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The detrital mineralogy of sandstones is affected significantly by depositional environment. The imprint of the environment is recorded as the relative abundance and size of individual detrital species. Differences between environments within any one basin are accompanied by concomitant changes in detrital-sediment composition. Petrographic analyses of Holocene and ancient sedimentary sequences reveal that the environmentally produced compositional changes are process dependent, and are interrelated closely with textural variations. Fluctuations in rock-fragment content among environments indicate that the relative survivability of unstable detrital grains can influence the overall composition of sandstones.
A total of 300 samples of varied grain sizes was selected from contrasting environments in three Holocene sedimentary sequences: (1) the Galveston barrier complex, Texas; (2) the lower alluvial valley of the Mississippi River; and (3) the Mississippi delta. In each sedimentary sequence, bivariate and multivariate analyses of petrographic data demonstrate the environmental sensitivity of both composition and texture. A comparison of data from these three Holocene sequences demonstrates that the most effective environmental segregation based on composition and texture alone is in depositional systems characterized by intermediate or high energy--in systems with a significant proportion of sand-size detritus. Four ancient sedimentary sequences, of different age and geographic location, w re selected to complement the Holocene sequences: (1) Wilcox (Eocene) fluvial rocks, Texas; (2) Wilcox (Eocene) deltaic rocks, Texas; (3) Lower Jurassic (Toarcian) marine rocks, England; and (4) Lower Cretaceous Muddy Sandstone marine, transitional, and nonmarine rocks, northeast Powder River basin, Montana and Wyoming.
Petrographic data from each sequence demonstrate that the composition of recent detrital sediments and ancient sandstones is environmentally sensitive. Composition is also sensitive to textural changes. In sediments with a wide range of grain sizes, the relation between the abundance and size of detrital monocrystalline quartz is curvilinear. Compositional and textural data derived exclusively from thin-section analyses can be used jointly to segregate depositional environments within a single sedimentary basin, providing no source changes occur, and providing that certain basic sampling restrictions are adhered to. Thus, small samples such as sidewall cores and cuttings may be used in the delineation of depositional environments. Such small samples are available in abundance in the p troleum industry and, if analyzed petrographically, they can render invaluable assistance in the search for stratigraphic traps.
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