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
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,
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