Chapter 7.5, Page 2 of 4

Hydrogeologic Units

Bedrock in the fractured, crystalline-rock aquifers consists of igneous and metamorphic rocks of Precambrian age and Tertiary age igneous rocks. A thin veneer of soil, with moderate to high permeability, generally less than 5 feet thick, overlies the crystalline bedrock. The surficial deposits are not extensive enough to yield suitable quantities of water, but are an important unit for recharge and shallow, seasonal ground-water discharge. A conceptual model of the fractured, crystalline-rock aquifer system is shown in Figure 7.5-3.

Figure 7.5.3 Conceptual model of the aquifer system in fractured, crystalline rock.

Intrusive igneous rocks of Tertiary age are subdivided on the basis of composition, such as basaltic and rhyolitic, whereas extrusive (volcanic) rocks are divided by their physical characteristics such as ash flows, lavas, breccia, and tuffs. The physical characteristics of volcanic rocks vary greatly. Chemical composition, mineralogy, volatile content, temperature, and mode of extrusion greatly affect their hydraulic characteristics. The San Juan Mountains represent one of the larger volcanic areas, where hundreds to thousands of feet of ash-flow, air fall, and water-laid tuffs have been deposited.

This outcrop at the western base of Wolf Creek Pass exposes part of the
thousands of feet of ash-flow tuffs deposited in the San Juan Mountains.
Photo by R. Topper, CGS

Many of these rocks are extensively contorted through folding and faulting, producing joints and fractures that provide openings for storage of water. The fracture porosity of crystalline rocks is very low; as a general rule, less than 1 percent. Fractures provide the only significant porosity and flow conduits within the unweathered crystalline rocks of Colorado. Ground-water discharge and storage in crystalline rocks predominantly occurs in fracture networks. Vertical or steeply dipping fractures provide recharge, while near-horizontal fractures provide storage capacity and some degree of hydraulic continuity.

This granitic cliff in western Jefferson County is the real world expression
of the fractured, crystalline-rock aquifer system.
Photo by J. Cain, USGS

Chapter 7.5, Page 2 of 4