Geology is all around us, scarcely thought of as we go about our lives. Yet, it affects everything we do as a civilization, as a society and as individuals. While barely appearing to change from day to day, it works to alter the course of evolution. Preserving a record of creatures and landscapes both ancient and forgotten, the story of our past is written in stone and waiting to be read. I offer a view of how I see our world and its inhabitants, both past and present, as seen through my lens.
Sunday, May 29, 2011
The Enigma of Upheaval Dome: Diapiric Salt or Ground Zero?
GETTING OUR BEARINGS Carved on horizontal strata and situated amongst the canyons, mesas, buttes, fins and arches of CanyonlandsNational Park in southeast Utah, Upheaval Dome is a most unusual structural feature. Although superbly exposed, its geological origin has been debated for almost a hundred years and remains controversial to this day.
Looking east from our Cessna Skyhawk II at an elevation of about 1,000 feet (the surrounding plateau is barely 5,300 feet), the concentric rings of Upheaval Dome are clearly visible. Note a small stretch of the Green River at the bottom left and Upheaval Canyon leading into the center of the dome (right of center). Beyond Upheaval Dome is the Island in the Sky District of Canyonlands and the La Sal Mountains at the horizon. (Click photo for larger image)
This is also an easterly Google Earth view of Upheaval Dome (red arrow) from the approximate perspective of the aerial photo (above) only framed at a steeper angle and with an exaggerated vertical angulation (2x). Note the concentricity of Upheaval Dome which is readily recognizable.
Topographic map of the Upheaval Dome area (USGS)
Upheaval Dome is a roughly circular structure that forms a “bull’s-eye” pattern nearly three miles across, large enough to be seen from space. While the surrounding plateau is largely undeformed, the dome consists of strongly deformed and uplifted rocks surrounded by a structurally depressed ring syncline.
3-D view of Upheaval Dome from the south (Modified from Mcgraw-Hill)
Stucturally, it's a dome, but topographically, it's a crater. Simply stated, the entire structure appears as an eroded, 5.5 km (3.4 mi.) diameter crater surrounded by concentric rings composed largely of siltstone and sandstone. The central portion of the dome is a topographic depression eroded 350 m below the surrounding escarpment, which is ringed by a syncline and breached by a canyon cut through its west wall. The innermost portion of the crater has a central uplift or peak with a complex sequence of folded and faulted strata with an imbrication of thrust slices piled against the central peak and splayed towards the southeast.
Stratigraphically, Permian age White Rim and Cedar Mesa Sandstones of the Cutler Group lie at the center, the oldest exposed rocks of the dome. Upheaval’s rocks are progressively younger from its center to the rim. Like the layers of an onion, outwardly lie Triassic strata of the Moenkopi and Chinle Formations, Jurassic age Wingate Sandstone, Kayenta Formation and Navajo Sandstone. The outer rocks of Upheaval Dome dip outward, anticlinally, in all directions from the central peak. Non-resistant formations such as the Kayenta and the Chinle are eroded into strike valleys that encircle the center. Resistant sandstones stand tall as circular ridges, outermost of which is the Navajo. Upheaval Dome is located within the boundaries of the Paradox basin, and is therein underlain by the salt-bearing Pennsylvanian Paradox Formation.
Are there any clues to the aforementioned geological structure that suggest the process(es) that formed it?
Geologic Map of Upheaval Dome
Erosion of Upheaval Dome exposed progressively older strata from the center of the structure to the rim, which reflects the layering of the stratigraphic column in Canyonlands (Jackson and others, 1998). A more encompassing map and stratigraphic column of CanyonlandsNational Park is available at: http://www.nature.nps.gov/geology/inventory/publications/map_graphics/cany_map_graphic.pdf
Stratigraphy in the vicinity of Upheaval Dome. Note the location of the salt-bearing Pennsylvanian Paradox Formation.
Geologic Map of the central dome of Upheaval Dome showing stratigraphic units and the faults.
(Jackson and others, 1998)
WHAT GEOLOGICAL PROCESS FORMED SUCH AS UNUSUAL STRUCTURE?
WAS THE PROCESS GRADUAL, CATASTROPHIC OR A COMBINATION OF BOTH?
Hypothesis #1: A crypto-volcanic eruption
Early geologists (Bucher, 1936) suspected a subterranean crypto-volcanic eruption, a volcano that erupted but never made it to the surface. But, no volcanic rocks are exposed in the region or subsurface resulting in a consensual abandonment of this theory.
Hypothesis #2: A salt dome
Under the tremendous pressure exerted by the weight of a large overburden of sedimentary rock, salt behaves plastically, like a squeezed tube of toothpaste. The overburden forces the ductile salt to flow vertically in a subsurface pillar called a diapir, especially if stress is released by faulting or surface erosion. In the case of Moab, Salt and other valleys not far to the east, deeply buried salt from the Pennsylvanian-age Paradox basin (formed by evaporation of its enclosed body of water) flowed upwarp along long faults. Topographic, anti-clinal bulges were created by the rising salt that lifted the overlying strata. Synclinal valleys formed where the bedded salt was eventually depleted. Since Upheaval Dome also lies within the confines of the Paradox basin, salt was contained deep in the subsurface (confirmed by a USGS geophysical study along with basement faults).
A rather obvious assumption is that Upheaval was a salt dome, a structure like the elongate salt-valleys found to the east, only circular. That would seemingly explain the domal uplift and concentric synclinal-structure of Upheaval Dome (Mattox, 1968; Shoemaker and Herkenoff, 1983). A big problem is that there isn’t another diapiric structure like it anywhere within the confines of the Paradox basin.
Notice the boundary of the Paradox basin, Upheaval Dome within it, and numerous, elongate salt-intruded valley structures.
(After Doelling, 1985, and Jackson and others, 1998)
Hypothesis #3: A pinched-salt dome A corollary to the salt dome hypothesis is the development of a pinched-off salt diapir. Here, workers (Jackson and others, 1998) propose that an overhanging diapir of partly extrusive salt was pinched off from its stem amd subsequently eroded. Their inference is supported by synsedimentary structures that indicate Jurassic growth of the dome over at least 20 million years. They infer that a pancake-shaped salt-glacier spread from a passive salt-stock during Late Triassic and Early Jurassic time. During Middle Jurassic time, the allochthonous salt spread into a glacier, inferred to be 3 km in diameter. Diapiric pinch-off may have involved inward, gravitational collapse of the country rocks, which intensely constricted the center of the dome. Sediments in the axial shear zone beneath the glacier steepened to near vertical. The center of the dome comprises radial thrusts and sedimentary dikes, indicative of high fluid pressures.
Hypothesis #4: An extraterrestrial bolide impact
“Impact origin confirmed” is the title of a recent article (Buchner and Kenkman, 2008), attributing the dome’s provenance to an extraterrestrial bolide impact, although the impact theory originated decades ago (Boone and Albritton, 1938). Unfortunately, there exists no telltale meteoric debris or ejecta from a massive collision. To explain the absence of “smoking gun” evidence, proponents speculate that what we now see at Upheaval Dome is actually the deep-seated core of an impact structure. The original crater formed perhaps as much as a mile above the present surface and was eroded down to the present level with the uplift of the Colorado Plateau. Erosion would have removed any evidence of debris which may have vaporized on impact.
The debate centers on the interpretation of microstructural, shock-metamorphic indicators. Proponents demonstrate microscopic, impact-diagnostic, planar deformational features from the shock of collision, whereas antagonists argue that deformational microstructures are related to rate-dependent plastic deformation from a rising diapir. Based on their analyses, proponents (Kenkmann et al, 2005; Scherler et al, 2006) further hypothesize an oblique impact from the northwest. The age of the impact has not been determined, since datable strata deposited immediately after the impact have been removed by erosion.
Impactor-advocates believe that the destruction of the crater by the inflow of material during its collapse coupled with a rebound of the central peak produced the ring structure. That classifies Upheaval as a small complex crater. Additional revealing features are the numerous listric normal faults which allowed rocks along the perimeter of the crater to glide inward and upward on the rebounding central peak. As the rocks impinged on the rapidly rising central peak, the early arrivals were overrun by later arrivals, producing a stack of outwardly dipping imbricated thrust sheets on the flanks of the peak. Also, clastic dikes are widespread within the sharply domed central peak, an indication of the dynamic injection of fluidized material.
Hypothetical model of impact scenario before and after erosion. It is thought that the land surface at the time of impact was near the top of the Carmel Formation.
(From CIT website, John Louie principal investigator)
Advocates for an impact hypotheses for the origin of Upheaval Dome suggest a collision with a 0.3 to 1.2 km meteor which formed a wide surface crater. Today, Upheaval Dome does not represent the original impact crater but an eroded dome that is beneath the original crater. Slumping of the crater along listric faults forced a central peak-uplift in the shattered center of the structure by crustal rebound. Uplift of the Colorado Plateau eroded ~1 km of Mesozoic cover, exposing the roots of the impact structure.
Hypothesis #5: A combined impact-salt diapiric event
More recently, researchers (Daly and Kattenhorn, 2010) have theorized a combined impact-salt diapiric event. They believe that the deformation styles at Upheaval Dome actually represent a meteorite impact that decreased the pressure of the overlying rocks and subsequently triggered the vertical flowage of salt. Salt flow may have bulged the Paradox Formation later in a ring surrounding the center of the impact without significant salt diapirism. Little is actually known about the effect of a meteorite impact into layers of salt, since this is the only known scenario of such an event.
The debate goes on.
OUR HIKE TO THE RIM OF THE CRATER OF UPHEAVAL DOME
Back on the ground, I accompanied Wayne Ranney, geologist, accomplished author, and reknown guide (visit Wayne at http://www.wayneranney.com) to Upheaval Dome in order to view this enigmatic structure firsthand. Leaving our vehicle at the parking area, we hiked a short distance on the Overlook Trail up to the rim of the crater situated in the Navajo Sandstone. Along the way we observed postulated overturned-beds of the Kayenta Formation. Upon reaching Whale Rock in the Navajo Sandstone at the rim, we were afforded a fantastic view of this very intriguing rock formation, as we peered into the crater of the dome.
This panorama was created with three sequential photos. Taken from atop Whale Rock in the Navajo Sandstone, it looks to the northwest. Notice the internal strike valley and central dome. The Google Earth image (below) was captured from the same perspective. Click the panorama for a larger image.
This Google Earth image shows the same perspective as the panoramic photo above looking northwest. The arrow at the bottom indicates the car park and just above it is Whale Rock. Notice the concentric rings of sandstone with the Wingate outlining the central crater and the Navajo forming the resistant outer crater. Sandwiched in-between is a strike valley composed of the Kayenta Formation. Within the central crater lies a central peak mainly of Chinle and Moenkopi Formations. Lastly, note the Upheaval Canyon, the outlet canyon that drains the central crater leading to the Green River.
We then hiked a short distance clockwise along the outermost rim along the Upheaval Dome Trail to a fenced-in overlook that provided a great view of the central dome.
The perspective in this panorama is from the fenced-in overlook on the south rim of Upheaval Dome. I'm taking these three photos while perched on the outer crater's rim of Navajo Sandstone. Again, just within is a Kayenta strike valley and an inner crater of Wingate Sandstone, both of which are ill-defined in this panoramic perspective. Click the photo for a larger image.
The central peaks have an upturned and imbricated stratigraphy of the Cedar Mesa and White Rim Sandstones and the Moenkopi Formation. An eroded strike valley of the Chinle Formation concentrically lies outside the central peak.
The enigma of UpheavalCanyon certainly has captured my imagination. Although the disagreement about its origin prevails, the causative options seem to have converged upon a core of impactors and diapirists. Check it out the next time you’re in Canyonlands and weigh-in on your hypothesis!