SEDIMENTATION AND TECTONICS OF THE GILA GROUP CONGLOMERATE IN DUNCAN BASIN, GREENLEE COUNTY, SE ARIZONA

The Mid Miocene to Recent Gila Group Conglomerate in the Duncan basin, SE Arizona, records the sedimentation and unroofing history of its bounding structural blocks. Alluvial fan architecture studies of the Gila Group illustrate interfingering relationships between an axial fluvial system, low energy floodbasin deposits, and smaller marginal hanging-wall alluvial fans. The NNW trending basin is uniquely situated at the southeastern margin of the Transition Zone near the triple junction of the Colorado Plateau, Basin and Range, and Rio Grande Rift physiographic provinces. Fault-bounded uplifted structural blocks, common physiographic features of the Transition Zone, promote alluvial fan deposition in adjoining basins. Excellent exposures of alluvial fan deposits in the Duncan basin, having been incised since early Pleistocene by the modern Gila River, permit evaluation of the depositional history that resulted from the tectonic events controlling basin formation.

Forty-one partial stratigraphic sections that span five 7.5’ quadrangles were used to create a lithofacies architecture model of the fan deposits. Lithology, degree of lithification, sedimentary structures, vertical and lateral relations, paleocurrent, and provenance data were integrated to determine numerous lithofacies in the basin. Clast and matrix supported debris flows, debris flow sandstones, streamflood conglomerates, cross-stratified fluvial deposits, tabular sheetflood deposits, floodbasin deposits, loosely consolidated tuffaceous cross-stratified sands, and paleosols are the lithofacies recognized in Duncan basin. Based on distribution and depositional associations, the lithofacies define three depositional units in the Gila Group of the Duncan basin: 1) unconsolidated upper basin fill, 2) axial channelized cross-stratified fluvial deposits, and 3) small marginal hanging-wall fans.

⁴⁰Ar/³⁹Ar dates (22–28 Ma) obtained from 3 volcanic ashes found in the upper basin fill indicate that the ashes were reworked from older volcanics rimming the basin. As such, the reworked ashes were deposited in younger sediments along the margins of one of several playa lakes that formed in the southern part of the Duncan basin during the Plio-Pleistocene.