CONSTRAINING THE ORIGIN OF CLIMACTICHNITES FOUND IN FLOAT AND POSSIBLE IMPLICATIONS FOR THE IGNACIO FORMATION, SOUTHWESTERN COLORADO, USA

In 2019, a slab of sandstone containing a Late Cambrian trace fossil, Climactichnites, was found in float near outcrops of the Tamarron Member of the Ignacio Formation at Electra Lake in southwestern Colorado. This is problematic, as the Ignacio Formation has most recently been proposed to be of Devonian age, after over a century of debate. The goals of this study were to take advantage of unprecedented low lake levels to document newly exposed stratigraphy, to use these stratigraphic data to establish a depositional history of the lower portion of the Tamarron Member, and to determine if the fossil-bearing slab is indeed derived from the Ignacio Formation at Electra Lake. From three outcrops, 36m of stratigraphic data were collected and compiled into a stratigraphic section. Petrographic analyses of thin sections cut from Tamarron Member strata were performed to characterize lithologies. Lithofacies groups were established based on lithology and sedimentary structures, and macrofossils and trace fossils were identified to help constrain depositional settings. A distinctive marker bed containing Skolithos burrows was used to correlate the three outcrops.

The lower portion of the Tamarron Member of the Ignacio Formation at Electra Lake is interpreted as a wave and tide influenced shoreline deposit, a facies that is not dissimilar to the tidal flat deposits in which Climactichnites is found. Trace fossils, such as Skolithos, Planolites, and Gordia are common. Poorly preserved phosphatic brachiopods were equivocally identified as Dicellomus, Lingullella, or Obolus. Sericite cement found in the Climactichnites-bearing slab and several lithologically similar samples may be related to a mineralized fracture that crosscuts several beds of the Tamarron Member and contains euhedral barite and weathered sulfides.

No further specimens of Climactichnites were found. However, similarities in lithology, petrographic similarities such as a two-phase diagenetic history, and similarities in depositional environments between the Climactichnites-bearing slab and the Tamarron Member of the Ignacio Formation suggest it is plausible that the fossil may have originated from Electra Lake, and that the Devonian age of the Ignacio Formation may therefore require revision.