Paradox Exposed

Sparky is awestruck by the Onion Creek salt diapir.

Onion Creek is short little creek, flowing less than 10 miles before entering the Colorado River northeast of Moab, Utah.  It was named for its fragrance, most notable at Stinking Spring.  The drainage was not always so short.  Fisher Creek, which flows off the north side of the La Sal Mountains, used to turn and continue west along this route.  Then about 3 million years ago, it was blocked by a rising dome of salt.

Modern-day Fisher Creek flows northeast through Cottonwood Canyon to the Dolores River, leaving Onion Creek to drain the area to the west.  Base map from ArcGIS Online (click to view details).

Salt tectonics, aka halokinesis

Salt has played a prominent role in the spectacular scenery of southeast Utah and adjacent Colorado.  It has moved around underground, enough to produce surface features such as domes, ridges and valleys.  This is the phenomenon of “salt tectonics”, but it isn't necessarily associated with the more common type of tectonics -- movement of crustal plates.  In fact, salt can migrate and change the landscape even in areas of tectonic quiescence.  Perhaps it’s better to use the term "halokinesis" -- “a fine word” as Ole Nielsen pointed out in his fine post on Salt Tectonics.  Halokinesis also was featured at Evelyn Mervine’s Geology Word of the Week last summer.  (She briefly discussed psychic halokinesis -- moving salt with one’s mind -- but we won’t go there in this post!)

Halokinesis happens because salt doesn’t compact, in contrast with most sediments.  Sand, silt, etc. become more dense under pressure, due to loss of pore space, but the crystalline structure of salt means there is little pore space to lose.  While other strata increase in density as sediment accumulates above, a salt layer becomes relatively more buoyant.  Under pressure, it can become plastic and flow, initially laterally but then upward as well, due to greater buoyancy.  If the salt layer is sufficiently thick, it will change the landscape.

Cross-section through the Salt Valley anticline (uplift) north of Arches National Park, Utah, pushed up by moving salt.  The crest was breached, forming a valley.  Modified from Fillmore 2010.

Salt is highly soluble and will disappear quickly if groundwater gets to it.  Salt anticline valleys are uplifts that later collapsed when the underlying salt was dissolved (diagram above, photo below).

Paradox Valley, a collapsed salt anticline in southwest Colorado.
Photo by Geotripper; for details, see his Yellow Line Fever.

Paradox Formation (Pennsylvanian)

There is no shortage of underground salt in southeast Utah and southwest Colorado.  This was the Paradox Basin 300 million years ago, an extensive downwarp that developed southwest of the Uncompahgre Uplift of the Ancestral Rocky Mountains.  Seawater periodically flooded restricted basins, followed by evaporation and deposition of evaporites. The cycle was repeated many times producing thick beds of evaporites, mostly salt, that would be come the Paradox Formation.

Ancestral Rocky Mountains showing Paradox Basin and Uncompahgre Uplift; after Trudgill 2011. 

Salt anticlines (uplifts) are especially common in the vicinity of the Colorado - Utah state line, where the Paradox Formation is thickest (below, after Gutiérrez 2004).

Onion Creek salt diapir

The Onion Creek salt diapir is located about 17 air miles east-northeast of Moab, Utah, just south of the Fisher Towers.  Here the migrating Paradox Formation gathered into a blob, creating a dome on the surface.  The uplift subsequently collapsed, revealing underlying evaporites.  The exposure is about 2 miles in length and 3/4 mile across.

Pale area lower center is the Paradox Formation along Onion Creek.  From Google Earth, click to view details.

Portion of the Geologic map of the Fisher Towers quadrangle (Doelling 2002).  The Paradox Formation (IPp, light purple) is in the lower right quadrant.  Click to view.

This is a halokinetic feature we all can enjoy, as a decent dirt road runs through it.  The visuals are spectacular!  Then there is the memorable fragrance of Onion Creek, best appreciated at Stinking Spring, which reeks of sulfuric acid.

Looking east into the Onion Creek salt diapir.  From here, the road quickly drops down to the canyon bottom.

Scenic drive through Paradox strata, Onion Creek on left.

Deformed rocks of the Paradox Formation.

Rocks of the Paradox Formation along Onion Creek include gypsum, anhydrite and darker shales from the cap rock of the diapir.  Salt was the most abundant mineral originally, but it was dissolved and carried off by groundwater, leaving the resistant cap rock collapsed and deformed.

Right:  hints of sulfur in cap rock.  Both gypsum and anhydride contain sulfur.  This explains the aroma of Stinking Spring, where sulfur-reducing bacteria produce hydrogen sulfide gas.  For more on cap rock minerals, see the recent post by Sandatlas on Sulfur, gypsum, and hydrocarbons.

Below:  Section through a salt diapir showing sulfur-rich cap rock.  From Damon Mound at Geocaching, no source given.

Rising salt is powerful enough to deform the rigid rocks it moves into, causing fractures and folding.  Sloping beds of the Cutler Formation, tilted by the salt diapir, are often visible on the north side of Onion Valley (see photo at beginning of post).

Looking west down Onion Creek.  Tilted red Cutler strata are visible on the right side of the valley in the distance.  In the center of the photo is a large block that has slid down from the left.

The Onion Creek salt diapir is young, only 2-3 million years old, and there is evidence that there was a pulse of activity just 250,000 years ago.  Apparently the 300-million-year-old Paradox salt hasn't stopped moving, and is going to continue to affect the landscapes of southeast Utah.

Fisher Valley

Heading east, the Onion Creek road leaves the drainage bottom and follows narrow ridges up to Fisher Valley.  This basin contains over 400 feet of fill, deposited after the salt dome dammed Fisher Creek.  After driving through the tortured strata along Onion Creek, the flat landscape is striking!

Fisher Valley has all the amenities -- flat land, water and arable alluvial soil.  Combine this with great scenery and you have a lovely place for a ranch (note center pivot and ag fields in aerial photo above, about halfway through post).  But the Fisher Valley of today is not long for this world, for little Onion Creek is steadily eroding headward, excavating the basin. Given no more interference from halokinesis, it will reach Fisher Creek in less than a mile, and connect it once again with the Colorado River, ending its three-million-year-long diversion.

Headwaters of Onion Creek, looking southeast; tip of La Sal Mountains on horizon.  Click photo to view fields of Fisher Valley Ranch in distance -- little Onion Creek is going to take them all!

For additional information:

For more information on the Onion Valley salt diapir, including how to get there, see the Utah Geological Survey's Geosights page.

For detailed descriptions of strata, structure and history of the diapir and surrounding area:
Doelling, H.H.  2002.  Geologic map of the Fisher Towers quadrangle, Grand County, Utah.  Utah Geological Survey.  22 p., 2 pl.  Available at http://geology.utah.gov/maps/geomap/7_5/pdf/m-183.pdf

For thorough and interesting discussions of salt structures of the Colorado Plateau, including the Onion Creek diapir:
Fillmore, Robert. 2011. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado. Salt Lake City: University of Utah Press.

Evelyn Mervine's post on halokinesis includes a glimpse at the power of underground salt. In an exploratory well in Louisiana, Texaco drilled through an underground salt mine (oops!), allowing water in.  It dissolved the salt, of course, causing all kinds of havoc -- the Jefferson Salt Mine disaster.

Salt and sediment:  A brief history of ideas at Hindered Settling is an interesting account of the history of salt tectonics.

While planning my trip to the Paradox Basin last spring, I put together a post about it: Field Trip Plans -- the Land of Paradoxes.

Other literature cited:

Gutiérrez, F.  2004.  Origin of the salt valleys in the Canyonlands section of the Colorado Plateau Evaporite-dissolution collapse versus tectonic subsidence.  Geomorphology 57: 423–435.

Trudgill, B.D.  2011.  Evolution of salt structures in the northern Paradox Basin: controls on evaporite deposition, salt wall growth and supra-salt stratigraphic architecture.  Basin Research 23: 208-238.