Abstract

The Middle to Late Devonian lower Genesee Group of New York has been the focus of sedimentologic, stratigraphic, and paleoecologic studies for over 150 years. Documentation of fluvial discharge events in this succession supports the growing realization that mudstone sedimentation in shelf seas is much more dynamic than commonly appreciated. Although this process has been observed on modern shelves, so far only few examples have been reported from ancient shelf strata. Integrated sedimentologic, sequence stratigraphic, and geochemical data suggest that organic-rich mudstones of the Geneseo Formation were strongly influenced by fluvial-discharge events that carried fine-grained clastics and phytodetritus as turbulent flows from river-mouth regions onto the shelf. These discharge events appear to occur most abundantly during initial flooding at parasequence boundaries, a depositional context that resulted in increased coastal erosion processes and enhanced continental runoff.

In this study we explore the potential relationships between flooding events and depositional processes that govern the transport and deposition of organic-rich, fine-grained clastics in shelfal settings. In addition, we document how stable carbon isotopes and X-ray fluorescence analyses allow chemical fingerprinting of fluvial input events in an ostensibly monotonous succession of organic-rich mudstones. Isotopically, the organic material within hyperpycnal layers shows a dominantly terrestrial signature (δ13Corg = –26.5‰) and contrasts strongly with the dominant marine signature (δ13Corg = –30‰) of the Geneseo organic-rich succession. The less negativeδ13Corg values of hyperpycnal layers (organic-lean, gray silty mudstones) are associated with elevated Zr/Al, Ti/Al, and Si/Al ratios, decreased FeT/Al, and lower trace-metal concentrations. Intervening intervals of organic-rich, sparsely to weakly bioturbated, pyritic and banded black mudstones show more negativeδ13Corg values, and are associated with decreased Zr/Al, Ti/Al, and Si/Al ratios, increased FeT/Al, and trace-metal enrichment. Recognizing wave-aided hyperpycnal input hundreds of kilometers away from the paleoshoreline suggests that deposition of basinal organic-rich mudstones occurred within storm-wave base and preservation of organic-matter richness is dominantly controlled by dilution.

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