Aeromagnetic and marine-seismic prospecting across the southern Swain Reefs and adjacent waters, central Queensland, Australia, preceded the drilling of two stratigraphic tests in 1967 and 1968.

Outcropping rocks onshore are Silurian(?) to Permian eugeosynclinal sediments and volcanic rocks, intruded by granite of Carboniferous to Mesozoic ages. This rock complex is overlain unconformably by 20,000 ft of Mesozoic nonmarine or shallow marine clastic and volcanic rocks in the onshore part of the Maryborough basin, 100 mi south. Onshore Tertiary rocks are thin nonmarine terrigenous clastic strata and basalt flows. Tertiary trachyte plugs are present locally.

Geophysical surveys and drilling have outlined offshore structural elements tentatively named (1) Swain Reefs high, an area of shallow Paleozoic(?) basement overlain by undated volcanic rocks and coinciding with the Swain Reefs at the southern end of the Great Barrier Reef, and (2) the Capricorn embayment, a half-graben 20-70 mi wide, trending NNW along the southwestern side of the Swain Reefs high. Normal faults separate the two features. The embayment contains 0-8,000 ft of Tertiary marine shale, marlstone, lignitic shale, and sandstone; 3,100 ft of Cretaceous(?) sandstone, shale, and conglomerate, possibly equivalent to the Lower Cretaceous Maryborough Formation; and more than 1,200 ft of conglomerate and volcanic rocks of the Upper Jurassic-Lower Cretaceous Graham's Creek Formatio . This section is present at the southeast end of the graben near its intersection with the continental shelf. The graben is bounded on the southwest by the Bunker-Capricorn high composed of Mesozoic volcanic rocks of unknown thickness. The volcanic rocks are overlain unconformably by thin marine Tertiary strata which thicken eastward into the Capricorn embayment. Mesozoic sedimentary and volcanic rocks of the Maryborough basin are present southwest of the high and are separated from it by normal faults. These features are the result of block faulting during the Tertiary controlled by pre-Mesozoic deformation with a strong NNW grain.

Capricorn No. 1-A was abandoned at 5,609 ft in Mesozoic volcanics; Aquarius No. 1 was abandoned at 8,696 in Paleozoic(?) metamorphics. There were no shows of oil or gas.

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