L015 Perth Basin seismic reflection traverse at Gingin, WA, 1955

Brief description

A seismic reflection traverse was surveyed across the Perth Basin through the township of Gingin. The traverse was planned to give information on the thickness of the sedimentary section, the structure of the Basin, and the aplicability of the seismic method as a tool for both regional and detailed investigation.
The traverse was chosen to cross a major negative gravity anomaly (-110 milligals at Gingin) and to cross the
so-called "Hill River Fault". A considerable thickness of section was indicated (in excess of 17,000 feet) and tectonic structure, not apparent in the surface geology, was shown. A deep-seated anticline, the Gingin anticline, was discovered.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: The Gingin area is about 45 miles north of Perth, and lies approximately in the centre of the Perth Basin, which extends
from the vicinity of Cape Lecuwin in the south to Geraldton in the north. The Perth Basin has been recognised as sedimentary since before 1850 (Pairbridge, 1949) but has generally been considered to be a shallow basin.

The Bureau of Mineral Resources, Geology and Geophysics first carried out gravity observations at Watheroo in 1949 and at Bullsbrook in 1951, while socking sites for water bores. As a result of the interest aroused by these observations, the Bureau then made a reconnaissance gravity survey of the entire Basin during portions of 1951 and1952 (Thyer and Evaringham, 1956). The results of that survey were very important. Bouguer gravity anomalies up
to -140 milligals were recorded. Steep gradients covering the greater part of the anomalies occur within a strip 4 to 6 miles wide, running along, and covering, the Darling Scarp. It was difficult to account for such large anomalies with such steep gradients without assuming a thickness of fairly light sediments of the order of 30,000 feet on the western side of the scarp.

There was no known geological evidence to suggest the existence or non-existence of such a, thickness of sediments. Consequently, at the first available opportunity, the Bureau moved a seismic party to the Perth Basin with the principal object of verifying, if possible, the interpretation of the gravity anomaly in terms of a thick sedimentary section. The results obtained by that party from the first traverse form the subject of this report.