THE MARYBOROUGH BASIN

Abstract

The Maryborough Basin, occupying an area of 9,500 square miles, developed as a structural entity in the Mesozoic. Onshore regional mapping has shown that subsidence is controlled by ancient lineaments which have a north-westerly trend. These structural lines are characterised by normal fault movements with downthrow to the east. The eastern margin of the basin has been interpreted from geophysical data as a shallow platform. The sediments of the basin were severely folded in a post-Aptian orogeny.

A maximum thickness of 31,000 feet is known in the post-Permian section. Marine conditions prevailed in the Lower Triassic, and again in the Aptian when 7,000 feet of shallow-water quartzose sediments were deposited. The remainder of the section comprises continental sediments. Currents flowing from the east in Aptian time point to an emergent high in this direction. Such a high may have been downfaulted during the post-Aptian orogeny. Four thousand feet of trachyandesitic pyroclastics and tuffaceous sediments which were deposited during the Neocomian may have been derived from an oceanic eruptive zone. This possibly corresponds to the eastern margin of the basin, and also to an interpreted vental belt in the Swain Reefs area immediately to the north-west.

Mesozoic and Tertiary marine rocks are possible sources of hydrocarbon. Porous Jurassic orthoquartzite has reservoir potential, but this unit may only be tested by very deep drilling. Much of the onshore Cretaceous section is known to be tight. In the offshore area, the flattening of the northwesterly plunge of the fold structures in the Cretaceous rocks, with the possibility of the development of closures, together with the known thickening of porous, marine Tertiary sediments, enhance the prospect of hydrocarbon accumulation in this area.