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Open File Report 87

Atlas of transported overburden

Robertson, I.D.M., Koning, A.E.M., Anand, R.R. and Butt C.R.M.

The Atlas has an introductory review of the various types of transported overburden on the Yilgarn Craton, and its margins, including their age, stratigraphy, distributions, depositional environments and provenances. The sediments have been classified by an extension of the scheme developed initially for lateritic and other ferruginous materials. This is followed by a discussion of the general procedures for chemically characterising and discriminating various types of transported overburden and discriminating between sediments and weathered basement.

The Atlas proper contains detailed descriptions of the principal sediments that comprise the transported overburden types at selected, well-characterised sites. At each site, their field relationships, field appearances, microfabrics and chemical and mineralogical compositions are characterised and illustrated. Where possible, aspects of the distributions, thickness and thickness variations of the sediments have been determined and documented, to assist in mapping regolith stratigraphy from drill cuttings and core. For each type of sediment, the principal criteria are given to distinguish it from the underlying residual regolith. These criteria tend to be site-specific but they indicate what could be applied to make this very important distinction.

It is estimated that as much as 50% of the prospective terrain of the Yilgarn, and substantially more in some districts, is concealed beneath transported overburden which, itself, commonly overlies highly weathered residuum. Thus, it presents one of the most significant challenges to exploration in the region. Effective exploration for mineralisation within and beneath these sediments requires reliable recognition of the transported overburden. The overburden ranges in age from Permian to recent; the older sediments have shared the intense weathering that has affected the basement rocks, so that discrimination between the transported and residual units of the regolith can be very difficult, particularly from drill cuttings. In many places, two or more sedimentary sequences, of quite different ages, may overlie the basement.

This Atlas provides an overview of the types of transported overburden, environments of deposition, relationships to the main period of weathering and a scheme for classification. It documents examples of the main types of transported overburden, provides stratigraphic and distribution information, mineralogy and chemical compositions. Possible means of making the important basement-cover distinction are provided for each site.

Examples of the principal types of transported overburden were collected from many sites used for other investigations in the Project (Figure 1). Their degree of transportation varies from materials such as palaeosols and lateritic residuum, in which only minor movement has occurred by settling and colluvial mass flow during their formation, to polymictic, alluvial sediments of varying ages, with diverse, distal provenances.

Types of transported overburden include Permian fluvioglacial tills (Lancefield - Laverton), collapsed lateritic residuum (Red Lake), collapsed ferruginous saprolite (Bronzewing), lateritic conglomerate (Ora Banda), vallye fill clays and sands of palaeochannels (Greenback and Peak Hill), lateritic colluvium (Mt Magnet and Peak Hill), gravelly sediments (Golden Delicious and Fender), valley calcrete (Lake Way and Yeelerie) and dune sand (Laverton).

Last updated: Friday, July 21, 2000 04:24 PM


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