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Sedimentologic and Stratigraphic Evolution of the Middle Devonian Geneseo Formation of New York, USA: Implications for Mud Transport in a Shallow Epeiric Sea


Characterization of mudstone-dominated systems has undergone a paradigm shift with recent advances in experimental sedimentology. Flume experiments have demonstrated that even at flow velocities that transport medium sand in bedload, fine-grained sediments (grain-size diameter >62.5 μm) are prone to flocculate and accumulate in bedload as migrating ripples. Detailed facies analysis and petrography of the Geneseo Formation in the Northern Appalachian Basin has revealed a wealth of sedimentary textures and fabrics that suggest mud deposition was not a result of suspension settling in a “quiet”, stagnant, anoxic water body. Instead, this interval reflects deposition of organic-rich muds in a storm-dominated system, with graded beds, current-and combined-flow ripple cross-lamination, and varying degrees of bioturbation indicating the environment was energetic as well as dynamic. At Geneseo time, Acadian uplift supplied fine-grained detritus from the east to be incorporated in a growing delta. As sediment was shed from the hinterland, autogenic processes coupled with a general rise in sea-level significantly controlled the distribution of mudstone facies. The Geneseo Formation has been separated into three lithostratigraphic members, representing an initial HST (lower Geneseo Mbr.), a LST and TST (Fir Tree Mbr.), and another HST (upper Geneseo Mbr.). The vertical assemblage of mudstone facies observed in this succession indicates an overall shallowing-upwards with multiple modes of sediment transport and deposition. The water-column became more oxygenated upsection as indicated by an increase in benthic fauna diversity (e.g.Leiorhynchus and Orbiculoides), increasing bioturbation (e.g. Chondrites, Planolites, Teichichnus, Zoophycus, Paleophycus, and Zoophycus), and a decrease of organic-carbon content (via oxidation and consumption). Our observations do not support the stratified-basin model proposed previously for the Geneseo. Mud deposition appears to have taken place in a shelf setting within storm-wave base, and the environment was comparatively energetic with frequent interludes of lateral sediment transport by bottom currents. The latter view is supported by the small scale heterogeneity in mudstone facies and sedimentary features observed. Seasonal stratification and marine algal blooms may have contributed to the organic-richness of this succession (up to 8% TOC), but clastic dilution appears to be the primary control on overall organic content.