DIAMICTONS ON INTERFLUVES NEAR THE SUMMIT OF THE FRONT RANGE—ANCIENT TILL OR REMNANTS OF MIDDLE TERTIARY ALLUVIUM AND DEBRIS-FLOW DEPOSITS?

Diamictons cap ridges and fill cols in places along the summit of the Front Range and other northern Colorado ranges. Many of these deposits have been mistaken for till, and they are the reason several early studies concluded that an ancient ice cap, much more extensive than later valley glaciers, existed prior to the cutting of deep canyons. Subsequently, the marine oxygen-isotope record revealed that global ice volume during late Pliocene and early Pleistocene glaciations was less than during most glaciations since 800 ka. Nevertheless, bouldery diamictons continue to be interpreted as till in recent publications, even though they are high above or far beyond the limits reached by Bull Lake and Pinedale glaciers.

In high mountains, glacial, alluvial, and mass-wasting processes all are capable of producing bouldery, matrix-supported, unstratified deposits. However, a glacial origin is least likely for most of the diamictons in question because the spatial relations required for their accumulation and ablation areas are impossible to reconcile with the existing terrain. Although the timing and magnitude of Cenozoic uplift is controversial, none of the hypotheses suggest that the location and altitude of the Front Range Summit were not similar to the present by 2.5 Ma, the time that large ice sheets first formed on the continent. Also, most bouldery diamictons are much thicker than till in ground moraine, and they are much more voluminous than comparable areas of end- and lateral-moraine systems constructed by middle and late Pleistocene valley glaciers. In the Front Range, ground moraine of any age is thin (typically 2-8 m), and few till deposits older than Bull Lake are as much as 2 m thick. In contrast, a diamicton on Niwot Ridge, near the summit of the Front Range in western Boulder County, is about 0.7 km wide, 2.2 km long, and as much as 36 m thick, and a similar deposit on a ridge in the Tenmile Range in southernmost Summit County is at least 1 km wide, 4.5 km long, and 30-40 m thick.

The diamictons in question probably are remnants of middle Tertiary (Oligocene and Miocene) alluvium and interbedded debris-flow deposits on valley floors that were abandoned and isolated in the landscape when deep canyons were incised in late Tertiary and Quaternary time.