THE STRATIGRAPHY OF THE PERMIAN OF THE NORTHERN PERTH BASIN, WESTERN AUSTRALIA

The Perth Basin is a narrow, deep graben flanking the lower 400 miles of the west coast of Western Australia. It attains a maximum width of 60 miles and may contain more than 40,000 feet of sediments. Permian rocks crop out around the northern margin of the basin, between the Greenough Precambrian Inlier on the west and the Darling Fault on the east. Bores near the northern margin of the basin and down the west coast for some 120 miles south of Geraldton have encountered Permian rocks. Because of the limited nature of the outcrop, most of our information concerning variations of stratigraphy within the Permian comes from these bores. As in other basins in Australia, the Permian sedimentation in the Perth Basin started with a cycle of intense glaciation. Two units were deposited: the lower one is a submarine tillite consisting of boulders set in a sandstone and siltstone matrix; the upper unit is a fossiliferous marine shale. The thickest development of these units is adjacent to the eastern boundary fault of the basin. They thin rapidly to the west by sedimentary onlap on to the Beagle Ridge. The glacial cycle is followed by a continual cycle in which coal measures predominate. This unit shows a remarkable constancy of thickness, indicating very stable tectonic conditions during its deposition. The coal measures are followed by a monotonous marine siltstone unit which completes the Lower Permian (Artinskian) sequence in this basin. After a hiatus in which both tectonism (faulting and tilting) and erosion occurred, a series of estuarine lakes developed in uppermost Permian times, and the Wagina Sandstone was deposited. These rather restricted lakes were then followed by the more widespread Lower Triassic transgression in which the Kockatea Shale blankets most of the Permian. Significant signs of oil are seen in several of the Permian units-oil in the Coal Measures, oil and gas in the overlying marine strafes, and oil and gas associated with the Wagina Sandstone. Thus the Permian ranks as one of the best potential source and reservoir sequences for hydrocarbons in the Perth Basin.