Abstract The Middle Devonian Geneseo Formation and its lateral equivalents in the Northern Appalachian Basin are regarded as crucial secondary targets to the extensively explored Marcellus subgroup. High-resolution sedimentology, stratigraphy, and petrography have yielded differentiation of genetically related packages, comprised of distinct lithofacies with characteristic physical, biological, and chemical attributes. In addition, argon ion milling and nanoscale scanning electron microscopy of shale sections has shown that the pore structure of the Geneseo derives from pores defined by phyllosilicate frameworks, carbonate dissolution, and within organic matter. Intervals of silt-rich mudstones and muddy siltstones occur in multiple facies types and “interrupt” facies, reflecting background sedimentation. These deposits and their sedimentary features are interpreted as products of high-density fluvial discharge events. Pore morphology and distribution correlates with distinct mudstone lithofacies as a result of small-scale compositional and textural characteristics. Phyllosilicate framework pores are small triangular openings (100-1500 nm wide) and are the dominant pore type observed in hyperpycnites. Organic matter porosity is common (10-500 nm pore size) and dominates the organic-rich facies that represents “background” sedimentation with high organic content. Carbonate dissolution pores (50-500 nm wide) are observed in calcareous intervals and reflect partial dissolution of carbonate grains during catagenetic formation of carboxylic/phenolic acids.

The names Genesee, Portage and Chemung are indelibly linked to the early Upper Devonian stratigraphy of the Catskill delta in western New York. For over a century generations of stratigraphers struggled in the attempt to correlate these groups eastward from the Genesee Valley into sections around the delta-front at Cayuga Lake. Rapid thickening and intertonguing of facies were the chief difficulties, but the misinterpretation of ammonoids also played a major role in obscuring the facies relationships. Manticoceras sinuosum (Hall, 1843), the index fossil of the Portage Group, was mistaken by James Hall and J. M. Clarke and later workers for species of Ponticeras and Koenenites from horizons in the older Genesee Formation east of Canandaigua Lake. Some of these Genesee ammonoid horizons have been traced from the pelagic facies in the basin into the benthic facies of the delta-front. One of these horizons in the Upper Penn Yan and Middle Ithaca Members may record an interval of transgression or standstill in the progradation of the delta. The ammonoid horizons in the Genesee Formation at Cayuga Lake are stratigraphic markers which should aid in the taxonomic and biostratigraphic revision of the benthic groups and in the refinement of the paleoecological associations in the delta-front facies.