PROVENANCE AND GEOCHEMISTRY OF LARAMIDE VOLCANIC PEBBLES IN THE DENVER BASIN

Three stratigraphic units of the Upper Cretaceous and Paleocene Dawson Formation in the southwestern part of the Denver Basin contain volcanic pebbles nominally described as andesite. Recent mapping, supported by petrology, conserved element geochemistry, and paleocurrent measurements, indicate three sources for input of volcanic material into the basin. Volcanic input began in the late Upper Cretaceous with basaltic andesite introduced to the basin from the southwest in the vicinity of present-day Colorado Springs. Later, in the latest Upper Cretaceous and Paleocene, pebbles from a more differentiated volcanic source (basaltic andesite through dacite) were introduced into the southern end of the basin from an unidentified southern source. At roughly the same time, more primitive andesites were introduced from a western source in the vicinity of present-day Golden and Lakewood. The provenance and geochemistry of the volcanic component of the Denver Basin Laramide sediments are useful both as a stratigraphic tool and as a glimpse into the Laramide history of the Front Range from which these volcanics were eroded.