Neotectonic disruption of silicified palaeovalley systems in an intraplate, cratonic landscape: regolith and landscape evolution of the Mulculca range-front, Broken Hill Domain, New South Wales

Abstract

The landscape expression of a wide range of ancient and contemporary regolith materials in the vicinity of the Mulculca Fault demonstrates two important points: (i) tectonism can be a significant factor in the evolution of landscapes in some parts of the Australian craton; and (ii) ancient and young regolith-landform features can coexist within a tectonically active landscape. Tectonic activity along the Mulculca Fault has created a range-front near the Broken Hill Domain - Murray Basin margins. This tectonism defeated a now silicified palaeodrainage system that flowed from the area now occupied by the Barrier Ranges towards the present area of the Murray Basin. Stream defeat led to the development of lacustrine-overbank conditions within the area of the fault-angle depression, which was later breached by stream incision across the range-front. The area of lacustrine-overbank deposition is now dominated by alluvial (channel and overbank) deposits with minor colluvial and aeolian deposition. Silicification of palaeovalley sediments and adjacent saprolite has been occurring over a broad range of times during landscape development, including several stages of palaeovalley evolution and as minor red-brown hardpan development in the contemporary landscape. Ferruginised regolith has been developing at many different times during the evolution of the landscape, including: (i) prior to the defeat of the palaeodrainage system; (ii) during the sedimentary infilling of the fault-angle depression; and (iii) within the contemporary landscape. The variable preservation and, therefore, landscape expression of a wide range of regolith materials that formed over a long period of landscape evolution has persisted even though there has been tectonic activity along the Mulculca Fault. For example, topographically inverted, and therefore ancient, silicified alluvial deposits occur alongside contemporary colluvial and alluvial deposits along the tectonically active range-front. This is in contrast to simple models invoking long-term tectonic stability to account for the expression of ancient regolith and landforms in Australian cratonic landscapes.