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Maroon Bells |
The incomparable Elk Mountains of Colorado are home to the Maroon Bells (right), Cathedral Lake, Crater Lake, a rich late 1800's mining history, high-end ski resorts like Aspen and Snow Mass, billion-dollar estates, an airport chock-full of private jets, and strongly related to all of the above, a fetching brew of Paleozoic sedimentary and mid-Tertiary igneous geology.
The Elks are the westernmost range in the Rockies proper. To their west, the west-dipping Grand Hogback marks the transition to the Colorado Plateau, just as the east-dipping Dakota Hogback marks the transition to the High Plains on the east.
The Elks are unique among Laramide uplifts in that they lack a core of Precambrian crystalline basement, but they more than make up for it with their spectacular interplay of sedimentary rocks and igneous processes.
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Mount Sopris |
Over the last 510 Ma, many earth processes have combined to create the Elk Mountains we know and admire today, and they couldn't have done a nicer job. Here's what happened.
From Middle Pennsylvanian through Triassic times, sediments washed to the west off the Frontrangia uplift of the Ancestral Rocky Mountain Orogeny accumulated in the Maroon Basin (also known as the Central Colorado Trough) between the Uncompahgria and Frontrangia island ranges. Prominent among these were great thicknesses of Late Pennsylvanian through Permian dark red clays, sands and gravels now known as the Maroon Formation, but Pennsylvanian evaporites and Triassic redbeds also accumulated there.
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Maroon strata dipping steeply to the southwest at Cathedral Lake |
During the Latest Cretaceous through Early Tertiary Laramide Orogeny, the massive Precambrian-cored Sawatch Range block moved upward and to the west over Paleozoic and Mesozoic Maroon Basin sediments along the Castle Creek fault zone (CCFZ), a complex east-dipping high-angle Laramide thrust system exposed in Castle Creek valley, among other places along the western Sawatch front. West-dipping and overturned strata exposed in the Roaring Fork Valley just north of Aspen are part of the CCFZ's footwall syncline. (A syncline is a downward " rug fold" wherein the tops of the folded strata bend toward each other; a syncline folded up just ahead of an overriding thrust block is a footwall syncline, a structure common in the Laramide orogen). West of the syncline, continued advance of the Sawatch block buckled Maroon sediments into a large, thick anticline (an upward "rug fold" wherein the tops of the folded strata bend away from each other). This anticline would become the Elk Range.
This much seems clear, but how the Elk anticline ended up at least 5-6 km west of its initial position relative to the Sawatch block remains controversial. Additional facts to be explained include the following:
The Elk Range is the only major Rocky Mountain Laramide structure lacking a core of crystalline Precambrian basement.
The sedimentary strata of the Elk Mountains now dip steeply not to the west but to the southwest now, as seen in the photo above at Cathedral Lake. (Initial Elk folding due to westward thrusting of the Sawatch block along the CCFZ would have produced strata dipping to the east and west, not to the southwest.)
A popular gravitational collapse model holds that further uplift of both the Sawatch block and the Elk anticline eventually caused the latter to detach above its basement and slide off the Sawatch uplift to the southwest, in part along the Elk Mountain Thrust (EMT), a low-angle east-dipping thrust fault of Late Cretaceous to Early Paleocene age exposed along the west side of the Elk Range for more than 50 km. This model implies the presence of one or more large high-angle west-dipping normal faults of similar age east of the EMT. None of my geologic maps show such faults, but more detailed mapping presumably supports the model. How the Elk block came to fall off the Sawatch uplift to the southwest is unexplained in this model, but Elk strata might have ended up with a southwest dip if the block happened to spin counterclockwise in map view as it slid. The northern east side of the Elk Range contains a number of normal faults that might permit such a rotation, but they seem too short to do the job, even in aggregate.
Another school of thought considers the EMT a large, oblique western splinter of the CCFZ. In this view, the low-angle EMT continues down to the east below the Elk Range to merge with (root into) the higher-angle CCFZ somewhere beneath the Sawatch uplift, presumably near the brittle-ductile transition at the base of the upper crust. Riding the relatively small wedge between the EMT and the CCFZ, the Elk anticline simply shot out ahead of the massive Sawatch block in response to continuing Laramide contraction. No normal faults are required.
These models differ substantially regarding the eastern extent of the EMT, but both view its western segment as a southwest-directed low-angle thrust similar to many other Laramide thrusts. Fault-related folding along this western segment could impose a net southwest dip on overlying Elk strata in either model. Asymmetric folds commonly develop in rocks both above and below angled thrusts, with anticlines forming in the hanging wall above the thrust, and synclines in the footwall below the thrust. When an angled thrust fault cut its way up through initially flat-lying strata, the steepest fold dips develop in the strata closest to the fault. To simulate hanging wall folding, thrust your hand fingers-first across a table top, allowing your fingertips to curl under as they go. Elk strata moving up the thrust ramp to the southwest above the thrust segment of the EMT would curl the same way, acquiring a southwest dip in the process.
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Granodiorite boulder |
Around 34 Ma, Oligocene magmas intruding the Colorado Mineral Belt along lines of weakness left over from Late Proterozoic continent building hit the all-sedimentary Elk Mountain anticline and the adjacent Sawatch uplift particularly hard. In the Elk Range, magmas—mostly granodiorites (right) and quartz monzonites—spread out horizontally and unevenly along the Elk Mountain Thrust to pool in two large intrusive bodies known as the White Rock and Snowmass plutons. In doing so, the intrusions further uplifted the overlying Maroon Basin sediments in a process known as magmatic inflation.
Mid-Tertiary magmas reached the surface in great volume in the nearby West Elks and in the San Juan Mountains, but volcanism may or may not have occurred in the Elks. Any Elk Range volcanics that did erupt are long lost to erosion.
Continued high heat flows and a substantial gravity low along the Colorado Mineral Belt together s0uggest the ongoing presence of large, hot magma bodies at depth. These are presumably the deep reservoirs that fed the mid-Tertiary Elk Range intrusions, but the story may go deeper yet. Tomographic imaging reveals a broad region of unusually low seismic velocity known as the Aspen anomaly in the upper mantle beneath the Sawatch and Elk Ranges. Whether this hot mantle resides in the asthenosphere or in the lithospheric mantle remains unclear, but it may be generating melts either way.
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Mount Sopris |
Directly or indirectly, the Oligocene intrusions deserve much of the credit for the great heights, the sheer topography and the uncommon majesty that together set the Elks apart from other Colorado ranges. The tough, medium- to fine-grained, light-colored Elk intrusives tend to form prominent peaks like Mt. Sopris (12,660', right) in the north and Castle Peak (14,265') in the south. The intense heat and mineral-rich fluids the molten intrusions injected into the surrounding sediments altered the latter in several ways. The Mississippian Leadville Limestone acquired rich deposits of zinc, silver and gold, mostly in combination with the lead sulfide ore known as galena. In the late 1800s, this legacy turned Aspen and Leadville into world-famous mining centers. Near the town of Marble, the Treasure Mountain intrusion recrystallized gray Leadville Limestone into the exquisite gleaming white Yule Quarry Marble of Lincoln Memorial and Tomb of the Unknown Soldier fame. A large sill (slab-like horizontal magma body) intruding the Maroon Formation along the Elk Mountain Thrust baked and basted the normally dark red beds of the Late Pennsylvanian through Permian Maroon Formation into the hard, handsome maroon strata of the Maroon Bells and the durable gray-green hornfels of the Cathedral Lake basin.
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Maroon Bells above Maroon Lake |
A final and ongoing pulse of regional uplift over the last 10 Ma reinvigorated stream erosion throughout the Rockies and the Colorado Plateau. Uplift has been greatest around the intersection of two of the most profound structural trends in Colorado—the Colorado Mineral Belt and the Rio Grand Rift. In fact, most of Colorado's 55 Fourteeners cluster around that point (near Leadville), and many of the largest rivers in the West—in clockwise order from north, the North Platte, Laramie, South Platte, Arkansas, Rio Grande, San Juan, Gunnison, Colorado, White and Yampa—drain the resulting dome in a radial pattern. The Elk Mountains sit astride the Colorado Mineral Belt just 35-40 miles WSW of Leadville.
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Crater Lake, upper Maroon Valley |
The Elks were already deeply dissected when glaciers arrived to sculpt them into final form in Pliocene and Pleistocene time. The strong Elk intrusives and surrounding indurated metasediments proved to be ideal media to hold the sharp-edged cirques, sheer mountain faces and deep U-shaped valleys (right) that glaciers are wont to carve. This combination makes Elk Range alpine scenery second to none.
Elk Range Gallery |
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McClure Pass |
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McClure
Pass, western gateway to the Elk Range:
A stunning early morning drive up CO133 from Somerset
took us up over McClure Pass (8,755') and down into the Crystal River
basin toward Carbondale.
Looking ahead to the east from the pass (2nd frame), we got our first glimpse of the incomparable Elk Mountains. Dramatic canyon walls and road cuts exposing the Maroon Formation and Early Pennsylvanian sediments and evaporites graced the drive beyond. Looking back to the west (3rd frame), we glimpse the west-dipping Grand Hogback marking the western edge of the Colorado Rockies as a geomorphic province. The Grand Hogback is a bookend to the Dakota Hogback along the east flank of the Front Range and contains equivalent Mesozoic and Paleozoic strata. Beyond it to the west lie the dissected tablelands of the Colorado Plateau. |
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Mt. Sopris and vicinity, north of Aspen |
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Mt.
Sopris: The erosional remnant of a large Tertiary stock, Mt. Sopris
(12,660') dominates the northern tip of the Elk Mountains and much of
the scenery between Carbondale and Aspen as well. It's not a volcano per
se, but the frozen cylindrical magma reservoir now exposed on Mt. Sopris
may have fed a
vent or two in its day. The uniform light gray color of the quartz
monzonite stock immediately differentiates Mt. Sopris, Castle Peak and
similar Elk intrusions from the pink granites, the dark banded gneisses
and schists, and the red, orange and buff sedimentary strata of the Gore, Ten Mile and Front Ranges to the north and east. If you're coming
from either direction, the color alone tells you that something
different has gone on here.
The top frame in this series shows a group of Carbondale grade-schoolers taking in an outdoor Colorado history lesson. The remaining frames came from a spectacular drive along West and East Sopris Creek Roads, wide and well-maintained dirt thoroughfares skirting the north and east flanks of the mountain south of Carbondale, respectively. The route eventually joined CO82 north of Aspen and south of Basalt. We hit these fall colors in mid-September just before their peak, but we weren't complaining. Glacial outwash stream terraces are clearly visible at the foot of Mt. Sopris in the 5th frame. In the 6th frame, a pair of large high cirques loom over wooded slopes built on moraine. |
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Rough duty: The quaint little cabin behind me sits on moraine on the east flank of Mt. Sopris. It's badly out of focus here, in favor of yours truly, but it looked like rough duty from the road. |
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Basalt: Basalt-draped Triassic redbed ridges rise to the east from this turnout on East Sopris Creek Road. These Late Tertiary basalts probably postdate mid-Tertiary Elk intrusions like Mt. Sopris by 20 My or so. The town of Basalt, on CO82, is probably below them to the right. |
Castle Creek Valley, south of Aspen |
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Ashcroft: These ^near infrared shots capture a few of the buildings preserved at Ashcroft, an old mining town near the top of the paved portion of Castle Creek Road. |
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Upper Castle Creek Valley, Summer: [] |
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Upper Castle Creek Valley, Fall: Looking south from
vantages along the bumpy dirt road to the Cathedral Lake
trailhead, we see Castle Peak
looming beyond golden Aspen glades reaching
down into the upper reaches of Castle Creek Valley. Around 20 Ka, a large Pleistocene
valley glacier filled Castle Creek Valley from the high cirques in the distance
all the way down to Aspen, the town—a distance of 14 miles. Halfway down, it
merged with another large glacier filling Conundrum Creek Valley, the next big
drainage to the north.
Aspen trees are eager fire-scar pioneers. Here, their sinuous golden stands reveal the traces of ancient wildfires on the alluvial fans and moraines below tree line. This strategy allows them to compete with the much taller conifers for sunlight, water and nutrients—and ultimately for real estate. |
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Cathedral Lake |
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Cathedral Lake Trail: The turnoff to the Cathedral Lake
trailhead is ~1.4 mi up from Ashcroft and the Toklat Lodge on the west
side of Castle Creek Road. From there, the steep but rewarding route
climbs 1,986' over 3.2 mi to the glacial Cathedral Lake (11,866') and then on to
Electric Pass.
The trail climbs first through an aspen forest to reach a thinly-soiled pediment of light gray, fine-grained granodiorite intruded here ~34 Ma. The middle part of the trail follows Pine Creek along a narrow staircase defile (1st frame) cut into the hard granodiorite. Pine Creek drains the Cathedral Lake cirque to Castle Creek and takes a nice fall here. Near the top of the first set of switchbacks is the somewhat jumbled contact between the intrusion and overlying Maroon sediments. Along the contact, heat and fluids from the intrusion baked and basted the overlying normally dark red Maroon sediments into a hard gray-green metamorphic rock called hornfels that nevertheless retains the original SW-dipping Maroon bedding planes (2nd and 3rd frames). The trail then climbs relentlessly over moraines, talus slopes, steep rock walls and tundra along the south face of Leahy Peak (13,322') before crossing Pine Creek at the lip of the glacial bench and dipping into the hummocky terrain of the Cathedral Lake cirque basin. |
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Near the top: One of the many false summits on the trail afforded this SW view of the Elk Mountain crest (center) and a large rock glacier (lower left). Cathedral Lake sits behind the glacial bench beneath the sharp triangular peak (?? Malamute Peak, 13,348') at right center. |
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Cathedral
Lake: At 11,866', this classic glacial lake occupies a large,
deep cirque drained below the southern Elk Mountain crest. The moraine
damming the lake is behind us in the first frame.
Among the many tilted, angular gray-green hornfels spires surrounding the lake is Cathedral Peak (13,943', 2nd frame, behind the rock in the middle ground). The trail from the lake up to Electric Pass (~13,465', shown above the rock glacier below) looked intriguing, but we were running behind schedule and had no taste for more climbing at this juncture. A late lunch on the lake shore consisted of weary leftovers from our previous 4 day-hike lunches, but that didn't seem to dull its appeal. The big can of peaches I'd lugged all the way up the trail was worth every pound—especially the gulp of light syrup at the end. It was mighty tempting to linger, but we had to be on our way. It was already 3:30 pm, and we needed to make Twin Lakes on the other side of Independence Pass before we could bed down. Due to the steepness of the trail and the variable footing, the way down didn't seem much easier than the way up, but we managed to make good time. |
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Gee, wish we'd thought of that: This frequent Aspen visitor from Texas "hired" her eager companion Fred from a kennel near the airport. Fred's unbridled enthusiasm for anyone who came along had us missing our own dogs in no time. My Belgian sheepdog and border collie would have given a paw to come on this hike. |
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Rock glacier: A rock glacier snakes down toward the trail from the high ragged ridge connecting Cathedral Peak and Electric Pass (top center). Ice bound into the base of the rock mass serves as a lubricant, but the bulk of the glacier is rock debris, not ice. Nevertheless, rock and ice glaciers share many morphologic similarities. |
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Metamorphic
gradients:
Coming down from Cathedral
Lake, we spotted this near-vertical gradient in the alteration of Maroon
Formation rocks (1st and 2nd frames). Under the influence of heat and
fluids emanating from the magma intruded below, the gray-green rocks at the base of the hill
reached hornfels-grade metamorphism, while the more normal red rocks at the
higher in the column suffered less alteration.
Since the intrusive heat and fluids responsible for the alteration came from below, the vertical variation in metamorphic grade seen here makes sense. But horizontal metamorphic gradients also become evident as scans the rock colors around the Cathedral Lake cirque. Notice the marked short-range variation in the wall behind John. In the final frame, the trail crosses a talus slope exposing a jumble of variably-altered Maroon Formation debris. The horizontal metamorphic variations suggest two end-member scenarios: The contact between the intrusion and the overly Maroon sediments has quite a bit of relief, or the heat and fluids responsible for the alteration found some directions easier to move in than others. The reality is probably a mix of the two. |
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Sawatch
Range from the Elks: To the east and south, distant ridges of
the Sawatch Range from the Cathedral Lake Trail. The Sawatch is on
average the
highest range in Colorado, but it doesn't look all that exceptional from
here.
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Maroon Bells and Crater Lake |
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Maroon
Bells: No Colorado geology site would be complete without a
homage to the nearby Maroon Bells, the crown jewels of the Elk
Mountains and deservedly the most photographed
mountains in Colorado. Looking west across Maroon Lake in the first two
frames, we see Maroon Peak (14,156') on the left (south) and North Maroon
Peak (14,014') on the right. Maroon Lake looks for all the world like a
glacial lake dammed by a moraine, but a rockslide from the north wall of
the canyon is the real culprit. The bottom frame was taken from a meadow just above the
lake. The thinly bedded red Maroon Formation clays, sands, and gravels of the Maroon Bells accumulated in coalescing alluvial fans draping the Uncompahgre Uplift during Colorado's Late Paleozoic Orogeny. (They're equivalent to the Fountain Formation of the eastern Front Range.) Heat and silica-rich fluids from an underlying Oligocene intrusive sill hardened and added a gray cast to the normally deep red Maroon sediments. In places, the alteration advanced to a hard, lustrous dark green to gray-green metamorphic rock known as hornfels. The sill also lifted the Maroon Bells, but it isn't exposed there. |
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Granodiorite: The visible grains in this Oligocene granodiorite boulder encountered on the trail below Crater Lake tells of an injected magma freezing slowly well below the surface. The light gray color indicates a high silica content and viscosity and is typical of Elk stocks and sills. If this magma had erupted, it would probably have done so explosively. |
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Redbeds: Contorted Triassic redbeds form the north wall of Maroon Creek Canyon between Crater and Maroon Lakes. If these peaks had popped up anywhere other than next to the Maroon Bells, they'd be attractions in and of themselves. |
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Pyramid Peak: Seen here to the SE from the lower end of the trail to Crater Lake, Pyramid Peak (14,018') holds up the south side of Maroon Creek's upper canyon. Alluvial fans draping the slopes here support forest growth better than the intervening shallow alpine soils and hard rock surfaces. |
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Crater
Lake: In the 1st and 2nd frames, the Maroon Bells tower over misnamed Crater Lake,
which was dammed by a rockslide across Maroon Creek's upper canyon. The U-shaped valley above the lake clearly
harbored a glacier, but the dam at the opposite end isn't a glacial
moraine. Nor does the lake occupy a crater, volcanic or otherwise.
In the top left corner of the 2nd frame, a rock glacier spills down a hanging canyon left behind in the Pleistocene as the large Maroon Creek valley glacier outpaced the downcutting of its tributaries. In the 3rd frame, magenta flowers (I'm guessing fireweed) line the trail on the way down from Crater Lake. Alluvial fans of the north flank of Pyramid Peak rise to the left.The 4th frame looks down Maroon Creek to the east over Maroon Lake from the rockslide impounding Crater Lake. The far end of Maroon Lake is dammed by alluvial fans, not by moraine. |
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In addition to the references cited on the home page and in supporting articles, this article relies on the following sources, in alphabetical order by first author:
Unless explicitly attributed to another contributor, all content on this site © Jeremy McCreary. Comments and corrections to colorado@cliffshade.com. Nothing is bought or sold here. Spam will be forwarded to the proper authorities. |
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