Scientific Investigations Report 2008–5045
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5045
This section describes the hydrogeologic framework of the basalt and interbed hydrogeologic units that compose part of the ground-water system in the Yakima River Basin aquifer system. An understanding of the framework is important in determining the occurrence, availability, and movement of ground water within the aquifer system. The ground-water flow system that is composed of the basalt and interbed units is in turn, part of the larger Columbia Plateau regional aquifer system. A correlation chart of the regional generalized stratigraphy and hydrogeologic units is shown in figure 6.
The hydrogeologic units vary in extent and thickness, and each unit is discussed separately. The hydrogeologic units identified in this report do not necessarily correspond to geologic time-stratigraphic deposits. The ground-water flow system within the Yakima River Basin aquifer system is interconnected with the overlying basin-fill deposits and with the Columbia Plateau regional aquifer system.
Knowledge of the geologic structure that exists within the Yakima River Basin aquifer system (fig. 5) also assisted in mapping of the hydrogeologic units. The structural setting helps to explain the depositional sequences, thickness variations, and segmentation of the ground-water movement or anomalous water-level distributions within the area. For example, a predominantly fine-grained or non-porous unit could be vertically offset and juxtaposed with a coarse-grained or porous unit, thereby truncating the lateral ground-water movement along a fault and also offsetting the water-level distribution on either side of the fault. Although such ground-water flow issues are not addressed in this report, the structural history has provided information for mapping the hydrogeologic units that will assist in the development of numerical ground-water flow models.
The Saddle Mountains hydrogeologic unit is the youngest and least extensive of the basalt units. The unit is located in the southeast and south-central part of the Yakima River Basin aquifer system (pl. 1) and encompasses an area of about 2,290 mi2. Most of the unit area, about 1,800 mi2, lies beneath the basin fill deposits described in Jones and others (2006). Surficial outcrops of the Saddle Mountains Basalt (SMB) make up about 460 mi2 of the Saddle Mountains unit and about 30 mi2 of surficial outcrops of the older Wanapum Basalt (WB) and Grande Ronde Basalt (GRB) are within the mapped unit extent. The outcrops of the SMB are located in the southwest generally south of the Toppenish Ridge near Dry and Satus Creeks, and along the flanks of Horse Heaven Hills Structure, Rattlesnake Hills Structure, Ahtanum Ridge, Yakima Ridge, and the Umtanum Anticline (fig. 5, pl. 1).
The Saddle Mountains unit predominantly contains the basalt and interbed members associated with the SMB. The SMB is composed of at least 13 named flows and 5 interbed members (Meyers and Price, 1981). The SMB flows’ texture and composition differ greatly throughout its extent. The sedimentary interbeds contained within the SMB are common, relatively thick (often 50 ft or greater) and range in composition from clay to sand and gravel (Drost and Whiteman, 1986). The Saddle Mountains unit also may contain some of the younger basalts present in the unit extent due to the inability to delineate between the basalts based on the available well record information and the minimal data available in some areas. This is true particularly in the area near Naches (pl. 1) where Quaternary basalt is mapped at the surface and in places that directly overlie the SMB, but due to a lack of data to effectively separate the basalts they were combined. The hydrogeologic framework could be improved as more detailed data become available at depth and for areas were data are sparse.
The top of the Saddle Mountains unit ranges from a maximum altitude of 4,290 ft where it is exposed at land surface to a depth of 1,840 ft below land surface. The mean and median depths to the top of the Saddle Mountains unit are about 200 ft and 120 ft below land surface, respectively. The highest altitudes of the Saddle Mountains unit are located along the southern boundary of the study area and along the western part of the Horse Heaven Hills Structure where the unit reaches altitudes above 4,000 ft. The unit is at its greatest depth below land surface in the area northwest of the city of Yakima where the depth to the top of the unit exceeds 1,800 ft. Thickness of the Saddle Mountains unit, based on wells that completely penetrated the unit ranged from about 0 to 1,110 ft, with a mean and median thickness of about 550 ft and 560 ft, respectively. The distribution of the well-record information used to delineate the depth to the top of the Saddle Mountains unit is shown in figure 7.
The Mabton hydrogeologic unit is the sedimentary interbed between the overlying SMB and the WB and is informally called the Mabton Member of the Ellensburg Formation (Drost and Whiteman, 1986). The Mabton unit is located in the southeast and south-central part of the Yakima River Basin (pl. 2) and encompasses an area of about 2,210 mi2. Most of the unit area, about 2,180 mi2, lies beneath the Saddle Mountains unit and about 30 mi2 of the older WB and GRB are within the mapped unit extent (pl. 2). No surficial outcrops of the Mabton unit are present within the study area and the extent is assumed to be within the extent of the Saddle Mountains unit. The Mabton unit might extend beyond the boundary of the Saddle Mountains unit, but it is difficult to delineate between the basin-fill deposits and the Mabton deposits at depth by only using available well-record information.
The Mabton unit generally consists of clay, shale, claystone, clay with basalt, clay with sand, and sandstone, but also may contain small amounts of sand and sand-and-gravel, based on the interval interpretations from available well records.
The depth to the top of the Mabton unit ranges from 80 to 2,000 ft below land surface. The mean and median depths are about 730 ft and 640 ft below land surface, respectively. The unit is at its shallowest depth along the southeastern flank of the Yakima ridge near the north end of Rattlesnake Hills Structure (fig. 5, pl. 2). The areas where the unit is at its greatest depth below land surface are in the areas northwest and northeast of Yakima, south of Wapato and north of Toppenish, where the depth to the top of the unit exceeds 1,800 ft. Thickness of the Mabton unit, based on wells that completely penetrate the unit range from about 0 to 250 ft, with a mean and median thickness of about 70 ft. The distribution of the well record information used to delineate the depth to the top of the Mabton unit is shown in figure 8.
The Wanapum hydrogeologic unit is located in the northeast, central, south, and southeastern part of the Yakima River Basin aquifer basin (pl. 3) and encompasses an area of about 3,450 mi2. Most of the unit area (2,760 mi2) lies beneath the basin-fill deposits, Saddle Mountains unit, or Mabton unit. Surficial outcrops of the WB make up about 660 mi2 of the Wanapum unit and about 30 mi2 of the surficial outcrops within the mapped unit extent are older Grande Ronde Basalt. The outcrops of the WB are located predominantly in the northeast, north of and along the Yakima Ridge; and in the southwest, along and south of the Toppenish Ridge. Less continuous WB outcrops occur along the Ahtanum Ridge, Rattlesnake Hills Structure, and Horse Heaven Hills Structure (fig. 5, pl. 3).
The Wanapum unit contains predominantly the basalt and interbed members associated with the WB. The WB is composed of at least six named flows and two interbed members (Meyers and Price, 1981). The WB flows’ generally are medium-grained to moderately plagioclase-phyric, olivine bearing, and relatively high in iron and titanium oxides. The clay to sand-and-gravel sedimentary interbeds in the WB are less common than those in the SMB and generally are only a few feet thick (Drost and Whiteman, 1986). But the Wanapum unit also may contain some of the younger basalt, particularly in areas where the available well records occur along the margins of the unit extent and where the younger basalt abuts or overlies the WB.
The top of the Wanapum unit ranges from a maximum altitude of 5,680 ft where it is exposed at land surface to a depth of 2,050 ft below land surface. The mean and median depths to the top of the Wanapum unit are about 420 ft and 260 ft below land surface, respectively. The highest altitudes of the Wanapum unit are located along the southern boundary near Satus Creek, where the unit reaches altitudes above 5,000 ft (fig. 5, pl. 3). The areas where the unit is at its greatest depth below land surface are east of Ellensburg and northwest of Yakima where the depth to the top of the unit exceeds 2,000 ft. Thickness of the Wanapum unit, based on wells that completely penetrated the unit, ranged from about 0 to 1,180 ft, with a mean and median thickness of about 600 ft and 490 ft, respectively. The distribution of the well-record information used to delineate the depth to the top of the Wanapum unit is shown in figure 9.
The Vantage hydrogeologic unit is the sedimentary interbed between the overlying WB and the GRB that informally is called the Vantage Member of the Ellensburg Formation (Drost and Whiteman, 1986). The Vantage unit is located in the northeast, central, south, and southeastern part of the Yakima River Basin aquifer system (pl. 4) and encompasses an area of about 3,090 mi2. Most of the unit area (3,050 mi2) lies beneath the Wanapum unit and about 40 mi2 of the older GRB are within the mapped unit extent (pl. 4). No surficial outcrops of this unit within the study area are present and its extent is assumed to be within the extent of the Wanapum unit. The Vantage unit might extend beyond the boundary of the Wanapum unit, but it is difficult to delineate between the basin-fill deposits and the Vantage deposits at depth by only using well-record information.
The Vantage unit consists of clay, shale, sandstone, tuff with claystone, and clay with basalt, but also may contain small amounts of sand and sand-and-gravel. A few well-record interpretations also indicate that the Vantage unit is not present in the southeastern part of the Yakima River Basin aquifer system along the eastern boundary and near the Cold Creek Syncline and Rattlesnake Hills Structure (fig. 5, pl. 4).
The depth to the top of the Vantage unit ranges from 40 to 2,790 ft below land surface. The mean and median depths are about 1,150 ft and 820 ft below land surface, respectively. The unit is at its shallowest depth in the northeastern part of the Vantage unit extent along the eastern boundary, in the area where the WB outcrops at the surface (pls. 3 and 4). The areas where the unit is at its greatest depth are an area north of Toppenish and an area northeast of Rattlesnake Hills Structure, where the depth to the top of the unit exceeds 2,600 ft. Thickness of the Vantage unit, based on wells that completely penetrated the unit, ranged from about 0 to 135 ft, with a mean and median thickness of about 30 ft and 20 ft, respectively. The distribution of the well-record information used to delineate the depth to the top of the Vantage unit is shown in figure 10.
The Grande Ronde hydrogeologic unit is the oldest and most extensive of the basalt units. It underlies most of the Yakima River Basin aquifer system (pl. 5), except for an area along the western boundary and the northwestern part of the basin where generally older bedrock units outcrop at the surface (fig. 4). The extent of the Grande Ronde unit encompasses about 5,390 mi2 with most of the unit, 3,790 mi2, present beneath the basin-fill deposits, Vantage unit, and Wanapum unit. Surficial outcrops of the GRB make up about 1,550 mi2 of the Grande Ronde unit and about 50 mi2 of the surficial outcrops within the mapped unit extent are older bedrock deposits. The outcrops of the GRB are located predominantly along the northeastern and western boundary of the unit extent. Less continuous GRB outcrops occur along Yakima Ridge, Rattlesnake Hills Structure, and in the south along Satus Creek (fig. 5, pl. 5).
The Grande Ronde unit predominantly contains the basalt and interbed members associated with the GRB. The GRB is composed of at least 30 flows and perhaps as many as several hundred individual flows (Meyers and Price, 1981; Drost and Whiteman, 1986). The GRB flows’ are aphyric with microphenocrysts of plagioclase and clinopyroxene. Olivine is generally present only in the groundmass, and typically totals less than 0.5 percent of the flow volume (Drost and Whiteman, 1986). Sedimentary interbeds within the GRB generally are rare and where present are only a few feet thick (Meyers and Price, 1981; Drost and Whiteman, 1986). These sedimentary interbeds range in composition from clay to sand-and-gravel.
The top of the Grande Ronde unit ranges from a maximum altitude of 6,900 ft where it is exposed at land surface to a depth of 2,800 ft below land surface. The mean and median depths to the top of the Grande Ronde unit are about 1,300 ft and 980 ft below land surface, respectively. The highest altitudes of the Grande Ronde unit are located along the western boundary near Cowiche Mountain and along the northern boundary on the northern part of the Naneum Ridge Anticline, where the unit reaches altitudes above 6,500 ft. The areas where the unit is at its greatest depth are an area north of Toppenish and an area northeast of Rattlesnake Hills Structure, where the depth to the top of the unit exceeds 2,600 ft (pl. 5). Thickness of the Grande Ronde unit was not determined. The distribution of the well-record information used to delineate the depth to the top of the Grande Ronde unit is shown in figure 11.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5045
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