Salt Valley - Arches National Park EarthCache
Salt Valley - Arches National Park
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Would you think that salt is responsible for all beautiful arches in this park? If you answered yes, you are correct. Salt was very important at the beginning of process leading to formation of most features you can see here today.
The park lies atop an underground salt bed, which is basically responsible for the arches and spires, balanced rocks, sandstone fins and eroded monoliths that make the area so beautiful. Thousands of feet thick in places, this salt bed was deposited across the Colorado Plateau when a sea flowed in the region and eventually evaporated. Over millions of years, the salt bed was covered with residue from floods and winds and the oceans that came and went at intervals. Much of this debris was compressed into rock. At one time this overlying layer of rock may have been more than a mile thick.
Salt Valley Formation
Sandstones and shales bury the salt-rich Paradox Formation. The layers are parallel until pressure change at depth causes the relatively plastic, buoyant salt to begin to flow upward through the rock column. The sedimentary strata above the salt are deformed into an anticline. Extensional fractures and normal faults develop along the crest and limbs of the anticline and allow water to seep downward through the cracks. When groundwater comes into contact with the salt, the salt dissolves. With dissolution, the overlying strata collapse and a graben valley is formed bounded by a normal fault.
The strata in the hinge zone of the collapsed structure roll over into the graben, and large joints form to accommodate the movement. This process is illustrated along parts of Salt Valley, especially in the area of Fiery Furnace.
Salt dissolution and salt flowage continue today. Unequal loading on the salt beds will cause salt to flow. Salt flowage may seal fractures, “heal” fault planes, fill in voids created by previous salt dissolution, and cause bulges to form at the surface. Because salt movement is slow, groundwater usually dissolves the salt before it reaches the surface. Today, the Colorado River controls salt dissolution by controlling the depth to which it allows fresh water to reach the salt, which, in turn, is controlled, by the depth to which the river has cut its canyon.
Paradox Formation
The Paradox Formation within the Pennsylvanian Period (286- 320 Ma) comprises the oldest rocks exposed in Arches National Park. In general, the Paradox Formation is made up of a series of cyclic deposits containing anhydrite (found in evaporite deposits), silty dolomite, black shale, and halite (table salt). Groundwater dissolves salt near the surface so that the Paradox Formation exposed in Arches consists of a cap rock of contorted gray shale, carbonaceous shale, some gypsum beds, and thin limestone beds that have remained in place following the dissolution of interbedded salts. This cap rock overlies thick salt deposits of anhydrite, halite, and other types of salt that have not been exposed to intense groundwater leaching. The Paradox Formation is more than 3,048 m (10,000 ft) thick in some areas of the park such as under Salt Valley, Cache Valley, and Moab Valley. Individual salt beds can be more than 274 m (900 ft) thick in areas where the Paradox Formation is very extensive. While the Paradox Formation salt underlies large areas of Arches, only the cap rock is exposed at the surface. In the middle of Salt Valley, west of Devils Garden, cap rock crops out along the Klondike Bluffs access road. Contorted thin limestone, gray shale, and other insoluble rock are well displayed along the sides of the Salt Valley Wash.
Questions to answer (email me your answers, do NOT post them in your log):
1. How many years ago had many layers of salt been deposited here?
2. What minerals (other than halite = table salt) does the salt bed contain?
3. What was formed after the salt encoutered a fault?
You can log your find immediately after you send your answers to me. If there is any problem with them, I will contact you.
Optional task is to post a picture(s) from earthcache location. (DO NOT INCLUDE THE SIGN IN YOU PICTURE).
PREVIOUS VISIT DOES NOT COUNT!!
"FOUND IT" logs will be DELETED WITHOUT FURTHER NOTICE if the answers are not received within seven days of the post date.
sources: Arches National Park, Geologic Resource Evaluation Report; Geologic Resources Division Denver, Colorado
http://www.nps.gov/arch/index.htm
http://en.wikipedia.org/wiki/Arches_National_Park
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