Roles of Eustasy and Tectonics in Development of Silurian Stratigraphic Architecture of the Appalachian Foreland Basin

Abstract Refined intrabasinal correlation of medial-Silurian strata has led to recognition of discrete eustatic and tectonic controls on the stratigraphic architecture of the Appalachian foreland basin. Major unconformities and disconformities are used to define third- and fourth-order sea-level cycles (depositional sequences and sub-sequences). Although they are asymmetric, most unconformities are present along both basin margins, and their timing corresponds with sea-level lowstands in other Silurian basins. The correlation with apparent, global, sea-level lowstands suggests a eustatic component to Silurian Appalachian basin unconformities. Silurian sequences are divisible into systems tracts that are correlatable across the basin. Transgressive systems tracts are laterally correlative, retrogradational, carbonate and sandstone successions that onlap unconformities on northwestern and southeastern margins of the basin, respectively. Highstand systems tracts are thickest in the basin center and thin laterally toward each margin. They are divisible into early highstand phases, typified by aggradational, fine-grained siliciclastic and argillaceous carbonate successions, and late highstand (or regressive) phases, that characteristically exhibit a general upward-coarsening (progradational) pattern. These regressive deposits are typically divisible into two or more sub-sequences whereas the transgressive and early highstand systems tracts comprise a single subsequence. Smaller-scale, discontinuity-bound, stratal packages that are interpreted to record fifth- and sixth-order sea-level changes, are analogous to parasequences sets and parasequences, respectively. Many of the small-scale cycles can be mapped basin-wide or unti 1 they are truncated under marginal unconformities. High-frequency, eustatic, sea-level changes or climatic oscillations are plausible mechanisms to explain the pervasiveness of these cycles. Documented tectonic controls on stratigraphic architecture include lateral migration of the foreland-basin axis and uplifts along both the cratonic arch and the tectonically active eastern basin margin. These tectonic signatures are imprinted on marginal unconformities and are also recorded by progressive lateral shift in the locus of thickest accumulated sediment and deepest facies. Tectonically imprinted unconformities are more fully developed on one basin margin and are distinguished from purely eustatically generated unconformities by their asymmetry. Consequently, large-scale sedimentary cycles, that are bounded by unconformities and correlate with sea-level changes in other basins, record an interplay between foreland basin geodynamics and eustatic processes of similar rate. In contrast, smaller-scale, sedimentary allocycles, that have recurrence intervals that outpace rates of foreland-basin flexure, more clearly record high-frequency, low-amplitude, eustatic sea-level changes or climatic oscillations.