Sedimentary Rock Characteristics Are Determined By Twenty-One Depositional Medium/Process Associations

A. Eugene Fritsche

Sedimentary rock textures and associated sedimentary structures are the direct result of the depositional medium and the depositional process 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 medium/process associations or settings within which all sedimentary rocks have their origin.

Crystalline sedimentary rocks are deposited in only three medium/process settings, either as pure chemical precipitates in water (Pw, e.g., evaporites), pure chemical precipitates in air (Pa, sublimates), or biochemical (organic) precipitates in water (Ow, reef structures). Granular sedimentary rocks fill the remaining 18 medium/process niches: suspended-load deposition in air loess), in water (Sw, offshore mud), or in ice (Si, ablation moraine); saltation- or bouncing-load deposition in air (Ba, aerial sand sheets) or in water (Bw, sand beaches); rolling-load deposition in air (Ra, desert pavement) or in water' (Rw, gravel beach); turbid-flow deposition in air (Ta, basal layers from a nuee ardente) or in water (Tw, graded portion of Bouma sequence); debris-flow deposition in air (Da, final movements in an avalanche), in water (Dw, mudflows), or in ice (Di, lodgement till); grain-flow deposition in air (Ga, lee slopes of aerial dunes) or in water (Gw, lee slopes of subaqueous dunes); liquefied-flow deposition in air (La, convolution in aerial dunes) or in water (Lw, convolution in subaqueous deposits); and mixing from bioturbation in air (Ma, filled subaerial burrows) or in water (Mw, filled subaqueous burrows).

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California