Conodonts and the Devonian–Carboniferous transition in the Dyer Formation, Colorado

D Jeffrey Over,James W Hagadorn,C James Bullecks,Zackery Wistort,Linda K Soar

doi:10.24872/rmgjournal.56.2.51

Abstract

ABSTRACT Conodont assemblages from shallow platform carbonate strata of the Dyer Formation in northwestern Colorado indicate that the unit spans the Devonian–Carboniferous transition, and include four new taxa, described herein. The underlying Parting Formation and most of the Broken Rib Member of the Dyer Formation are in the Palmatolepis expansa Zone. The upper portion of the Broken Rib Member and the lower part of the Coffee Pot Member of the Dyer Formation are in the Bispathodus aculeatus Zone to the Bispathodus ultimus Zone, and associated with an ~6‰ positive δ13C shift identified as the end-Devonian Hangenberg excursion. Upper Coffee Pot Member conodonts are equivocally Tournaisian, and cap the δ13C excursion. Collectively, these observations suggest that the Devonian–Carboniferous boundary is in the upper Coffee Pot Member of the Dyer Formation, and thus the overlying Gilman Sandstone is Carboniferous.

Highlights

The end-Devonian–earliest Carboniferous (D–C) was a time of significant global change marked by glaciation in the Americas—southern Laurentia and western Gondwana (Isaacson et al, 1999, 2008; Brezinski et al, 2010; Etten‐ sohn et al, 2020), and sea level fluctuation (Myrow et al, 2013)
In the Dyer Formation, conodonts from 24 productive horizons spread across five measured sections provide biostratigraphic constraint for a measured carbon isotope excursion consistent with the Hangenberg event
Bulk 2+ kg samples were collected for conodonts and δ13Ccarb chemostratigraphy

Summary

Introduction

The end-Devonian–earliest Carboniferous (D–C) was a time of significant global change marked by glaciation in the Americas—southern Laurentia and western Gondwana (Isaacson et al, 1999, 2008; Brezinski et al, 2010; Etten‐ sohn et al, 2020), and sea level fluctuation (Myrow et al, 2013). These changes were associated with the multiphasic Hangenberg Crisis interval that includes a major mass ex‐ tinction (Walliser, 1984; Kaiser et al, 2016). The conodonts and carbon isotope shifts allow placement of the Devonian–Carboniferous boundary within a shallowwater carbonate platform setting

Methods

Results

Discussion

Conclusion