Unroofing of the Southern Front Range, Colorado: A View From the Denver Basin
KELLEY, Shari A., Dept. of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801; RAYNOLDS, Robert G., Denver Museum of Nature and Science, 2001 Colorado Blvd., Denver, CO 80205

Recently, zircon fission-track dating of detrital grains in sedimentary basins has been used to constrain the exhumation history of mid-crustal rocks in rapidly deforming orogenic belts such as the Olympic Mountains in Washington and the Alps in Europe. In this study we use both apatite and zircon fission-track analysis of detrital grains in synorogenic sediments in the Denver Basin to examine the unroofing history of a mountain range that cooled over a more protracted interval and experienced less overall denudation compared to these more active belts.

Eleven core samples from Kiowa 1, a research well drilled jointly by the Denver Museum of Nature and Science and the NSF in the Denver Basin, were examined. Kiowa 1, which is ~670 m deep, penetrates two packages of synorogenic sediments (D1 and D2), the Arapahoe and Laramie formations, the Fox Hills Sandstone, and a sandy interval at the top of the Pierre Shale. Paleocurrents from nearby surface outcrops suggest that the synorogenic sediments were derived from the south end of the Front Range.

The general sequence of rocks that existed on the Front Range prior to Laramide deformation is well know from exposures in the hogbacks along the southeastern and eastern margins of the range. Ordovician to Cretaceous or Pennsylvanian to Cretaceous sedimentary rocks overlie Proterozoic basement rocks. Volcanic rocks that originated from Cretaceous to Paleocene-aged intrusive centers along the Colorado Mineral Belt likely covered most of the mountain range shortly after deformation began. In addition to the sedimentary record, another marker is preserved in the basement of the southern Front Range. Previous apatite fission-track (AFT) work has revealed the presence of an apatite partial annealing zone (PAZ) in the basement, which separates AFT ages >100 Ma from those with cooling ages of 50 to 70 Ma.

Consequently, the expected trends in detrital zircon and apatite fission-track (FT) age populations upsection through a synorogenic package would be as follows: (1) the Phanerozoic cover should contribute recycled zircon and apatite from within the PAZ to the oldest sediments as deformation began; (2) a large influx of volcanic grains in both the detrital zircon and apatite FT age populations should be recorded as volcanoes dominated the landscape; (3) the middle part of the sequence should contain a mix of volcanic grains, recycled grains from the Phanerozoic cover, and perhaps a minor basement component; and (4) the younger portion of the sequence should contain primarily metamict zircon grains and 50 to 70 Ma apatite grains derived from the Proterozoic basement below the base of the PAZ.

In general, these are the trends that we observe in our data, although, not surprisingly, there are complexities superimposed on the simple pattern. This study demonstrates the power of analyzing both apatite and zircon when examining detrital grains derived from a basement cored uplift where basement resided at shallow levels of the crust (~4 km) prior to deformation.