Traces from the past: the Cenozoic regolith and intraplate neotectonic history of the Gun Emplacement, a ferricreted bench on the western margin of the Mt Lofty Ranges, South Australia

Abstract

The Gun Emplacement is a small but distinctive bench on the Eden–Burnside Fault Escarpment near Anstey Hill, in the northeastern suburbs of Adelaide, South Australia, occurring at an elevation of ∼210–220 m asl. It is underlain by Middle Eocene North Maslin Sand and is capped by resistant, ferricreted colluvium. Paleomagnetic dating of hematitic mottles in the ferricreted colluvium, immediately underlying the emplacement, returned a Pliocene/Early Pleistocene age. This age is equivalent to that obtained for summit surface weathering. Fault scarps and exposures, including slickensides and fault gouge material, suggest that the Eden–Burnside Fault at this location has a strong en échelon pattern developed in response to reverse-sinistral oblique-slip faulting, reflecting continental stress fields. Remnants of ferricrete cappings forming stranded benches on the Eden–Burnside Fault Escarpment at elevations up to 25 m above the Gun Emplacement demonstrate recurrent tectonism of the South Mt Lofty Ranges related to intraplate deformation. There are at least four distinct ferricrete benches preserved on the eastern side of the active fault leading up from the Gun Emplacement surface. These benches demonstrate alternating periods of stability and tectonic activity disrupting and uplifting the ferricreted surfaces. A fresh surface rupture occurs and may be related to a recent seismic event.

Key Words:

• Cenozoic
• ferricrete
• intraplate deformation
• Mt Lofty Ranges
• neotectonics
• paleochannel deposits
• paleomagnetic dating
• weathering

Acknowledgements

We wish to acknowledge the Department of Environment and Heritage for permission to collect samples for analysis from the Anstey Hill Conservation Park, and Cemex for permission to access the formerly quarried Middle Eocene Sand. We are grateful to Neville Alley, John Cann and Rob Fitzpatrick for their constructive comments on an early version of the paper. Discussions with Wolfgang Preiss were rewarding. Journal reviewers, Steve Hill and Cliff Ollier, made valuable contributions.