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A Geologic History of Fishlake
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A Geologic History Of Fishlake Basin
A Literature Review by Lorna Campbell (Geoscientist)

Edited by Lance Oditt

Overview:
Utah is internationally recognized for its outstanding geological displays, largely focused on the rocks exposed and preserved in the National parks and monuments in the southeast of the state.  Utah is also home to the Pando Tree, the world’s largest tree. This review details the literature of the geologic characteristics of Fishlake Basin, Pando's home.

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.

regional map of utah showing various ecological boundaries
Figure 1. Physiographic provinces of Utah and location of Fish Lake (from Utah Geological survey.

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).

table showing geologic layers of southern utah
Figure 2. Schematic cross section of the Rocks layers of the Colorado Plateau (from National Park Service brochure)
figure 3 illustration of geologic features in capitol reef national park
Figure 3. Schematic cross section through the Waterpocket fold (from National Park Service brochure)

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.

 

 

basin and range geology example
Figure 4: Example of the characteristic "accordion folds" of the Basin and Range land. The Mountains are roughly North to south ranges while basincs in between are the sediment basins.


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.

map m- 1:250,000 scale area map of geology of fishlake basin
Figure 5. Geologic map of the Salina Quadrangle (from Utah Geological Survey), 1:250,000 scale. The green / blue colors are the older Mesozoic and Paleozoic sedimentary and metamorphic rocks of the Colorado Plateau, while the pinks / purples are younger Tertiary volcanic rocks. Fish Lake is located in the center-left. The Waterpocket fold of Capitol Reef is also annotated.
cross section map emphasizing fishlake basin
Figure 6. West to East geologic cross section showing the Thousand lake fault zone, which juxtaposes Tertiary volcanics (pinks) against Jurassic sediments (green) (from Utah geological survey Lyman quadrangle). Approximate line of section shown on map in Figure 5.

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.

 

map showing history of volcanic eruptions in southwest americas