Latest Devonian (Famennian) global events in western Laurentia: Variations in the carbon isotopic record linked to diagenetic alteration below regionally extensive unconformities

Abstract

Integrated analysis of the sedimentology, stratigraphy, and chemostratigraphy of the uppermost Devonian Chaffee Group of Colorado reveals the presence of two 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 an ~ 5 m thick, transgressive, cross-bedded, shoreline sandstone unit that locally overlies a 2.5-m-thick paleokarst breccia. δ^(13)C values shift lighter across the formation contact, in some cases by as much as 5‰. Oxygen isotopic values are extremely variable between measured stratigraphic sections, in cases invariant across the contact, and in other cases covarying with the δ^(13)C values. At Ouray, CO, δ^(18)O covaries with δ^(13)C throughout the section, and reaches extreme values (< − 30‰) below the unconformity. 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 ALFIE is highly variable across western Laurentia, and that important carbonate chemostratigraphic variations result from diagenesis that is clearly linked to a regional unconformity and associated relative sea-level fall. This lowstand may be a signal of eustatic fall associated with the Dasberg Event, a late Famennian marine extinction event. Similar isotopic patterns exist for strata below and above a paleokarst breccia in the upper Dyer Formation that we link to the globally significant latest Famennian Hangenberg Event, which includes a eustatic lowstand and subsequent transgression. Similar to the Parting–Dyer contact, both carbon and oxygen isotopes in strata below this regional unconformity surface show the variable nature of diagenetic alteration of carbonate units during lowstand conditions. Our data also suggest that correlatable δ^(13)C chemostratigraphic shifts can be diagenetically produced during lowstands across a regionally widespread (western U.S.) basin, and that these δ^(13)C shifts may be expressed within outcrops that show no macroscopic sedimentological signature of subaerial exposure. This has broad implications for the evaluation of δ^(13)C data in the rock record, particularly the assumption that extensive correlatable isotopic anomalies reflect global marine signatures.

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