Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Guy W. Adema Roy M. Breckenridge Kenneth F. Sprenke Idaho Geological Survey University of Idaho Technical Report 07-2 Moscow, Idaho 83844-3014 ISBN 1-55756-514-6 Contents Abstract ...........................................................................................................................1 Introduction .................................................................................................................... 1 Geologic Setting ............................................................................................................. 2 Basement Rocks .............................................................................................. 2 Tertiary Geology ............................................................................................. 3 Quaternary Geology ........................................................................................ 4 Previous Geophysical Investigations ............................................................................. 7 Seismic Surveys .............................................................................................. 7 Gravity Surveys ............................................................................................... 9 Gravity Data Collection ................................................................................................. 9 Purves Data ..................................................................................................... 9 Cady and Meyer Data .................................................................................... 21 New Observations ......................................................................................... 21 Gravity Data Reduction ............................................................................................... 23 Instrument Calibration Correction ................................................................ 23 Tidal and Instrument Drift Corrections ......................................................... 23 Terrain Corrections ........................................................................................ 24 Latitude and Elevation Corrections ............................................................... 25 Bouguer Corrections ..................................................................................... 25 Correlation of Data Sets ................................................................................ 25 Removal of Regional Trend .......................................................................... 26 Gravity Data Modeling ................................................................................................ 27 Gravity Model Interpretation ....................................................................................... 30 Conclusion ................................................................................................................... 34 Acknowledgments ........................................................................................................ 34 References .................................................................................................................... 34 Figures Figure 1. Location of the Rathdrum Prairie ................................................................... 2 Figure 2 Maximum extent of the Cordilleran ice sheet ................................................. 4 Figure 3. Maximum terminal extent of the Lake Pend Oreille lobe .............................. 5 Figure 4. Locations of geophysical work performed on the Rathdrum Prairie ............. 7 Figure 5. Seismic reflection profile ................................................................................ 8 Figure 6. Locations of eight gravity measurements ..................................................... 26 Figure 7. Bouguer anomaly map of the Rathdrum Prairie ........................................... 27 Figure 8. Bouguer gravity map showing regional trend .............................................. 28 Figure 9. Residual Bouguer gravity map of the Rathdrum Prairie .............................. 29 Figure 10a. The Idaho Road (Washington) profile....................................................... 30 Figure 10b. The Corbin Road profile ........................................................................... 31 Figure 10c. The Idaho Road (Idaho) profile ................................................................ 31 Figure 10d. The Idaho Highway 41 profile .................................................................. 32 Figure 10e. The Hayden Avenue profile ...................................................................... 32 Tables Table 1. Principal Gravity Station Data .................................................................. 10-20 Table 2. Principal Facts for Primary Benchmark .................................................... 22-23 Table 3. Digital Elevation Models ............................................................................... 25 Table 4. Modeled Aquifer Characteristics .................................................................... 33 iii Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Guy W. Adema1 Roy M. Breckenridge1 Kenneth F. Sprenke2 ABSTRACT the Coeur d’Alene Mountains to the east. The aquifer is identified as the sole-source water supply for the greater The Rathdrum Prairie overlies part of a regional Coeur d’Alene and Spokane metropolitan areas. Our ground-water source, known as the Rathdrum Prairie- study will focus on the 615-square-km part in Idaho, the Spokane Valley aquifer, that covers 1,055 square km in Rathdrum Prairie aquifer. Spokane County, Washington, and Kootenai and Bonner counties, Idaho. The aquifer is considered a sole-source In 1978, the aquifer was designated “sole source,” water supply for the greater Coeur d’Alene and Spokane which qualified it for protection under the 1974 metropolitan areas. The 615-square-km part in Idaho is Federal Safe Drinking Water Act. Since then, the called the Rathdrum Prairie aquifer. It extends from Lake area’s rapid growth in both population and irrigation Pend Oreille south and west to the Idaho-Washington demand continues to strain the aquifer’s capability. state line. The aquifer occupies a glacially scoured Consequently, to answer civic concerns about the aquifer trough filled with highly permeable, coarse-grained, requires reconstructing the broader geologic history of catastrophically deposited glacial outwash. the region. This must begin with a clear understanding of the subsurface geology of the Rathdrum Prairie; Five geologic cross-sections of the valley have been however, developing an accurate predictive model of created using 630 gravity measurements, 146 of which its structure has been difficult because of its complex were collected specifically for our study to complement history. Nonetheless, knowing what the limits are for existing data. The data were modeled in 2¾ dimensions, this critical aquifer depends on scientific analysis that and the regional trend caused by crustal thickening measures its actual extent, recharge potential, and future to the east has been removed. The models present a expectations. Determining the depth and geometry of generally smooth valley floor with an incised channel the bedrock surface is the first step. Our study brings in the ancestral, subsurface valley in the western half together new geophysical information and an improved of the prairie. The bedrock-sediment interface appears interpretation technique to more fully analyze the basin to be slightly sloped to the east with the deepest point geometry and its influence on hydrologic assumptions. between Idaho Road (Idaho) and Hayden Avenue where sediments may extend over 350 m below the surface. The Rathdrum Prairie aquifer is primarily composed Westward, near the state line, the sediments appear to of valley-fill deposits of glaciofluvial origin. Extensive thin to a thickness of 216 m. gravels, deposited by catastrophic floods from Glacial Lake Missoula, fill the ancestral valley of the Rathdrum INTRODUCTION River (Breckenridge, 1989). The Missoula Floods, as these events are known today, were caused by the periodic The Rathdrum Prairie-Spokane Valley aquifer (Figure 1) impoundments and sudden releases of water from Glacial is a valley-fill aquifer that extends from the southern end Lake Missoula, which was reestablished many times of Lake Pend Oreille, south to Coeur d’Alene Lake, and during the Pleistocene when the Pend Oreille lobe of the west to Spokane. The valley is bound by Mount Spokane Cordilleran ice sheet repeatedly blocked the Clark Fork to the north, the Mica Peak uplands to the south, and River (Bretz and others, 1956; Bretz, 1959). The latest 1Idaho Geological Survey, University of Idaho, Moscow, ID 83844 2Department of Geological Sciences, University of Idaho, Moscow, ID 83844 Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho Adema, Breckenridge, and Sprenke Adema, Breckenridge, and Sprenke Figure 1. Location of the Rathdrum Prairie, Idaho (from Wyman, 1994). periods of lake-emptying cycles occurred from 12,000 to These new data target coverage to specific profiles and 17,000 years ago (Waitt and Atwater, 1989). Proglacial validate the previous work. The combined data set was and flood processes deposited clasts with lithologies reduced and modeled with techniques