Abstract

Published interpretations of paleochannel patterns and my own interpretation of fluvial styles in the Paleozoic sequence of Pennsylvania both confirm an evolution of the style of rivers in mid-Paleozoic time. Of the more than one hundred published interpretations, virtually all pre-Silurian rivers were braided, whereas Silurian and younger rivers were either meandering or braided. Clouding this conclusion, however, are differences in tectonic, geomorphic, and climatic settings of the various depositional sites, the use of potentially different diagnostic criteria, and the paucity of interpretations of pre-Silurian rivers.

The Central Appalachian Paleozoic sequence exposed in central Pennsylvania contains eight formations of fluvial origin, ranging in age from Ordovician to Pennsylvanian. These accumulated on aggrading foreland plains in molasse phases of Appalachian clastic wedges. Consistent criteria were used to categorize the fluvial styles as fine-grained meanderbelt, coarse-grained meanderbelt, channeled-braided, or sheet-braided. The three older Paleozoic units (Bald Eagle, Juniata, Tuscarora) were deposited with a sheet-braided fluvial style; the five Devonian and Carboniferous units (Catskill, Pocono, Mauch Chunk Pottsville, Llewellyn) accumulated principally in fine- and coarse-grained meanderbelts, with downslope changes in fluvial style. In Early Paleozoic time only braided styles existed, under conditions that in the Later Paleozoic led to the development of meandering.

Likely cause of this middle Paleozoic fluvial style change was the advent of land vegetation, which altered flood peaks and sediment yields, enhanced production of finer sediment, and stabilized stream banks.