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The basic elements of an idealized or model cyclic deposit in the Pennsylvanian strata of the Appalachian region are the commonly occurring rock types--siltstone, shale, sandstone, coal and associated "seat rock," ironstone, and limestone. These rocks can be considered as members of genetic classes which reflect dominance of either "physical" or "chemical" depositional processes. Siltstone, shale, and sandstone are mainly the product of physical deposition of solid particles from suspension, whereas coal, "seat rocks," ironstone, and limestone wholly or in part originate from biochemical or physical-chemical processes. Pennsylvanian sequences consist of alternating layers of these "physical" and "chemical" deposits of varying thicknesses and such alternations comprise, pe haps, the only completely unequivocal manifestation of cyclic sedimentation. Specific differentiation of lithologic types beyond this simple "physical"-"chemical" dichotomy leads to increasing complexity in describing the cyclic deposits. Stratigraphic sections commonly show "chemical" units which may include only one or all three of the common chemical rocks. Likewise, physical units may include one or more rock types and the sequence of these rock types differs from cycle to cycle.
Detailed studies at lateral variation of completely exposed small cyclic deposits and of larger cycles with a small rate of lateral variability suggest the generalized model shown diagrammatically. This diagram, shown
with "chemical" rocks as limiting members, represents a cross section of a Pennsylvanian cycle extending from seaward to landward (left to right) extremities. Examples show that this simple model is modified by local as well as regional conditions.
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