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

Remote sensing, extensive field studies and petrographic analyses have revealed a previously unknown cluster of impact structures in the Colorado Front Range Mountains. Eight circular structures ranging from 10 km to 30 km in diameter have been identified in the Front Range west of I-25 between Denver and Colorado Springs. Shock metamorphic features definitive of an impact origin are present within each structure. Each impact structure is 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 11 08/15 11, Lost Creek 12 39/26 52, Lake George 00 34/19 34, Perry Park 17 29/00 45, and Bear Creek 38 35/13 50.

Shatter cones are present within each circular structure and range from less than one cm to more than 18 m long. 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 cones are smaller parasitic ones formed in large muscovite phenocrysts on the mega cone fracture surface.

Petrographic analyses of samples from the structures indentified a variety of microscopic scale, shock-metamorphic forms including diapletic glass, impact melt glass, and Ballen quartz. Parallel deformation lamellae in quartz grains are developed in samples from several structures and are consistent with and strongly indicative of PDFs.

Further impact evidence includes kink bands in feldspar, macro and micro breccias, and pseudotachylite. Crater fill materials are preserved within the Lake George impact structure and consist of a glassy impact breccia and an overlying clast-rich melt rock. A prominent ringed magnetic anomaly is associated with the 30 km diameter Buffalo Creek impact structure.

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. Several impact structures are cut by Laramide faults (K- Eocene) giving a minimum age limit for the impact event.