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Criteria for Recognition of Cold-Water Carbonate Sedimentation: Berriedale Limestone (Lower Permian), Tasmania, Australia
C. Prasada Rao
The Permian Berriedale Limestone is an alternating sequence of limestone and calcareous shale associated with marine glacial terrigenous sediments. It was deposited at a paleolatitude near 80° S during the Gondwanan Permo-Carboniferous Ice Age. Dropstones commonly disrupt laminae, range widely in size and composition, and reveal glacially-derived shapes and textures under the SEM. There is a lower diversity of fauna (mostly bryozoans, brachiopods and pelecypods) and non-skeletal grains (mostly intraclasts) than in warm-water limestones.
The vertical variation in depositional microfacies, through shale-limestone-shale beds, indicates that the shallow-marine limestones formed during the lowering of sea level in colder periods. As the sea level rose during warmer periods, a shower of dropstones preceded shale deposition. For this mason dropstones are more numerous around the contacts between limestone beds and overlying shale beds but rare in the middle or base of limestone beds. Icebergs, calved from adjacent glaciers, were floating, and the surface water temperature was near freezing during limestone deposition. Upwelling cold currents provided nutrients and maintained calcium carbonate at saturation.
Well-developed rhombs in micrite, microspar and spar, and length-slow elongate rhombs and poikilotopic rhombic plates (both formed by accretion of rhombohedra) indicate low Mg-calcite precipitated from seawater at a temperature of <3° C. These cements are overlain by micrite, marine-mud, dropstones and geopetal fillings of well-sorted internal sediments, and are cut by borings, all proving both synsedimentary ages and submarine origin of these cements. The calcium carbonate content in these cold marine waters fluctuated, ranging from undersaturated (destructive diagenesis: pits, etchings, rhombic pores, and dissolution features) to saturated (constructive diagenesis).
As the sea level rose during warmer periods, large volumes of melt-waters mixed with marine waters. The mixing zone cements. succeeding marine cements, are cloudy, inclusion-rich, coarse plates (up to a few mm). These crystals grew, embedding or nucleating over early-formed crystals and previously-cemented (both fillings and encrustations) bioclasts. The mixing zone cementation is preceded by erosion of early-formed crystals. The eroded-crystals now occur as inclusions in mixing zone cements.
The field, microfacies, and diagenetic criteria evolved in this study might be used for recognition of a wide spectrum of cold-water carbonates (temperate, subpolar, and glacio-marine).
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