Aquifers in pre-Miocene rocks (fig. 46) consist of un-differentiated volcanic rocks, undifferentiated consolidated sedimentary rocks, and undifferentiated igneous and metamorphic rocks that are distributed throughout Segment 7, principally in the mountainous areas. In some places, the thickness of the volcanic rocks might be as much as about 5,000 feet and that of the consolidated sedimentary rocks might be as much as about 15,000 feet. The thickness of the igneous and metamorphic rocks is unknown. East of the Cascade Range, the aquifers in pre-Miocene rocks generally yield freshwater but locally yield saltwater. Within the Cascade Range and west of it, these aquifers commonly yield saltwater. Fresh ground-water withdrawals are used mostly for domestic and commercial purposes.
In the volcanic rocks, water is present primarily in joints and fractures as in the Pliocene and younger and the Miocene basaltic-rock aquifers. In the consolidated sedimentary rocks, water is present primarily in solution cavities and joints in carbonate rocks (fig. 47) and in fractures, faults, and intergranular pore spaces in clastic rocks, such as sandstone and conglomerate. In igneous and metamorphic rocks, water is present primarily in fractures, faults, and weathered zones that developed on exposed surfaces (fig. 48). The aquifers in all rock types generally yield only from 1 to 100 gallons per minute of water to wells. In all rock types, but especially in igneous and metamorphic rocks, yields of wells tend to decrease as depth increases and open spaces become fewer, smaller, or are filled with secondary minerals; for example, there generally are few open spaces in igneous and metamorphic rocks below a depth of about 300 to 400 feet.
In places, particularly in western Oregon and in Washington west of the Cascade Range, the consolidated sedimentary rocks are of marine origin. At depth, these rocks contain saltwater that can contaminate overlying freshwater aquifers. Locally, the saltwater can move upward through open spaces, particularly faults, and either mix with the freshwater in overlying aquifers or discharge to the land surface as springs. Such discharge can adversely affect the quality of water in the surficial aquifers that contain freshwater.
Saltwater contamination of freshwater aquifers also can occur in coastal areas if withdrawals from wells are sufficiently large to induce saltwater movement from the ocean or other saltwater bodies into the freshwater aquifers. Because salt-water is denser than freshwater, saltwater contamination generally is restricted to the basal part of the freshwater aquifers.
