REINTERPRETATION OF THE DEVONIAN IGNACIO FORMATION AND ELBERT FORMATION AS AN INCISED VALLEY SEQUENCE: SAN JUAN BASIN, COLORADO

The Cambrian(?)-Devonian Ignacio Formation and the Devonian Elbert Formation of Southwest Colorado represent the lowest succession Paleozoic strata in the San Juan Basin of Colorado and New Mexico. This study reinterprets the Ignacio Fm. as Devonian in age and finds the two units represented a laterally continuous depositional system of fluvial and estuarine environments (Ignacio Formation), prograding shoreline environments (McCracken Sandstone Member, Elbert Fm.), and tidal flats and shallow marine shales (Upper Member, Elbert Fm.).

The present study is based on lithofacies analysis, 137-m of measured stratigraphic sections at 11 locations, provenance analysis from 24 representative thin sections, 77 paleocurrent measurements, and photomosaics. Lithofacies analysis identified 14 lithofacies that are organized into fluvial channel, tidal channel, tempestite, and tidal flat sequences.

This study suggests that the Ignacio and Elbert formations may be reinterpreted an incised valley sequence. Evidence for this conclusion include the conformable contact between the Ignacio Formation and the McCracken Sandstone Member, lithofacies associations and ichnology which confirm the shift from fluvial to estuarine to marine environments, and the apparent onlap of these Paleozoic sedimentary rocks onto Precambrian basement paleohighs. It is suggested that initial accommodation space was provided by paleotopography on the Precambrian basement surface. Later accommodation space was provided by relative sea-level rise, and the paleovalleys were backfilled and later overtopped by marine sediment.