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ABSTRACT

A depositional process defines the parameters of sediment behavior at the moment of deposition. A depositional current, on the other hand, such as a turbidity current, is a mechanism of sediment transport within which the velocity, depositional rate, and grain concentration can change sufficiently to alter the depositional process through time. Sedimentary rock textures and associated sedimentary structures are the direct result of the depositional process and the depositional medium that existed at the moment of deposition. The depositional media for sedimentary rocks include only air, water, and ice. These three, when combined with various depositional processes, produce 21 process/medium associations or settings within which sedimentary rocks have their origin. Crystalline sedimentary rocks are deposited in only three process/medium settings, either as pure chemical precipitates in water (CW, e.g., evaporites), pure chemical precipitates in air (CA, sublimates), or organic (biochemical) precipitates in water (OW, reef structures). Granular sedimentary rocks fill the remaining 18 process/medium niches: traction-flow suspended-load deposition in air (SA, loess), in water (SW, offshore mud), or in ice (SI, ablation moraine); traction-flow bouncing- or saltation-load deposition in air (BA, eolian sand sheet) or in water (BW, sand beach); traction-flow rolling-load deposition in air (RA, desert pavement) or in water (RW, gravel beach); turbid-flow deposition in air (TA, basal layer of a pyroclastic flow) or in water (TW, Bouma graded Ta deposit); debris-flow deposition in air (DA, pyroclastic flow), in water (DW, mudflow), or in ice (DI, lodgement till); grain-flow deposition in air (GA, lee slopes of aerial dunes) or in water (GW, lee slopes of aqueous dunes); fluidized-flow deposition in air (FA, gas-escape structures in air-fall tuff) or in water (FW, convolution in aqueous deposits); and mixing from bioturbation in air (MA, filled aerial burrows) or in water (MW, filled aqueous burrows).

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