Geologic Map of the Beacon Rock Quadrangle, Skamania County, Washington by Russell C
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Geologic Map of the Beacon Rock Quadrangle, Skamania County, Washington By Russell C. Evarts and Robert J. Fleck Pamphlet to accompany Scientific Investigations Map 3367 Beacon Rock along north shore of Columbia River, Hamilton Mountain in background. 2017 U.S. Department of the Interior U.S. Geological Survey Contents Introduction.....................................................................................................................................................1 Geographic and Geologic Setting ......................................................................................................1 Previous Geologic Investigations in the Western Columbia River Gorge ...................................1 Acknowledgments ................................................................................................................................3 Synopsis of Geology ......................................................................................................................................3 Cascade Volcanic Arc ...................................................................................................................................3 Stratigraphic Relations and Nomenclature ......................................................................................3 Oligocene and Early Miocene Strata Older than the Lava Flows of Hamilton Creek ................5 Lava Flows of Hamilton Creek ............................................................................................................5 Eagle Creek Formation .........................................................................................................................7 Intrusive Rocks ......................................................................................................................................8 Metamorphism and Hydrothermal Alteration ..................................................................................9 Columbia River Basalt Group .......................................................................................................................9 Grande Ronde Basalt .........................................................................................................................11 Analytical Issues ........................................................................................................................11 Wapshilla Ridge Member .........................................................................................................14 Grouse Creek Member ..............................................................................................................14 Ortley Member............................................................................................................................14 Winter Water Member ..............................................................................................................15 Sentinel Bluffs Member ............................................................................................................15 Wanapum Basalt .................................................................................................................................16 Frenchman Springs Member ...................................................................................................16 Rocks of the Boring Volcanic Field ...........................................................................................................16 Alluvial and Mass-Wastage Deposits ......................................................................................................18 Landslide and Talus Deposits ...........................................................................................................18 Holocene and Pleistocene Alluvium of Tributary Streams ..........................................................19 Holocene Outburst-Flood Deposits ..................................................................................................19 Structural Features ......................................................................................................................................19 Faults... ................................................................................................................................................. 20 Geologic Resources ................................................................................................................................... 20 Geologic Evolution ...................................................................................................................................... 20 References Cited......................................................................................................................................... 21 Appendix 1.................................................................................................................................................... 26 Figures 1. Regional setting of Beacon Rock 7.5’ quadrangle.....................................................................2 2. Simplified geology of Beacon Rock quadrangle showing distribution of rocks of ancestral Cascade volcanic arc, Columbia River Basalt Group, and Quaternary volcanic rocks ...............................................................................................................................4 3. Chemical characteristics of volcanic rocks from Beacon Rock 7.5’ quadrangle (analyses recalculated volatile-free) ........................................................................................6 4. Cliff composed of upper Eagle Creek Formation on southeast flank of Hamilton Mountain ........................................................................................................................................7 5. Plot of Sr/Y versus Y for igneous rocks in Beacon Rock 7.5’ quadrangle .............................8 6. Typical jointing patterns exhibited by lava flows of the Columbia River Basalt Group .....10 i 7. Stratigraphic nomenclature, ages, and magnetic polarities of the Columbia River Basalt Group ................................................................................................................................12 8. Chemical characteristics of Grande Ronde Basalt flows in the Beacon Rock 7.5’ quadrangle ...................................................................................................................................13 9. Paleomagnetic directions for Grande Ronde Basalt flows in the Beacon Rock 7.5’ quadrangle ............................................................................................................................15 10. South face of Hamilton Mountain showing ~200-m-thick intracanyon Grande Ronde Basalt flow overlying basal foreset-bedded pillow breccia of same flow on the left and Eagle Creek Formation on the right ...............................................................16 11. Selected chemical characteristics of Pleistocene volcanic and subvolcanic rocks in the Beacon Rock 7.5’ quadrangle .............................................................................17 12. Paleomagnetic directions for Quaternary volcanic rocks in the Beacon Rock 7.5’ quadrangle ...................................................................................................................................18 13. Three Corner Rock, a mass of basaltic andesite that occupied the throat of a now-eroded, approximately 1-Ma scoria cone .....................................................................19 Tables 1. Chemical and modal analyses of igneous rocks of the ancestral Cascade volcanic arc, Beacon Rock 7.5’ quadrangle, Skamania County, Washington ..................34 2. Chemical analyses of the Columbia River Basalt Group, Beacon Rock 7.5’ quadrangle, Skamania County, Washington ..........................................................................45 3. Chemical and modal analyses of Quaternary volcanic rocks, Beacon Rock 7.5’ quadrangle, Skamania County, Washington ..........................................................................55 4. Summary of 40Ar/39Ar incremental-heating age determinations, Beacon Rock 7.5’ quadrangle, Skamania County, Washington ...................................................................60 5. Paleomagnetic data for Columbia River Basalt flows and Quaternary volcanic rocks, Beacon Rock 7.5’ quadrangle, Skamania County, Washington ..............................61 ii Introduction report and accompanying map were the first to describe the major stratigraphic units visible in the walls of the gorge. He Geographic and Geologic Setting distinguished four major rock units: tuff and conglomerate that he named the Eagle Creek Formation, Miocene Columbia River The Beacon Rock 7.5′ quadrangle is located approximately basalt, cobbly gravels assigned to the Satsop Formation, and 50 km east of Portland, Oregon (fig. 1), on the north side of young andesitic and basaltic lavas and tuffs, which he called the Columbia River Gorge, a scenic canyon carved through the the Cascades Formation. All except the post-basalt gravels are axis of the Cascade Range by the Columbia River. Although present in the Beacon Rock area. Despite marked changes in approximately 75,000 people live within the gorge, much of geologic concepts and terminology in the past century, later the region remains little developed and is encompassed by the workers have largely retained the basic stratigraphic framework 292,500-acre Columbia River Gorge National Scenic Area,