Field Analysis:Why Sedimentary and Secondary

From SASSA

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[edit] Why record sedimentary and secondary features?

Primary sedimentary structures result from the way in which the material was deposited and include laminations, cross-bedding and ripple marks, secondary structures result from post-depositional modification of the deposits producing convoluted bedding and liquefaction features.

Sedimentary features such as bedding and fining sequences are the result of process of deposition, and hence provide information about the depositional environment. Where sedimentary structures are preserved they may provide valuable information about the mode and environment of deposition, and about post-depositional modification processes. Individual beds (strata) represent a single phase of deposition under constant physical conditions. Layering (stratification) results from the alternation of depositional and standstill or erosional events. Cross bedding suggested deposition on a slope, for example in a dune, river bar, alluvial fan or delta. Horizontal bedding by contrast is more characteristic of overbank deposition on floodplains. Graded bedding contains a vertical gradation in grain size, usually this is a fining of grain size up through the profile. Such fining sequences can occur over scales a few mm’s in laminae in floodplain deposits, or over several metres. They not only provide information about the changing depositional environment, but can also act as useful way up indicators in redeposited sediments. Surface ripple marks may also be present and may information on direction, depth and intensity of water flow in alluvial settings.

Secondary, or deformation, features include cracks, rainsplash, folding and convoluted bedding, faulting, and liquefaction structures. These structures provide information about environmental conditions in non-depositional phases and about post-burial alteration.

The nature of any sedimentary features should be recorded, wherever they are present as they provide invaluable information about the nature of the depositional environment and rates of accumulation.

More information on recording sedimentary and secondary structures can be found here.