THE COLORADO CATACLYSM: AN IMPACT CRATER CLUSTER IN THE FRONT RANGE

Remote sensing and extensive field studies have revealed a previously unknown cluster of impact structures in the Colorado Front Range Mountains. Seven circular structures ranging from 10 km to 30 km in diameter were identified with USGS LIDAR as well as Google Earth satellite imagery in the Front Range west of I-25 between Denver and Colorado Springs. Shatter cones were found at each site and confirm an impact origin for the circular structures. Each impact structure was named for a nearby geographic feature and are as follows (with lat./ longs. in 39N 105W): Aspen Park (35 54/17 17), Deer Creek Canyon (30 60/08 33), Buffalo Creek (08 12/16 37), Goose Creek (08 12/19 38), Lost Creek (12 39/ 26 52), Lake George (00 34/19 34) and West Creek (11 08/15 11).

Shatter cones range from less than one cm to more than 18 m long. Most are developed in coarse-grained granite, granite pegmatite and vein quartz. Mega shatter cones ranging from 10 m to over 18 m long were discovered at one site in the Aspen Park impact structure. The most unique and best preserved cones are smaller parasitic cones formed in large (>3 cm) muscovite phenocrysts on the mega cone fracture surfaces.

Petrographic analyses of samples from each site have indentified multiple sets of parallel deformation lamellae in quartz grains that are consistent with and strongly indicative of PDFs. Most lamellae are highly decorated. Further evidence commonly associated with impact structures has been observed and includes kink bands in muscovite, macro and micro breccias, pseudotachylite, and fracture cleavage in quartz. A prominent ringed magnetic anomaly correlates with the 30 km diameter Buffalo Creek impact structure.

Minimum diameters of the impact structures indicate that they are complex craters. The impact structures are clustered and overlap one another with at least one portion of their rims crosscutting and terminating the ringed character of the adjacent structure. Clustering strongly suggests that the impacts were essentially simultaneous and therefore formed during the same impact event. A maximum age of the impact structures is established by shatter cones found in sandstone of the Cambrian Sawatch Formation at the northwest end of the Woodland Park graben. Several impact structures are cut by Laramide faults (K- Eocene) giving an upper age limit for the impact event.