Relevance of Cratonic Erosional Unconformities and Sedimentary Veneers to Mineral Exploration in Weathered Terranes: ABSTRACT

Abstract

On stable platforms, erosional intervals may persist for great lengths of time either as continuously exposed planation surfaces or, when buried by sedimentary covers, as unconformities. On the West Australian craton, for example, erosional unconformities and thin sedimentary veneers are closely comparable in attitude and altitude. Repeated cycles of weathering, stripping or exhumation, and burial of shields constitute a morphogeodynamic pattern, a cratonic regime, which accounts for the slow but progressive lowering of cratonic erosion surfaces. Because phases of intense chemical weathering initiated in the later Mesozoic and continuing in Tertiary times tend to mask the presence of buried paleogeomorphic surfaces, specialized techniques are required for detection of deg aded (weathered) unconformities. Application of stratigraphic principles to weathered zones and micromorphological analysis of paleosolic and weathered rock fabrics, as well as interpretation of geochemical and sedimentological data, facilitate reconstruction of paleoenvironments. Stone lines, saprolitic fabrics, gravel-clay interfaces, reverse weathering differentials, and etched or embayed skeleton grains showing the effects of epidiagenetic alteration are key to the detection of unconformities in strongly weathered cratons. Differentiation of soil-stratigraphic layers from sedimentary deposits requires proof of pedogenic existence and is in large part based on interpretation of boundaries between them, i.e., pedologic, lithologic, and geomorphic discontinuities. Paleogeomorphic recons ructions incorporating unconformities have practical application in mineral exploration End_Page 568------------------------------ because they provide a rational basis for pedogeochemical sampling techniques applicable to the search for base metals, the detection of deep leads (buried stream channels containing placer deposits), and telethermal uraniferous deposits associated with calcreted valley fills. End_of_Article - Last_Page 569------------