Geologic Model Units


Generalized geologic model units recognized in this investigation include Overburden, Saddle Mountains Basalt, Mabton Interbed, Wanapum Basalt, Vantage Interbed, Grande Ronde Basalt, and Older Bedrock. These correspond directly to the hydrogeologic units defined by Kahle and others (2009), with minor exception (described in section, “Overburden Unit”). The precise relation between geologic units and geologic model units are shown by a geologic column (fig. 2). 


Basalt and interbed units are described in detail in Drost and others (1990), Whiteman and others (1994), and Jones and Vaccaro (2008). The surficial distribution of the sedimentary portion of the mapped Overburden unit, the three Columbia River Basalt units, and the Older Bedrock unit are shown in figure 1A. This map was developed by grouping surficial geologic units into selected model units on the basis of geologic age and stratigraphic position. 


Overburden Unit


Sedimentary deposits cover portions of the CRBG across the study area (fig. 1A). These deposits are of variable thickness, either filling topographic lows or forming a relatively thin mantle on bedrock highs. For this study, the focus is on identifying thick sedimentary deposits in valley bottoms that are regionally important for storing and transmitting groundwater. Thin, discontinuous deposits are ignored in the final model-generated Overburden unit.


As defined by Kahle and others (2009), the Overburden unit consists of undifferentiated unconsolidated to semi-consolidated sedimentary deposits and post-CRBG volcanic deposits of very limited extent ranging from Miocene to Holocene in age. In an effort to define hydraulically important units, Kahle and others (2009) delineate areas where the thickness of the Overburden unit likely exceeds 100 ft, with some wells penetrating thicknesses as much as 1,300 ft.


For the current study, the post-CRBG volcanic deposits were not explicitly grouped with the Overburden unit because post-CRBG lava flows are expected to have significantly different hydraulic properties from sedimentary overburden, and ultimately this model is to be used as the foundation for a groundwater flow model. The geologic modeling process allows for post-CRBG volcanic deposits to be grouped with Overburden if they exist in sediment-filled valleys and CRBG units when they are deposited in contact with CRBG units (see section, “Geologic Modeling Methodology”). This grouping preserves the important hydraulic character of the uppermost unit for use in the groundwater flow model. Because these post-CRBG volcanics were not used explicitly in the modeling process, the locations where post-CRBG volcanics are mapped as surficial geology are left blank in figure 1A.


Saddle Mountains Basalt Unit


The Saddle Mountains Basalt unit, the least extensive and youngest formation of the CRBG, consists of the Saddle Mountains Basalt and interbed members. Most of the unit is in the west-central part of the study area, with less continuous occurrences in the Blue Mountains and eastward into Idaho (fig. 1A). The Saddle Mountains Basalt unit has an estimated areal extent of about 8,000 mi², and the elevation of the top of the unit ranges from about 4,000 to –280 ft. Thickness of the unit, based on well log data from 351 wells that fully penetrate this unit, ranged from about 0 to 990 ft (Kahle and others, 2009).


Mabton Interbed Unit


The Mabton Interbed unit is the sedimentary interbed between the overlying Saddle Mountains Basalt unit and the underlying Wanapum Basalt unit. The Mabton Interbed unit consists of the Mabton Member of the Ellensburg Formation and is mostly in the west-central part of the study area. Limited surficial outcrops of the Mabton Interbed unit are present in the study area and the extent is assumed to be within the extent of the Saddle Mountains Basalt unit. The Mabton Interbed unit generally consists of clay, shale, claystone, clay with basalt, clay with sand, and sandstone. Thickness of the Mabton Interbed unit, based on well log data from 242 wells that fully penetrate this unit, ranges from about 0 to 520 ft (Kahle and others, 2009).


Wanapum Basalt Unit


The Wanapum Basalt unit, composed of basalt and interbed members of the Wanapum Basalt, is in most of the north-central part of the study area (fig. 1A), and has an estimated areal extent of about 25,000 mi².The elevation of the top of the Wanapum Basalt unit ranges from about 3,400 to –1,000 ft (Kahle and others, 2009). Much of the unit lies beneath the Overburden and Saddle Mountains Basalt units. Thickness of the Wanapum Basalt unit, based on well log data from 738 wells that fully penetrate this unit, ranges from about 0 to 1,200 ft (Kahle and others, 2009).


Vantage Interbed Unit


The Vantage Interbed unit is the sedimentary interbed between the overlying Wanapum Basalt unit and the underlying Grande Ronde Basalt unit. Over most of the study area, this unit consists of the Vantage Member of the Ellensburg Formation; however, this unit includes sediment of the Latah Formation in the northeastern part of the study area. Limited surficial outcrops of this unit are present in the study area and the extent is assumed to be within the extent of the Wanapum Basalt unit. The Vantage Interbed 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. Interpretation of a limited number of well logs also indicates that the Vantage Interbed unit is not present in the southeastern part of the Yakima River basin and near the Cold Creek Syncline and Rattlesnake Hills Structure (Jones and Vaccaro, 2008). Thickness of the Vantage Interbed unit based on well log data from 444 wells that fully penetrate this unit ranges from about 0 to 320 ft (Kahle and others, 2009).


Grande Ronde Basalt Unit


The Grande Ronde Basalt unit is the oldest and most extensive of the basalt units. This unit underlies most of the study area (fig. 1A). The estimated areal extent of the Grande Ronde Basalt unit is about 42,000 mi² (Kahle and others, 2009).


The Grande Ronde Basalt unit contains the basalt and interbed members associated with the Grande Ronde Basalt as well as the less extensive Prineville, Picture Gorge, and Imnaha Basalts, and several stratigraphically consistent basalts mapped as undifferentiated in the compilation by Kahle and others (2009). Sedimentary interbeds in the Grand Ronde Basalt unit generally are rare and only a few feet thick, where present. The elevation of the top of the Grande Ronde Basalt unit ranges from 4,300 to –2,100 ft based on well log data used by Kahle and others (2009). Except near the periphery of the Columbia Plateau, where the unit thins and is fully penetrated by a number of water wells, thickness of the unit is largely unknown. Near the central part of the basin, the thickness of the unit is estimated to be greater than 15,000 ft.


Older Bedrock Unit


The Older Bedrock unit that borders and underlies the CPRAS is composed of various rock types older than the CRBG (Kahle and others, 2009). In Washington and Idaho, the rocks bordering the CPRAS consist mostly of sedimentary and granitic rocks. In Oregon, the CPRAS is bordered by sedimentary, volcaniclastic, volcanic, plutonic, and metamorphic rocks (Drost and others, 1990).


First posted February 25, 2011