The Ordovician-Silurian boundary in the western Valley and Ridge province is in an unfossiliferous, continental sequence, and has been traditionally placed to separate red Juniata Formation from the overlying white Tuscarora Formation. The systemic boundary, thus based on color differences, has been assumed to represent a time-parallel surface of considerable paleogeographic significance. Detailed field and mineralogic data now indicate that this assumption leads to ambiguities in correlations, and that a new means of defining the boundary is necessary.

Sedimentologic aspects of rocks adjoining the color boundary provide primary control on diachroneity. Westward-prograding and eastward-transgressing fluvial regimes generated a sequence of distinct conglomerate, sandstone, and shale lithofacies recognizable over wide areas and occupying the 3,500-ft interval between datable Upper Ordovician and Middle Silurian fossiliferous marine units. Boundaries based on lithofacies approach more closely than other kinds of boundaries to time-parallel surfaces. The color boundary, for example, fluctuates as much as 400 ft vertically, both parallel with and across depositional strike, irregularly and unpredictably crossing lithofacies boundaries. Zones of constant clay, heavy, and opaque mineralogy within lithofacies are likewise randomly truncated y the color boundary.

These data strongly suggest that redefinition of the Ordovician-Silurian systemic boundary in terms of the newly recognized lithofacies sequence is in order. Such revision depends fundamentally on precise time correlations of specific lithofacies with age-established marine units outside the Valley and Ridge area. In the absence of such rock units and of meaningful ichno-fossil data, exact location of the boundary may not be possible.

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