Sunday, June 30, 2013

A Curious Intra-Formational, Angular Unconformity within the Chinle Formation: Part II – The Salt of the Earth

“The same regions do not remain always sea or always land,
But all change their condition in the course of time.”
Aristotle, Meteorologica

In my previous post entitled “Part I – A Conspiracy of Events”, I posed two questions about an intra-formational, angular unconformity within the Chinle Formation of Moab Canyon, Utah. "What events conspired to create this unconformity?" and "What can it tell us about the ancient landscape?"

Those events (read about them here) incIude global plate tectonics, intraplate orogenesis, Pangaean climatology, australly-induced glacioeustasy, Milankovitch-influenced solar forcing and cyclical, basin evaporite sedimentation. But there’s one critical detail left to discuss. It's the physical behavior of salt when placed under a load or halokinesis for short.

This intra-formational, angular unconformity within the Chinle Formation
re-appears at the bottom of this post with its bedding drawn in.

We’re standing on the banks of the Colorado River over 4,000 feet above sea level in east-central Utah, where the view is nothing less than spectacular. For reference, the Grand Canyon is over 275 miles downriver in Arizona. This is Richards Amphitheater at the entry to Fisher Valley. The solitary spires belong to Fisher Towers that are eroding from the valley’s north flank (left). The rampart on the valley’s south flank (right) is a cluster of mesas that separates Fisher from Castle Valley.
Both valleys are flat-bottomed and are curiously oriented northwest to southeast. Counter-intuitively and visually-contradictory, Fisher Valley resides on the crest of an elongate anticline. 

From here, the angular unconformity is ten miles downriver, but there are actually many regionally, and not just within the Chinle. On this late Spring day, the Oligocene laccolithic La Sal Mountains retain their winter snows. The derivation of their name dates back to the Spanish who called them the "Salt Mountains", a hint at what lies buried beneath the valley floor.

Ascending the mesa (left), it is composed of the Cutler, Moenkopi and Chinle Formations.
The cliff-former is Wingate Sandstone with a vegetated-cap of Kayenta.

The Navajo Sandstone has eroded back on the mesa-tops.

Just out of view, the Colorado bends to the right nudging past Fisher Valley, and in succession, transects three more valleys that are essentially parallel in their orientation. The valleys are anticlines that have collapsed along their crests.

An anticline is formed from stratified rock that has folded upward with its beds sloping downward from the crest, thereby creating a landform with positive relief. Yet here, we have the opposite geomorphology, a negative relief landform with a flat floor. The reverse in architecture from what is anticipated is a result of collapse along the axis of the anticline. Buried salt is the culprit, but I’m getting a little ahead of myself. 

View southeast across Richards Amphitheater from Utah Highway 128 known locally as the River Road.
Fisher Towers (center) is eroding from the adjoining mesa.
The opposite mesa (right) defines the southern flank of Fisher Valley
with the badlands of the Onion Creek diapir interposed.
We just passed Castle Valley off to the right.
The Cutler Formation and Quaternary fill blankets the valley floor.

Facing southeast, the region is within the Paradox basin of Pennsylvanian and Permian time. The marine basin has been filled in for about 300 million years, but its buried contents have profoundly altered the contemporary landscape, and still do!

The Colorado River can be seen entering from the lower left, the location of my Fisher Valley photo. It then noses across Fisher, Cache and Castle Valleys before plunging into Moab Canyon, the location of the Chinle unconformity on the lower right. Upon its emergence (not seen), it crosses Moab Valley, the fourth anticlinal landform, rather than follow the more logical path down the axis of the valley. Geologists have been trying to make sense of these valleys and the river’s transecting course for 150 years in this land of geological enigmas, paradoxes and contradictions.

Notice that Cache Valley (bottom center) appears to be in an earlier stage of development than the others. Its NW-SE orientation has not yet fully developed nor has its fully-collapsed, flat bottom. Interestingly, a shallow syncline is evident amongst the mesas between the Fisher Valley and Castle Valley anticlines. Also, observe that the strata flanking Fisher and Castle Valleys forms two long escarpments by turning upward (red arrows) with the mesa dipping to the north above Fisher Valley and the opposite below Castle Valley.

The stratal geometry implies a once-continuous, anticlinal trajectory that existed over the intervening landscape. Might there be a formative relationship between the anticlinal valleys, their orientation, the up-turned strata, and even the unconformity downriver? Think salt.

If the majestic pinnacles of Fisher Towers appear remotely familiar, it’s because they’ve been the backdrop in scores of movies and advertisements. The tallest spire is Titan, topping out at 900 feet, a favorite of rock climbers. The lithology displayed in the towers and adjoining mesa typifies the stratigraphy of the region and tells a geological story of a long-vanished mountain range. And the geological story of the valleys is equally incredible.

The towers are weathering out from the mesa on Fisher Valley’s north flank. They are hewn from purplish-brown, coarse-grained arkosic sandstones and conglomerates of the Permian Cutler Formation and capped by knobby, dark brown Early Triassic Moenkopi sandstones and shales. Higher up on the mesa (left), upper Moenkopi beds merge with slopes of overlying Late Triassic Chinle conglomerates and sandstones.

These Permian and Triassic clastic deposits came off the Uncompahgre highlands to the northeast, one of a series of mountain ranges belonging to the Ancestral Rocky Mountains that reached its greatest intensity between the Middle Pennsylvanian and Early Permian. Being so close to the mountain-front, the Cutler is extremely thick and contains sizable Precambrian clasts derived from the uplifted-core of the once great range, a veritable geological signature of their existence. Read about the Ancestral Rockies here.

Following the Triassic, a transition to increased and prolonged aridity witnessed the deposition of the Glen Canyon Group's sandstone threesome. Overlying the Chinle slopes, cliffs of black desert-varnished, reddish-brown Early Jurassic Wingate Sandstone (upper left) rise in the mesa and are capped with a veneer of ledgy, fluvial Kayenta Sandstone. On the mesa-tops (not seen), the eolian, Sahara-esque Jurassic Navajo Sandstone has eroded well-back. The Cretaceous and Neogene successions of the Western Cretaceous Seaway have completely unroofed from the region, a consequence of Colorado Plateau uplift.

Check out Fisher Towers in this Citibank video here. The stratigraphy is somewhat out of order, but the scenery is all there.

After Fisher Valley, the Colorado River skirts the head of neighboring Castle Valley, flowing right to left (northeast to southwest) along the cliff-line in the distance. Our perspective is exactly opposite that of the Fisher Valley photo. This time we’re in the foothills of the La Sal’s with the mountains at our backs looking northwest instead of southeast. The prominent spire of Castleton Tower is weathering from the cluster of mesas that separates Castle from Fisher Valley. 

The stratigraphy and geomorphology, with a few noteworthy exceptions, is the equivalent of Fisher Valley. It's because the valleys share a common genesis by the rise and subsequent collapse of their initial anticlinal structures. 

On closer inspection, Castleton Tower (a.k.a. Castle Rock) bears a similarity to Fisher Towers and its parent mesa, only composed of formations slightly higher in the stratigraphic column. The valley floor and base of the mesas flanking Castle Valley largely consist of the Cutler Formation in addition to a flotsom and jetsom of Neogene fill. The Cutler is separated from the slopes of the overlying Early Triassic Moenkopi Formation by a thin, white bed of gypsum, thought to be an eolian sand sheet. Look for it in the photo.

Ascending the slope, the Moenkopi is separated from the Late Triassic Chinle Formation by its basal Shinarump Conglomerate Member, which is faintly visible on the profiled-slope just below Castleton Tower. Rising above the Triassic slopes, Castleton Tower and neighboring Parriott Mesa are held up by cliff-forming Wingate Sandstones with a thin cap of Kayenta Sandstone. The mesas, buttes and plateaus of Castle Valley are concordant with those of Fisher Valley, meaning they are in agreement structurally and stratigraphically.

Notice the profound inclination of Parriott Mesa and its bedding. The pattern of dipping and stratal geometry on the various mesas within the valleys and their flanking escarpments suggest a genetic link. What is the landscape trying to tell us about its past? Hint: The geological process responsible for the landscape's formation and deformation are related to the behavior of compressed salt, a process referred to as "salt tectonics." The process is regional, but without a global and astronomical interplay, it never would have occurred! 

Here's a television commercial featuring Castleton Tower and a 1964 Chevrolet.

Back in Middle Pennsylvanian through early Permian time, the Paradox basin extended from eastern Utah into western Colorado and a small slice of northwestern New Mexico. The epeirogenic (land-based) marine-basin formed contemporaneously with the Uncompahgre highlands, which was a NW-SE-trending mountain range on the southwestern flank of the long-gone Ancestral Rocky Mountains. The Ancestral's were a mosiac of about 20 basement-cored arches and adjoining basins that virtually uplifted from the Panthalassic sea from Texas up into Idaho, roughly in the same locale as the modern Rocky Mountains, their namesake.  

As the Uncompahgre highlands tectonically-uplifted, the adjacent Paradox basin reflexively-subsided, thereby creating an asymmetrical, ovoid trough with its deepest part nearest the Uncompahgre fault. The basin remained in intermittent communication with the open sea on the west and south. As global sea levels rose and fell during Pennsylvanian time, tied to glacial cycles of freezing and melting at the South Pole, the Paradox basin was flooded with great regularity an astounding 33 times. Again, the details are all in Part I here.

Middle Pennsylvanian (315 Myr) Paleo-view of the Ancestral Rocky Mountains
Note the location of the Uncompahgre highlands UH) and the Paradox basin (PaB).
The red dot depicts the locale of the Chinle unconformity near the center of the basin.
Modified from Ron Blakey and Colorado Plateau Geosystems, Inc.

The 12,000 square mile (190 by 95 miles) extent of the Paradox basin (red line) is defined by evaporite (salt) deposits of the Middle Pennsylvanian Paradox Formation.  The character of the rocks within the basin was contingent on their proximity to the rising front of the Uncompahgre highlands AND their locale within the basin. Sedimentation kept pace with subsidence which maintained a consistently shallow depth to the basin.

From the Middle Pennsylvanian through the Early Permian, the developing basin was deepest and received thick successions of boulder to pebble clastics along the Uncompahgre front (Gateway, CO for reference), which is represented along the modern Uncompahgre Plateau. The siliciclastics are the “undivided” (“undifferentiated”) Cutler Formation’s conglomerates and sandstones that are strongly progradational to the southwest.

Away (proximal) from the front (the locale of Fisher Valley), the basin received thinner and finer “undivided” Cutler clastics deposited over evaporites of the Hermosa Group’s Honaker Trail and deeper Paradox Formation’s evaporites, cyclically bedded with black shale, dolomite and anhydrite. The Hermosa Group's third and lowermost member is the Pinkerton Trail Formation occupying the bottom of the basin.

Further southwest into the (medial) basin (beginning with the locale of Castle through Moab Valley, into Canyonlands and beyond), the Cutler Formation assumes Group status with it becoming thicker and multi-formational (such as the Cedar Mesa, Organ Rock and White Rim Formations). Beneath the Cutler Group, the deeper basin received the Paradox Formation's evaporite-dominated deposits derived from the sea. Furthest from the front (distal) on the basin’s shallow shelf, the Paradox Formation developed cyclical carbonate-dominated sedimentation with petroleum-containing algal bioherms.

Map of Uncompahgre Highland-Paradox Basin System
The boundary of the basin (red) is defined by the evaporite deposits of the Paradox Formation.
Basin-fill consists of carbonate (shelf) and evaporite (center) facies of the Paradox Formation, mixed siliciclastics of the overlying Honaker Trail Formation and siliciclastics of the Cutler (“undivided” Cutler Formation proximally and Group status medially and distally). Note the location of Fisher and Castle Valleys, and the Chinle unconformity (red dot), relevant to our discussion.
Modified from Barbeau, 2003

The following stratigraphic column reflects the varied lithology of the Paradox basin. To the right are deposits closest to the front, while to the left, it progresses through the basin center to the shelf. The four valleys in our discussion (and the Chinle unconformity), reside within the proximal and the beginning of the basin. Of interest are the buried evaporites within the Paradox Formation (yellow).  

Modified from Barbeau, 2003 and Gradstein, 2004

An excellent Utah Geological Survey map and detailed discussion of the stratigraphy of the Fisher Towers Quadrangle is located here.

During the Permian, the Cutler Group succeeded in filling the Paradox basin. The depositional environment in the basin has progessed from marine to terrestrial. As the Uncom
pahgre highlands wore down and its uplift-intensity diminished, the basin's rate of subsidence likewise diminished, and eventually ceased, but not before blanketing the filled-in basin with Moenkopi and Chinle fluvial and lacustrine clastics during the Triassic.

The non-marine, low-gradient alluvial and coastal-plain environment in which they were deposited extended over 100 km to the sea. The Triassic sediment sources, in addition to the nearby Uncompahgre highlands, included the Mogollon highlands to the southwest and the newly-formed southern Appalachians to the southeast. The sediment sources and the river patterns are evident on the following hypothetical paleo-map.

Late Triassic Paleo-view of the remnant Ancestral Rocky Mountains
and the filled-in Paradox Basin, blanketed by
Early Triassic Moenkopi and Late Triassic Chinle clastics.

Our Chinle unconformity is at the red dot.
Modified from Ron Blakey and Colorado Plateau Geosystems, Inc.

By the Jurassic, the Paradox basin (pierced and bulged by its buried Paradox salt!) was deeply buried. Millions of cubic miles of sedimentary cover from the Western Cretaceous Seaway was totally stripped from the region by the Laramide event that gradually-uplifted and gently-flexed the region. The once-lofty Uncompahgre highlands and parent Ancestral Rocky Mountains would only be identifiable by their telltale sediments distributed across the contemporary landscape. Deciphering the details of the vanished range is an amazing piece of geological detective work, still in progress.

It is the unique behavior of Paradox salt under pressure that has dictated the evolution of the landforms within the basin (and our unconformity). In the strictest sense, salt is the pure mineral halite, NaCl in its crystalline form. In the depositional environment of the Paradox basin, "salt" includes additional evaporites such as anhydrite, gypsum and potash. These "salts" precipitated directly from sea water as it was reduced by evaporation, and in a highly predictable sequence.
Halite crystallizes after 90% of the water has evaporated. 

As the evaporites formed, they settled to the bottom of the basin, interbedded with shales and dolomites. The immense hypersaline lake of the Paradox basin could not have developed were it not for the climatic conditions of warmth and aridity on the nascent, un-uplifted, marine-communicating Colorado Plateau of western Pangaea during the Pennsylvanian through Jurassic.

It is likely that the moment salt within the Paradox Formation became subjected to the weight of the arkosic Cutler overload from the front and the Honaker Trail Formation's load from above it began to move within the subsurface. Behaving like toothpaste under pressure, it buoyantly lifted the Honaker into the earliest beginnings of an anticline. The movement of salt under pressure, called halokinesis, is attributable to its physical properties of low density and low strength which does not increase with burial, it being essentially uncompactible.

As subsequent sediments accumulated upon the Paradox salt, the density of compactible, non-halite overburden increased, far exceeding that of the underlying salt. Under increasing pressure, the salt moved vertically toward the path of least resistance fed by salt in adjacent regions that flowed laterally. As networks of subsurface salt-ridges gradually coalesced, the rising salt formed diapirs, Lava lamp-like blobs of ascending salt.

Adjacent to the salt anticlines on the surface, complementary synclines developed in response to the lateral flow of subsurface salt that was evacuated to feed the ascending diapirs. Back on the surface, Permian and Triassic strata was being “shunted” from the crests and inclined slopes of the anticlines to the downwarped-troughs of the synclines. As the rising diapirs forced its way through the overburden, faults and fractures developed in the cap rock that acted as conduits for the entry of water from the surface (meteoric water). In some circumstances salt diapirs may have actually pierced the surface, as the overburden on the anticlines thinned. Rising salt was beginning to affect the geomorphic evolution of the landscape!

Another distinctive physical property of salt is its high solubility. Upon contacting the salt diapir, meteoric water initiated its dissolution causing the unsupported overburden to collapse into the void. As dissolution and collapse progressed, the anticlines widened forming flat-bottomed valleys or grabens (German for “grave”) along the axes of their crests. As collapse continued, rimmed escarpments of upturned, resistant sandstone, that initially blanketed the pre-collapsed anticlines formed at the flanks of the valleys. We saw precisely that on the Google Earth "Big Picture" above.

Modified from wikipedia

Interestingly, in spite of the fact that salt drove the ascent of the anticline, it is never seen at the surface where it is rapidly dissolved, even in today's arid climate on the Colorado Plateau. Gypsum, however, one of the interbedded evaporites that formed within the proximal basin of the Paradox Formation, IS found at the surface, exposed as light gray mounds on valley floors. Its persistence is attributable to its reduced solubility. 

The following hypothetical and over-simplified schematic represents a likely scenario in the development of a salt valley over the axis of a collapsed, salt-cored anticline.

Following a period of evaporite deposition: (A) Rising salt diapir elevates the overburden deforming it into an anticline and complementary (rim) synclines laterally; (B) Dissolution-induced subsidence occurs along the axis of the anticline's crest; (C) Salt withdrawal triggers faulting and foundering of blocks of overburden into the "void" creating a salt-cored, collapsed anticline.

The Stages of in the Evolution of a Salt-Cored, Collapsed Anticline:
(A) Diapiric intrusion; (B) Dissolution; (C) Collapse
The following USGS cross-sectional map of the contemporary landscape slices through Fisher Valley from north to south. A diapir ("salt wall") of Paradox salt in its ascent has forced the Permian, Triassic and Jurassic overburden to elevate into an anticline. Following the dissolution of salt, the overburden faulted and collapsed into the void, thereby creating a "collapsed, salt-cored anticline."

There are many noteworthy features of interest. With the collapse of the structure along its crest, long escarpments of upturned strata flank the anticline (recall the "Big Picture"). The graben has developed a listrically-faulted (curved downward on one side) architecture. This faulting additionally facilitated the penetration of surface water to the diapir and its dissolution. Unique to Fisher Valley, the Onion Creek diapir on the valley floor is both currently active and accessible for examination. It developed during the Pliocene-Pleistocene, and in its ascent, has chaotically folded the Paradox Formation into a badlands-jumble of gypsum, limestone and shale. Again, salt is never found at the surface, but gypsum, the less soluble interbeds of the original sequence, is.

Modified USGS Cross-Section

With the initial deposition of the Honaker Trail Formation over the first halite bed within the Paradox Formation, the most active phase of salt movement began in the Pennsylvanian through the Triassic, a period of about 75 million years. 

The NW-SE orientation of the Ancestral Rocky Mountains and their collection of subsidiary  uplifts and basins are likely related to extension with the craton caused by the supercontinent of Rodinia as it broke up starting a billion years ago. Subsequently, during the first halokinetic phase, these regional extensional fractures within the basement structure likely accommodated initial movement of salt. I discussed this tectonic inheritance in Part I here.

Phase two, from the Jurassic through the Early Cretaceous spanning 125 million years, involved diapiric rise, cap rock penetration and salt dissolution. Phase three, from the Late Cretaceous through the Late Tertiary, lasting 90 million years, involved subsidence and burial. The final phase of activity began about 10 million years ago through the present and is dominated by further dissolution and collapse. 

Once educated to their presence, the salt features most easily recognized on the surface are the flat-bottomed valleys and their complementary synclines. Additionally, as the landscape faulted, folded and buckled under the strain of ascending salt, runoff from the limbs of the anticlines drained into the troughs of the synclines shunting Triassic deposits to them.

This can be seen when travelling Utah Highway 128 along the Colorado River through Moab Canyon between Castle and Moab Valleys. The canyon exposes the strata through the axis of the Courthouse syncline. Both the Moenkopi and Chinle undulate in thickness and rise and fall in relation to the river reflective of their relationship to the syncline (below). The Chinle can expand to a thickness of 700 feet and in other areas completely pinch out. Even the Wingate Sandstone at the top of the steep escarpments that form the walls of the valleys and Moab Canyon display vertical cracks and scallops indicative of the buckling effects of salt-intrusion on the landscape.

Modified from Doelling, 2001
Perhaps the most dramatic demonstration of the movement of salt is the recording of local angular unconformities within buried beds of the limbs of anticlines and synclines. As horizontal sedimentary beds (A) are intruded by an ascending diapir of salt, the overburden arches into a syncline (B). Halokinesis was rapid but sporadic, allowing the overlying strata time to erode to a flat plain on the landscape (C). With further horizontal deposition (D), an angular unconformity has developed. The angular unconformity (E, enlarged red ellipse) is a confirmation of the time when the salt actually moved the strata after the deposition of the cap rock but before the deposition of the more recent beds. 

As rising salt deformed the landscape into anticlines and synclines during the first phase of halokinesis, it dragged the overdurden upward forming angular unconformities in the process. Typically, they exist within the beds of the Moenkopi and Chinle and are readily seen along the River Road for a three or four mile stretch within Moab Canyon. The Chinle unconformity in the photo is in a region called the Big Bend of the Colorado River. 

There, angulated lower beds within the Chinle Formation, referred to as "lower mottled strata", are amongst the oldest in the region, exposed in an isolated outlier along Moab Canyon between Castle and Moab Valleys. Not to be confused with the ubiquitous basal Shinarump Conglomerate Member of the Chinle (although both are roughly-time and facies-equivalent), the basal unit (below) was tilted about 10º early in the Late Triassic while the salt was inititating its ascent, likely caused by salt removal from the Courthouse syncline (see cross-sectional diagram above) as it was laterally shunted to the rising diapir. The tilted bed was then truncated by erosion and subsequently covered by normal, flat-lying Chinle strata after the resumption of deposition. Voila! An intra-formational, angular unconformity. 

Incidentally, as the region was buckling under the ascent of salt, three different units have been recognized at the base of the Chinle that are solely regional and not found elsewhere. The diversity is due to the movement of salt that created isolated depo-basins adjacent to the anticlines.

The rise and collapse of the salt-anticlines are thought to have stimulated this localized, basal deposition even prior to the deposition of the rest of the Chinle above the Tr-3 unconformity that everywhere else on the Colorado Plateau separates the Moenkopi and Chinle. 

One more finding. In some places where the Chinle has been "dragged" upward by rising salt, it rests on underlying Cutler rather than Moenkopi, and even elsewhere on steeply tilted beds of the Paradox Formation. And further east within the Uncompahgre highlands, the Chinle rests directly on the Precambrian igneous and metamorphic rocks that served as a core for the Ancestral Rocky Mountains. This is yet another manifestation of the Great Unconformity of 1.5 billion years!

Intra-formational unconformities such as within the Chinle are far less common outside of the Paradox region and indicate a rapid rate of deformation during the Late Triassic. The time gap of angular unconformities is typically on the order of tens to hundreds of millions of years, as plate tectonic forces gradually alter the landscape. The Chinle unconformity, being a product of salt tectonics, is on the order of many thousands to perhaps a few million years, as salt gradually rose and deformed the landscape.

The intra-formational angular unconformity within the Chinle Formation of Moab Canyon is a manifestation of rising salt and its effect on the landscape. The process encompasses the interplay of events that occurred regionally, globally and astronomically. It never ceases to amaze me. 

"Ancient Landscapes of the Colorado Plateau" by Ron Blakey and Wayne Ranney, 2008.
"A Traveler’s Guide to the Geology of the Colorado Plateau" by Donald L. Baars, 2002.
"Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado" by Robert Fillmore, 2011.