Conductivity-depth imaging in the Ord Basin, Western Australia

The Ord Basin contains three asymmetrical synclines encompassing Cambrian and Devonian strata overlying the Kalkarindji Large Igneous Province, which extends across the Western Australian - Northern Territory border in northern Australia. Previous understanding of the basin was based mostly on surface mapping and a few mineral and petroleum drillholes. No seismic data are available in western part of the basin, and the airborne electromagnetic (AEM; AusAEM 02 in 2019-2020) survey becomes the first geophysical method to profile the basin down to a depth of 600 m. A large portion of the basin, including the Hardman Syncline and southern part of the Kalkarindji Large Igneous Province is covered by 10 west-east AEM flight lines, in which contrasts of conductivity are highly consistent with outcrop geology and the Cambrian section in Okes-Durack 1 drilled in 1923-24. The survey shows Devonian sandstone and conglomerate are more resistive than underlying Cambrian siltstone in the western part of the syncline. The midCambrian section especially is dominated by shale and siltstone of the Panton Formation and Nelson Shale, and shows a higher conductivity than younger coarser grained units. Intermediate and high conductive sections associated with Cambrian outcrops appear to shallow and thicken towards the east, indicating a Cambrian depocentre at least 50 km east of the Devonian outcrops. The AEM data, in conjunction with aeromagnetic data, also indicates that most outcrops mapped as Windoo Sandstone in the south of the basin underlie the lower Cambrian Antrim Plateau Volcanics, rather than overlie as previously interpreted. These outcrops are most likely associated with the Neoproterozoic Timperley Shale of the Wolfe Basin, but may locally include other Precambrian units.