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.