Petrographic analysis can be a powerful tool in the recognition and delineation of depositional environments of detrital sediments. A study of varied fluvial-deltaic, and interdeltaic environments in the Eocene of the Gulf Coast and the Cretaceous of Montana-Wyoming shows that individual environments may be segregated by applying univariate and multivariate statistical techniques to petrographic data (composition and texture).

These results are supported by data collated from Holocene environments in the Mississippi alluvial valley and delta, and Galveston barrier island. Holocene environments that can be segregated with a minimum of 80% reliability are point bar, natural levee, lake, bay, lower shoreface, middle shoreface, upper shoreface-beach, dune, and lagoon. Consequently, full diameter cores may not be necessary for precise environmental interpretation everywhere in the subsurface. Small samples, such as side-wall cores, taken throughout an interval, may yield significant and reliable environmental information. An example is provided by the Lower Cretaceous Muddy Sandstone of Montana in which the subenvironments of the petroliferous barrier-bar complex at Bell Creek field can be recognized from thin-section analysis and verified by a study of full-diameter cores. The presence of the Bell Creek barrier bar can be predicted through thin-section analyses of sediments in dry holes which surround the bar. If thin-section data are integrated with paleogeographic knowledge of the Lower Cretaceous, the relative position and strike of the barrier system may he ascertained.

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