of Episodic Orogenic Activity on the Front Range
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.
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.
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.