Buried inset-valleys in the Eastern Yilgarn Craton, Western Australia: Geomorphology, age, and allogenic control

A contributory network of narrow, fluvially incised valleys filled with a distinctive sequence of Tertiary continental to shallow marine sediments is widespread throughout the eastern Yilgarn Craton in southwestern Australia. Previously referred to as "deep leads,"or "paleochannels,"these are herein termed "inset-valleys" to more accurately reflect their geomorphic expression and emphasize their subordinate and entrenched position with the bedrock surface of a much broader and older system of "primary valleys." Owing to minimal denudation and tectonic deformation during the Late Cenozoic, the inset-valleys are excellently preserved and can be readily traced in boreholes and open-cut mines throughout the eastern Yilgarn Craton and possibly also adjacent parts of southwestern Australia. With a combined catchment areal extent of at least 1.2 x 10(6) km(2), this makes the inset-valleys among the largest of paleo-valley systems known globally. Six major inset-valley networks occur within the eastern Yilgarn Craton, each comprising up to seven or more orders of tributaries arranged in a subdendritic pattern controlled by the initial slope provided by the primary valleys and, to a lesser degree, by lithological and structural discontinuities in the weathered Precambrian bedrock. The inset-valleys typically have a broad U-shaped form, a width-to-depth ratio of 10 - 15, and a maximum width and depth of about 1.5 km and 120 m, respectively. Inset-valley incision occurred in the early Middle Eocene in response to lowered geomorphic baselevel and increased stream gradients affected by epeirogenic uplift of the Yilgarn Plateau. Promoting inset-valley incision was a widespread mantle of deeply weathered bedrock and less certainly a marked reduction in fluvial sediment supply induced by climate change at the Middle/Late Eocene boundary.