Facies analysis and sequence stratigraphy of the Middle Devonian (Givetian) Geneseo Formation of New York: Implications for accommodation during a eustatic sea-level rise

Facies analysis of the Middle Devonian Geneseo Formation has yielded differentiation of packages of shale in a seemingly monotonous succession. This Middle Devonian shale succession of New York was deposited as part of the westward prograding Catskill Delta complex. Acadian uplift supplied sediments from the east to be delivered for delta growth, and coincides with a general rise of eustatic sea-level. Depositional parameters and lithofacies analysis were inferred using a combination of optical petrography, scanning electron microscopy, and core description. The Genesee Group (Middle to Upper Devonian) represents the onset of the third tectophase of the Acadian Orogeny and consists of the Geneseo Formation, the Penn Yan Formation, the West River Formation, and their correlative coarser facies (i.e., Sherburne Formation, Ithaca Formation). Organic-matter enrichment appears to be focused in the lower Geneseo Formation, where finer-grained pyritic and banded black shales are dominantly observed with small scale truncations, current ripples, graded silt-clay couplets, and cryptobioturbation with a low-diversity Leiorhynchus and Styliolina assemblage. Upsection, banded black shales grade into dark gray silty mudstones with abundant wave ripples, current ripples, graded silt-clay couplets, evidence of extensive reworking and erosion, and increased bioturbation/biodeformational structures with small brachiopods (Ambocoelia, Orbiculoides, and Leiorhynchus). In the middle Geneseo, a silt-rich calcareous mudstone is observed, and contains abundant wave ripples, current ripples, with increased bioturbation/biodeformational structures and is rich in auloporid tabulate corals, ostracodes, and small brachiopods (Abocoelia and Devonochonetes). Above this concretionary horizon, dark gray silty mudstones are observed and display abundant wave ripples, current ripples, graded beds, evidence for extensive reworking and erosion, and increased bioturbation/biodeformational structures. Dark gray silty mudstones grade upsection into dark muddy siltstones with abundant wave and current ripples, deep scours, and extensive bioturbation with auloporid tabulate corals. Mudstone properties in the Geneseo succession are highly variable, and exhibit an overall shallowing upwards trend that corresponds to the westward progradation of the Catskill delta. High-resolution stratigraphy has yielded differentiation of genetically related packages, comprised of distinct lithofacies with characteristic physical, biological, and chemical attributes.;In addition, nano-scale microscopy of ion-milled shale sections has led to identify three distinct pore-types in the Geneseo Formation. It appears that 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 (50--1000 nanometers wide) that are observed throughout the Geneseo succession, however, are better developed in the dark gray mudstones where pressure halos of compaction resistant grains (sand, silt, pyrite, etc.) surround and prevent compression of primary clay fabric. Organic matter porosity is also observed (10--500 nanometers wide), and dominates the lower Geneseo pyritic and banded black shale facies where organic content is highest. Carbonate dissolution pores are observed in the calcareous intervals within the Geneseo (50--500 nanometers wide), and probably reflect partial dissolution of carbonate grains (calcite or dolomite) during catagenetic formation and migration of carboxylic and phenolic acids.