Paleoclimate interpretation from a petrographic comparison of Holocene sands and the Fountain Formation (Pennsylvanian) in the Colorado Front Range

In at least four areas of the Colorado Front Range remarkable similarity exists in the detrital quartz populations in the Fountain Formation and Holocene sand from modern day streams draining Precambrian crystalline rocks and located close to the Fountain outcrops. Near Colorado Springs and Boulder both the Fountain and Holocene quartz populations were derived principally from granitic source rocks; in the vicinity of Loveland and Golden metamorphic rocks provided most of the quartz. Because both the Fountain sandstone and Holocene sand in these four areas came from common parent rocks and accumulated under similar conditions, plots of frequency percent resistant framework grains as a function of size for the two deposits can be used to compare Pennsylvanian and Holocene climates for the east flank of the Front Range following the approach of Young et al. (1975). In all four areas such comparisons suggest that the Fountain was deposited in a climate substantially more humid than that which has existed during the Holocene. A humid climate is consistent with the inferred near equatorial, southern hemisphere location of the Ancestral Rocky Mountains during the Permian and Pennsylvanian. An orographic effect would have been produced as warm, moist southeasterly trade winds, blowing off the mid-continent sea, ascended the Ancestral Rockies. Precipitation would have been concentrated on the east flank, but more arid conditions would have prevailed in western Colorado on the opposite flank.