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Evidence of Episodic Orogenic Activity on the Front Range
RAYNOLDS, Robert G., Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, CO 80205-5798

Our research on the Laramide synorogenic strata of the Denver Basin confirms the hypothesis that these sediments accumulated as two unconformity-bounded sequences termed D1 and D2. Considerable efforts have gone into trying to refine the age of the D1 sequence. Radiometric dating, palynology, and magnetostratigraphy have all been applied.

The combined evidence from these dating efforts indicates onset of D1 deposition at about 67.5 MY in both the Castle Pines and Kiowa cores. In the Kiowa core, the data, weighted by a nearby radiometric date, suggest termination of D1 deposition at about 64 MY. The D1 depositional event therefore lasted about 3.5 million years. Within this time span, the Front Range was uplifted, stripped of overlying Paleozoic and Mesozoic sediments, and witnessed a flare of andesitic volcanism. The volcanoes were largely removed by erosion prior to the completion of D1 deposition. Sedimentation ceased for up to 6 or 8 million years and the Denver Basin Paleosol was formed.

The D2 Sequence: Renewed uplift of portions of the Front Range: Much less control exists to constrain the age range of the D2 sequence. A single dated volcanic ash, some pollen analyses, a limited megaflora, and a
fragmentary Coryphodon tooth all suggest an early Eocene age for D2. The duration of the sediment pulse is unknown, although comparisons to D1 suggest that it too may have been relatively short-lived.

There remains a logical step to infer that the Front Range orogenic activity was directly synchronous with the observed sediment accumulation record. If this step is taken, we document a startlingly episodic character for Laramide deformation in the Front Range.