The Middle Devonian (Eifelian and Givetian) Hamilton Group in western and central New York, composed largely of fossil-rich marine shelf and slope facies, is subdivisible into repetitive facies sequences of varying thicknesses. Minor cycles include 1- to 3-m-thick, upward coarsening shale to siltstone hemicycles, resembling sixth-order punctuated aggradational cycles ( PACs), in central New York; probably correlative (0.5-1.0 m) shale to argillaceous, nodular limestone couplets occur in western areas. Major cycles in western New York include subsymmetrical 3- to 5-m-thick packages with medial limestone beds, analogous to "upper limestones" of Pennsylvanian cyclothems. The center of each cycle marks a regressive maximum and grades vertically, above and below, into black shale or dark gray mudstone, corresponding, in a facies sense, to cyclothemic "core shales" in the midcontinent Pennsylvanian. These widespread units appear to be allocyclic in nature and approximate fifth-order transgressire-regressive cycles, based upon average unit thickness relative to the encompassing, well-zoned, Hamilton sequence. Correlations show that most key regressive events marked by limestones in western New York are expressed by equivalent siltstone or fine sandstone units in central New York. Most cycles, which are nearly symmetrical in western New York, grade eastward into 5- to 20-m upward coarsening (PAC motif) hemicycles. These asymmetric cycles apparently reflect rapid sediment aggradation to limits imposed by wave energy in a region of abundant sediment supply. Further minor shallowing was accompanied by winnowing and bypass of fine-grained sediments, generating condensed, capping deposits of coarser silt and sand, reworked shells and, commonly, phosphatic nodules. Ensuing sea level rises were accompanied by initial sediment starvation followed by sedimentation in deeper water. Hence asymmetrical, upward shallowing hemicycles (PACs) appear to grade offshore into thin but symmetrical cycles. Both PAC and symmetrical motifs are local responses to the same, probably allocyclic, sea level fluctuations. We suggest that the PAC motif is partially facies controlled, being largely restricted to shallow shelf settings with relatively high rates of sediment accumulation.
