Rising abruptly from the plains east of Interstate 25, between Colorado Springs and Pueblo, Colorado, are cone-shaped hills of limestone and shale known as the Tepee Buttes. These distinctive features formed from carbonate precipitation around spring vents on the sea floor during the Late Cretaceous Period — between 75 and 76 million years ago (Kauffman et al., 1996). The Tepee Buttes, ranging from a circular form to a more common elliptical shape, can be up to 60 meters wide and rise as high as 10 meters above the plains. A limestone core or pipe in the center supports each butte. The central core of each limestone pipe is vuggy (full of holes) and generally no more than a few meters wide.
|Figure 1. View of Tepee Buttes. These features are aligned along early Laramide faults that parallel the Front Range. Image by Clyde Hoadley, used with permission.|
Fault zones control the placement of the buttes, with butte fields commonly aligned in clusters along block faults or fracture zones formed during the Laramide uplift (Howe and Kauffman, 1985). The faults and fracture zones created submarine springs and seeps on the seafloor that vented methane and nutrient-rich fluids from the underlying Pierre Shale and Niobrara Formations (Arthur et al., 1982). Methane gas also bubbled out along some of the fractures and made the long ascent to the surface of the vast Cretaceous Interior Seaway.
Chemically enriched waters, produced by vents, supported chemosynthetic communites (Collom, pers. comm.). Mats of oxidizing bacteria, living on chemical energy contained in compounds such as methane and hydrogen sulfide, blanketed the sediments around the vents (Howe and Kaufman, 1985). Aerobic bacterial oxidation of methane is thought to cause carbonate precipitation and lithification of the mounds (Kaufman et al., 1996). The fossils associated with the Tepee Buttes indicate these structures were formed at a depth of 30 to 100 meters (Kaufman 1967; Howe 1987). These springs were intermittently active over a period spanning 1.25 million years (Kaufman, 1984).
|Figure 4. Aerial photograph of a Tepee Butte. Small format aerial photography (SFAP) provides low-height, large-scale imagery using a lightweight 35mm camera format. Photo date 11/00 by S. Veatch|
|Figure 5. Low-oblique view of Tepee Buttes aligned along a fault. The airplane used for the project was a Cessna 172 P, flown over the site high enough to capture the target in a single frame. Photo date 11/00 by S. Veatch.|
|Figure 6. Aerial overview of Tepee Butte field in southern El Paso County. Photo date 11/00 by S. Veatch|
A repeating pattern of rocks and fossils developed around the limestone mounds. The vent core rocks contain few fossils and are a vuggy limestone formed from carbonate mud with fecal pellets from marine organisms. Fossilized tube worms are attached to rocks, just as with modern tube worms around present-day seafloor vents. Tube worms, with their feather duster-like appendages, lived in tiny calcareous tubes near the vents. The tube worms adapted to the mineral-rich waters, formed a symbiotic relationship with bacteria, and thrived. Only their hollow tubes remain in the Buttes.
A dense limestone coquina of shells occurs around the vuggy core of the limestone pipe.
|Figure 7. Nymphalucina occidentalis is the dominant fossil around the vent core. Horizontal length is 3.2 cm. Image courtesy of Wayne Itano, used with permission.|
Nymphalucina occidentalis, a fossil lucinid bivalve, is the dominant fossil around the vent core. This marine creature thrived in conditions around the margin of the methane vents. Modern lucinids are not known to occur in such high density populations, making this occurrence unusual (Kaufman et al., 1996).
A richly fossilferous limestone, containing different kinds of molluscs, adjoins the coquinas (Kaufman et al., 1996). These massive deposits, which hold up the core, also contain Inoceramus, a genus of large and somewhat flat Mesozoic pelecypods (clams). These clams had a distinctive shell with concentric wrinkles. Inoceramus went extinct near the end of the Cretaceous.
|Figure 8. This baculite (uncoiled ammonite) was found in the lower part of a Tepee Butte. This specimen, from the Rudy Weber collection, measures approximately 5 cm. Image by Mike Estlick.|
|Figure 9. A trombone-shaped ammonite, Solenoceras sp., coils back on itself. The part that coils back, like a hairpin, is missing. Image by Mike Estlick.|
Flank breccias, the most distant rock from the core, formed from marine cements and slump block material. Bacterial mounds and tube worms are also found here. The fossil remains of foraminifera and radiolaria—single-celled organisms—occur in the rocks forming the buttes (Howe and Kauffman, 1985). The Pierre Shale, containing only a few fossils, surrounds the vents, thus the mounds were a type of "oaisis" on the sea floor.
Tepee Buttes are carbonate mounds formed around methane springs and vents on the seafloor of the Cretaceous Interior Seaway. The buttes are aligned in linear fields along Laramide faults.
At the center of each butte is a vuggy limestone core, partially filled with cements. A large Nymphalucina occidentalis paleocommunity surrounds the limestone core that supports the mound. The flanks of the mound hosted a diverse paleocommunity of bacteria, algae, and molluscs. The Pierre Shale, with few fossils, surrounds the Buttes. Few ancient vent sites have been studied, as they are relatively rare. The recognition of Tepee Buttes in Colorado as vent sites provides paleontologists the opportunity to study paleocommunities that are found around these ancient methane spring deposits.
This paper stems from a field trip through the Fossil Study Group of the Colorado Springs Mineralogical Society. The first trip was made April 8, 2000 under the direction of John Harrington. A number of subsequent field trips have been made to this site. I thank Christopher Collom, Mount Royal College, Calgary for informative discussions at the field site and Beth Simmons, Metropolitan State College, Denver for valuable reviews of the manuscript.
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