The All-Important Haycock Mountain Exposures
Any geologist familiar with the volcanic stratigraphy of southwest Utah would stand atop Haycock Mountain and with utter confidence declare that the Isom Formation, a densely welded, 27- to 26- million-year old ash-flow tuff that forms its resistant caprock, is undisturbed and in-place. Several have done just that. But while mapping that area, I came across several exposures—preciously small and hidden by mountain mahogany, pinyon, and juniper trees—of the base of the Megabreccia that no geologist had ever seen. There, the lower few tens of feet of the Isom Formation were brecciated and locally pulverized to rice-size pieces and then resilicified, grading upward into undeformed Isom at the crest of the mountain. The rock is technically called a cataclasite, having formed through extensive fracturing of the parent rock; it everywhere overlies a thin layer, typically one inch to one foot thick, of what looks like concrete. This thin basal breccia represents broken up, over-pressured debris that the gravity slide rode on and that was injected as dikes at the base of the Megabreccia. The base of the Megabreccia is a sharp, planar surface with striations, grooves, and small-scale brittle microfabrics including fractures known as Riedel shears, all of which serve as directional indicators, telling us that the Megabreccia was emplaced from north to south.
The Haycock Mountain exposures are the “trunk” of our mythical beast, something that no geologist had previously seen or understood. The exposures are important because the cataclasite, basal breccia and clastic dikes, and brittle microstructures provide strong evidence of catastrophic emplacement by gravity sliding, not by slow gravitational spreading or creep nor by seismically cycled thrust faulting. Further, these exposures unequivocally demonstrate south to southeast transport of the Megabreccia, not northward transport as originally inferred. This is the kind of evidence that most of us can only dream about finding (and which commonly comes about only after others have appreciably narrowed the search for instructive exposures!).
Geologists designate type sections of rock formations, a place where characteristic features of the rocks are well developed and can be readily studied. John Anderson designated a 2-mile stretch along Utah Highway 143, just east of Panguitch Lake, as his Markagunt Megabreccia reference section. Given what was known at the time, John and his colleagues reasonably interpreted the caprock of Haycock Mountain as in-place—part of the lower plate, undisturbed volcanic Isom Formation. But what he could not have known is that the true size of the Megabreccia was even larger than he imagined. Ironically, his reference section turned out to include just the uppermost part of the Megabreccia—a fuller story of the Megabreccia awaited discovery in exposures just a few miles to the south at Haycock Mountain.
In their defense, early mappers of the Megabreccia started out in puzzling northern exposures, in essence high on the back of the elephant, so it wasn’t readily apparent exactly what kind of creature they had. Mapping of the frontal margin of the Megabreccia, where critical exposures are best preserved, came last, and fell into my lucky hands. Yet still, as described in the recently open-filed geologic map of the Panguitch 30′ x 60′ quadrangle (UGS Open-File Report 599), we remain uncertain about several aspects of the Markagunt Megabreccia: its full northern extent, the location of its flanking faults, and certain features of its southern exposures, including possible debris avalanche deposits south of Cedar Breaks National Monument.
When did the Megabreccia form?
The age of emplacement of the Markagunt Megabreccia is constrained by the age of its underlying and overlying rocks. The Megabreccia was originally thought to be overlain by the apparently undisturbed 22.8-million-year-old Haycock Mountain Tuff, but we now recognize that this tuff simply rode along on the back of the great slide as a mostly undisturbed block many square miles in extent. During our recent mapping, we discovered exposures of the Megabreccia that overlie the 22.0-million year-old Harmony Hills Tuff and stream gravel deposits that contain rounded cobbles of eroded Harmony Hills Tuff. Thus, the Megabreccia must be younger than 22 million years old.
Unfortunately, we lack overlying, post-Megabreccia rocks to significantly constrain its upper age. However, because the Megabreccia is preserved in grabens at the west margin of the Markagunt Plateau, we infer that emplacement of the Megabreccia predates the main phase of basin-range deformation, which resulted in the present topography and which began about 10 million years ago at this latitude. The Megabreccia was thus emplaced between 10 to 22 million years ago, and likely about 20 million years ago as described next.
How did the Markagunt Megabreccia form?
Geologists don’t know for sure what triggered the gravity slide that led to formation of the Markagunt Megabreccia. This is a particularly vexing problem because the northern part of the Megabreccia, and areas even farther north, are mostly volcanic mudflow deposits of the Mount Dutton Formation; it is difficult to know where these rocks are part of the Megabreccia and where they may postdate and thus bury the Megabreccia. But given what we do know, there are two plausible explanations: (1) doming and subsequent southward sliding of roof rocks off the Iron Peak laccolith or related intrusions in the northern Markagunt Plateau, or (2) related to inflation of the crust due to emplacement of intracaldera intrusions following eruption of the 21- to 20-million year-old Mount Belnap caldera.
Utah’s Miocene landscape looked very different than that of today; Utah occupied the east side of the Great Basin altiplano, a high-elevation plateau studded with volcanic mountains and intervening basins, analogous perhaps to the modern Altiplano of South America. The oldest volcanic rocks in southwest Utah belong to the Brian Head Formation, clay-rich volcaniclastic rocks and rhyolitic ash beds that spread across the southwest part of this high-elevation region. Brian Head strata are overlain by several aerially extensive, densely welded ash flow tuffs that erupted from calderas near the Utah-Nevada border, which in turn are overlain by volcanic mudflow deposits and lava flows that erupted from vents on the northern Markagunt Plateau and in the southern Marysvale volcanic field. The foundation on which at least the southern part of the Marysvale volcanic field rests is thus non-resistant, clay-rich, fine-grained volcaniclastic strata of the Brian Head Formation that even today are highly susceptible to landsliding. This weak foundation is key to either explanation of the Megabreccia’s origin.
Given our current understanding of the Megabreccia, our favored trigger is the 20-million-year-old Iron Peak laccolith, an idea first suggested by USGS geologists Florian Maldonado and Ed Sable in the mid-1990s. The laccolith was emplaced as molten rock from deep within the earth moved upward via vertical dikes into the Bear Valley and Brian Head Formations, where it spread out into a shallow, mushroom-shaped intrusive dome. Although modern exposures of the Iron Peak laccolith appear too small to have created a dome large enough to produce the Markagunt Megabreccia, only a small part of the Iron Peak laccolith is preserved—it must have been much larger. Evidence for its larger size includes numerous dikes in Claron strata immediately to the west of the laccolith; these were likely feeder dikes, suggesting that large parts of the laccolith must have overlain this block before being removed by erosion. An even larger laccolith can be envisioned if we postulate that part was faulted down to the west and buried by basin-fill deposits of Parowan Valley. Aeromagnetic anomaly maps and well data also suggest the Iron Peak laccolith is part of a much larger intrusive complex that underlies the Red Hills, northern Parowan Valley, northern Markagunt Plateau, and the valley north of Panguitch. In this intrusive complex, most if not all intrusions are laccoliths. Inflation of this larger complex, or several individual laccoliths within it, may have triggered catastrophic sliding of the Megabreccia. The 20-million-year-old Iron Peak laccolith is the correct age as a trigger for the Megabreccia.
It is also possible that the Markagunt Megabreccia resulted from collapse of the southwest part of the Marysvale volcanic field. In 1993, University of Arizona geologist George Davis and USGS colleague Pete Rowley proposed a “two-tiered” model wherein the southeast part of the volcanic field spread and collapsed under its own weight, creating southward-directed thrust faults rooted in evaporite strata of the Middle Jurassic Carmel Formation. These thrust faults are part of the Ruby’s Inn thrust fault zone on the adjacent Paunsaugunt Plateau, which displaced Upper Cretaceous strata over early Tertiary Claron Formation. They also envisioned the Markagunt Megabreccia to be a surficial part of this process, perhaps triggered by near-surface laccolith emplacement and consequent doming and catastrophic failure of overlying strata. Collapse of the volcanic field could also have resulted from inflation of the 21-million-year-old Mount Belnap caldera in the southwest part of the Marysvale volcanic field. If so, the Megabreccia would be at least twice as long and nearly three times the aerial extent of what we now envision (this is comparable to but still the junior of the famous 1300-square mile Heart Mountain detachment in northwest Wyoming, the World’s largest terrestrial gravity slide).
Thus, we have several possibilities but lack a definitive trigger for the formation of the Markagunt Megabreccia. Of the possibilities, gravity sliding off the Iron Peak laccolith seems the most likely. This idea is supported by similar gravity-slide deposits in the Pine Valley Mountains, which are tied to shallow igneous intrusions that domed up overlying strata, leading to catastrophic failure on oversteepened slopes (see Survey Notes, v. 34, no. 3, p. 1–3).
The densely forested, high elevation Markagunt Plateau has long been a refuge for those seeking both winter recreation and a cool respite from the summer heat of the valleys below. We all see this landscape somewhat differently. Botanists and wildlife enthusiasts revel in the diversity of plant and animal life and its profound changes with elevation along its 5000-foot-high western escarpment, culminating with spruce forests, isolated groves of ancient bristlecones pines, wildflower-filled meadows, and plentiful elk. Those whose life’s work revolves around water will see the plateau as the ultimate watershed that sustains life in the dry basins below. Even those of us with no specific bias enjoy the scenic diversity and open space the plateau offers. But who else besides a few geologists have ever really seen and understood the rocks that cap much of the northern Markagunt Plateau? Who knew about the collapsed remains of an ancient volcanic center that covers an area at least as large as the entire Salt Lake Valley? It’s exciting to think that such spectacular geologic phenomena remain to be discovered and understood.