LATE DEVONIAN CHAFFEE GROUP: AGE REVISION AND RECOGNITION OF GLOBAL OCEANOGRAPHIC EVENTS

The Chaffee Group of west-central Colorado yields sedimentological and chemostratigraphic evidence that indicates the existence of globally recognized geochemical events within these late Devonian strata. The Chaffee Group is subdivided into the Parting Formation and overlying Dyer Formation, the latter of which consists of the Broken Rib and Coffee Pot Members. Parting Formation strata are predominantly fine-grained and record deposition within a shallow, oxygen-stratified, marginal marine setting. The uppermost strata within the Parting Formation record a transgressive episode that abruptly flooded these shallow-water deposits and created a stable middle shelf environment, characteristic of the Broken Rib Member of the Dyer Formation. The overlying Coffee Pot Member records a regression that renewed deposition in a possible oxygen-stratified basin. A shallowing-upwards succession in the uppermost strata of the Coffee Pot Member leads to a sequence boundary at the contact with the overlying Leadville Formation.

Chemostratigraphic data from the Parting Formation display significant variation, with highly negative carbon isotope values being attributed to a restricted depositional environment and/or secondary alteration of these rocks during diagenesis. A distinct positive trend is observed in the strata from the Broken Rib Member and is similar to the positive-drifting values recorded in previously published carbon isotope curves for the Early expansa standard conodont Zone. This increasing trend is capped by very negative values in the lower Coffee Pot Member strata that are locally derived from secondary alteration due to the existence of a thick paleokarst interval (48.68-51.70 m) at this horizon. Immediately above this paleokarst, the d13C values begin a very distinctive positive excursion that ends in highly positive values, ranging from 4.1-5.3 per mil (VPDB). This large positive excursion is interpreted as representing the late Famennian Hangenberg Excursion. Such a correlation would imply that the paleokarst interval from 48.68-51.70 m records a very large and previously unknown unconformity, and this in turn would require the age of the Dyer Formation in Colorado to be much younger, namely Middle to Late praesulcata Zone of the uppermost Devonian.