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.

FISHER VALLEY, UTAH
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.

LANDSCAPE ARCHITECTURE IN REVERSE
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.

THE "BIG PICTURE" FROM WAY UP
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.

FISHER TOWERS
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.

CASTLE VALLEY
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. 

CASTLETON TOWER
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.

THE MIDDLE PENNSYLVANIAN PARADOX BASIN
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 FACIES OF THE PARADOX BASIN

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.

ALONG THE FRONT...

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.

PROXIMAL TO THE FRONT...

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.

MEDIAL TO LATERAL 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

STRAT STATS

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.

BURIED BUT NOT FORGOTTEN
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 Uncompahgre 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.

GOT SALT?
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.

HALOKINESIS
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.

SALT TECTONICS
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!

EVAPORITE-DISSOLUTION COLLAPSE-STRUCTURES
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. 

A SCHEMATIC GEOMORPHIC SCENARIO
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

  
IT'S ALL ON THE MAP
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



FOUR PHASES OF HALOKINESIS 
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. 

SALT DISSOLUTION FEATURES OF A DIAPIR-GENERATED TOPOGRAPHY
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

   
SALT TECTONICS AND THE GENERATION OF AN UNCONFORMITY
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. 

THE INTRA-FORMATIONAL, CHINLE ANGULAR-UNCONFORMITY WITHIN MOAB CANYON
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.

IN CONCLUSION
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. 

VERY INFORMATIVE RESOURCES
"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.

A Curious Intra-Formational, Angular Unconformity within the Chinle Formation: Part I - A Conspiracy of Events

Within Moab Canyon on the Colorado River between Castle and Moab-Spanish Valleys, the Chinle Formation possesses a spectacular angular unconformity. Its distinctiveness resides both in its intra-formational locale (rather than between two lithologically distinct formations) and the tectonic context in which it originated. What events conspired to create this curious deformational feature within the Chinle? What can it tell us about the ancient landscape?  The answer is contained in the interplay of events that occurred regionally, globally and even astronomically.

WHAT’S AN ANGULAR UNCONFORMITY?
If the successive, horizontal deposition of sedimentary rock layers is interrupted, say by erosion of a layer or a failure of deposition, the gap in time between the strata of different ages is called an unconformity. Unconformities are extremely common in the rock record and generally indicate a regional or even global geological event.

Angular unconformities occur where an older, underlying package of sediments has been uplifted, tilted and truncated by erosion, followed by a younger package that was deposited horizontally on the erosion surface. This gap in the rock record generally occurs from a regional tectonic event which changes the altitude and attitude of the bedding before sedimentation resumes. Compare the diagram below with the photo above. 

WHERE ARE WE?
We’re on the Colorado Plateau in east-central Utah within the Paradox basin of late Paleozoic time. Paleogeographic reconstructions place us between 5º and 15º north of the paleo-equator during the Triassic, the time of deposition of the Chinle Formation.  The town of Moab and Canyonlands National Park are off to the southwest, while Arches is just to the north.

The unconformity is east of town within Moab Canyon along the Colorado River across from Scenic Byway 128. Running from the northeast to the southwest, the Colorado transects a succession of NW-SE-trending, salt-generated, anticlinal valleys (first Onion-Fisher-Sinbad, then Salt-Cache, Castle-Paradox Valley) before entering Moab Canyon (the location of our unconformity and others), and then emerges from the canyon into another salt-intruded anticline at Moab-Spanish Valley.

The Colorado River flows NE to SW through a succession of salt-intruded valleys.
The Chinle unconformity in the photo is exposed at river level within Moab Canyon.
It is displayed at numerous locations throughout the basin.
Google Earth

Once again, what processes are responsible for the formation of the unconformity? Hint: The region’s many anticlines, synclines and the unconformity share a common genesis.

THE PENNSYLVANIAN AND PERMIAN PERIODS OF THE LATE PALEOZOIC
The Pennsylvanian and Permian Periods herald the close of the late Paleozoic, a time of expansion for marine invertebrates, gigantism amongst arthropods, the diversification of terrestrial stem tetrapods, and the advent of the amniote egg. Pennsylvanian coal forests in eastern North America’s more northerly paleo-latitudes attest to swampy, humid conditions, while western paleo-equatorial North America was largely arid. At the South Pole, extensive glaciation repeatedly waxed and waned causing global sea level to successively rise and fall. The wide range of climatic extremes was related to the development of a supercontinent, when things came together tectonically.

Pangaea before the initiation of break up in the Early Permian (280 Myr)
Note the orogen within the Laurussian-Gondwanan collision zone
and the South Polar continental ice sheet.
Ron Blakey and Colorado Plateau Geosystems, Inc.

Near the end of the Mississippian Period, the majority of our planet’s landmasses began to assemble into a supercontinent called Pangaea (Greek for “all lands”). It spanned the poles and was surrounded by a vast global sea called the Panthalassic (Greek for “all oceans”). Pangaea was largely the unification of the megacontinents of equatorial-situated Laurussia (North America and Eurasia) and australly-situated Gondwana (most of the modern South Hemisphere continents), and lasted for over 100 million years.

GLOBAL AND REGIONAL OROGENESIS
When continents tectonically collide, there’s nowhere to go but up. Orogeny (literally “mountain creation”) occurs when landmasses converge. The competition for space within the Laurussian-Gondwanan collision zone created a Himalayan-esque, trans-global mountain chain. Today, the eroded remnants are distributed amongst Pangaea’s globally-rifted siblings, and in North America, form the Appalachians.

The unification of Laurussia and Gondwana brought Africa into contact with North America’s eastern margin (using contemporary coordinates) along the Appalachian-Caledonian-Herycnian suture, which extends through Greenland into western and northern Europe. Along the collision zone to the southeast, South America accreted at the Ouachita-Marathon-Sonoran suture, building mountains from Arkansas and Texas into Mexico.

Curiously, the South American collision is thought to have created a second mountain system further to the west of the suture within Laurussia’s interior called the Ancestral Rocky Mountains (circled on the map below). 

The red dot depicts the location of the future Chinle unconformity.
Late Pennsylvanian paleomap (300 Myr ago)
Modified from Ron Blakey and Colorado Plateau Geosystems, Inc.

ENIGMATIC ORIGINS
Traditionally, the uplift of the Ancestral Rocky Mountains has been ascribed to a continent-continent collision of the conjoined masses of Laurussia and Gondwana. But not all tectonic aficionados agree with the intraplate geometry of a South American collision from the southeast having raised a range that trends NW-SE and so far-afield from the effects of the Ouachita-Marathon convergent margin. They also find fault (pun intended) with the extensionally-derived, “pull apart” structure of the marine basins that also formed as a part of the Ancestral Rockies. Opponents advocate for a volcanic arc-collision occurring somewhere from the southwest, likely within Mexico, which fits better with the Ancestral’s orientation and the compressionally-derived, foreland structure of its basins. 

The arrow indicates the traditional collision vector from the southeast.
Modified from Wood (1987) and Houch (1998)

A third hypothesis (and there’s undoubtedly more) evokes pre-existing weaknesses within the craton that, when compressed, uplifted the range along deep Proterozoic basement lineaments, a Precambrian "inherited" defect, if you will. In "Canyonlands Country" by geologist Donald Baars, he says "These deep-seated Precambrian faults set the geological stage, and will come back to haunt us throughout geologic time."   

TECTONIC INHERITANCE
Rodinia was the supercontinent that preceded Pangaea by half a billion years, give or take. When Antarctica separated from Rodinia’s southwest paleo-shore in the Late Proterozoic-Early Cambrian, the rifting event sent extensional shockwaves through the craton. Notice the orientation of the normal faults within Rodinia's interior (below). The NW-SE trend of the Ancestral’s ranges and basins reflects these deep-seated, basement-penetrating structures.

These zones of structural weakness were predisposed to future re-activation during Pennsylvanian-Permian compressional tectonics and even Cretaceous-Tertiary age Laramide contractional structures (such as monocline orientation). Tectonic inheritance of structural features in continental cycles, especially with intraplate orogenesis, is a recurring theme in the science of plate tectonics. We’ll see inheritance resurface later (literally) in our discussion of the Chinle unconformity.

Incidentally, the Late Proterozoic rifts that formed throughout Rodinia when it fractured apart likely induced "inversion" tectonics (extensional faults rejuvenating contractionally) in cratonic platforms of its rifted siblings worldwide.

ANCESTORS OF THE ROCKIES
The Ancestral Rocky Mountains, named after the modern Rockies that would eventually reside in roughly the same locale, rose from the sea in western equatorial Pangaea beginning in the Late Mississippian, reached their greatest intensity in the Middle Pennsylvanian, and ended their ascent in the Early Permian. An enigma to this day, they rose amagmatically (without volcanism) in an intra-cratonic and intra-plate setting far from any known plate boundary (1,500 km).

They consisted of a collection of crystalline, Precambrian basement-cored, NW-SE-trending ranges (referred to as highlands and uplifts) and paired fault-bounded depressions (referred to as basins and troughs) from Chihuahua, Mexico, through Oklahoma, Texas, Colorado, Utah and up to British Columbia, Canada. Initially, the many basins were in communication with the marine waters of the Panthalassic Ocean. 

Middle Pennsylvanian (300 Myr ago) paleograph of Pangaea’s Southwest
Illustrating the uplifts and basins of the Ancestral Rocky Mountains.
Note the future location of the Chinle unconformity (red dot) within the Paradox Basin.
Modified from Ron Blakey and Colorado Plateau Geosystems, Inc.

THE UNCOMPAHGRE UPLIFT AND THE PARADOX BASIN
On the southwest flank of the Ancestral range, the Uncompahgre (UH) highlands (alternately called an uplift) was bordered on the east by the Central Colorado basin (CCB) and on the west by the elongate Paradox basin (PaB). Tectonically associated with the highland’s rise, the Paradox basin rapidly subsided and assumed an asymmetric profile 200 miles in breadth and as much as 33,000 square miles (about the size of Maine). As the entire range ascended, erosion worked to bring it down, shedding deposits into the waters of the intervening basins in large debris fans. The Paradox basin's relationship with the sea became intermittent but with astounding regularity.  

Map of the Paradox Basin, the extent of which is delineated by salt of the Paradox Formation.
The red ellipse encloses the region of our unconformity.
Modified from Nuccio and Condon, 1996.

CYCLIC SEDIMENTATION
Closest to the rising front, 16,000 feet of the Uncompahgre’s arkosic, Precambrian sediments were shed into the Paradox basin as it rapidly subsided (northeast in diagram). Moving away from the highlands, the high seas poured into the deepest portion of the basin from the north and south. When the cyclically-oscillating global seas dropped low enough, the basin’s shallow shelf (labelled southwest) prevented the entry of sea water.

Cut off from the sea, the basin became a hypersaline lake as water evaporated within the restricted basin in the hot, arid Pennsylvanian climate of western Pangaea. Salt brines precipitated from the briny solution and settled to the deepest depression of the basin where they accumulated. The depositional scenario reversed when sea level cyclically rose again, reentered the basin and diluted the briny concentrate. And so on.



Schematic cross-section through the Paradox basin with carbonate shelf facies (pink) to the southwest, evaporite facies (olive) in the basin center and northeastern clastic facies against the Uncompahgre highlands. Notice that the Uncompahgre highlands and their parent Ancestral Rocky Mountains are cored by Precambrian basement rocks (gray) that were shed back into the basin subsequent to the range's tectonic uplift.
Modified after Stevenson and Baars, 1986

These events repeated an amazing 33 times with pulse-like regularity and are recorded within the multiple evaporite-cycles of the Paradox Formation, called cyclothems. The deepest portion of the basin received as much as 6,000 feet of evaporite-dominated sequences and is the location of our Chinle unconformity. For the record, the broad, shallow outer-shelf of the Paradox basin was teeming with marine life (note the algal mounds above) to the south and southwest. This region of the basin accumulated carbonate-dominated deposits that were also affected by the global oscillations of the sea. The basin sequences are found within the Paradox Formation of the Hermosa Group.

From geomechanics.geol.pdx.edu

The Paradox Formation was conformably succeeded by the alternating terrestrial eolian and fluvial, and marine shales and limestones of the Honaker Trail formation, the uppermost unit of the Hermosa Group within the Paradox basin. Like the Paradox Formation, the Honaker Trail Formation continued to record cyclic sea level fluctuations but contained no evaporites.

ABSAROKA HIGH SEAS
The rising Pennsylvanian and Permian seas that flooded the Paradox also inundated other neighboring basins and low-lying regions both regionally and worldwide. Called the Absaroka transgression, it was not a smooth event but progressed with sea levels that constantly rose and fell, withdrawing and advancing onto land and communicating basins.

From earthscienceinmaine.wikispaces.com

For the record, the Absaroka wasn’t the first marine highstand to flood the planet. It was actually the fourth of six complete transgressive-regressive cycles during the Phanerozoic. Why global changes in sea level occur, called eustasy, is a complex process partially involving tectonoeustasy (with the shallowing of ocean basins in rift zones) and glacioeustasy (as climate triggers glaciation and deglaciation).

PENNSYLVANIAN POLAR ICE
Pangaea lasted about 100 million years from the Late Mississippian period until the Late Triassic, when it ultimately fragmented apart. Like previous supercontinents, its enormous landmass profoundly influenced the Earth’s geosphere, atmosphere, hydrosphere and biosphere. With progressive cooling, Pangaea was thought to possess extensive continental glaciers at the South Pole that locked up a substantial portion of the planet’s water, enough to lower the level of the global seas. Conversely, deglaciation flooded the seas and basins with which the seas communicated. We are witnessing this process today in reverse as deglaciation adds water to the planet’s hydrologic budget and triggers a rise in sea level.

From wikipedia

GLACIOEUSTASY
Thus, the basins of the Ancestral Rockies received marine waters that cyclically fluctuated with the waxing and waning of glacial ice, estimated to range from 100 to 230 meters of sea level change. Spanning 60 million years, the late Paleozoic ice age was the most severe glaciation in the Phanerozoic, far exceeding the more familiar ice ages of the Pleistocene in the northern latitudes.   

Why South Polar glaciation was triggered during the late Paleozoic has a great deal to do with the formation of Pangaea. Stretching from pole to pole, ocean and atmospheric circulation was drastically altered. Mountain ranges were uplifted that altered wind patterns and precipitation. Climate determinants, however, were not only terrestrial but extra-terrestrial.

MILANKOVITCH CYCLES
Cyclic sedimentation in Pennsylvanian rocks is not unique to the Paradox basin but has been recognized in basins around the world. After all, the Absaroka transgression was a global event that affected all low-lying regions in communication with the sea. The consensus is that the sea level changes were caused by regular climate fluctuations that triggered the alternating accumulation and melting of glacial ice in Pangaea’s South Polar region. While the waxing and waning of Pennsylvanian polar ice is the source of the cyclic changes in sea level, the cause of the fluctuations of the climate is thought to be extra-terrestrial or astronomical.

Our planet derives its energy from the sun, but the amount of energy we receive is not always the same. The late Paleozoic sun was less bright than it is today, 3% less than modern values. But solar luminosity (the amount of energy that reaches us) is also related to sunspots and the Earth’s orbit. The Earth gyrates and wobbles in its solar orbit such that the amount of sun reaching our planet varies. Milutin Milankovitch, a Serbian geophysicist in the 1920’s and 30’s, hypothesized that climatic fluctuations are related to the position of the Earth as it travels about the sun.

Orbital factors such as precession (axis wobble), obliquity (axis tilt) and eccentricity (roundness) effect the amount of light reaching the Earth’s surface (solar insolation), and hence affect the planet’s climate. Each of these motions possesses a time period, the sum of which affects climate by driving the hot and cold cycles that produce glaciation. Orbital variations clearly had a substantial impact on Pangaean ice volume. Within the cyclothems of the Paradox basin, the repetitive successions (cyclicity) of Pennsylvanian marine and non-marine sediments are considered to be the stratigraphic signature of orbitally-controlled ice volume fluctuations during the late Paleozoic. 

From windows2universe.org

Why are the effects of the Milankovitch cycles “suddenly” seen in the late Paleozoic? The cycles have likely been occurring over a vast period of geologic time, but conditions were optimal for recording the changes with Pangaea sprawling across the South Pole, a climate perfect for glaciation and deglaciation, and shallow marine conditions within the basins of the Ancestral Rocky Mountains. Small periodic changes in sea level profoundly affected evaporite sedimentation and cyclization within the Ancestral’s basins.

PARADOX BURIAL
The entire process of mountain-uplift, basin-subsidence, oscillating sea level and cyclic salt deposition continued throughout the Middle Pennsylvanian and into the Early Permian. During the Permian, highland uplift and basin subsidence continued but at a declining rate as deposits of the Cutler Group (strat column above) derived from the Uncompahgre uplift blanketed the cyclic deposits of the Hermosa Group. Eventually, the Paradox basin was overtopped as the Panthalassic shoreline made a final wavering westward retreat.

Although greatly eroded in the Triassic, the remnants of the Ancestral Rockies (assisted by the Mogollon highlands to the south and the distant Southern Appalachians to the east) covered the Paradox basin in its entirety with the Lower Triassic Moenkopi Formation’s deep red mix of tidal flat and coastal plain sandstones, mudstones and shales. The Triassic closed with sandstones, siltstones, conglomerates, mudstones and limestones of the Upper Triassic Chinle Formation deposited within an alluvial and lacustrine environment. Like the Moenkopi, the Chinle was derived regionally from the same sources especially the much-reduced Uncompahgre highlands.


Paleographic reconstruction of Pangaea's Southwest
during deposition of the Owl Rock Member of the Chinle Formation.
The Chinle's source is from the Uncompahgre highlands, the Mogollon highlands
and the distant Southern Applachians to the east.
Modified from Blakey and Gubitosa, 1983 and Fillmore, 2011

The angular unconformity within the Chinle Formation is located several miles west of the uplifted front of the Uncompahgre highlands in the shadow of its eroding flanks and within the confines of the deepest portion of the infilled Paradox basin. And let the truth be told, the beds of the underlying Moenkopi Formation and the even-deeper Cutler Formation also possess similar unconformities from the same regional geological scenario, which has yet to be discussed.

As for the once precipitous Ancestral Rockies, it wasn’t until the Jurassic that eolian sediments finally buried the once great range. Deposition and burial continued with the epeirogenic inland seas of the Cretaceous and Early Tertiary, further entombing the detritus of the Ancestral Rockies, the only remaining record of their existence.

THE BIG PICTURE BEGINS TO TAKE SHAPE
In summary, a complex relationship likely exists between Rodinia’s fragmentation, tectonic inheritance and Ancestral Rocky Mountain orogenesis; and between the Pangaean climate, astronomical solar forcing, cyclical South Polar glaciation, Absaroka glacioeustasy and cyclical evaporite sedimentation.

But there’s more to the story, and I’ve run out of space. We still must explain the genesis of the intra-formational, angular unconformity within the Chinle Formation, and if you haven't guessed by now, it has to do with salt.

Stay tuned for Part II.

VERY INFORMATIVE RESOURCES
"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.

The Ancestral Rocky Mountains and their Eroded Remnants

“First there is a mountain, then there is no mountain, then there is.”

Buddhist proverb and 1967 Donovan’s song

OLD ROCKIES AND NEW ROCKIES

North America has experienced two lofty mountain ranges called the Rockies during the Phanerozoic Eon, the time period in which multicellular life existed on Earth (the last 542 million years). The first was the Ancestral Rocky Mountains that has long since eroded away. We know of its existence because of its eroded remnants. The second, the modern Rocky Mountains in all their majesty, are also eroding, but are still in a state of uplift.

The details of the origin of the Ancestral Rocky Mountains remains a mystery to this day. What explanation we have is based on circumstantial evidence, namely their remnants and other geological clues. Of interest to this discussion are those remnants. "How did they get there?" "Where can they be seen today?" To bring greater meaning to the geological significance of these deposits, let's explore the process of formation of both mountain ranges.

THE BIG PICTURE IN THE EARLY AND MIDDLE PALEOZOIC

The ancient supercontinent of Rodinia rifted apart in the Late Proterozoic, spawning the smaller, but no less diminutive, supercontinents of Laurentia and Gondwana in the process. Amongst other effects, that event left a long passive margin on the western seaboard of Laurentia. Beginning with the Cambrian Period, extensive sedimentation took place on this long and broad, coastal margin, where subsidence prevailed for over 200 million years. The orogenic rise of the future Ancestral Rocky Mountains and the modern Rocky Mountains would occur from this sea level locale, but both ranges were more cratonically positioned. During the early and middle Paleozoic, Laurentia gradually morphed into Laurussia by the accretion of various magmatic arcs and micro-continents along its eastern seaboard. Laurussia’s passive, western seaboard became active in the late Paleozoic, and continues as such to this day. The new tectonic regime changed the face of western North America and the future Colorado Plateau, while its eastern seaboard has remained passive with the spreading of the Atlantic Ocean.

LOTS OF PLATE CONVERGENCE BEGINS IN THE LATE PALEOZOIC

Both the ancestral and modern ranges were formed as a result of the interactions of converging tectonic plates. In the case of the Ancestral Rocky Mountains, the convergence involved two massive plates, the Gondwana and the Laurussia plate. That formed a continent-continent boundary in the late Paleozoic. The modern Rocky Mountains, on the other hand, formed from the convergence of an ocean-continent boundary between the Farallon and the North American plate, respectively, in the Mesozoic and early Paleogene.

Modern plate tectonic theory establishes the location of orogeny or mountain-building in association with the converging plate boundaries. In the case of ocean-continent plate convergence, mountain-building is where the collision zone replaces the consuming margin. This produces subduction, destruction of ocean lithosphere, earthquakes, and a line of very active volcanoes. For continent-continent plate convergence, a powerful collision occurs. This produces intense compression, folding and faulting of rocks, and deformation extending into the plates' interiors. In either case, mountain ranges are generated in association with the plate margins. In contradiction to tectonic-tenets, both the Ancestral and modern Rocky Mountains occupied decidedly intraplate-locations and at significant distances from their converging plate boundaries. The specifics of the collisions and the types of structural deformation that formed both mountain ranges differ greatly.

GONDWANAN-NORTH AMERICAN PLATE CONVERGENCE: A CONTINENT-CONTINENT COLLISION

In the case of the Ancestral Rocky Mountains, the austral-polar supercontinent of Gondwana converged upon the plate of the low- to mid-latitude supercontinent of Laurussia during the Pennsylvanian and Early Permian Periods. The ensuing continent-continent collision, approximately at today’s eastern seaboard of North America, created the lofty Appalachian Mountains at the juncture of the converging plates. That event is referred to as the Alleghenian Orogeny. That massive collision put the finishing touches on the assembly of a new, massive supercontinent called Pangaea. It wasn't until Pangaea was torn apart by rifting from the Late Triassic through the earliest Paleogene that the Appalachian chain would remain on North America's eastern margin.

The supercontinents of Gondwana (visible in lower right) and Laurussia (nascent North America)

are on a collision course during the Late Devonian (365 Ma).

Note that western North America

is largely submerged.
From Ron Blakey, NAU Geology and Ancient Landscapes by Blakey and Ranney

A Middle Pennsylvanian (307 Ma) depiction of the Gondwanan-Laurussian supercontinental collision 

showing the uplifted Appalachian Mountain chain.
Note the Ancestral Rocky Mountains in the southwest.
From Ron Blakey, NAU Geology and Ancient Landscapes by Blakey and Ranney 

THE BIRTH OF THE ANCESTRAL ROCKY MOUNTAINS AND THEIR MANY UPLIFTS AND BASINS

Gondwana was an amalgamation of the modern continents of South America, Africa, India, Australia and Antarctica at the time of its collision with Laurussia. Gondwana being so large, several orogenies occurred from the collision at various times and locations. One in particular, the Ouachita-Marathon Orogeny, resulted from the South American portion of Gondwana striking the future Gulf Coast region of North America. Far to the west of that suture-line and situated in an intraplate location, the Ancestral Rocky Mountains were created in parts of Colorado, New Mexico, Texas and Oklahoma from the late Mississippian to early Permian.

Competing hypotheses exist in attempts to explain the widespread deformational event (also referred to as the Ancestral Rocky Mountain Orogeny) being so far from the site of the Gondwanan collision and with structures oriented obliquely to the "known" compressional forces. No general consensus has been reached, although the  Gondwanan collision from the southeast is favored. Those advocates focus on pre-existing crustal weaknesses along fractures in the basement rocks in association with strike-slip faulting. More recent explanations invoke a tectonic collision from the southwest, a more logical explanation from a compressional mountain-building perspective. In such a scenario, the orientation of the Ancestral Rockies and their basins would be appropriately oriented perpendicular to the stress that formed them. Advocates of this hypothesis are looking at what was then the southwest margin of Mexico for a telltale subduction zone and ancient volcanic arcs.  

Regardless of a universally agreed upon tectonic genesis, the Ancestrals began their rise and created a very complex paleogeography that dominated sedimentation for the next 100 million years, give or take.

A Late Pennsylvanian (300 Ma) close-up depiction of the Ancestral Rocky Mountains

and their associated basins, currently inundated by marine highwater.
Note the uplifts in Texas and off to the southeast, located closer to the converging plates. 

Modified from Ron Blakey, NAU Geology and Ancient Landscapes by Blakey and Ranney

The Ancestral Rocky Mountains consisted of a series of mountain ranges (uplifts and highlands) and deep, associated, assymmetrical bounding basins (troughs). Tectonically-induced block-faulting was responsible for the formation of the intracratonic ranges which trended north- to northwest and radically affected sedimentation into the respective basins, especially marine shale, carbonates and coarse-grained arkosic detritus. Over this region, basin subsidence and basement uplift were approximately synchronous. Relevant to this post were the Uncompahgre Uplift and its neighboring Paradox Basin to the west, the Central Colorado Basin, the Front Range (Frontrangia) Uplift and the Denver Basin to the east. As the Ancestrals eroded, they shed their sediments into the basins filling them with thousands of feet of red, arkosic sandstone and shale. In regard to the Uncompahgre Uplift, sedimentation extended onto far reaching regions of the future Colorado Plateau. By the end of the Permian, the uplifts had completely eroded away, reduced to subdued, low hills and plains. 



A diagram of the numerous uplifts (red) and basins of the Ancestral Rocky Mountains.

Pertinent to our discussion, note the Uncompahgre Uplift (32), its associated Paradox Basin (44),

the Central Colorado Trough (36), and the Front Range Uplift (30) and its associated Denver Basin (31).
From www.colorado.edu/GeolSci/Resources/WUSTectonics/AncestralRockies/introduction.html

A diagrammatic view showing the Ancestral Rocky Mountains, its uplifts and basins.
The three red dots correspond to my discussion of erosional remnants at the end of this post:
Fisher Towers (near Moab, UT), the Maroon Bells (near aspen, CO) and the Flatirons (Boulder, CO).
Modified from Lindsey et al (1986)

 

A Middle Pennsylvanian close-up view of the Ancestral Rocky Mountains,

and the uplifts and basins germane to this post:

Paradox Basin (PaB), Central Colorado Basin (CCB) and Denver Basin (DnB).

Modified from Ron Blakey, NAU Geology

FARALLON-NORTH AMERICAN PLATE CONVERGENCE: AN OCEAN-CONTINENT COLLISION

The Ancestral Rocky Mountains were completely eroded away by the time the modern Rocky Mountains formed. The western migration of the North American plate, driven in part by the rifting Atlantic Ocean, converged with the oceanic Farallon plate. The ensuing subduction of the more dense, Farallon plate beneath the more buoyant, continental North American plate, approximately at what is today the western seaboard of North America, created crustal thickening and associated magmatism at the continental margin. This subduction event is known as the Sevier Orogeny (from the latest Jurassic to the Eocene). The Sevier is distinguished by having a second “phase” called the Laramide Orogeny (from the Late Cretaceous through the Eocene). The two are basically one continuous orogenic event with arbitrarily separated-phases occurring over a considerable overlap in time. Their differing effects on the landscape  are attributable to changing geometries over time associated with the subducting Farallon plate.


THE BIRTH OF THE MODERN ROCKY MOUNTAINS

Unlike the Sevier event, the Laramide Orogeny penetrated deeply into and profoundly affected the craton. It was the greatest mountain-building episode to affect the western U.S. That uplift was at considerable distance from the plate boundary and created the modern Rocky Mountains, and uplifted a fair share, if not most of, the Colorado Plateau. Uplift of the plateau is commonly linked with its erosional denudation. Thus, many of the existing erosional features of the Colorado Plateau were created such as its canyons, mesas, buttes, arches, bridges, hoodoos, spires, pedestals and towers. More on that later!

Interestingly, the Ancestral Rocky Mountains and the modern Rocky Mountains bear a vaguely similar position within the continent, near present-day Colorado. Laramide uplifts in many cases coincide with the location raised by the Ancestral Orogeny. This is no coincidence of location, indicating the "susceptibility" of the continental crust, once broken, to future re-activity. A subject for another discussion, preexisting Precambrian folds and faults comprising the cratonic basement exert a long-lasting affect.       



THE EROSIONAL REMNANTS OF THE ANCESTRAL ROCKY MOUNTAINS

No sooner had the Ancestrals begun to assume their lofty status than erosion began to wear them down. The  rocks that formed the core of the Ancestral Rocky Mountains were Precambrian metamorphic and sedimentary rock, the latter from the vast seas of the early Paleozoic, both forming the basement of the western North American continent. As the Ancestrals eroded throughout the late Paleozoic, they left extensive deposits of rock that was a signature of their core and cratonic basement (confirmed by dating techniques of detrital zircon geochronology). Those remnants can be viewed at Fisher Towers near Moab, Utah, the Maroon Bells of Aspen, and the Flatirons above Boulder, Colorado.

THE FISHER TOWERS NEAR MOAB, UTAH

The Uncompahgre Uplift dominated sedimentation into its associated Paradox Basin and throughout large areas of the Southwest during the Pennsylvanian and Permian Periods. Closest to the Uncompahgre Mountains, thick deposits of coarse-grained arkose were formed on huge alluvial fans and their flood plains, built against the mountainous front and stretching from eastern Utah to northern New Mexico. Uplift of the Colorado Plateau region during the Laramide Orogeny triggered the erosion that sculpted the towers, spires and pedestals of Fisher Towers.

Fisher Towers is located in Fisher Valley (a collapsed salt-anticline), which is about 20 miles northeast of Moab. On display are various shades of red-brown, red-purple and maroon sedimentary rock. Several of the upper, darker parts of Fisher Towers are capped by the lower sandstone remnant of the Triassic Moenkopi Formation. The Moenkopi is more resistant to erosion than the softer, underlying layers and, therein, helps to form the pedestals and towers of Fisher. The middle and lower parts of the towers are sandstone, mudstone and conglomerate of the Permian Cutler Formation. These rocks were deposited within rivers and streams flowing south from the Uncompahgre Uplifts that formed at the beginning of the Pennsylvanian Period (about 320 million years ago). By the end of the Permian Period (about 250 million years ago) the highlands of the Uncompahgre had succumbed to erosion, being reduced to low hills and plains. 

The conglomerate of the Cutler Formation contains cobbles and pebbles of quartz, feldspar, mica, granite, schist and quartzite derived from Precambrian crystalline rocks that were eroded into the Paradox Basin from the Uncompahgre Uplifts of the Ancestral Rockies. The coarseness of the conglomerate in the low cliffs is indicative of the nearness to the source of the sediments. Contained within the red, sandy-matrix of the Cutler Formation are Middle Proterozoic crystalline-clasts that look identical to the Vishnu-Zoroaster complex at the bottom of the Grand Canyon. These clasts represent the basement-core of the long-eroded Ancestral Rocky Mountains. The uplift of the Colorado Plateau that began 80 to 50 million years ago carved the erosional features of Fisher Towers. By the end of the Permian period, the highlands no longer existed but their erosional remnants remain as a signature of their presence.

Fisher Towers illuminated by the late day sun not far from Moab

THE MAROON BELLS NEAR ASPEN, COLORADO
There are two peaks in the Elk Mountains of Colorado, southwest of Aspen, both of which are over 14,000 feet. They are the Maroon Bells, called the "Deadly Bells" by the US Forest Service, owing to the "rotten and unstable" rock  for climbers that comprises the Maroon Formation. Between the Uncompahgre Uplift and Front Range Uplifts of the Ancestral Rocky Mountains existed an intervening basin called the Central Colorado Trough. It is here where the Pennsylvanian and Permian dark red clays, sandstones and conglomerates shed from the eroding Ancestrals accumulated known as the Maroon Formation. During the Laramide Orogeny, the Maroon Formation was folded and thrust westward over itself and over younger strata. Mesozoic rocks that lay above the Maroon Formation have largely eroded away.

Note that in central Colorado further to the east in the modern Eagle Basin, the Pennsylvanian Minturn Formation along the eastern margin of the Central Colorado Basin reflects a similar depositional lithofacies of largely arkosic, fan-delta and open marine deposits in a tectonically active setting. Similarly, the Sangre de Cristo Formation formed to the southeast in the Central Colorado Basin and the contiguous Taos Trough.



Looking up a classic, U-shaped glacial valley near Aspen at the Maroon Bells in the distance. 

THE FLATIRONS ABOVE BOULDER, COLORADO

The Flatirons form the most recognizable feature of the Boulder backdrop, soaring upward at an angle of over 50 degrees. The mudstone, sandstone and arkosic conglomerates of the Early Pennsylvanian to Early Permian Fountain Formation were deposited in the Denver Basin in the erosional shadow of the Front Range (Frontrangia) Uplift of the Ancestral Rocky Mountains. Faulting during the much later Laramide Orogeny is responsible for the extreme, near-vertical uplift, angulation and erosion of the strata.

The escalloped and vegetated Middle Permian Lyons Formation can be seen at the base of the Flatirons. The Lyons Formation visually tends to merge with the Fountain Formation, on which it lies. The dunes that lithified into the Lyons were blown from stream channels descending from the low remnants of the Ancestral Front Range.

The upthrown Flatirons shine in the morning sun above Boulder

IN SUMMARY
Fisher Towers, the Maroon Bells and the Flatirons bear a common thread. They all tell the story of an orogeny that happened long ago, of mountains that rose from the sea and towered over the region,  eventually succumbing to the forces of erosion and the ravages of time. If it wasn't for the Ancestral's erosional signature, we might well not have known of their existence. Such is the geological evidence. Our knowledge is partial and biased, constructed only from the fragmentary evidence that has been preserved. Yet, an incredible story is told. The beauty of it all.