Mudrock sequence stratigraphy: A multi-proxy (sedimentological, paleobiological and geochemical) approach, Devonian Appalachian Basin

Delineation of stratigraphic sequences and their component systems tracts in mudrock-dominated facies is generally difficult due to the relatively homogenous, fine-grained nature of the strata. In this study, we apply a multi-proxy analytical approach to a thick Devonian mudrock-dominated succession through detailed analysis of sedimentologic, paleobiologic, and geochemical data through 600. m of mudrock-rich facies. Varied combinations of proxies prove to be most useful in delineating sequence development in anoxic-, dysoxic-, and oxic-dominated mudrock settings, and in mixed mudrock-carbonate and mixed mudrock-sandstone successions. These interpretations are tested against an established sequence stratigraphic framework for 11 Middle to Upper Devonian (mid-Eifelian to lower Famennian) sequences in the Appalachian Basin. The sequences presented here further detail and refine global Devonian T-R cycles Id to IIe of the well known Johnson, Sandberg and Klapper sea-level curve.The usefulness of proxies in delineating depositional sequences and systems tracts varies dependent on depositional, paleoceanographic, paleoecologic, and early diagenetic conditions. Those proxies that show a range of variations in specific settings, such as grain size, degree of bioturbation, and concentrations of TOC and elements/elemental ratios (e.g., CaCO₃, Al, Ti, Mg, Sc, Si, Mo, Ni, V; Si/Al and Ti/Al) may help delineate depositional dynamics related to redox conditions, condensation, dilution, and clastic, biologic, and/or authigenic sediment sources.In fine-grained, anoxic-dominated facies, interpreted to represent basinal settings, sequences and systems tracts are best delineated by anoxic-related proxies TOC and Mo. In intermediate, dysoxic-dominated settings, TOC, Mo, bioturbation, and Al remain effective indicators of sequence development. In relatively oxygenated, mudrock-rich and carbonate poor sequences, bioturbation may function as the most effective proxy for recognizing systems tracts.For mixed fine-grained siliciclastic-carbonate successions, concentration and type of CaCO₃ (e.g., benthic macroskeletal, pelagic styliolinid/dacryoconarid, and micritic/calcisilt) are useful in identifying position within cycles. In more proximal, carbonate-poor successions, fine- and coarse-grained fractions become increasingly differentiated; these can be distinguished by relatively high Si/Al ratios (Si/Alca. 5).Elemental ratios indicative of coarser clastic input (e.g., Si/Al, Zr/Al and Ti/Al) are applicable to identifying position with a sequence, but they may also be affected by input from eolian, volcanogenic, or biogenic sources. In addition, fluxes of siliciclastic, carbonate, and TOC sediment types may dilute the concentration of the others. Multiple lines of evidence should be examined in interpreting relative depth and position within a sequence.