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Introduction

The Colorado Plateau is a nearly circular highland covering 13,000 mi² centered around the four corners region of Utah, Colorado, Arizona and New Mexico (figs. 1 an 2). Regional elevations range from ~5000 to ~14,000 feet.   The surface of the Plateau is warped by  broad, asymmetrical folds and fault-bound uplifts that create numerous local plateaus, such as the Kaibab Plateau, Paunsaugunt Plateau, Circle Hills Upwarp, Monument Upwarp and so on.  Hence, province is commonly referred to as the Colorado Plateaus, as originally named by John Wesley Powell, to emphasize the fact the Plateau is not a uniform, flat structural surface.   The geology of the Plateau's flat to gently inclined sedimentary layers is detailed, but relatively straight forward. It's landscape,  etched by water,  illustrates the great variety of landforms that can be sculpted from sedimentary rocks.

The plateau derives its name from the Colorado River, which cuts diagonally across it's surface from the northeast to southwest. Because of its unique and inspiring landscape, remarkable geology, and numerous archeological sites, the Colorado Plateau has 9 National Parks,  17 National Monuments, and nearly 30 wilderness areas--more than any other province in the United States. 

These parks are a good place to start our study for several reasons.  First, they are geologically and esthetically magnificent in their display of brightly colored rocks, and geomorphic features--such as deep canyons, arches and high mesas;  second, their rocky layers  are minimally deformed, facilitating the interpretation of their story; and third the arid climate bares the landscape, leaving the rocks largely uncluttered by thick vegetative cover.  Take it from someone who has mapped rocks while crawling on her stomach through dense spruce groves--this is a geologist's paradise!

 Before reading on, vlew this video from the NPS Canyonlands website. It's a great introduction to the Colorado Plateau, the variety of landforms you will see, and its cultural history.

Geologic History  - Setting the Stage

At the time the rocks forming the Colorado Plateau were deposited, the tectonic, continental, and climatic conditions were far different than present and constantly changing. For nearly 300 million years--extending from the late Precambrian through the Paleozoic, the Colorado Plateau region rested on the southwestern passive continental margin of Laurentia (proto-North America), where it received tens of thousands of feet of sediment.  From the Cambrian through the Permian periods, tropical seas rose and fell along the continental margin, laying down seemingly rhythmic layers of coastal sandstone, nearshore shale, and deeper water limestone that covered thousands of square miles.   During the Mesozoic Era the sea withdrew and the landscape fluctuated between several environments--including a fluvial plain, tropical forest, and vast arid desert--each leaving behind a signature layer of strata.  Towards the end of the Mesozoic, during the Cretaceous Period, another sea transgressed--this time from the east, blanketing portions of the plateau with sandstones, shales, and locally thick coal deposits. 

All of these rocky layers were stacked, sometimes inter-fingering, but mostly just one on top of another--like a finely layered cake. The sediment pile may have resembled that supporting the modern Atlantic Coastal Plain and Mississippi Delta region.  As each layer was deposited the land beneath subsided under the load, maintaining the surface close to sea level. As thick as this sedimentary package was, its flat gently dipping surface barely extended above sea level. It would have to be uplifted before rivers could dissect it.  Stream gradients were too low--incapable of scouring the canyons we see today.  However, this would change.

During the Late Cretaceous Period and into the Tertiary Period, compression, driven by subduction farther west, forced the region to rise several thousands of feet, and the Colorado Plateau was born.  Initially the greatest amount of uplift occurred in the southwest, across central Arizona, gently tilting the Plateau's surface northeast (fig. 5).   Otherwise deformation was broadly uniform with local zones of mild crumpling and faulting--adding variety to the erosional landforms that would developed across the Plateau.  The Laramide tilting and uplift initially caused rivers and streams to drained northeast, opposite to the modern southwesterly drainage.  By most accounts, the present southwesterly drainage of the modern Colorado River appears to be a relatively recent development--forming during the last 6 million years in the history of the Colorado Plateau. However, this will be discussed later.

Renewed uplift over the last 20 million years, coupled with rifting and opening of the surrounding Basin and Range Province, resulted in a rearrangement of the plateau's drainage and its recent incision by the modern Colorado River and it's tributaries.  Deep canyons, mesas, buttes and a variety of other landforms evolved as streams rapidly carved into and episodically stripped off layers of strata. The vast and varied landscape of the modern Colorado Plateau is the culmination of a long history of sedimentary deposition during the Paleozoic and Mesozoic Eras, followed by pulsating episodes uplift and erosion during the Late Cenozoic Era.

Formations, Features, Structures and Processes

Read the discussion below and follow the embedded links.  Make sure you understand the material presented on the content exercises at the end.
In the process of rapid down-cutting by streams, a spectrum of erosional geomorphic landforms--including large canyons, slot canyons, mesas, buttes, arches , natural bridges, hoodoos, cuestas, hogbacks, and badlands, have been carved into the Plateau's many layers. These features are called geomorphic or "rock"formations, not to be confused with stratigraphic formations.  Before continuing I need to clarify the usage of these terms. 

A stratigraphic  formation is defined as identifiable, mapable rock layer, representative of an interval of time. The lithology (character of the rock formation) contains features representative of a local or regional depositional (or volcanic) environment.  For example the Navajo Sandstone Formation is a Jurassic-age quartz-rich sandstone containing diagnostic sedimentary features created by migrating sand dunes in a desert environment.  Stratigraphic formations may be subdivided into members or grouped into a package of similar stacked formations.   An example would be the Glen Canyon Group that includes the Wingate, Navajo, and Kayenta formations, exposed in the walls of Glen Canyon, upstream from the Grand Canyon.  A sequence of stratigraphic formations in an area is represented by a stratigraphic column. (See Stratigraphy of Grand Canyon National Park.)  The pattern (dashed lines, brick, and dotted) indicates composition (shale, limestone, sandstone and respectively), and the profile of each unit reflects resistance to weathering and erosion. More resistant rocks, which are cliff formers, protrude more to the right.

The erosional features listed above, such as buttes, hoodoos, etc., are geomorphic formations, artifacts of a landscaped etched by erosion.  They are features shaped from the rock by processes of weathering and erosion. Towering buttes, like those in Monument Valley, and the fanciful hoodoos in Bryce Canyon are geomorphic rock formations.  The shapes of such formations are determined by the stratigraphic formations being eroded, their structure, and the processes responsible for shaping them. Because rocks vary in strength, texture, and structure they respond differently to weathering and erosion. Rocks that are finely layered with abundant clay or held together by soluble cement are quickest to disintegrate, leaving behind those rocks that are stronger.  Geologic structures are features that affect or cut across rock layers. Commonly they are the result of deformation.  Folds, fractures (e.g. joints and faults) and unconformities are examples.  They help develop the framework and pathways of erosion that influence the development of geomorphic formations.

Surface water flowing as runoff across rock surfaces is directed into rills, fractures, and other weak zones that are further excavated.   Erosion by water is the dominant process shaping the landscape. Although rainfall is only 10 inches/year or less over the Colorado Plateau, erosion is facilitated by brief intense storms occurring over sparsely-vegetated and unprotected land.   More importantly however, is the Plateau's high regional relief (ave.6000 feet) and steep stream gradients--without which rivers would be powerless to carve deep canyons.  While canyons deepen, runoff and mass-wasting attack the slopes.  Each rock layer exposed along a slope is eroded back in accordance with its strength. The final shape a landform takes is also strongly controlled by its lithology and structure. 

Each park that we visit on the Colorado Plateau has a different element of structure and lithology controlling its landscape.  These create the resisting framework that erosion progressively modifies.  Horizontal strata wear differently than folded strata, or fractured rock.   Towering flat-topped mesas, buttes, and pinnacles are created by the progressive erosion of horizontal strata (fig. 3) topped by resistant sandstone or limestone caprock.  Slopes retreat in a parallel fashion until the final slab of caprock is undercut and removed.  The remaining unprotected layers disintegrate into rounded hills unless a lower resistant layer is uncovered and a new caprock established.  If weak shale and mudstone lack resistant beds to protect them runoff carves  a fine network of rills and rounded hills known as badlands topography. Some formations on the Colorado Plateau typically develop badlands--like the Chinlee Formation in Petrified Forest/Painted Desert National Park in Arizona, and the lower portion of the Clarion Formation at Bryce National Park in Utah. Of course the best example of badlands topography is Badlands National Park in South Dakota where runoff  etches the soft shales that were deposited in the depths of an interior seaway during the Cretaceous Period.

Figure 3. Landforms created by the erosion of horizontal strata.   A caprock of resistant sandstone or limestone protects underlying weak shale and the slope progressively retreats parallel to itself. Eventually erosion causes collapse of the caprock and dissection of underlying units.

Rocks inclined and fractured by folding form another characteristic set of erosional landforms.  Cuestas, hogbacks, strike valleys, and flatiron-slopes are distinctly linear features eroded from dipping strata on the flanks of folds and monoclines--such as the Waterpocket Fold exposed in Capitol Reef National Park, Utah.   Where thick strata are cut by joints and faults, water diverted into the fractures excavates deep valleys--like the slot canyons seen in Zion National Park and many other area on the Plateau.  Continued erosion of canyon walls produces narrow rock fins, which can be seen in Bryce, and Arches National parks.

Why so much incision?

In addition to buttes and mesas, the Colorado Plateau is noted for its dramatic canyon landscapes. During the Late Cretaceous Laramide Orogeny the Colorado Plateau was pushed up several thousands of feet--a process that continues today in the High Plateaus section.  There is little doubt that the entire Colorado Plateau has been subject to more than one great pulse of uplift.  Uplift provides the steep gradients and energy required to carve deep canyons. 

Relief and base level are concepts that are intimately related and requisite to understanding erosion and canyon formation.  Relief is the difference between the highest and lowest elevation in an area of reference.  For example, the vertical relief of the Colorado Plateau measured from sea level to it highest point in the High Plateaus region (+11,000 ft) is over two miles.  However, the local relief on the surface of the Plateau ranges from 2,000 - 6,000 feet.  Relief limits the vertical boundaries of erosion.

Base level is the elevation where a stream discharges--above it erosion occurs, below it deposition occurs.   The higher the land is above base level the greater its erosion.  If the relief of a stream's drainage basin, from it's uppermost tributary to base level, is only 10 feet you wouldn't expect it to carve a 500-ft canyon.   The Colorado River is not the only river responsible for carving deep canyons. Many of its tributaries, such as the Green, Virgin, Paria, and San Juan have also incised magnificent canyons.  The tremendous relief (measured from the surface of the Colorado Plateau to sea level) produces steep gradients. Such gradients empower the Plateau's rivers with the energy to carve through its thick layers.   This is why there are so many deep canyons carved in the Colorado Plateau, and so few within tectonically dormant provinces, such as the Laurentian Uplands, Midwest, New England, and the Atlantic Coastal Plain.

The ultimate base level for most large river systems, including the Colorado River, is the ocean.  Exceptions however, exist in arid regions housing interior basins. Water does not flow to the ocean, but escapes through evaporation from the basin floor. Base level for these drainage systems is either a saline lake, playa, or dry basin floor.    (Great Salt Lake in Utah, and Death Valley in southwestern California are interior basins.)   So drainage systems can be classified as either external (flowing to the ocean) or internal (flowing to a internal basin).

During the Miocene the ancestral Colorado River may have flowed into an interior basin, trapped on the Plateau's surface, or off to some other basin in the Basin and Range. Regardless of where it once flowed, the river now descends steeply from the plateau into the Gulf of California. The Grand Canyon is the product of this adjustment.  In addition, the Plateau seems to still be rising, forcing streams to vertically carve while readjusting to base level. Once uplift ceases, the youthfull landscape will age as streams direct their energy to lateral erosion widening valleys and reducing the Plateau's surface.

Basement and Marginal Structures

What makes the Colorado Plateau unique relative to its neighboring provinces, such as the Basin and Range and Rocky Mountains, is its relative immunity to complex deformation.  It has always shifted as a coherent block. The plateau simply rose during the compressional Sevier and Laramide events, while its neighbors were sliced by compressive faulting.  Later, when tensional stresses began ripping apart the Basin and Range, the Colorado Plateau again behaved like a durable knot in a plank of splintering wood--rising and rotating but remaining coherent.   The cause of this enigmatic behavior is not entirely understood, but geoscientists hypothesize that the ancient metamorphic and granitic basement beneath the Plateau's sedimentary cover is simply thicker and stronger than the basement underlying surrounding provinces.  (Read This Old Continent: Constructing the Basement of North America by Brad Ilg)

The Plateau's crystalline basement and overlying cover of sedimentary and volcanic rocks reveal nearly 2 billion years of Earth history. In the late 19th century exploration of the plateau by such notable geologist as John Wesley Powell, Grove Karl Gilbert, and Clarence Dutton played a key role in the development of modern geology.  Scientists from the world over are still drawn to the classic geological localities of the Plateau region.

The Colorado Plateau is bounded by normal fault zones that isolate it from the adjacent Basin and Range, and Rocky Mountain Provinces. On the west side the Plateau is separated from the Basin and Range Province by the Grand Wash, Hurricane, Sevier and Paunsaugunt faults (fig. 2), which were reactivated during the Tertiary-Quaternary Basin and Range deformation, but have their origin linked to earlier Late Precambrian or Paleozoic deformation. The Mogollon Rim, the northeast retreating scarp of the Kaibab (limestone)-Coconino (sandstone) Plateau of the Grand Canyon region, is bordered on the south by the Verde Fault and Transition zone.   The Mogollon Rim is the remains of a mountainous highlands that was uplifted during the Laramide Orogeny and then torn apart by rifting during the formation of the Basin and Range.  The compressional Uinta Range lies to the north, and the Oligocene Rio Grand Rift defines the plateau's eastern margin. What makes the Colorado Plateau unique structurally is that it has not yet suffered the degree of internal fragmentation as adjacent provinces.  Nevertheless, rift zones are wrapping around the Plateau and slowly tearing it apart.

In addition to the normal faults, the Plateau is also warped by gentle folds.   The most prominent fold structures on the Plateau are broad anticlinal upwards and monoclines.  Many folds are related to normal faults yet to break the surface.  The unique topography  of Capitol Reef National Park is attributed to the Waterpocket Fold, a 100-mile long monocline formed between 50-70 ma during the Laramide Orogeny. The Kaibab Uplift cut by the Grand Canyon is an broad doubly plunging anticline.

The six subdivisions of the Colorado Plateau

Figure 5. Above is a geologic cross-section drawn from the Grand Canyon(right) Northward to Zion and Bryce canyons (left) illustrating the structure of the Colorado Plateau. Over 2 billion years of history are recorded in the plateau's sedimentary layers and underlying basement. Warping and differential erosion of layers north of the Grand Canyon created a series of terraces and cliffs resembling a large staircase, known as the Grand Staircase. Terraces mark where easily eroded shales were stripped from the surfaces of  thick layers of resistant sandstone and limestone. Riser mark the headward retreating scarp of these resistant layers. The recently activated faults that cut the plateau have a long and varied history. Warping of sedimentary rocks over these basement faults created folds, such as broad anticlines (upwarps) and monoclines, adding to the variety of features found on the plateau.