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Ancient Fluvial Systems
Proterozoic Stream Deposits: Some Problems of Recognition and Interpretation of Ancient Sandy Fluvial Systems
Fluvial deposits of Proterozoic age (2.5 – 0.6 Ga) have been reported from several continents. They include the products of deposition in alluvial fan, braided stream and possibly meandering stream environments. The braided stream deposits include both conglomeratic and non-conglomeratic sequences. Many of the non-conglomeratic, sandy, fluvial deposits, which may be several kilometres thick, are characterized by a paucity of fine-grained clastic material, whether as matrix or as associated mudrocks. In many cases, thick arenaceous sequences of Proterozoic age have been interpreted as fluvial by some authors and non-fluvial by others, with little conclusive evidence for either interpretation. Fossils are generally unavailable as evidence, except perhaps in the uppermost Proterozoic. Even stromatolites, where present, may have developed in nonmarine settings. Differentiation may in some cases be achieved by examination of grain size characteristics, in association with the type, scale and abundance of sedimentary structures, their directional attributes and vertical and lateral associations. Other criteria include sand-body geometry, lithofacies association, petrology and, in some examples, colour. A comparison can be made between Proterozoic and Phanerozoic fluvial environments, although the absence of vegetation must have had profound effects on the style of deposition in Proterozoic fluvial systems. The paucity of fines in many Proterozoic fluvial systems can be related to differences in hydraulic regime, dominance of bedload type streams, the extreme vulnerability of overbank flood deposits to later fluvial and aeolian erosion, and possibly to removal of fines from the fluvial systems as wash load.
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