Hartsel Geology

By Donald McGookey, Author of Geologic Wonders of South Park, Colorado

Driving through Hartsel, the area appears to be a simple geologic setting. But if you study of the surrounding hills and the available subsurface data, they tell a much more interesting and unique story.

Hartsel is located at an elevation of 9000 feet above sea level near the center of South Park. The town lies just east of a sandstone ridge in the middle of a very large (40 miles EW by 60 miles NS) intermountain basin. South Park is one of a string of four broad intermountain valleys down the middle of the Colorado Rocky Mountains. They include North Park near the Wyoming State line, Middle Park centering around Kremmling on the Colorado River, South Park and Alamosa Valley that straddles the New Mexico line around. All of these parks are surrounded by high mountains; in South Park's case, the mountains are the Collegiate and Mosquito Ranges on the west, the igneous intrusive mountains along the Continental Divide on the north, the Front Range on the east and the Thirty-Nine Mile volcanic pile on the south.

The South Park fault, which bisects the Park just east of Reinecker Ridge, is a continuation of a fault that extends from Breckenridge on the north, to south of the Arkansas River. It was active during the Early and Late Tertiary, and thankfully, shows no sign of recent movement.

Most basins are simple downwarps of the earth's crust with a bathtub shape. The exposed rock around Hartsel tells a much different story. In the middle of the basin, immediately southwest of Hartsel is a large hill of Precambrian Metamorphic rocks. This type of old rock is usually at the bottom of the basin, not exposed in the middle. Thus, you might notice looking at the following map that in reality, South Park Basin is actually two basins. There is a Paleozoic basin west of Hartsel and a Late Cretaceous -early Tertiary basin to the east. The Mesozoic sands and shales (yellow and black on cross section) are the eroded remnants of a blanket-like sequence of non-marine and marine clastic rocks deposited over the entire area.

Around Hartsel the rocks overlying the Precambrian rocks include the Mesozoic age Entrada Sandstone, Morrison Formation or Dakota Sandstone with no indication of Paleozoic rocks here or east of Hartsel. Traveling west of the Precambrian exposures, around Antero Junction (US 24 and US 285), the basin is very deep. There is about 16,000 feet of Late Paleozoic age Maroon Formation underlain by thin sections of Cambrian though Mississippian early Paleozoic rocks. A cross section of the basin tells the story.

Next, on the following map, you see Late Paleozoic uplifts and basins (troughs) [from the Rocky Mountain Association of Petroleum Geologists, Geologic Atlas (1972)]. It shows the hill southwest of Hartsel to be low on the west flank of a low relief mountain range. (F=Fairplay, H=Hartsel) The Maroon Formation's red sediments, like the sandstones of the Red Rocks Park west of Denver, are stream deposits from rock fragments eroded from these mountains. Now comes the hard part to visualize! The western half of these old mountains had to be downwarped over 16,000 feet to form a deep basin in early Tertiary time, while the Front Range was being uplifted!

The middle Tertiary part of the story begins with the area having been eroded to very low relief and the basin drainage flowing south to the Canon City area. Then a series of volcanoes formed along the south side of the basin culminating in the large (16 mile diameter) Guffey Volcano. The extrusive rocks from these volcanoes dammed the rivers and formed a large lake over most of South Park. A new South Platte River draining this lake cut a canyon across the Front Range.

The extrusive rocks were mainly mud flows (lahars) carrying large blocks of lava. The northern end of preserved flows is along the east side of Highway 9 just 3 miles southeast of town (20 miles from the volcano). The large blocks originated as volcanic bombs blasted high the air, which landed high on the flanks of the volcano and mixed with ash. With the next heavy rain, the combination of bombs and ash would flow rapidly down slope for many miles. These deposits can be seen in road cuts almost all the way from Hartsel to Guffey.

Here are a few more photos of the area's geological wonders:

For a more detailed description of the geology of this area see:
Geologic Wonders of South Park, Colorado, 2001, by Donald P. McGookey. (Note: This book is out of print, but is available at libraries and universities.)
Geology and Mineral resources of Park County, Colorado: 2001 L. A. Scarbrough, Colorado Geological Survey Publication RS-40. Web: dnv.state.co.us/geostore. $30.00 Excellent geologic map at scale of 1:100,000.
Colorado Geologic Map. Web: dnv.state.co.us/geostore.