Reinterpretation of the Ignacio and Elbert Formations as an Incised Valley Fill Using Facies Analysis and Sequence Stratigraphy; San Juan Basin, Southwest Colorado

• Main Author: Maurer, Joshua Thomas
• Format: Publication
• Language: English
• Published: Bowling Green State University / OhioLINK 23.07.2012
• Subjects: Colorado; Devonian; Elbert Formation; Facies Analysis; Ignacio Formation; Incised Valley Sequence; San Juan Basin; Sequence Stratigraphy

The 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 Formation-Ouray Limestone as Devonian (Famennian) in age and finds that the succession represented a laterally continuous depositional system of fluvial and estuarine environments (Ignacio Formation), prograding shoreline environments (McCracken Sandstone Member, Elbert Formation), tidal flats and shallow marine shales (Upper Member, Elbert Formation), and marine limestone and shales (Ouray Limestone).The present study is based on lithofacies analysis, 137-m of measured stratigraphic sections at 11 outcrops, provenance analysis from 24 representative thin sections, 72 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 variations in thickness, facies distribution, 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 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 by fluvial - estuarine sediments and later overtopped by marine sediment.