ANETH - UTAH, USA
FEB 10TH 2020
The area around Hovenweep has been affected by the Laramide Orogeny 70 - 40 million years ago, which was the last major mountain building event that affected the Rocky Mountains. Hovenweep is located in the Blanding Basin which was folded during the Laramide Orogeny and is sloping gently to the Southwest. The Monument Uplift lies to the west and the Defiance Uplift to the South, both are Laramide age and border the Blanding Basin.
The Sleeping Ute Mountains are visible on the Horizon of Hovenweep National Monument. They were formed by a laccolith igneous intrusion, which was injected in-between layers of sedimentary rocks. This can only happen if the magma volume is high enough to fold the strata above, this gives the intrusion a dome-like shape. The igneous rocks were intruded during the Tertiary period 66 million years ago, possibly even younger.
The McElmo Dome is thought to be igneous in origin, igneous rocks were found at the bottom of an oil well drilled during the 1960s. The rocks uncovered in the McElmo Dome were very similar to the Ute Mountains, although the dome lies several hours North. The McElmo Dome is also associated with the Paradox Salt Embayment and may have influenced salt movement in Arches National Park. Faults in the area are associated with the Ute Mountains and the McElmo Dome, most are steeply dipping Normal Faults but none are visible within the National Monument.
The Hovenweep Ruins with the Sleeping Ute Mountains in the Distance
Hovenweep is in the centre of a network of thirteenth-century communities and most likely started out by seasonal use before year-round settlement towards the end of its lifetime. The Hovenweep ruins are apart of the larger Mesa Verde archaeological region built over the years by the Ancestral Puebloans, the area was once populated from 900 A.D to the late1200 A.D (10,000 Years ago) and supported around 2,500 people, although the ruins were only built around 2,000 years ago when the area was permanently settled.
The ruins of the village are located within a network of farming communities in a arc of sustainable farming areas as the soil was and still is ideal, this arc is located between 5,500 and 7,200 feet in elevation. Once the elevation band drops below 5,500 feet the rainfall average is too low to sustain any type of dry-farming, above the 7,200-foot mark the growing season is far too short to sustain any farming at all.
The Morrison Formation exists under the Burrow Canyon Formation but is less noticeable as it's older and lies lower in the sequence of strata. The formation consists of shale and clay, it is well known for its Bentonite which is a mix of altered volcanic ash and clay. The Morrison Formation was deposited in the Late Jurassic and is split into two members, the Brushy Basin Member and the older Salt Wash Member. The younger Brushy Basin Member is the only one visible in the Monument, it's around 150 million years old. The Member is a mix of altered volcanic ash, the clay was most likely formed from ashfall which has not been transported or reworked. The ash most likely originated in Utah, the exact location is unknown.
Burrow Canyon Formation
The Burrow Canyon Formation was deposited during the
Cretaceous 136 to 100 million years ago, it is comprised of a mix of Green shale, conglomerate, Mud-rock and Sandstone. Pebbles of Chert and cobbles are interbedded within the layers with solidified Limestone and Quartzite. Surprisingly no fossils have been found within the formation.
The Burrow Canyon Formation is generally 114-180 feet thick (35-55 meters). The formation was deposited in a braided stream system that once flowed toward the Western Interior Seaway from the East, the sandy braided bars moved across the landscape laterally and deposited most of the formation. The shale was formed by abandoned backshore swamps as the stream system moved across the landscape.
The Dakota Sandstone caps the Cajon Mesa which is what the Hovenweep ruins were built on. The formation was deposited during the closure of the Western Interior Seaway during the late cretaceous 108 to 94 million years ago, the middle layer of the unit consists of shale while the upper unit consists of Evenly-bedded Sandstone. The past environment that deposited the upper sandstone was once an ancient shoreline, it is possible that it was once a receding Mainland beach or possibly a Barrier beach system.
The lower/middle portion of the formation was most likely formed from marine clastic sediments, and stream channel systems. The fluvial deposits were formed in brackish waters as indicated by the local fauna of the time period and location.
Slope failures are very common within the canyon, as the Dakota Sandstone overhangs collapse and fall into the canyon below. The canyon has widened over the years as the Dakota Sandstone is undermined by wind and water erosion. The Brushy Basin Member of the Morrison Formation is prone to collapse, especially when it's located on a slope.
Alluvial Fan deposits are located under the overhangs and are comprised of clastic sediment. The alluvial fan's in the canyon accumulate over time after every collapse and continue to expand as more sediment accumulates.
The most estranged ruins in the canyon are the Square Tower Ruins which were built on a bolder of collapsed Dakota Sandstone. It's a three-story building balanced on an eroding bolder next to a stream, the rock is eroding relatively fast as honeycomb weathering is visible in the poorly cemented sandstone.
Square Tower Ruins located in the lower portion of the canyon
Collapsed Dakota Sandstone overhang under the Hovenweep Castle
Geologic Resource Evaluation Report
Natural Resource Report NPS/NRPC/GRD/NRR—2004/002
National Park Service U.S. Department of the Interior
Natural Resource Program Center
ELMER S. SANTOS
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New Mexico Geological Society
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V. H. Zabel
Four Corners Geological Society
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Dcyartnlent of Geology, Bringham Young University, Berkeley geochronology Center, Geological Survey Federal Centre, Geological Survey National Center
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Published: January 01, 1991
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