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  • Prepared by the Military Geology Branch

  • 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.

  • The seismic survey was carried out at the request of the U.K. Ministry of Supply. The purpose of the survey was to disclose the geological structure and, if possible, the physical rock characteristics at the Maralinga testing ground. The 11,000 ft/sec layer at a depth of about 200 ft, probably a sandstone-shale formation, and the 19,000 ft/sec formation at a depth of about 1350 or 1800 ft (according to the method of computation used) were successfully mapped. The subsurface information to a depth of about 200 ft was derived mainly from shallow drill holes and up-hole shots. An experimental spread indicated a very low Poisson ratio for the sandstoneshale formation.

  • The Giralia and Marrilla Anticlines are in the north-western part of the Carnarvon Easin, North West Division, Western Australia. The Giralia Anticline extends southward for 80 miles from Exmouth Gulf to Salt Lake; the maximum outcrop width across the two anticlines is 18 miles. The Giralia and Marrilla Anticlines are maturely dissected asymmetrical folds of low relief which consist of sediments deposited in Cretaceous and Tertiary time in a shelf area possibly contiguous with the geosyncline of Timor and East Celebes. The Giralia Anticline is the second largest of twelve closed anticlines in the post-Palaeozoic sediments of the Carnarvon Basin north of the Gascoyne River. The vertical closure of the Giralia Anticline on the top of the Boongerooda Greensand is 700 feet, and the closed area is 230 square miles. The anticlines were folded at four different periods, viz. between upper Eocene and lower Miocene, lower Miocene, between lower Miocene and Pleistocene, and Pleistocene or post-Pleistocene.

  • Deep alluvial leads of Recent, Pleistocene, and Tertiary age have been of considerable economic importance in Australian mineral production. Numerous basalt-covered and alluvium-covered auriferous leads have been worked in New South Wales and Victoria, but are quite rare in other states. However, important stanniferous deep leads have been worked at Herberton in North Queensland. Several geophysical methods can be applied to the investigation of alluvium-covered leads, the seismic refraction method being generally the most suitable. In the case of basalt-covered leads the geophysical problem is more difficult, although satisfactory results have been obtained by the magnetic method at Gulgong, New South Wales, and at Herberton. More recently (1948-49) the seismic refraction method has been applied to problems of this nature at Kalgoorlie, Western Australia, and Wellington and Ardlethan in New South Wales. At Kalgoorlie, geological evidence indicates that deep auriferous leads may extend for some distance to the south of the main Kalgoorlie lodes. At the request of the Western Mining Corporation, seismic refraction profiles were obtained along two 7-mile traverses. The first traverse (A) was about 15 miles south of the mines and the second (B) about 2 miles to the south of the "golden mile". The refraction profile revealed two deep channels crossing traverse A and two channels crossing traverse B; the depth to bedrock on both traverses was, 'however, much greater, than anticipated. For some years alluvial gold has been dredged along the banks of the Macquarie River near Wellington, New South Wales. In order to plan the course of the dredge, a considerable amount of advance drilling and sampling has to be done by, the company concerned. It was considered that the seismic method might be employed in order to reduce the amount of test drilling which has to be done. In 1949, the method was tried in this area but it was found to be too costly in this -application, due to the poor transmission qualities of the overburden which necessitated the use of -large charges or the drilling of deep shot holes. However, the limited amount of work which was done did indicate the unexpected presence of deep ground in a part of the area which had not been tested by boring. In the past, small stanniferous lodes have been worked about 6 miles to the north-west of Ardlethan. The geological evidence suggested that alluvial tin, shed from the Bygoo deposits, might be found at shallow depth. The seismic refraction method was used once again, but it was found that the depth to bedrock was much greater than had been anticipated. However, the presence of a deep gutter, running in a north-easterly direction from the old workings, was fairly well established and drilling recommendations were made to the company concerned.

  • During 1949 and 1951, whilst investigating possible underground water supplies, staff of the Bureau of Mineral Resources surveyed two east-west gravity traverses near Watheroo Observatory (120 miles north of Perth) and Bullsbrook (20 miles north-north-east of Perth). Results of these investigations suggested the presence of a major fault a few miles to the east of both places and gave evidence of a large thickness of sediments underlying the coastal plains. It was decided, therefore, to extend the gravity investigations to a reconnaissance survey of the whole of the coastal region between Geraldton in the north and Cape Leeuwin in the south, extending inland from the coast for distances of 100 to 150 miles. This bulletin gives results of the survey, which was made in 1951 and 1952 over an area of about 43, 000 square miles and comprised gravity observations at 650 stations. The survey revealed a negative gravity anomaly of large magnitude and areal extent. Bouguer anomaly values, with a minimum of more than 130 milligals, form a trough about 400 miles long and up to 55 miles wide. Steep gravity gradients on the flanks of the anomaly suggest the existence of faults which have resulted in relatively light sediments lying adjacent to denser basement rocks. Such steep gradients indicate that the major part of the anomaly is due to an accumulation of sediments which at their maximum development may be about 30,000 feet thick. The Darling and Dunsborough Faults show up prominently on the gravity pattern, but other faults deduced from geological evidence are not apparent probably because the vertical movement on these faults has been too slight to have affected the gravity pattern. On the other hand, the gravity results indicate the presence of other, faults previously undetected. Two minor synclines are indicated, diverging from the main synclinal axis, and the importance of anticlinal axes associated with these synclines is stressed. The gravity pattern indicates that although the Perth Basin narrows to the north, between converging faults, it continues in that direction, to join the Carnarvon Basin. Recommendations are made for more detailed gravity surveys to be made in certain localities and for seismic surveys of selected traverses across the basin.