By Lorna Campbell (Geoscientist)
Fishlake Basin is located in the Fremont Range of the Fishlake National Forest. Fishlake Basin straddles a volcanically land that spans the physiographic boundary of the Colorado Plateau, the Basin and Range and the Middle Rocky Mountains. This Colorado Plateau extends to the east into the Four Corners region of Utah, Colorado, Arizona and New Mexico. The Middle Rocky Mountains extend to the north and the Basin and Range province extend from the center and toward the west. (Figure 1). Each of these provinces display different physiographic characteristics and show the complexity of different times in the Earth’s history. The Fish Lake area (used throughout to refer to Pando’s location) sits in a broad transition zone between the Colorado Plateau and the Basin and Range province, where features of each domain can be observed.
The Colorado Plateau
The Colorado Plateau was uplifted in several pulses beginning around 70-80 million years ago (Laramide orogeny) which built many of the mountains across the west (including the Rocky Mountains) and again more recently beginning ~10 million years ago. Despite these tectonic pulses deforming much of the west as they did creating the Rocky Mountains, the Colorado Plateau remained as an intact block despite being lifted upward more than 10,000’ above sea level in some areas. One exception to the generally undeformed geology of the region is the Waterpocket fold which formed 35-75 million years ago. The primary feature of Capitol Reef National Park just 40 miles southeast of Pando, this “buckle” as it is known colloquially, runs north-south for almost 100 miles. The more recent uplifting of the plateau led to erosion of the top part of the fold by great rivers, rain and wind, exposing its inclined time layers at the surface (Figures 2 and Figure 3). The oldest rocks observed in the center of the fold are around 248 Million years old (Permian Period).
The Basin and Range
Pando also lies the Basin and Range Province. The province is extends west to California, North into Oregon and south into Mexico. The “Basin and Range” takes it name from the characteristically “accordion like” succession of north-south oriented mountain ranges separated by intervening broad basins partially filled with sediment. The “range” or mountain portions were formed when older underlying rock from the Paleozoic (>240 million years old) were lifted upward and broken into huge fault blocks by east-west extensions of the land between 17 to 23 million years ago. The wide, flat basins between each range have been filled with sediments eroding from the ranges and were further, shaped and modified in some areas by glaciers and lakes that periodically filled the valleys. The land mass and topography is readily recognized on satellite images of Nevada and western Utah, as shown in Figure 4.
Pando: Life on Violent Boundary
The land Pando calls home spans a boundary between the Colorado Plateau and Basin and Range provinces. A transition zone that is is
complex and highly variable along its north-south extent. Figure 5 is a 1:250,000 scale geologic map of the region and provides a
detailed look at the ages and types of rocks exposed at the surface and their relationships. In this area, the transition has been mapped as a large fault, which places Jurassic Period rocks (~200 million years old) at the surface to the east, directly adjacent to younger Tertiary rocks (<66 million years old) to the west (Figure 6). The fault is known as the Thousand Lake fault. Other fault zones also extend further to the north.
As we zoom in more closely to the Fish Lake region, perhaps the most striking geologic feature is the amount of volcanic rock that appears at the surface (pink / purple colors on Figure 5, 6 and 7). Prolific volcanism (eruptions, lava flows, fissure eruptions etc) occurred in much of the western United States and Mexico (Figure 7) beginning in the Oligocene (~33 million years ago) and continued sporadically through Basin and Range deformation to recent Holocene times (0.5 million years ago). The earliest volcanic rock are part of the Great basin ignimbrite flare up, where more than 200 explosive eruptions distributed ash and lava flows over thousands of miles. More than 42 surface calderas (eruption sites) are observed today. Geologists interpret this widespread activity to have occoured due to a change in the angle of tectonic plate subduction occouring further to the west which led to increasing melting of the subducting plate and subsequent extrusion of volcanics (lava, ash) at the surface.
The Marysvale volcanic field (Figure 8) is one of the larger areas of volcanic activity and deposited a significantly thick volcanic sediments layer over the high plateaus of central Utah. Around Marysvale the volcanic pile exceeds 1.8 miles thick (3,000m). The Fish Lake plateau lies on the eastern edge of the area. Several major extrusion points have been mapped, including the Monroe Peak caldera which lies 13 miles west of Pando. As the eruptions continued, a vast amount of material was ejected from the molten magma pool below the earth’s surface, creating a void. The walls of the volcano eventually collapsed, creating a depression. This feature is very large, extending 14 miles east-west and 11 miles north-south. (Figure 8, below)
The characteristics and precise dating of the different volcanic deposits in the region provides a useful age constraint for major
geological events in the area. For example, the Osiris Tuff (dated at 23 million years old) erupted from the Monroe caldera and forms the cliffs and ledges at Fish Lake Hightop. This same tuff appears in valley floors further to the west, indicating downward displacement of several thousand feet across more recent normal faults, as the Basin and Range continues to undergo extension.
Multiple episodes of land movement (compression or extensions) between 5.3 million and 2.58 million years ago (Pliocene) and again between 2.5 million and 11,000 years ago (Pleistocene) set up a north-northwest and northeast trending ”horst and graben” system in the high plateaus of Utah (see Figure 12 for visualization). Fish Lake sits in one of these grabens and forms the headwaters of the Fremont River. Underlying the lake and forming the sides of the graben is a sequence of volcanic deposits from the Marysvale system (Figure 8 and Figure 10).
In addition to recent volcanism and faulting, the surface geology of the Fish Lake area has been modified by glaciation (Figure 11). According to the Utah Geologic survey, most of Utah’s highest mountain ranges and plateaus, those over 8,200’ elevation in the north and over 10,000’ in the South, show evidence of glaciation during the Last Glacial Maximum, LGM, ~32,000 to 14,000 years ago (Figure 11). During the Last Glacial Maximum, Canada was covered by the 1.8 miles thick Laurentide-Cordilleran Ice sheet complex. The presence of this ice sheet had profound impacts on the climate in North America, altering atmospheric circulation patterns and moisture sources. While there were no ice sheets further south, alpine glaciers were common across the western US and reached their maximum extent around this time and were sustained by local climate conditions.
Alpine glacial deposits are remarkably hard to date, and the geologic record is often incomplete as glaciers advance and retreat, destroying evidence of earlier episodes. The Fish Lake plateau preserves evidence of at least two glacial advances; the most recent one dated at 21,100 years ago. Eight small alpine paleoglaciers have been mapped in the Fish Lake area, identified by their moraines marking the glaciers’ extents, and glacial outwash deposits (See Figure 12 dotted line). The glaciers progressed east to southeast from the Fish Lake Hightop Mountain (11,634ft) via side canyons and valleys, terminating in the basin created by the Fish Lake graben. Glaciation does not appear to have been widespread in the Fish Lake Basin itself and there is no evidence of paleoglaciers at the southern end where Pando is located today. A sedimentary layer of soil does cover the underlying rock formation on the southern end of the lake which could be glacial in origin and it has been suggested Fish Lake once flowed to the South, but evidence for how or why has not been independently verified.
The Pieces that Fit and the Puzzles that Remain (Lorna Campbell and Lance Oditt)
The land Pando calls home has been shaped volcanoes, glaciers, active fault zones and erosion. Although we lack focused and comprehensive studies of the geologic complexity beneath Pando’s land mass and need to learn more, we can draw from what we know about trees to understand why Pando thrives in this landscape. We know Trembling Aspen like Pando thrive is disturbed lands, the places where earthquakes, landslides, avalanches and floods give the fast growing and light hungry tree species unrivaled advantage when other trees are destroyed. As Trembling Aspen prefer acidic soils, Pando’s location atop a complicated volcanic landscape provides a mineral rich homestead. As Pando prefers well drained soils, its home along the steep east-facing slope ensures a well-drained residence. As Aspen prefer loamy soils, the underlying volcanic rock, glacial history and topography of the high mountain abode supports soil development that helps sustain the tree. Today, what we know about the land Pando’s calls home is painted in broad strokes and by looking back. Only a few studies exists about the general landscape and no known works detail the immediate underlying geology of Pando’s land mass. What would could learn about Pando and other lifeforms like it is tantalizing to consider. Geomorphological studies could help us understand what shaped conditions for this unique lifeform to take hold not because it is the only one of its kind, but very likely because is not. In discussions with the author of this review over the last 5 months, we came up with a list of questions we believe will be critical to working from the present tense and downward into the land instead of looking backward across its range. In a future update we will detail a list of foundational studies needed in geology and geomorphology that might help us understand Pando and its home better.
Williams and Hackman, 1971. Geology
of the Salina quadrangle (1:250,000)
Willis and Doelling, 2019.
Geologic Map of the Burrville quadrangle (1:24,000)
Biek et al, 2021. Geologic
Map of the Lyman quadrangle (1:24,000)
Capitol Reef National Park geology www.nps.gov/care/planyourvisit/upload/Geology-big-pictureDec15-accessible.pdf
Best et al., 2013. Introduction:
The 36–18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms
of subduction-related supervolcanoes. Geosphere. GeoscienceWorld.
Rowley et al. 2002. Geology and Mineral
Resources of the Marysvale Volcanic Field, Southwestern Utah GSA 2002. Rocky Mountain Section Annual Meeting.
Rozier, 2006 (unpublished undergraduate work,
Carleton College). Pyroclastic Geology of the Johnson Valley Reservoir Andesite.
Bergman, 2007 (unpublished undergraduate work,
Carleton College). Late Pleistocene Glacial History and Reconstruction of the
Fish Lake Plateau, South-Central Utah: Implications for Climate at the Last
© 2021 | Design Hope Smith BA / Lance Oditt