At any single locality in the Helderberg Group of New York State, most formation and member boundaries coincide with boundaries of punctuated aggradational cycles. In each case of coincidence, the boundary is a discontinuity that was produced by a rapid rise of relative base level, rather than a facies contact representing gradual migration of contiguous paleoenvironments. Apparently diachronous formation boundaries in the sequence are not single continuous surfaces; instead, each diachronous boundary is actually a stratigraphic series of isochronous surfaces that are separate punctuated aggradational cycles (PAC) boundaries produced by distinct geologically instantaneous base-level rises. These conclusions about the origin of formation boundaries suggest that the concept of the formation as a fundamental unit of stratigraphic description and interpretation may need revision if the stratigraphic record accumulated episodically on a small scale.

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