A little east of Buttes, Colorado, just off I-25, rise cone-shaped hills that resemble giant anthills. These hills, comprised of limestone and shale, are called Tepee Buttes.
These distinctive features formed around 75 million years ago from carbonate precipitation around hot water vents on the sea floor during the Late Cretaceous Period (a period spaning from 100 to about 66 million years ago.) At the time almost all of Colorado was covered by a relatively shallow sea although at its greatest depth in the Denver or Colorado region it was 600 feet deep.
The ocean which existed here is referred to as, variously, the Cretaceous Western Interior Sea, the Western Interior Seaway, or just the Cretaceous Sea. It extended from the Arctic to the Gulf of Mexico, covering nearly all of Colorado and Kansas, as well as eastern Utah. It would exist, as a sea, until around 70 million years ago, when it receded.
Over this period of some 30 million years marine deposits in the sea compacted to form what is now called Pierre Shale, 8,000 feet of hardened ocean-bottom ooze and mud. Faults and fracture zones in the underlying formations created submarine springs and seeps on the seafloor that vented methane and nutrient-rich fluids. Methane gas also bubbled out along some of the fractures and made the long ascent to the surface of the vast Cretaceous Interior Seaway.
The chemically enriched waters were used by certain bacteria to synthesize carbohydrates from carbon dioxide and water. Mats of oxidizing bacteria, living on chemical energy contained in compounds such as methane and hydrogen sulfide, blanketed the sediments around the vents. Aerobic bacterial oxidation of methane is thought to cause carbonate precipitation and lithification (the processes by which loose sediment is hardened to rock) of the mounds. The fossils associated with the Tepee Buttes indicate these structures were formed at a depth of from about 100 feet to 300 feet. The springs were intermittently active over a period spanning 1.25 million years.
A repeating pattern of rocks and fossils developed around the limestone mounds. The vent core rocks contain few fossils and are a type of limestone that contains small cavities, often with a mineral lining of different composition from that of the surrounding rock The core, generally no more than a few feet in diameter, 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.
To claim a find please e-mail me (do not post them in your log) the answers to the following questions:
1. How many buttes do you see from the posted coordinates? (Don't forget to look behind you.)
2. Look to the southeast and locate the tallest butte - how high do you think this one butte rises above the surrounding land?
3. Look at the rocks on the ground around you at the posted coords and find the darker ones, and the whitish ones - what is the difference in texture between the two? Which rock would you say is shale, and which is limestone? What suggests this area was once under a vast inland sea?
4. Optional: post a picture of yourself with your GPSr.
References:
Russell Shapiro,Henry Fricke, Tepee Buttes: Fossilized methane-seep ecosystems, Colorado College Document
Neal L. Larson, Jamie Brezina,Neil H. Landman,Matthew P. Garb and Kimberly C. Handle, Hydrocarbon seeps: unique habitats that preserved the diversity of fauna in the Late Cretaceous Western Interior Seaway, Geological Society of Wyoming Field Guide 2013. Publication
Kauffman, E.G., Arthur, M.A., Howe, B., and Scholle, P.A. 1996. Widespread venting of methane-rich fluids in Late Cretaceous (Campanian) submarine springs (Tepee Buttes), Western Interior Seaway, U.S.A. Geology 24: 799-802 Abstract