern Georgina Basin. Working hypotheses contain the following elements: (1) Where angular unconformities between the basement and lowermost strata are preserved, the pre-existing basement foliation can be shown to have been steeply to moderately dipping. Faults interpreted as having initiated during rifting have strikes and spacing similar to that of basement foliation domains. These observations suggest the pre-existing basement fabric likely affected the response to Neoproterozoic rifting. (2) Mapped inverted fault zones contain 10 m-thick panels of quartz-hematite breccia, overlain by lowermost Georgina Basin strata lacking brecciation and veining. Microstructures and outcrop relationships indicate deformation involved both hydraulic fracturing and cataclasis during dilational shear strain, which suggests an earlier stage of movement – possibly during the initial rifting event – was accompanied by significant fluid flow. (3) Contrast in geomechanical properties across the NW-SE breccia-wall rock contacts would have made them likely surfaces for reactivation during Devonian contraction, forming oblique faults expressed as NW-SE trending open folds within the basin.

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