CHAPTER 10: ON THE STRATIGRAPHY, SEDIMENTOLOGY AND STRUCTURE OF UPPER FRASNIAN TO FAMENNIAN DEVONIAN STRATA IN SOUTHWESTERN NEW YORK STATE

Our detailed field studies (conducted 1989-2005) at over 2,000 sites in the upper Frasnian and Famennian strata of southwestern NYS encouraged revisions to the Rickard (1975) stratigraphic chart, including: abandoning 2 formations, proposing 4 new stratigraphic units, reinstating 6 units that Rickard (1975) omitted, and revising 7 other units. These stratigraphic units record the transition from Frasnian deeper-water shales and turbidite sandstones through Famennian shallower, offshore-to-nearshore depositional environments to terrestrial sands and shales as the Acadian Foreland Basin filled. The unsteady westward march of the shoreline was controlled by the usual interplay of 1) basin subsidence and fault block motion, 2) sediment supply, and 3) eustatic sea-level variation.

Recent biostratigraphic and isotope work (e.g., Bush et al, 2017) indicates that Pepper and Dewitt (1951), whom we followed, might have been incorrect by one black shale cycle when they carried the black shale stratigraphy from western NYS across a region of no-outcrop into our study area. However, the local variations in the stratigraphy ascribed to structural influences and observed structures reported here are not affected by this more regional “’irregularity”. Further, the black shales show thickness anomalies across major faults no matter which set of regional correlations is used.

We recognized literally hundreds of multiply-reactivated faults by integrating detailed studies of fractures, stratigraphy, lineaments, soil gas, seismicity and seismic reflection profiles (e.g., Jacobi and Fountain, 1996). Devonian growth-fault geometries and facies changes occur across N- and NW-striking faults. The Hume black shale thins over a repeated Dunkirk black shale, suggesting syndepositional thrusting initiated in the Neoacadian. Support for Neoacadian thrusting is the bedding-restricted pencil-cleavage zones with sandstone clasts that display ductile/brittle transition structures.

Paleoflow indicators associated with the black shales and the intervening units are generally from the east in the study area. Thus, the model that the black shales were deposited on the starved western margin of the basin, upslope from the basin axis, requires revision. The black shales may indeed straddle the basin axis.