Abstract

In order to better understand the relationship of clastic influx and organic-matter accumulation and preservation in the Middle Devonian Acadian foreland basin, we conducted a high-resolution provenance analysis of a composite section of the Hamilton Group. The Hamilton Group includes the organic-rich Marcellus Shale, one of the most lucrative unconventional shale gas plays in the world, and the overlying Mahantango Formation. Geochemical analysis of 121 samples from two nearby wells in Monongalia County, West Virginia, reveals subtle changes in clay provenance throughout deposition of the Marcellus Shale and the Mahantango Formation. Major-element and trace-element geochemistry suggest derivation from a felsic igneous, granodioritic, upper-continental crustal source with additional influx from a recycled quartzose sedimentary source. All weathering indices indicate increasing chemical weathering intensities throughout deposition of the Marcellus Shale, followed by consistently moderate chemical weathering associated with accumulation of the Mahantango Formation. Sm-Nd isotopic analysis of ten samples throughout the Hamilton Group yield ε_Nd values ranging from –7.06 to –11.75, and Nd depleted-mantle model ages (τ_DM) ranging from 1.63 to 1.85 Ga, with ages becoming younger upsection. Our results suggest that the extrabasinal detritus of the Hamilton Group originated from a mixed sediment source, with clay influx from both rocks associated with the Superior Craton to the north and northwest (τ_DM > 2.7 Ga) and Grenville-sourced sediments derived from the adjacent Acadian fold–thrust belt to the east (τ_DM ∼ 1.4–1.6 Ga). Older Sm-Nd model ages, felsic composition, and evidence of sediment recycling suggest little to no contribution from the Acadian volcanic arc, indicating that volcanic tuff of the Tioga ash beds represent isolated episodes of volcanic input into the basin. Model ages, ε_Nd values, and trace-element geochemistry indicate increased sediment influx from the Acadian fold–thrust belt throughout deposition of the Hamilton Group, with the highest sediment influx having occurred during Mahantango Formation deposition. Increased clay influx from sediment shed off from the Acadian fold–thrust belt associated with marginal marine regression increased delivery of clastic detritus, diluting the flux of organic matter and likely disrupting basin water stratification that aided in preservation of organic matter in the lower Marcellus Shale. The Middle Devonian Acadian Basin serves as an example of the influence of detrital influx on organic-matter accumulation and preservation in mud-dominated depositional systems.