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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 . 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 he Henry Mountains
appear to be “springing abruptly from the desert.” (G.K. Gilbert, 1880)


While traveling through ’s backcountry in May, my good friend, geologist and author Wayne Ranney (WayneRanney.com and http://earthly-musings.blogspot.com/) 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 just north of . We mapped out a roughly 500-mile, ellipse and lifted off early in the morning to catch the best light on the terrain.
The are located on the Colorado Plateau in south-central . 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 complex intruded into the   (also ), a synclinal landform of the same age. The basin’s topography varies from steep, rugged terrain in the 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 . 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, and the Glen Canyon National Recreational Area.

Who was Henry?

Beginning with the renown geologist John Wesley Powell in 1869, the remote and unexplored wilderness of southern and northern along the became a source of great scientific and exploratory fascination. Seeing the Henrys, Powell called them the “” 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 , 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 (11,506 feet), (11,371 feet) and (10,723 feet). The southern group, also called the Little Rockies, includes (7,930 feet) and (8,235 feet). On our flight, we flew between the northern and southern group. That gave us a great view of .

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 .

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 . 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 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.

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.

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.

(Modified from Emplacement and Assembly of Shallow Intrusions, Field Guide, Horsman et al, 2010)

Uplifted and reflected overburden
The following cross-sectional diagram of 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.

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 , about 65 miles due south of the Henrys. 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 to the west of the Colorado Plateau (~30 km). And to the east at the , 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
Having crossed the Circle Cliffs Uplift at 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 . is located furthest to the north (left), followed by and its lesser summits. Photo Below:

We then turned to the south and followed the monocline, eventually banking due east toward the . 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: 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 is almost seen to the right. Photo Below: from the south
We headed between the northern and southern groups of the . The photo faces north towards ’ 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 leaving its denuded igneous core exposed. At the southern and southeastern base of , 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 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: ’ upturned dikes and sedimentary strata from the southeast

In this view, we have flown around the southern extent of . 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.

The steeply-dipping, deflected beds of Jurassic Navajo Sandstone provide photogenic evidence of the primary space-making mechanism for the magma of ’ central intrusion that of "roof-uplift." The oldest sedimentary unit that is exposed on the southern flank of 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: eastern flank

We’re now facing the eastern 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 bysmalith and Maiden Creek sill of Mt. Hillers
From this vantage point on the eastern slope of 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 complex and are thought to have been emplaced from weeks to years. Photo Below: Mt. Ellen and Bull Mountain from the east

Having turned the corner on our flight around the , we’re now heading north. The last of the Henry's peaks, (9,187 feet), can be seen from the southeast. The intrusive center of 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.


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.
Posted by Dr. Jack Share at 7:42 PM Labels: Capitol Reef, Circle Cliffs Uplift, Colorado Plateau, Farallon Plate, Henry Mountains, Intraplate Volcanism, Laccoliths, Laramide Orogeny, Western Interior Seaway
