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× 10 6 km2, this makes the inset‐valleys among the largest of paleovalley 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.