Recognition of Facies, Bounding Surfaces, and Stratigraphic Patterns in Foreland-Ramp Successions: An Example from the Upper Devonian, Appalachian Basin, U.S.A.

Abstract

ABSTRACT A detailed stratigraphic study of the Upper Devonian Lock Haven Formation in the Appalachian basin provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the foreland-ramp setting. The common occurrence of graded beds, hummocky cross stratification, and shell lags indicates that storm processes played an important role in deposition. Gradational-based, coarsening-upward shoreface sequences of interbedded sandstone and mudstone formed during gradual fall in relative sea level. In contrast, sharp-based regressive sequences consist of laminated sandstone facies overlying a submarine erosion surface that developed in response to more rapid fall in relative sea level. Transgressive sequences become finer grained upward from conglomerate lag, which was deposited on a basal transgressive surface, to heterolithic facies and mudstone facies. A combination of eustasy and tectonic subsidence of the foreland ramp produced changes of relative sea level. These fluctuations provided a means for transporting sand onto the ramp and for producing repetition of stratigraphic patterns in the Lock Haven Formation. The Upper Devonian clastic succession, which includes both the Lock Haven Formation and the Catskill Formation, represents northwestward progradation of shoreline to nonmarine environments as sediment supply to the foreland ramp exceeded the rate of formation of accommodation. Compared to passive-margin shelves, thick fine-grained successions such as the Lock Haven Formation are more likely to form in foreland basins, where tectonic subsidence provides a mechanism for creating accommodation. Sediment influx from uplifted areas and absence of a slope break on the foreland ramp results in a higher rate of fine-grained clastic sedimentation than occurs on shelves of passive margins.