Colorado’s Geology and Aquifers
The occurrence and distribution of Colorado’s water resources are directly linked to the state’s geography and underlying geology. This geographic variation is expressed in three major physiographic provinces that roughly trend north-south through the state – the Great Plains, Southern Rocky Mountains, and Colorado Plateau provinces. A basic understanding of the geology of Colorado is essential to understanding the nature and occurrence of ground water in the state, since the rock layers are also the aquifers in which water is stored. The structural deformation episodes associated with the uplift of the Rocky Mountains have produced Colorado’s varied and complex geology. A generalized geologic map showing the surficial distribution of Colorado’s major geologic units is presented as Figure 1.1. This map separates Colorado’s geologic units by eras within the geologic time scale. This time scale places the oldest rocks on the bottom and the youngest at the top. Table 1.1 depicts the geologic time scale and highlights the major geologic events occurring within Colorado.
Geologic units consist of either unconsolidated sediments or consolidated rock. A geologic unit containing interconnected pore spaces or crevices that are filled or saturated with water is termed an aquifer. Colorado’s principal aquifers are categorized into the following: (1) unconsolidated Quaternary age alluvial aquifers associated with major river systems, (2) poorly consolidated or unconsolidated sediments, (3) consolidated sedimentary rock aquifers, and (4) volcanic and crystalline rock aquifers. Alluvial deposits consist of unconsolidated silt, sand, and gravel that have been deposited during recent geologic time by a stream or river. Unconsolidated to poorly consolidated aquifers consist of sediments that were deposited by wind, water, and gravity often filling intermontane valleys, such as the San Luis Valley. The major sedimentary rock aquifers in Colorado consist predominantly of sandstones and limestones of varying ages. Many of these aquifers are located in structural basins that contain multiple geologic units. Precambrian granites and gneisses and geologically recent volcanic and igneous intrusive rocks form the backbone of Colorado’s major mountain ranges. These rocks represent the fractured-rock aquifers that supply much of the domestic needs in the mountainous portion of our state. The location and areal distribution of these aquifers or structural basins is depicted in Figure 1.2.