--> Some Controls on the Distribution of Slurry-Flow Deposits

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Some Controls on the Distribution of Slurry-Flow Deposits

Abstract

Slurry flows, aka transitional flows or hybrid flows, widely redistribute sediment within deep-water basins. Their deposits show that both flow turbulence and flow cohesion played roles in sediment transport and deposition and their mud-rich deposits can significantly reduce the reservoir potential of deep-water sands where they are extensively developed. There remains considerable controversy about the mechanics of slurry flows, the source(s) of the mud, and the environmental controls on their distribution within larger turbidite systems. The general consensus is that slurry-flow deposits are most abundant in downslope lobe and lobe fringe deposits. However, we would emphasize that slurry-flow deposits can occur in virtually all deep-water environments. The occurrence of slurry-flow deposits within channels is exemplified by deposits in the Upper Cretaceous Cerro Toro Formation in the Magallanes Basin, southern Chile. These units, which are commonly 5-15 m thick, include a basal division of clast-supported pebble to cobble conglomerate and an upper division of matrix-supported conglomeratic mudstone. Except for scale, they are similar to so-called linked debrites. In the Cerro Toro Fm., as well as the Oligo-Miocene Puchkirchen Formation of the Austrian Molasse Basin, these deposits are interbedded with coarse conglomerate and sandstone within deeply erosional foreland-basin axial channels. In the Pennsylvanian Jackfork Group of the Ouachita Mountains, Arkansas and Oklahoma, slurry-flow deposits are common in the lower parts of thick-bedded sandstone intervals that overlie major mudstone units. These slurry-flow units appear to mark avulsion splays in a more mid-fan setting that abruptly diverted large volumes of sand from channels through levees onto surfaces previously dominated by mud accumulation. Both the Jackfork and the Eocene Tyee Formation of Oregon also show widespread slurry-flow sequences deposited in more downslope, unconfined, lobe and lobe fringe settings. The development of slurry-flow deposits in virtually every type of sediment-flow-dominated deep-water setting but their paucity in many mud-rich deep-water basins argue that the properties of mud during flow formation and evolution rather than a specific depositional setting provides a major control on the formation of slurry-flows. This also helps to explain the disparate development of slurry-flow deposits in basins with otherwise similar depositional settings and mud abundances.