Abstract

A Carboniferous terrestrial conglomerate near Barraba, in northern New South Wales, is considered to be a tillite, and a detailed examination shows that lithologic and fabric criteria authenticate its glacial origin. The tillite, of probable Middle Carboniferous age, occurs near the top of the Spion Kop Conglomerate, a lenticular terrestrial conglomerate formation reaching a maximum thickness of 1800 feet. The tillite is nonsorted and nonstratified, and is associated with stratified disrupted-framework conglomerates believed to be fluvioglacials. When compared with other rocks in the sequence, it is unique in that it contains no volcanic detritus, and is composed of mostly angular clasts of sedimentary, metasedimentary, and acidic igneous intrusive origin, some of which reach 4 m in length. Some smaller clasts are striated and even faceted. The lower boundary of the tillite is gradational with the underlying fluvioglacials, which include a compacted silty layer and varve-like layers containing dropped pebbles. Fabric data suggest that the tillite was mostly an ablation moraine, and that the glacier flowed toward the east. Fine-grained layers (probably varves) overlie the tillite, and it is considered that the onset of proglacial lacustrine sedimentary conditions protected the moraine from reworking by glacial meltwaters. The known paleogeography and restricted occurrences of glacial deposits in the Carboniferous rocks in this region of New South Wales suggest that the glaciation was alpine rather than ice-cap glaciation.

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