PRESERVED SEDIMENTARY STRUCTURES IN THE PROTEROZOIC COAL CREEK QUARTZITE AS INDICATORS OF TECTONIC BASIN DEVELOPMENT, CENTRAL FRONT RANGE, COLORADO: A MULTI-DISCIPLINARY APPROACH

The early Proterozoic (~ 1.8 Ga) schists and gneisses, popularly but incorrectly known as “Idaho Springs Formation”, of the central Colorado Front Range have been interpreted as metamorphosed units of a volcanic arc sequence in the accretion of arc terranes onto the Wyoming craton. Recent and current research is attempting to identify individual arc sequences and sequence boundaries of the Colorado Province in order to understand the assembly process. Quartzite units are rare within the accreted province, but may hold important clues to interpretation of basin tectonics and definition of sequence boundaries.

The Coal Creek Quartzite is a metamorphosed unit of sandstone with intercalated conglomerate and shale layers. A multi-disciplinary approach combines a team of a metamorphic petrologist and a sedimentologist/stratigrapher in order to better to examine and interpret this important unit. Numerous primary sedimentary structures are preserved in spite of metamorphic overprint, such as cross-bedding, graded bedding, channels, scour surfaces, and imbricated pebbles. Initial analysis suggests interpretation of protolith as braided stream, channel, and possible debris flow deposits. Deposition appears to have occurred in an asymmetric basin. This interpretation changes our view from an idealized to a more realistic model of tectonic basin development in the central Front Range. This may result in the identification of tectonic boundaries in the Colorado Province.

The early Proterozoic Tarkwaian Group of West Africa has many similarities to the Colorado central Front Range. Similar metasediments in West Africa occur in asymmetric basins and are interpreted as half-grabens formed in a back-arc basin extensional regime. Multi-disciplinary and comparative studies will prove important to the understanding of these and other Precambrian terranes.