The age of evaporitic units in the Kidson Sub-basin of the Canning Basin have been assigned to be of Late Ordovician to Early Silurian age through the dating of the sedimentary sequences within the Mallowa Salt. The Frome Rocks 1 exploration well was drilled in 1959 on the Jurgurra Terrace on the southern margin of the Fitzroy Trough and encountered 600m of salt down to a total depth (TD) of the well at 1220mMD (1137mTVDSS). Based on seismic data, this salt sequence is interpreted to be part of a diapiric structure that resulted from the rapid deposition of Carboniferous/Permian sediments over a thick evaporitic sequence. It is generally accepted that the evaporite sequence encountered in the Frome Rocks 1 well was deposited contemporaneously with the Mallowa Salt in the nearby Kidson Sub-basin. This paper presents an alternative model that suggests that an evaporitic salt sequence over the Jurgurra Terrace was deposited during the initial opening of the Fitzroy Trough during the Middle Devonian and prior to the deposition of the carbonate/claystone sequences of the Gogo and Pillara Formations. The presence of several evaporitic deposits within the Mellinjerie Formation of Middle Devonian age encountered in wells surrounding the Fitzroy Trough supports this age interpretation, while global indications for a major sea level drop in the Early to Middle Devonian allow for the development and generation of evaporitic conditions in an isolated shallow marine basin that predates the opening of the Fitzroy Trough. Ongoing work includes palynological and radiogenic dating of samples of the salt and overlying sediments from several recent mineral holes drilled on the Frome Rocks salt diapir. The confirmation of evaporites of Middle Devonian age would significantly increase the hydrocarbon prospectivity of the Jurgurra Terrace structural province, with good quality reservoirs of Devonian age proven in this area.

Open-Access Online Publication: Feb 24, 2023