THE HANGENBERG CARBON ISOTOPE EXCURSION (HICE) ACROSS THE DEVONIAN-CARBONIFEROUS BOUNDARY IN THE GREAT BASIN

The Hangenberg Event at the Devonian-Carboniferous (D-C) Boundary is one of the severe biotic crises in the Phanerozoic. Associated with this biotic crisis is a positive carbon isotope (δ¹³C) excursion with a magnitude up to 5‰, known as the Hangenberg Carbon Isotope Excursion (HICE). Biostratigraphically, the HICE spans the latest Devonian Bi. costatus-Pr. kockeli interregnum (ckI) and Pr. kockeli conodont zones and continues into lowermost Carboniferous Si. sulcata zone strata in some sections. In the southern Great Basin, the HICE is generally thought to be missing due to an unconformity at the D-C boundary, attributed to forebulge uplift and migration during the Antler Orogeny.

Carbon isotope analyses of late Devonian-Early Carboniferous strata confirm the presence of the HICE in some sections of the southern Great Basin. In South Burbank Hills (Utah), a positive δ¹³C excursion with peak values up to 5‰ is present in a 28-m-thick interval of the upper Pilot Shale, between the pre-Hangenberg Bi. ultimus ultimus conodont zone and an unspecified Mississippian conodont fauna at the base of the Joana Limestone. A positive δ¹³C excursion is also present in oncolitic wackestones at Fox Mountain (Nevada) and Mountain Home (Utah) and in biostratigraphically equivalent beds at Bactrian Mountain (Nevada). Available biostratigraphic data indicate that the oncolitic wackestone at Fox Mountain contains Si. sulcata, and strata exhibiting the excursion at both Bactrian Mountain and Fox Mountain contain a Syringothyris brachiopod fauna correlated with the Pr. kockeli zone. Previous studies also report a positive δ¹³C excursion in the Confusion Range in an oncolite bed that hosts the Syringothyris fauna, the Pr. kockeli zone trilobite Pudoproetus priscus, and ckI-Si. sulcata zone Acutimitoceras (Stockumites) ammonoids. The close association of a positive δ¹³C excursion, the Pr. kockeli zone fauna, and the oncolitic limestones in the Great Basin supports recent assertions that latest Devonian oncolitic limestones—commonly used as marker beds for stratigraphic correlation in Nevada, Utah, and Montana—were deposited during the HICE.