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Ancient environments may be interpreted, at least in part, from primary structures in sedimentary rocks. Because such structures are largely developed during deposition, they provide information on the processes involved and on the general geologic and climatic setting. Unfortunately, many sedimentary structures are poorly understood; numerous data are required from the observation of modern sediments and of controlled experiments before positive conclusions can be drawn concerning rock genesis.
Principal types of stratification and cross stratification include (1) flat or horizontal bedding, (2) low-angle, simple or planar cross-stratification, (3) tabular-planar cross-stratification of intermediate angle, (4) high-angle, wedge-planar cross-stratification, (5) ripple lamination, (6) graded bedding, and other less common varieties. Some of these structures are typical of more than one environment; other structures are represented by two or more varieties in a single environment. Information on natural combinations or associations of these structures is especially valuable for paleoecological interpretations.
Environments of deposition that have been studied, in greater or lesser degree, with respect to the characteristic type or types of stratification are dunes, river channels, river floodplains, alluvial fans, delta cones, tidal flats, foreshore beaches, backshore beaches, and barrier bars. Very little information is yet available concerning primary structures representative of the various offshore marine environments.
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