Periglacial structures and palaeoclimatic significance of a late Precambrian block field in the Cattle Grid copper mine, Mount Gunson, South Australia

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Abstract
A tabular body of quartzite breccia at least 5 km2 in area and up to 20 m thick is preserved beneath the aeolian Whyalla Sandstone of late Precambrian age at the Cattle Grid copper mine on the Stuart Shelf W of the Adelaide Geosyncline. The breccia (Cattle Grid Breccia) exhibits numerous periglacial structures and is interpreted as a fossil block field formed through in situ frost shattering of silicified sandstone (Pandurra Formation) during the Marinoan Glaciation ca. 680 Ma ago. Periglacial structures displayed by the breccia include: primary sand‐wedge polygons; anticlines and tepee‐like structures, some cut by reverse faults, ascribed to frost heaving and frost thrusting; and truncated earth mounds. Sand wedges, drop involutions, periglacial injections, and frost‐heaved blocks also occur within the basal few metres of the Whyalla Sandstone, indicating that periglacial activity continued after burial of the block field. During formation of the breccia the mean annual air temperature probably was no higher than 0°C. The sand‐wedge polygons developed under more severe, arid conditions when the mean annual air temperature may have reached ‐20°C or lower and mean monthly temperatures perhaps ranged between < ‐35 and + 4°C. This cold, strongly seasonal periglacial climate prevailed at a relatively low altitude and at a low palaeomagnetic latitude.