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.

Kimberlites in the eastern United States contain two suites of megacrysts/inclusions that are mineralogically similar but compositionally distinct. One suite (olivine, garnet, diopside, Cr-spinel) has higher Cr and Mg than the other (olivine, garnet, diopside, picroilmenite). Based on detailed petrologic studies of megacrysts from the Fayette County, Pennsylvania, kimberlite, Hunter and Taylor (1984) suggested that these two suites represent the crystallization products of separate magmas that mixed in the low-velocity zone (LVZ) to form kimberlite magma. Major and trace element abundances of individual garnet megacrysts from eastern U.S. kimberlites (i.e., from Kentucky, New York, Pennsylvania, and Tennessee) support the magma-mixing hypothesis but also indicate additional complications. Eclogite garnets have Cr 2 O 3 <0.3 wt.%, CaO >7 wt. %, and chondrite-normalized Lu/Hf <<1. Peridotite garnets have Cr 2 O 3 >2 wt.%, MG# >83, and chondrite-normalized Lu/Hf <1. Garnet megacrysts from Kentucky and Pennsylvania form two groups, one with TiO 2 <0.5 wt.%, and one with TiO 2 >0.5 wt.%. Both groups span a similar range in Cr 2 O 3 (≅ 1.0 to 9.0 wt.% Cr 2 O 3 ), but the high-Ti garnets may have Cr 2 O 3 as low as 0.1 wt.%. The low-Ti garnets have chondrite-normalized Lu/Hf <1 and are probably derived by the disaggregation of peridotite xenoliths and wall rock. The high-Ti garnet megacrysts have chondrite-normalized Lu/Hf ≥1 and are interpreted here as cognate “phenocrysts” that crystallized in a kimberlite or proto-kimberlite magma. Two suites of high-Ti garnet megacrysts are recognized: a low-Cr to very low-Cr suite (Cr 2 O 3 <4 wt.%) with flat to slightly positive heavy rare-earth element (HREE) slopes, and a high-Cr suite with steeply negative HREE slopes. These suites correspond to the “Cr-poor” and “Cr-rich” suites, respectively, defined by Hunter and Taylor (1984) for the Pennsylvania kimberlite. These data are consistent with the mixing of two magma batches to form kimberlite, as proposed by Hunter and Taylor (1984). Mixing prpobably occurred in the LVZ prior to eruption of the hybrid kimberlite magma.