Abrupt shifts from single widespread coal swamps to coarse siliciclastic alluvial channel deposits occur in at least five coal beds and zones within the Pennsylvanian Allegheny Formation. One of these, the Upper Freeport coal zone, was deposited over an area at least 200 x 200 km with a spacing of alluvial channels one-half to possibly one-fifth that of the immediately overlying coarse clastics. All these shifts occurred next to the rising Appalachian orogen, far from the eustatic effects of a marine shoreline. Recent models relating coal- swamp formation to isostatic warping of orogenic forelands by tectonic loads surely apply to this environment, but they seem to need an additional, more delicate mechanism to produce such abrupt but widespread switches in grain size and drainage spacing. We propose that irregularities in the advancing front of folds and blind thrusts caused temporary geomorphic diversions into the recessed areas and allowed a widespread coal swamp to form in the sedimentary shadow of the salients, a shadowing process that is occurring today in the central Zagros Mountains of Iraq and Iran.

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