The Ground Beneath
Manitou Springs Aquifer Hydrogeologic Setting
The geologic overview map shows a dramatic boundary along the Front Range between the very old granite and metamorphic rocks of the Rocky Mountain block, uplifted on the west side of a fault zone and the Great Plains block on the east. In an area of a few square miles, however, the small remnant of early sedimentary rocks at Manitou Springs, some of the oldest sedimentary rocks in the region are preserved on top of the Pikes Peak granite and metamorphic rocks in the upthrown Rocky Mountain block, west of the great bounding faults of the Front Range. Almost all of these sedimentary rocks have been eroded away over millions of years from the remaining area of the Rocky Mountain block.
The Manitou Limestone, and parts of the underlying sandstone and weathered granite, and overlying dolomite and limestone beds, constitute the Manitou Springs artesian aquifer, and the Fountain Formation serves as a sequence of low-permeability confining beds above it. The aquifer is the result of the dissolution of carbonate rocks by slightly acidic rainwater that seeps into the ground through fractures and joints. Once underground, the water slowly dissolves the soluble rocks into subterranean passages, caverns and caves. The result of this process is defined as a karst landscape. The mostly-dormant part of the Manitou Springs aquifer, now above the water table, is a cave system located north of the city called “Cave of the Winds”.
Carbonated Mineral Water
One of the most intriguing aspects of Manitou Springs is the carbon dioxide content of the gas emanating at the springs. The gas, is composed almost entirely of carbon dioxide, a characteristic not typical of low-temperature aquifers. By volume, the carbon dioxide content of gas at the springs ranges up to nearly 99 percent. In contrast, shallow groundwater containing dissolved gas originating from the atmosphere or soil typically has less than 1 percent by volume. Due to the purity of the gas composition, helium isotope evidence suggests that a significant portion of the carbon dioxide likely originates at depth from Earth’s outer mantle and migrates to the Manitou Springs aquifer along the Rampart Range and Ute Pass Fault Zones.
To the right is agraphic depicting the geologic stratigraphy of the area that tells the story of the type of environment that deposited each sedimentary layer.