Recognition of facies, bounding surfaces, and stratigraphic patterns in foreland-ramp successions: An example from the Upper Devonian, Appalachian basin, USA

A detailed stratigraphic study of the Upper Devonian Lock Haven Formation i n the Appalachian basin pro,ides new interpretations applicable to understa nding sedimentation and stratigraphic architecture in the foreland-ramp set ting, The common occurrence of graded beds, hummocky cross stratification, and shell lags indicates that storm processes played an important role in d eposition. Gradational-based, coarsening-upward shoreface sequences of inte rbedded sandstone and mudstone formed during gradual fall in relative sea l evel. In contrast, sharp based regressive sequences consist of laminated sa ndstone facies overlying a submarine erosion surface that developed in resp onse to more rapid fall in relative sea level. Transgressive sequences beco me finer grained upward from conglomerate lag, which was deposited on a bas al transgressive surface, to heterolithic facies and mudstone facies, A com bination of eustasy and tectonic subsidence of the foreland ramp produced c hanges of relative sea level. These fluctuations provided a means for trans porting sand onto the ramp and for producing repetition of stratigraphic pa tterns in the Lock Haven Formation. The Upper Devonian elastic succession, which includes both the Lock Haven Formation and the Catskill Formation, re presents northwestward progradation of shoreline to nonmarine environments as sediment supply to the foreland ramp exceeded the rate of formation of a ccommodation. 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. Sedime nt influx from uplifted areas and absence of a slope break on the foreland ramp results in a higher rate of fine-grained elastic sedimentation than oc curs on shelves of passive margins.