A DICE-Y SITUATION: CONSIDERING THE DRUMIAN CARBON ISOTOPE EXCURSION (DICE) AT THE DRUMIAN GSSP, WESTERN-CENTRAL UTAH

The base of the Drumian Stage of the international Geologic Time Scale (504.5 Ma; GSSP) is defined by the First Appearance Datum of the agnostid trilobite Ptychagnostus atavus in slope-basinal deposits of the Wheeler Formation on Stratotype Ridge in the Drum Mountains near Delta, Utah. The Drumian Carbon Isotope Excursion (DICE) is a moderate negative δ¹³C_carbonate excursion associated with the GSSP. It is used as a global correlation tool, especially in the absence of biostratigraphy and limited geochronological control in certain Cambrian strata. However, the robustness of the DICE for global correlation requires further testing.

The DICE at Stratotype Ridge is shown by very few carbon isotope values across a very thin (2m) stratigraphic interval. Previous work uses only four δ¹³C_carbonate values (between -1 and -2.5‰) to define the DICE and our reanalysis resulted in similar values. In the nearby House Range of Utah, this excursion is recorded in similar facies and is also defined by four δ¹³C_carbonate values (between -0.5 and -2.0 ‰) across <5 m in thickness. Both sections have multiple subtle sequence boundaries and are structurally complex.

Stratotype Ridge contains substantial calcite veining on a mm to dm scale and the GSSP itself is offset by a m-scale fault. Additionally, the section is ~0.5 km from an attenuation fault that thins and removes lower-middle(?) Cambrian strata. Isotopic values from calcite veins that crosscut the strata are presented, along with local lateral δ¹³C_carbonate profiles of the DICE-bearing interval and a structural map of Stratotype Ridge.

If this lateral and diagenetic analysis suggests that the DICE records primary open seawater values at the GSSP, it bolsters its utility as a potential global correlation tool, especially for the approximate location of the Drumian Stage boundary in transgressive shallow-water platform deposits that lack age-diagnostic trilobites. However, multiple negative carbon-isotope excursions of similar magnitude are observed in a single section of this general time interval. The integration of radioisotopic geochronology, biostratigraphy, chemostratigraphy, and structural geology may be needed for further understanding of the DICE.