Through the eons, the geologic processes of erosion, deposition, uplifting and subsidence have formed the landscape of the Lake Fork of the Gunnison River Watershed.
With a total area of 277,000 acres or 432 square miles, the landforms of the watershed vary greatly and include rugged mountain terrain in the upper reaches, the relatively broad flood plains of Capital City, Sherman, Lake City and Gateview, and the steep canyons of lower Henson Creek and the Lake Fork.
Elevations range from a high of 14,306 feet at Uncompahgre Peak to Blue Mesa Reservoir at 7,519 feet.
Volcanism and Calderas
Today, the surface geology of the watershed is dominated by volcanics, primarily of the San Juan volcanic field. About 30 million years ago during the Laramide Orogeny, thousands of feet of andesitic were deposited, forming the San Juan Formation.
Two more episodes of volcanic activity resulted in the formation of the andesitic tuffs of the Silverton Volcanic Group and the rhyolites of the Potosí Volcanic Group. Two dominant volcanic structures are the Lake City and Uncompaghre calderas which are large circular features. The Uncompaghre caldera erupted 29 million years ago and the Lake City caldera erupted 27 million years ago. At the end of the volcanic era, subsidence of the calderas caused radial faulting.
Following the volcanic period, erosion along with glacial activity created today’s rugged mountains. Glacial activity and movement contributed to the formation of hanging valleys, cirques, moraines and rock glaciers.
Valley bottoms typically contain alluvial deposits washed from the surrounding high country. At the bottom of tributaries flowing from the higher peaks are alluvial fans adjacent to Henson Creek and the upper Lake Fork. Other Quaternary alluvium deposits occur between Lake City and Gateview, located on the Lake Fork just south of Blus Mesa Reservoir.
Underneath the volcanics are Precambrian granite batholiths and metamorphosed gneisses and schists. A few remnants of this batholith can be found between Gunnison and Lake City. In the lower part of the watershed below Red Bridge, Precambrian gneiss and schist can be seen. During the Paleozoic era, (570 to 230 million years ago) there were periods of deposition and lithification which gave way to the Mesozoic era.
Following the formation of the Uncompahgre, San Juan and later the Lake City calderas during the middle Tertiary, mineralization occurred in the middle to late Tertiary. Massive alunitization and weak molybdenum and copper mineralization occurred along the northeastern margin of the Lake City caldera.
The deposition of metals by superheated fluids created the ore bodies which have been mined since the late 1800s. Economically important metals include silver, gold, lead, zinc, copper, and aluminum. While silver and gold have been the primary metals mined, potential future mining could include the recovery of aluminum from alunite deposits.
The Slumgullion Earth Flow
A more contemporary geologic phenomenon is the Slumgullion Earth Flow. Approximately 700 years ago, a massive volume of hydrothermally altered volcanic rock slumped from a glacial cirque at 11,500 feet on Mesa Seco and quickly flowed nearly 4.5 miles downhill, damming the Lake Fork of the Gunnison River and creating Lake San Cristobal at an elevation of 9,015 feet.
A second earth flow began some 300 years ago, over-riding the older material. The younger slide is now 2.5 miles long, with an impressive 100 foot high leading edge, and continues to flow downward as much as 20 feet per year in some areas. This active movement can be seen in the strange angles of the trees, predominantly Engelmann spruce, that struggle to survive in the orange-yellow clay soils.
Because the Earth Flow is so sparsely vegetated, there is considerable sediment erosion, especially from the younger active flow. Most of this material is carried by Slumgullion Creek, which empties into the northeast corner of Lake San Cristobal, creating an expanding delta near the lake outlet. This creek was diverted from the main stem of the Lake Fork below the slide into the lake in the mid 20th century. In wet years large amounts of yellow sediment discolor the outlet area, and contribute to the ever growing quantity of sediment in the lake, much of which is high in metals such as arsenic.
The Slumgullion Earth Flow is home to a rare plant, the reflected moonwort (Botrychium echo).
Because of the earth flow’s size, ease of access, and continued active geologic movements, the slide is regularly measured and studied by the United States Geological Survey and academic researchers from around the world.
The National Park Service has designated the Slumgullion Earth Flow as a National Natural Landmark. The Earth Flow is also a designated State Natural Area by the Colorado Park Service’s Natural Area Program and managed as a United States Forest Service Special Interest Area and Bureau of Land Management Area of Critical Environmental Concern.
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