Onshore Geology Research Articles

GEOLOGICAL FRAMEWORK OF THE CONTINENTAL SHELF OFF NORTHWEST AUSTRALIA

A simplified synthesis of exploration results in permits covering a large area of the broad continental shelf off northwest Australia (between Bathurst Island, N.T. and the Monte Bello Islands. W.A.) is presented and a preliminary analysis of the geological framework of the shelf is offered. Companies with working interests in these permits are B.O.C. of Australia Limited. Shell Development (Australia) Pty. Ltd., Woodside (Lakes Entrance) Oil Co. N.L., Mid-Eastern Oil N.L., B.P. Petroleum Development Australia Pty. Ltd. and. in the permits In Western Australia, California Asiatic Oil Co.Reconnaissance aeromagnetic and seismic surveys have provided evidence of substantial thicknesses of sedimentary rocks, and revealed the presence of several major structural/ depositional features. The geological significance of these features has been partly rationalized by the integration of stratigraphic and seismic data, and by interpolation of established onshore geology. Ashmore Reef No. 1 revealed a substantially complete Tertiary-Upper Cretaceous carbonate- clay sequence, a thin early Cretaceous-late Jurassic section, containing detritus from underlying Upper Jurassic basic lavas, and part of a thick Triassic sedimentary sequence. Legendre No. 1 penetrated Tertiary, Cretaceous, and Jurassic sedimentary sequences, with gross stratigraphic similarities to sequences in parts of the Carnarvon basin.The shelf has a block-faulted basement substructure of Precambrian rocks, related in part to the extension of major structure in Precambrian rocks of the northwest Australian mainland. The geological superstructure of the shelf consists of Phanerozoic rocks whose essential structural framework is related to rejuvenated Precambrian structure. Warped and faulted Phanerozoic sequences have two major strike components, one sub-parallel and the other sub-normal to the present coastline. The effects of tectonics on sedimentation are outlined.

Seismic-refraction measurements in the Magellan Straits

The results of 22 seismic-refraction profiles in the Magellan Straits are presented, together with the correlation between seismic layers and the onshore surface geology of Isla Riesco and Peninsula Brunswick and one well in the Manantiales oil field. Five seismic layers were measured: A, 1.78–2.52 km/sec; B, 2.78–3.00 km/sec; C, 3.24–3.51 km/sec; D, 4.02–4.59 km/sec; and E, 5.18–5.87 km/sec. Layer A is identified as Quaternary-Pliocene; B, Miocene-Eocene; C, Cretaceous; D, Cretaceous and Jurassic; and E, basement. The results indicate zero sediment thickness at Peninsula Brecknock (southern end of Canal Magdalena) to 6600 meters of Cenozoic and Mesozoic sediments measured at Bahia Whitsand. A 200-km-long profile recorded between Cabo Cono in the Second Narrows and Peninsula Brecknock measured a sub-basement layer, velocity 6.70 km/sec, at an average depth of 17 km. The M discontinuity, velocity 7.95 km/sec, was measured at a depth of 29.6 km at Cabo Cono and 34.8 km at Peninsula Brecknock.

Seismic refraction measurements on the margins of the Gulf of Mexico

The data from 16 seismic refraction profiles in the Gulf of Mexico are presented in the form of three cross sections. Section A-B crosses the southern Florida shelf to the Florida escarpment, section C-D runs from Walker County, Texas, toward the south to the edge of the upper continental slope, and section E-F runs from the southern section of the Sigsbee deep to Progreso, Mexico. In all three sections correlations have been made between the refraction velocities and the known onshore geology. The Florida shelf and the Campeche bank are similar in structure, showing flat-lying beds over most of the sections. Identification of the strata is reasonably well established by drilling logs to the depth of the Upper Cretaceous, which is at about 0.6 to 1 km on the Florida shelf and 1 km on the Campeche bank. Section C-D, which passes through Galveston, shows a continuation seaward of the south-dipping sedimentary beds of the Gulf Coast geosyncline. This trend continues approximately 60 km south of the present shoreline, where a reversal of dip is indicated. At this point, possibly the axis of the geosyncline, approximately 13 km of Cenozoic sediments are found.