ZIRCON LASER ABLATION CHRONOLOGY HELPS REFINE PALEOGENE STRATIGRAPHY IN THE SOUTH DENVER BASIN, COLORADO

Zircon laser ablation results from twelve locations in the South Denver Basin assist the stratigraphic interpretation of Paleogene and younger strata. The data supports Castle Rock Conglomerate and Larkspur lithofacies (Koch, 2013) derived from two Front Range provenance areas, one to the west (Pikes Peak Batholith, 1.08 Ga) and one to the north (Routt Plutonic Suite (1.7Ga) and Berthoud Plutonic Suite (1.4Ga). Drainages from the west and north combined to deposit both the Castle Rock Conglomerate and the Larkspur conglomerate during the late Eocene (42-34 my). Zircon histograms from the West lithofacies contain dominantly Pikes Peak age zircons, those from the North lithofaces are characterized by bimodal 1.4 and 1.7Ga peaks with minimal Pikes Peak input. Other samples are clear mixes from both provenances.

The 30-70my zircon histograms of samples from the poorly consolidated conglomerates in Douglas County thought to be younger than Castle Rock Conglomerate (Ogallala?) showed no evidence of zircons younger than Wall Mtn Tuff (37my). The Ogallala Fm at Cedar Point near Limon contained the youngest zircon ages (31-28 my). A single sample from the topmost Dawson D1, just below the Denver Paleosol, showed zircon ages of 66-56my. Almost all paleogene intrusives of this age range lie in the Front Range west and north of Denver.