Friday, November 25, 2011

Hopi Corn, Kachina Rain and Lessons from the Past

“Over your field of growing corn
All day shall hang the thunder-cloud;
Over your field of growing corn
All day shall come the rushing rain.”

Last stanza of Korosta Katzina Song from the Hopi corn-planting dance

This thousand year old petroglyph at Hopi Clan Rocks in Northern Arizona depicts lightning and clouds with rain falling on a stalk of corn. The rock-carving was created by chipping through a thin veneer of desert varnish into the lighter colored, virginal surface of a displaced block of Wingate Sandstone.

The Hopi people or “peaceful ones” are thought to have migrated north out of Mexico around 500 B.C. Primarily living on a 1.5 million acre reservation in northeastern Arizona in the Four Corners area, the Hopi have the longest authenticated history of occupation of a single area by any Native American tribe in the United States.

The Hopi have no religion in the traditional sense. Hopi life IS Hopi religion. There is no separation of a religious life from all other activities of the Hopi. Planting corn is a religious activity, amongst others, that ensures the continuation of life.

For the Hopi, corn is viewed as a metaphor of life. The Hopi say, “Um hapi qaa’oniwti.” “People are corn.” Beginning as seeds, as in a womb, life emerges, blessed by light and nourished by family. A Hopi child is brought from the house on the twentieth day and receives corn as the sun emerges on the eastern horizon. Throughout life, Hopi live with corn as the mainstay of their diet. For Hopi, death is part of the cycle of life. Death does not end a person’s presence in the physical world, but marks a transition from one state of being to another.

The Hopi believe that it is through respect of nature and spirit essences of the world of the Katsinas that will bring the rains needed to support life. It is both a reciprocity of life and rain that makes the corn grow. It is also the cycle of the corn seed becoming both the food for Hopi and the seeds of the future, and of Hopi life itself. The Hopi emerge and live only to die, and yet continue as ancestral Hopi to support their offspring as the spirit essences that bring rain. At death and their emergence into the Fourth World, Maasawu, the god of death, instructs the people on how to farm the land, to use it only with humility and with good harmonious hearts. Arrogance, disrespect, greed and failure to maintain their obligations to the Creator would bring sparse rains and their labor would be in vain. 

The spirits of important Hopi leaders go to the San Francisco Peaks, north of Flagstaff. Each year, the spirits return to Hopi Land during the Kachina season as bearers of rain, riding within billowy, white clouds. They come in response to Hopi prayers and powers generated by their ceremonies. The rain brought by the Kachinas is essential to crops of the Hopi, as it augments their only other water supply, ground water, a shrinking resource today. The Hopi know that a drought can come at any time. They know that their conduct has a direct bearing on the amount of rain that comes. If the Hopi behave badly, the Kachinas will be displeased and refuse to bring rain. Without rain, nothing will grow, and there will be nothing to harvest in the fall.

Ancestral Puebloans, such as the Hopi, have been cultivating crops adapted to the arid climate of the Colorado Plateau for thousands of years. The Hopi, who have had a long and deep cultural relationship with the Southwest's aridity, use a practice called dry-land or un-irrgated farming by taking advantage of run-off and flood-water from mesas. They farm at the mercy of the spirits to answer their prayers.  

Historically, in the late 1200’s, a massive and prolonged drought forced most of the Hopi villages on the mesas to be abandoned. Perhaps after years of intensive use the land and its resources were depleted. In the face of environmental stress, social and political conflicts are thought to have arisen. For well over a decade, widespread and persistent drought conditions have again plagued the region. Climatologists predict an indeterminate length to these conditions both regionally and globally. Many predict worse. In response, the Hopi Tribe and Navajo Nation’s resource managers are developing a regional climate monitoring network and are discussing long-term climate change adaptation to better prepare for the climate of the future.

The lessons of geologic history teach us that western North America has experienced some of the most long-lived arid conditions in Earth’s history. Widespread eolian sandstones in the geologic record bear testimony to this fact. In the Glen Canyon region alone, seven different eolian units are exposed. Drawing the majority of their waters from snow melt in the Rockies, Lake Powell and Lake Mead have achieved record low levels. And in the Southwest, population growth and demands continue to increase. The notion that severe arid conditions are only temporary regionally or can’t be experienced globally should be entertained only with reckless arrogance and abandon. This is true independent of one’s philosophical position on the causes of climate change.

Hopi corn, as with most agricultural crops, can tolerate only a narrow latitude of temperature extremes, drought and flooding, and pathogen and pest resistance. Advances in agricultural knowledge, technology and science are critical to improving crop traits such as tolerance. Many believe agricultural science has gone too far in the use of recombinant DNA techniques to produce transgenic products that could adversely effect the environment and human health. Others believe that advances in genomics will play a critical role in traditional plant breeding as well as in genetically modified crops. Regardless, if the climatologists are correct, time is of the essence. It takes on average a decade and $100,000,000 to breed a new transgenic crop cultivar and for it to become available to farmers.

Many feel that climate change could result in destabilization and the escalation of conflicts as crop yields fall on both a regional and global scale. Southwestern archaeologists have interpreted signs of precisely that having happened with the Ancestral Puebloans in the face of widespread drought.

The world’s population has reached 7 billion. Statisticians tell us there’s a 1 in 7 chance that a person will be born hungry and that nearly 1 billion people go to bed hungry each night. Given predicted climate change scenarios, global food production is unlikely to satisfy future demand without making advances in crop improvement, better use of nutrients, stress tolerance, land management, control of greenhouse gas emissions and crop breeding.

"The corn grows up. The waters of the dark clouds drop, drop.
The rain descends. The waters from the corn leaves drop, drop.
The rain descends. The waters from the plants drop, drop.
The corn grows up. The waters of the dark mists drop, drop."

Fertility song of the Navajo Indians

Saturday, November 19, 2011

Memorable Places Here and There on the Colorado Plateau: Ribbon Falls

About eight miles down the North Kaibab Trail from the Grand Canyon's North Rim, a short detour off to the right beckons sun-parched backpackers to Ribbon Falls. Its irresistible mist is near impossible to forgo on a typically hot and dry day in the canyon, making this side excursion a necessity to visit. But what’s truly fascinating is the geological structure that the falls have produced. The action of ground water, by virtue of its mineral composition, has resulted in the formation of a spectacular travertine dome that's over thirty feet tall.

How did this colossal structure form? Water from the falls makes a 120 foot free-fall landing precisely at the apex of the moss-covered travertine dome. Calcium carbonate is in solution, being made soluble by the absorption of atmospheric carbon dioxide, which makes the water mildly acidic. Its acidity allows the carbonate to be “acquired” from limestone formations at higher elevations such as the Redwall and Muav. Subsequently, carbonate is “released” from the mineral-rich dripping water when it plunges over the falls and releases the carbon dioxide held in solution. The change in water chemistry causes the re-deposition of the carbonate in the form of travertine or tufa (softer and more porous) from the mineral-laden water. Gradually, the mound grows by re-crystallization, molecule by molecule. This landform is called karst, made possible by the dissolution of soluble bedrock. The identical process forms the more familiar stalagmites and stalactites in subterranean limestone-caverns.

Ribbon Falls is located in an amphitheater bounded by dark red cliffs of Shinumo Quartzite. The falls plunge over the ledge of a resistant diabase sill. Diabase is the intrusive equivalent of basalt. This sill is part of a system of Cardenas conduits and a massive basaltic outpouring of the same name that fed magma to the Earth’s surface. These rock formations, along with three others, are members of the Unkar Group, which comprise the lower Grand Canyon Supergroup. Beginning 1.2 billion years ago, the formations of the Unkar Group were deposited over a span of 100 million years and appear to have been associated with a continental collision event that culminated in the formation of the supercontinent of Rodinia.

This view is taken from behind the falls, looking out at the top of its verdant, mossy travertine dome. Vegetation such as the moss, and golden columbine, maidenhair fern and scarlet monkeyflower thrives in the oasis of the fall’s unique microclimate. These plants are not indigenous to the hot, arid climate of the Grand Canyon only a few feet away.

Thursday, November 17, 2011

Memorable Places Here and There on the Colorado Plateau: The Solitude of Nankoweap

Fifty-three miles downriver from Lees Ferry, the put-in for all trips heading into the Grand Canyon, the Colorado River makes a dramatic, sweeping S-turn where its gorge widens into an area called Nankoweap.

A thousand years ago, give or take, a large, flat delta built by numerous debris flows and flash floods, similar to what we see today, was an open invitation for Ancestral Puebloans to grow crops such as corn, one of their staples.

These Native Americans called Anasazi, which is actually a Navajo term meaning "enemy ancestors" or "ancient people who are not us," stored their grain high above the river in granaries etched into the cliffs, where this photo was taken. For scale, notice (above) the hikers descending a trail on the talus slope toward their raft. A few windows of the granary (below) can be seen from the trail.

Why are some regions of the Grand Canyon wide and open with a tranquil river such as Nankoweap and others narrow with towering rock walls and a river that's fast and furious? We know the Grand Canyon was carved by the action of the running water (or more appropriately its carried burden). Perhaps this is an overly simplistic statement, but true nonetheless. But, we must look for other variables to explain the differences in canyon architecture.

As the river downcuts into its bed, it encounters rock layers of variable resistance. Less resistant rock erodes more readily and laterally undercuts more resistant rock. This causes the overlying rock to collapse which widens the canyon. A direct relationship exists between canyon geometry and hardness of the rock strata. Thus, the canyon in the region of Nankoweap widens at the expense of the erodable Bright Angel Shale at its base that undermines and weakens the rock overburden. As the canyon widens, so follows its river bed. That slows the river's rate of flow and encourages the formation of those big deltas as the water releases its sediment. Perfect for farming! Fertile, irrigated and flat. 

Below the shale lies the Tapeats Sandstone which will come into view in another six miles, when the river dissects deeper into its bed. Above lies the Muav Limestone, the cliffs just above river level. These formations comprise the classic, transgressive triad of the Cambrian known as the Tonto Group, formed when the rising Panthalassic Ocean (or ancestral Pacific Ocean) began to lap across the region of the future Grand Canyon around 525 million years ago. The South Rim looms in the distance with the Middle Permian Kaibab Limestone at the top which means we’re viewing the near full extent of the Grand Canyon’s Paleozoic column of deposits.

Suggested Reading: Carving Grand Canyon by Wayne Ranney, 2005. 

Monday, November 7, 2011

Flight Plan: Part III - The Henry Mountains Laccolithic Complex on the Colorado Plateau

This is the third post on my recent aerial investigation of the geology of south-central Utah. For the earlier portion of the flight, please visit my first two posts entitled “Part I – Geology of the San Rafael Swell” and “Part II – Geology of the Circle Cliffs Uplift at Capitol Reef.”

Photo Above: The Henry Mountains
Framed by the Henry Mountains, Factory Butte's badlands are formed in the Blue Gate Shale Member of the Cretaceous Mancos Shale, and its summit at 6,321 feet is in the resistant Muley Canyon Member. Twenty miles to the south, Mt. Ellen at 11,506 feet of the Henrys is clad in late May snow. A faint image of Table Mountain lies directly in front of Ellen, while the snowless peak to the left (east) is Bull Mountain. From their isolated and remote position within the Henry Basin, the Henry Mountains
appear to be “springing abruptly from the desert.” (G.K. Gilbert, 1880)

While traveling through Utah’s backcountry in May, my good friend, geologist and author Wayne Ranney ( and suggested that we take to the air to investigate the geology. From the ground, it can be challenging to fully appreciate the scale and geological relationships of the Colorado Plateau’s massive landforms. From the air, the landscape takes on an unparalleled, big picture-perspective and provides some beautiful photos as well.

Taking off from Price, Utah, we flew south over the crests of the San Rafael Swell and the Circle Cliffs Uplift, better known as Capitol Reef, paying special attention to the geology of their monoclines, the San Rafael Reef and the Waterpocket Fold, respectively. On our return to Price, we circled the Henry Mountains just north of Lake Powell. We mapped out a roughly 500-mile, ellipse and lifted off early in the morning to catch the best light on the terrain.
Where are they?
The Henry Mountains are located on the Colorado Plateau in south-central Utah (38°06'36.04" N, 110°49'21.97" W). They project a good 6,000 feet above the contiguous terrane of the blue and red rock desert of the plateau making them a highly recognizable landmark from considerable distances. The range is surrounded by Laramide-age uplifts, while the Henry Mountain complex intruded into the Henry Basin (also Henry Mountains Basin), a synclinal landform of the same age. The basin’s topography varies from steep, rugged terrain in the Henry Mountains in the east to a series of dissected mesas and buttes, and eroded cuestas and hogback ridges along the western margin.

To the west of the basin lies the Circle Cliffs Uplift and its Waterpocket Fold, a portion of which has been set aside as the Capitol Reef National Park. The northern boundary is the badlands and slopes within the Henry Mountains Syncline, and beyond, the uplift of the San Rafael Swell and its monoclinal Reef. To the east and south is the Colorado River, and below it, the Monument Upwarp, Lake Powell and the Glen Canyon National Recreational Area.

The Henry Mountains are located at the far left of center highlighted in yellow. Other regional laccolithic complexes of the Colorado Plateau are highlighted as well. The Laramide-age uplifts, basins and monoclines, the Paradox Basin and the Colorado River system are also labelled. 
(Modified from Tectonics of the Region of the Paradox Basin, Guidebook, Kelley 1958a, 1958b)

Who was Henry?
Beginning with the renown geologist John Wesley Powell in 1869, the remote and unexplored wilderness of southern Utah and northern Arizona along the Colorado River became a source of great scientific and exploratory fascination. Seeing the Henrys, Powell called them the “Unknown Mountains,” and rightly so. They were the last mountain range in the lower 48 states to be explored. Upon his return in 1871, he officially named them the Henry Mountains, after Joseph Henry, a close friend, supporter and secretary of the Smithsonian Institution.

Two groups of high peaks
The Henry Mountains are a 56 mile-long and 19 mile-wide, isolated string of five rugged, high peaks. From north to south, the range is clustered into two main groups, each dome being 6-10 miles in diameter. The larger northern group consists of Mt. Ellen (11,506 feet), Mt. Pennell (11,371 feet) and Mt. Hillers (10,723 feet). The southern group, also called the Little Rockies, includes Mt. Holmes (7,930 feet) and Mt. Ellsworth (8,235 feet). On our flight, we flew between the northern and southern group. That gave us a great view of Mount Hillers.

The two groups of the Henrys lying in the Henry Basin are visible in this NASA image.
They lie on strike with the Waterpocket Fold, ten miles to the west.
The Colorado River can be seen in the south snaking its way to Lake Powell.
(Image Science & Analysis Laboratory, NASA)

The Henry Mountain Basin
The Laramide Orogeny, a continuation of Cretaceous mountain-building, provided compression on the Colorado Plateau that resulted in numerous high-relief uplifts separated by small intervening basins. The uplifts and monoclines that we flew over on the earlier portion of our flight (my Posts I and II) demonstrated these landforms. One such basin is the Henry Mountains Basin that received localized intrusions of magma into shallow crustal levels, and that "pushed up" the Henry Mountains. More on that later. 

Having been stripped of its Tertiary deposits, the synclinal basin’s surficial bedrock is composed largely of the multi-membered Mancos Shale (which has experienced numerous revisions). These marine mudstone, siltstone, shale and sandstone deposits were deposited during the initial transgression of the Western Interior Seaway during the Early Cretaceous. The sedimentary section in the Henry Mountains is dominated by sandstones and shales ranging in age from Permian to Cretaceous.

In western regions of the basin are found the mesa-capping, fluvial sandstones of the Tarantula Mesa Sandstone that border the Waterpocket Fold. As we shall see, exposures of strata underlying the Cretaceous deposits down to the Permian are dramatically revealed by the formation of the Henry Mountains.

Stratigraphic column for the Henry Mountains region at the time of emplacement (~25 Ma). The approximate structural levels of selected igneous intrusions are indicated in the margin to the right. The Emery Sandstone is now referred to as the Muley Canyon Member.
(From Jackson and Pollard, 1988)

How did the Henry’s form?
Powell assigned the geologist G.K. Gilbert the task of studying the Henrys, which he accomplished in two field studies in 1875 and 1876. Gilbert’s 1877 report became the first thorough and classic investigation of the Henry Mountains. In the 1950’s, the geologist Charles B. Hunt further studied them and offered his own interpretation of their formation.

Gilbert reported that the the mountains
“mark a limited system of disturbances, which interrupt a region of geologic calm, and structurally, as well as topographically, stand by themselves.” He was referring to the fact that the peaks that share a common geological genesis. They formed when large igneous bodies intruded the flat-lying stratigraphy of the Colorado Plateau. The emplacement domed the overlying strata into a mushroom-shape which eventually eroded from the summits of what we know as the Henry Mountains of today. Each peak is an intrusive complex consisting of a large central, concordant (forming parallel rather than cutting-across existing strata layers)  "floored" laccolithic intrusion and many smaller satellite intrusions in a manner similar to lava flows emanating from its parent volcano.

In his insightful analysis on the Henrys, Gilbert coined the term “laccolite” (from the Greek word for “cistern” or “pool”) for the igneous structure resulting from the emplacement process, an intrusive (rather than extrusive) phenomenon. It was a "ground-breaking" thought at the time (excuse the pun); however, his hypothesis has been challenged.

Early sketch from the field notebook of G.K. Gilbert in 1875 of his conceptual model of a laccolith
(From Hunt, 1988).

Gilbert eventually devised this “half-stereogram” of a laccolith that intruded between flat-lying rock layers and domed the over-lying strata. Notice the flat-floor of the intrusion. The rear panel shows how erosion has unroofed the sedimentary cover of the dome, thereby exposing its igneous core.

(Report on the Geology of the Henry Mountains, G.K. Gilbert, Department of the Interior,
USGS of the Rocky Mountain Region, 1877)

The geology and geometry of the Henrys
The core of each of five intrusive centers is a separate diorite-porphyry structure that, at the summit, is bordered by an irregular and enigmatic zone of shattered sedimentary rock, appropriately called the “shatter zone.” Rocks within this zone are a complex intermingling of sedimentary and igneous rock. Laccoliths typically arise from relatively viscous magmas such as the diorite found at the Henrys which is texturally designated as largely a sodium-rich plagioclase and hornblende porphyry. Diorite is a gray to dark gray intermediate intrusive igneous rock and results from partial melting of a parent mafic (high iron and magnesium) rock. As we shall see later, that's a significant point of interest regarding the mountain's intra-plate locale on the Colorado Plateau.  

Many of the intrusive centers are surrounded on their periphery by clusters of smaller laccoliths, bysmaliths, dikes and sills. Their partially exposed, eroded pieces and remnants were visible from the air, as we shall see. In varying stages of exposure and surrounding the base of the centers are the basin's exposed sedimentary rocks, ranging in age from Late Permian to Late Cretaceous, which have been uplifted and deflected by the igneous intrusions that have arched their overburden skyward. Erosion has unroofed the cover from the summits of the intrusive centers and differentially exposed the verticalized rocks around their bases.

Emplacement ages for the Henry Mountains intrusions are from about 31 to 23 Ma, which have been radically revised from earlier calculations. A clear pattern in terms of spatial migration of emplacement ages amongst the various intrusive centers does not appear to exist. The entire complex appears to have been assembled in less than one million years.

What is the most likely emplacment scenario?
The Henry's formation appears to have begun with tongue-shaped sills and thin laccoliths fed by vertical dikes  emplaced in successive stacks. With the thickening of a major laccolith, the faulting of bedding planes was induced which began to tilt the overlying sills. Peripheral dikes and faults formed as lateral growth of the laccolith ceased. Formed from multiple intrusions, the major laccolith began to thicken vertically. Vertical growth or domal uplift of the overlying Mesozoic host rocks provided the accommodation space, the space-making mechanism, for the intrusions. The overlying strata, responding to the vertical displacement, developed numerous faults cut by vertical dikes. Subsequent erosion has removed as much as 3-4 km of sedimentary overburden that bared the crystalline cores of the intrusive centers.

Mt. Hillers possesses the best exposures of intrusions and sedimentary rock contacts. Note the diorite-core (red) at the summit and at various sills, satellite laccoliths and bysmaliths on the peak’s periphery (especially at noon to two o’clock). We’ll see these on our fly around. Also notice the large shatter zone (pink) surrounding the summit and the upturned Mesozoic rocks surrounding its base, especially the highly recognizable Navajo Sandstone (yellow).
(Modified from Emplacement and Assembly of Shallow Intrusions, Field Guide, Horsman et al, 2010)

Uplifted and reflected overburden
The following cross-sectional diagram of Mt. Hillers shows its central intrusion having uplifted and deformed the overburden of the basin during its emplacement. The slope of the uplifted layers increases with proximity to the dome and has a “doubly-hinged shape” consisting of a “concave-upward lower hinge” and a “convex-downward upper hinge.” The hinges are connected by a central limb of almost constant dip. This is largely where (A) upturned strata are differentially exposed and eroded, remniscent of the monoclinal erosion we saw earlier in our flight. This is also the zone of peripheral volcanic intrusions (sills that were emplaced horizontally and later tilted) and networks of bedding plane-faults (that have accommodated the strains of bending and stretching).

Are the intrusive centers of the Henry Mountains laccoliths or stocks?
The architecture of the Henry's subterranean volcanic structure underlying the domes is not without controversy. Although they are represented in geology textbooks as classic laccolithic mountains (Gilbert’s concept), contemporary analyses have suggested that they are larger more complex intrusions called stocks (Hunt’s concept).

Structurally, laccoliths may be low in height and range from circular to tongue-shaped in form. Stocks have greater height and are cylindrical in shape. Laccoliths are fed by a dike or stock; whereas, stocks do not have a feeder since they are continuous at depth. Laccoliths grow from a thin sill that thickens, thus are floored and concordant (parallel to rock layers). Stocks grow upward through zone-melting or diapirically, thus are not floored and discordant (cross-cutting rock layers).

Gilbert hypothesized that sill intrusion preceded the inflation of an underlying laccolith. Hunt believed the central intrusions are cylindrical stocks that are sheathed with a zone of shattered sedimentary rocks and that laccoliths grew laterally as tongue-shaped masses from the discordant sides of these stocks. Recent findings have confirmed the presence of a floored laccolithic intrusion but have not ruled out a stock at depth.

Gilbert’s concept of laccoliths in Mt. Hillers: A, Cross section with diorite in black; B, Subsurface structure;
C, Idealized laccolithic intrusion with a narrow feeder at its base.
(Original modified from Gilbert, 1877. From Processes of Laccolithic Emplacement, Jackson)

Hunt’s concept of relationships between the stocks and uplift of beds of Mt. Hillers
            (Modified from Hunt 1953. From Processes of Laccolithic Emplacement, Jackson)

The Henrys are not alone
Between the Late Oligocene and Early Miocene on the Colorado Plateau, magmatism in the form of laccoliths occurred on the Colorado Plateau in seven laccolithic ranges, the largest of which are the Henrys (31 to 23 Ma). Their “magma-blister”, laccolithic-architecture is shared by other ranges such as the Abajo (29 to 23 Ma) and La Sals (28 to 25 Ma), and singular Navajo Mountain, about 65 miles due south of the Henrys. Navajo Mountain, still retaining its sedimentary rock cover and its crystalline core not yet exposed by erosion, is believed to represent an early stage in the intrusive process. All these laccolithic complexes possess a coincidence of timing (collectively 32.3 to 22.6 Ma), a style of intrusion, and the same or similar chemical signatures. That has given rise to the conclusion that they share a similar origin.

An enigmatic intra-plate locale
In viewing the aforementioned laccolithic centers as a group, one must also consider potential relationships to coeval igneous activity elsewhere on the Colorado Plateau, namely its east and west margins. The emplacement of the Marysvale volcanics (34 to 21 Ma) on the west and the San Juan volcanic field (32 to 23.1 Ma) on the east fall within the time frame of the laccoliths. All of the aforementioned volcanic centers are found in a rather incongruous location, when one considers that volcanic activity classically occurs at the boundaries of tectonic plates (excluding  intraplate activity at hotspots, not the case here). Furthermore, coeval igneous activity on the west and east margins of the plateau exists in sharp contrast to the locale of our intra-plateau laccolithic centers. I recall my exact thoughts when I saw these landforms for the first time. "What's the big picture? What’s going on tectonically? Is there a relationship of genesis?"

Looking below the surface
One explanation of Oligocene magmatism centers on the crust of the Colorado Plateau, thick (between 45 to 50 km) and largely undeformed, and on its mafic composition, stronger and more resistant to deformation than the more silicic crust to the west. This is in contrast to the thinner, fault-riddled crust of the Basin and Range Province to the west of the Colorado Plateau (~30 km). And to the east at the Rocky Mountains, the crust is thick but also highly faulted. It has been surmised that faults on both sides of the Plateau acted as conduits to facilitate the rise of magma to the surface. So how does that explain mantle-derived, intraplate magmatism at the laccolithic centers and their "sudden" timing during the Oligocene after such a long period of quiescence?

Modern geochronology and geochemistry to the rescue
Revised ages of the intrusions have made it clear that mid-Tertiary magmatism on the Colorado Plateau was part of voluminous regional magmatism in the North American Cordillera. The data suggests the existence of an essentially continuous, thousand-mile plus, intra-continental magmatic zone that extended from western Nevada through southern Utah to southwestern Colorado, and south to west Texas during the Oligocene to Miocene transition. As we shall see, the length of the zone and its perpendicular orientation to the trend of subduction along the western coast, help to explain Mid-Tertiary igneous activity. In addition, the isotopic geochemical signature of the Henry's rocks tells us that the magma was derived from partial melting of subducted oceanic crust in the mantle, the characteristic mark of a magmatic arc.

The Farallon Big Picture
The rapid subduction of the oceanic Farallon Plate beneath the continental North American Plate proceeded at a flatter trajectory in the region that drove the Laramide Orogeny. It was the presence of the Farallon Plate that provided the voluminous and widespread source for arc-related magmatism. But how? Plate convergence presumably slowed by 50 Ma, which drove the dense Farallon deeper into the mantle causing it to founder and break up. That allowed underlying buoyant, hot mantle to rise and heat the base of the crust resulting in its partial melting.

That's not all. As the less-dense melt ascended through the mantle, it pooled at the base of the silicic crust, owing to its greater mafic-density. That facilitated a silicic, crustal melt. The final result was the shifting of the original mafic composition of the mantle-derived melt to a melt with a more intermediate composition (our diorite!). That further retarded its journey of ascension, eventually stalling its rise into the shallow crust in a neutral-buoyancy state. Voila! That produced the magma that fed the Henry Mountains (and the other laccolithic-derived landforms of the Colorado Plateau), identifiable by its mafic, arc-like affinity. Amazing stuff. 

In summary, magmatism at the laccolithic centers is likely a consequence of the subduction of Farallon oceanic lithosphere. That exerted control over the composition, distribution and timing of magmatism after the Laramide Orogeny. The transport of relatively small volumes of magma within the laccoliths to shallow crustal environments indicates suppression by the unique physical properties of the high-strength lithosphere of the Colorado Plateau relative to contemporaneous magmatism in the Great Basin to the west and the San Juan Mountains to the east.

Photo Below: The Waterpocket Fold and the Henry Mountains seen from the west
Having crossed the Circle Cliffs Uplift at Capitol Reef National Park from west to east, we emerged at its plunging monocline, the Waterpocket Fold. This curvaceous portion of the monocline, seen below, depicts numerous strike valleys and exposed ridges formed by the variably resistant and susceptible strata to the forces of erosion. About ten miles away looms a portion of the northern group of the Henry Mountains. Table Mountain is located furthest to the north (left), followed by Mt. Ellen and its lesser summits.

Photo Below: Mt. Pennell and the Henry Basin badlands
We then turned to the south and followed the monocline, eventually banking due east toward the Henry Mountains. In this view, we are over the badlands just east of the monocline at the western portion of the Henry Mountain Basin and facing the western flank of Mt. Pennell. Its slope gradually drapes into the foreground until plunging into badlands of the heavily eroded Blue Gate Member of the Mancos Shale and capped by mesas formed in the Muley Canyon Member (formerly the Emery Sandstone Member). Pennell's flanks are covered with Quaternary colluvial deposits consisting of slide material, slumps and talus.

Photo Below: Mt. Pennell from the southwest
This view of Pennell’s southwest slope shows well the gradual drape of the sedimentary rocks out over the basin from uplift of the igneous dome. The slope is comprised of members of the Cretaceous Mancos Shale draped over by Quaternary colluvium shed from the mountains. The rising flank of Mt. Hillers is almost seen to the right.

Photo Below: Mt. Hillers from the south
We headed between the northern and southern groups of the Henry Mountains. The photo faces north towards Mt. Hillers’ southern flank. The northern three mountains are more mature intrusive centers than the smaller southern group, each with more component intrusions in a wider range of sizes and geometries. Of the intrusive centers of the northern group, Hillers has the best exposures of intrusions and sedimentary rock contacts. The peak is considered a more mature, later stage in the emplacement process. 

Recall that as the laccolith evolved, it elevated the overlying strata which have since eroded from the dome and flanks of Mt. Hillers, leaving its denuded igneous core exposed. At the southern and southeastern base of Mt. Hillers, Cretaceous and Jurassic strata, and various dikes can be seen to have been uplifted and deflected in a manner analogous to a trap door opening skyward. The crest is composed of diorite, and below it, the shattered zone, not well displayed owing to the distance. The lower flanks are composed of Glen Canyon Group deposits, and closer to the base, the deposits of the San Rafael Group. The slopes surrounding Hillers and all the peaks are marked by radial drainage patterns. It is estimated that almost 5,000 feet of Cretaceous sedimentary rocks that once covered the intrusions and Canyonlands country have been stripped away by erosion, exposing the igneous rocks that cored the intrusions.

Having just left the Waterpocket Fold, its upturned strata bear sharp contrast to the geo-dynamics operating at the upturned strata at the Henrys. At the fold, the uplifted and subsequent exposure of the sedimentary strata is related to its monoclinal drape over a Precambrian fault at depth. On the other hand, the uplift and exposure of the sedimentary strata at the Henry Mountains is related to laccolithic doming. In both circumstances, the forces of erosion have acted upon the strata. The tectonic commonality that the two share is the subduction of the Farallon Plate beneath the North American Plate. In the case of the Waterpocket Fold, the mechanism was Laramide compression; whereas, in the case of the Henrys, the mechanism was post-Laramide magmatism and emplacement related to Farallon foundering.

Photo Below: Mount Hillers’ upturned dikes and sedimentary strata from the southeast
In this view, we have flown around the southern extent of Mt. Hillers to its southeastern flank. Seeing Hillers in profile, the dramatic upturned nature of the Mesozoic strata is readily discernible. Again, notice the sedimentary strata draping away from the base of Hillers. This indicates the areal extent of the intrusion at depth, far greater than what is seen at the surface. Also, note the sharply upturned Early Jurassic Navajo Sandstone. Strata of the San Rafael Group lie circumferentially outside of it, upturned as well, but bending into the subsurface as part of a long, sloping-limb that is buried by colluvium. Vertical dikes can be seen cutting through the buff-colored Navajo Sandstone and running towards the summit. Numerous bedding plane-faults exist in order to accommodate the flexure of the  bedrock. Hillers and its satellite intrusions are thought to have been assembled within no more than one million years.

The steeply-dipping, deflected beds of Jurassic Navajo Sandstone provide photogenic evidence of the primary space-making mechanism for the magma of Mt. Hillers’ central intrusion that of "roof-uplift." The oldest sedimentary unit that is exposed on the southern flank of Mt. Hillers is the Permian Cedar Mesa Member of the Cutler Formation. This provides one with a sense of the depth of the Hillers’ intrusive center!

Photo Below: Mt. Hillers' eastern flank
We’re now facing the eastern flank Mt. Hillers. Amongst the various other peaks of the Henrys, the igneous intrusions exhibit varying stages of development. For example, early centers possess a sedimentary cover that not only dominates the margins but can be traced almost to the summit. In addition, satellite intrusions around main intrusive centers exist in a highly varied spatial variation, but the main intrusive center is consistently a laccolith with numerous dikes and sills above a large central pluton.


PHOTO BELOW: The Black Mesa bysmalith and Maiden Creek sill of Mt. Hillers
From this vantage point on the eastern slope of Mt. Hillers (just out of view to the left), we again see the peaks of Mt. Pennell (left) and Mt. Ellen (right). As seen on the western flank of the mountains, a moderate inclination of the sedimentary layers continues for several kilometers away from the intrusive centers at each peak, a testimony to the doming that has occured at depth.

Dominating the left center of the photo are a few of Hillers' well studied satellite intrusions, the cliff-forming outcrops of the Black Mesa bysmalith (center), and at bottom center, the Maiden Creek sill. Immediately out of view to the right is the Trachyte Mesa laccolith. Each of these intrusions provides a snapshot of the growth history of a small pluton during its progressive assembly, thought to have occured in multiple pulses, as magma input increased.

The Maiden Creek sill provides evidence of the first episode, the Trachyte Mesa laccolith records the first two stages, and the Black Mesa bysmalith (an overinflated, cylindrical intrusion that cross-cuts adjacent strata) records all three. These satellite structures developed on the margin of the Mt. Hillers complex and are thought to have been emplaced from weeks to years.

SW-NE Cross-section of Mt. Hillers
Notice the drape of the sedimentary layers away from the central intrusion, and the dikes and sills bent upward along with the strata by the doming. Also note the depiction of the Black Mesa bysmalith and the Maiden Creek sill fed by a system of dikes and sills.

Schematic cross-sections through three satellite intrusions illustrating their emplacement mechanisms.
(Emplacement and Assembly… Field Guide, Horsman et al, 2010)

Photo Below: Mt. Ellen and Bull Mountain from the east
Having turned the corner on our flight around the Henry Mountains, we’re now heading north. The last of the Henry's peaks, Bull Mountain (9,187 feet), can be seen from the southeast. The intrusive center of Bull Mountain is a bysmalith similar to Black Mesa.

Photo Below: The northern section of the Henrys from the east
Here's our final glance at the northern section of the Henrys from the east while flying over the mesas and benches around the Dirty Devil River. That's Table Mountain, also a bysmalith, at the far right (north) with Mt. Ellen, the South Summit Ridge and finally snowless Ragged Mountain to the south. Notice the omni-present, photogenic clouds hovering over the range "making" their own weather in this arid region of Utah.

I'll continue with my upcoming and final post on our flight, as we head back to Price, Utah.

Highly Recommended Reading: 
Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney, 2008.
Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado by Robert Fillmore, 2011.