UPPER FAMENNIAN CARBON ISOTOPIC EXCURSIONS IN WESTERN LAURENTIA LINKED TO DIAGENETIC ALTERATION DURING LOWSTANDS

Integrated analysis of the sedimentology, stratigraphy, and chemostratigraphy of the uppermost Devonian Chaffee Group of Colorado reveals the presence of regionally extensive unconformity surfaces associated with globally recognized extinction/eustatic events. The contact between semi-restricted, marginal marine, mixed siliciclastic–carbonate deposits of the Parting Formation, and open marine carbonate of the Dyer Formation is a major marine flooding surface across western Colorado. This flooding surface rests at the top of a ~5 m thick, transgressive, cross-bedded sandstone unit that locally overlies a 2.5-m-thick paleokarst breccia. This formation contact interval is correlated to the Dasberg Event, a late Famennian extinction event, based on conodont biostratigraphy and carbonate carbon δ¹³C chemostratigraphy. δ¹³C values shift from negative to positive across the formation contact, and in some cases to greater than +5‰. Oxygen isotopic values are extremely variable between sections, in cases invariant across the contact, and in other cases, they covary with the δ¹³C data. At Ouray, CO, δ¹⁸O covaries with δ¹³C throughout the section, and below the unconformity reaches extreme values (<-30 ‰). An isotopic shift in rocks of this age in Utah, coined ALFIE, was previously correlated to the Parting–Dyer contact. This study demonstrates that the carbon and oxygen isotopic record of this event is highly variable across western Laurentia, and that it is clearly linked to a regional unconformity and associated diagenetic alteration. The lowstand at this contact may be in part a signal of eustatic fall associated with the Dasberg event. Similar isotopic patterns exist for strata below and above a paleokarst breccia in the upper Dyer Formation. We link this interval to the globally significant latest Famennian Hangenberg Event, which includes a eustatic lowstand and subsequent transgression.