Sequences, cycles, and basin dynamics in the Silurian of the Appalachian Foreland Basin

Abstract Field stratigraphic analysis of the Early and medial Silurian strata of Ontario, New York and Pennsylvania permits recognition of six major sequences, reflecting 3rd-order cycles, in the Niagaran (Llandoverian-Ludlovian) of New York and adjacent regions. These sequences correspond approximately to the Medina, lower Clinton, middle Clinton, upper Clinton (two sequences) and Lockport Groups. Each is bounded by widespread unconformities produced by the interplay of eustatic sea-level drop and local tectonic uplift. In general, erosional sequence boundaries and transgressive surfaces are merged, with no record of the lowstand wedge. Each sequence is subdivisible into two to five sub-sequences that are marked by sharp, though generally non-erosional basal discontinuities. Investigation of the Clinton and Lockport Groups corroborates earlier suggestions that carbonate and siliciclassic tongues extended basinward from opposite margins synchronously in response to sea-level drops. Analysis of the thickness and basinward extent of the progradational tongues reveals the existence of multi-scale sedimentary cycles. Sequences and component sub-sequences are correlative basinwide and are synchronous within the resolution of bio- and event-stratigraphy. Major drops in relative sea-level are represented by the sequence-bounding discontinutiies. These surfaces are overlain by siliciclastic wedges near terrigenous source areas, or winnowed carbonate pack- and grainstones on the tectonically passive margin. Overlying highstand deposits, typically shales, calcareous mudstones, and limestones, are separated from lowstand or transgressive deposits by thin condensed intervals commonly marked by phosphatic or glauconitic material and/or surfaces of maximum sediment starvation. The relative highstand deposits of sequences and sub-sequences are developed in relatively deep-water facies and are, at most, slightly progradational. Hence, most sequences and sub-sequences are sharply bounded, roughly upward deepening (to slightly shallowing) successions of strata. In turn, sub-sequences are further divisible into widespread minor parasequences and parasequence sets, probably corresponding to 6th- and 5th-order cycles, respectively. The smaller scale units (0.2–5 mthick) commonly display an upward shallowing motif. The nested relationship of these cycles supports the hierarchical model of transgressive-regressive allocycles proposed by Busch and Rollins (1984). Niagaran cycles discussed herein are equivalent to third-, fourth- and fifth-order cycles and are traceable circumbasinally. Depocenters of successive sequences and sub-sequences display a pattern of eastward-westward-eastward basin-axis migration through the Silurian. During early Llandoverian, the Taconic (Queenston) deltaic complex began to subside, and the basin axis, depocenter, and eastern shoreline shifted eastward approximately 200 km over a 6 million year period. As the basin axis migrated eastward, the western basin ramp (forebulge) was repeatedly upwarded and eroded, resulting in the generation of regionally angular unconformities at the bases of sequences II, IV and VI. The pattern of unconformity (greatest to the west) suggests rise of the Algonquin Arch concomitant with eastward basin-axis migration. During the late Llandoverian to early Wenlockian, the Algonquin Arch subsided slightly, concurrently with a reversal in direction of basin-axis migration. During deposition of upper Clinton and Lockport-Vernon strata (sequences V, VI), the basin axis shifted approximately 300 km back to the west. Eastward migration of the basin axis corresponds with a time of tectonic quiescence and probable thrust-load relaxation. The abrupt reversal of migration in the late Llandoverian may be a signal of renewed thrusting in the hinterland at the onset of the Salinic Disturbance.