Chapter 6.5, Page 2 of 4

Hydrogeologic Units

The major hydrogeologic units in the region consist of an upper Mesozoic sandstone aquifer and a lower Paleozoic carbonate aquifer that are separated by a thick sequence of confining salt beds (Table 6.5-1). The Mesozoic sandstone aquifer is composed of a stacked sequence of about 10 sandstone and shale geologic units. The sandstones tend to be intermittently saturated, with perched water tables common. Springs frequently occur at the base of the more permeable units, such as the Navajo and Wingate Sandstones. The Mesozoic sandstone aquifers are often under the influence of surface water and they discharge to the Dolores River where it is deeply incised.

Table 6.5-1 Hydrogeologic Units of the Paradox Basin

The lower Paleozoic aquifer is primarily composed of relatively porous and permeable limestones and dolomites. Some ground-water leakage from the overlying salt units occurs along faults and fractures; therefore, most ground water in the lower aquifer is saline and unsuitable for human consumption. A generalized geologic section of the basin is presented as Figure 6.5-2.

Figure 6.5-2 Generalized geologic cross section through the Paradox Basin.
(Click image to zoom .8MB)
From Tweto, 1983.

Chapter 6.5, Page 2 of 4