CARBON ISOTOPE STRATIGRAPHY OF THE BASAL EL PASO GROUP AT CABLE CANYON, NEW MEXICO

Carbon isotope ratio (d¹³C) stratigraphic variation was determined from the Sierrite and Bat Cave members of the Lower Ibexian-aged El Paso Group at Cable Canyon, Caballo Mountains, New Mexico. Trilobite and conodont biostratigraphic zonations provide the basis for detailed correlation of d¹³C variations to key Lower Ibexian-aged sections in western Utah and Colorado. These correlations permit several observations to be derived from the Cable Canyon section.

Comparison of the Cable Canyon d¹³C stratigraphy to the well-studied Lawson Cove section (western Utah) suggest a nearly complete- but highly attenuated- stratigraphic succession from the basal Ibexian Cordylodus proavus conodont zone to near the base of the Rossodus manitouensis zone. Depositional rate appears to increase rapidly at this level, in conjunction with a sharp negative d¹³C signature. After a brief rise to d¹³C ~0 permil, d¹³C values drop in a broad negative excursion of ~1 permil within or near the Paraplethopeltis trilobite zone, where they begin a rapid 1 permil rise. The studied interval terminates just above the end of this rise.

The completeness of the d¹³C profile in this section provides an estimate for the duration of a hiatus within the Manitou Formation in the Front Range, Colorado. There, d¹³C results indicated that deposition resumed in conjunction with a rapid 1 permil rise, followed by a sharp fall in conjunction with the end of the R. manitouensis zone. However, the duration of the hiatus was unconstrained because no continuous (or near-continuous) d¹³C profile had been determined for the entire R. manitouensis zone. The new d¹³C profile from Cable Canyon strongly suggests that the Front Range Manitou Formation hiatus lasted for most of the R. manitouensis zone, with deposition resuming at approximately the level of the trilobite Paraplethopeltis. This observation provides important insights into the influence of eustatic sea level change and regional development in the southern Transcontinental Arch area during the Early Ibexian.