Abstract The Klamath Basin is located between the Cascade Mountain Range and the Klamath Mountains. The Cascades are formed by an offshore subduction zone, the Juan De Fuca Plate is subducting underneath the North American Plate. The Klamath Mountains consist of exotic terrains that were amalgamated onto the North American Plate via accretionary tectonics. The Klamath Basin is in the northwestern most part of the basin and range province. The upper Klamath Basin consists largely of Pleistocene lakebeds surrounded by ancient playa lake terraces. Preliminary geologic mapping in the basin has shown that basaltic volcanic centers are flanked by small sedimentary basins, along with active tectonic faults. Introduction to the Upper Klamath Basin Located between the Cascades and the Klamath Mountain ranges. Upper Klamath Basin is undergoing extension due to back arc spreading of the Basin and Range. The entire Klamath Basin extends from Southern Oregon to Northern California, where the Klamath River drains into the Pacific Ocean. The Upper Klamath Basin is comprised largely of Pleistocene lakebeds that rose above present day mash level by approximately 9 meters (Conaway 2000). Figure 1. Upper Klamath Basin (Photo from US Fish and Wildlife, Bush 2001) Table 2. Generalized bedrock in the Upper Klamath Basin (Table derived from Conaway 2000) Faults trending in NW/SE direction Faults comprised of horsts, grabens, and tilted fault blocks. Two stages of deformation is thought to occurred in faults. Ramping structures Dipslip displacement thought to have occurred in Pleistocene Two moderate M 5.9 and 6.0 earthquakes occurred in 1993, approximately 30 kmnorthwest of Klamath Falls (Conaway, 2000). Geologic Setting of the Upper Klamath Basin Prepared by: Dane Wagner Formation of the Upper Klamath Basin Exotic terrains, composed of volcanic ach sequences, traveled hundreds of miles from near the equator to the coast of Oregon. The terrains formed and traveled on the Pacific Plate until they where amalgamated to the North American Plate, creating the Klamath Mountains. After amalgamation the terrains underwent extension to near their present position. Today the Juan De Fuca Plate is subducting underneath the North American Plate. The subduction of the Juan De Fuca Plate created the Cascade Mountain Range. Today the Upper Klamath Basin is positioned between the Klamath Mountains on the left and the Cascades on the right. Figure 2. Subduction Zone of the Coast of Oregon (Courtesy of NOAA/PMEL) Diagram showing subduction and extension in Oregon Formation of the Upper Klamath Basin The Upper Klamath Basin’s present position between the two mountain ranges is in the northern most part of the Basin and Range Province. The Basin and Range is an area of ongoing extension. The Upper Klamath Basin thus is still experiencing extension due to the Basin and Range. The extension is characteristic of horst-graben faulting. Faults trend on a northwest / southeast axis. References Cited Bishop, Ellen Morris. “Sunset of Upper Klamath Lake”. ONRC Conaway, Jeffery. “Hydrogeology and paleohydrology in the Williamson River Basin, Klamath County, Oregon”. Portland State University. Portland, Oregon Hladky, Frank. “Geological Mapping in the Klamath Basin of Oregon”. Humboldt State University. Arcadia, California California’s Groundwater, Bulletin 118. “Upper Klamath Basin, Tule Lake Subbasin”. California Department of Water Resources Oregon Department of Geology and Mineral Industries, Smithsonian Institution. Global Volcanism Program, National Museum of Natural History. Washington DC Klamath Basin Klamath Falls Crater Lake N More active faults in red Figure 3. Faults in Oregon and Upper Klamath Basin (Derived form Oregon Department of Geology and Mineral Industries, 2001) Upper Klamath Basin Ancient Lakebeds and Terraces The Klamath Marsh is reminiscent of a ancient lake bed located in the Upper Klamath Basin. The Klamath Marsh has experienced at least three major fluctuations. Each fluctuation is a recognized change in climate or surface deposits. The oldest lakebed formed in the Pleistocene, named Lake Chemult, and represents pre-Mt. Mazama eruption. Contains reworked pyroclastic-fall deposits. The Middle Holocene lakebed contains undisturbed pyroclastic-fall deposits, thus meaning that the lakebed formed after the eruption of Mt. Mazama. The Middle Holocene lakebed volume is greater than that of the older Lake Chemult, due to a blockage of the Williamson River by a pyroclastic flow (Conaway, 2000). The lowest and youngest terrace of the Late Holocene may have developed during the early nineteen hundreds, due to agricultural expansion (Conaway, 2000). Water LevelExtentVolume Klamath Marsh 99 km  10 8 m 3 Late Holocene water body 289 km  10 8 m 3 Middle Holocene backflooding 560 km  10 9 m 3 Pleistocene Lake Chemult 390 km 2 Not calculated Figure 4. Klamath Lake Table 1. Extent and Volume of lakebeds (Photo derived USGS)(Chart From Conaway 2000) Bedrock Geology of the Upper Klamath Basin The Upper Klamath Basin bedrock geology is composed mainly of Pleistocene and Holocene basalts, with variations of lake deposits and volcanic sediment. Basalts varying from largely vascular, jointed, porphyritic, and fractured. The differing characteristics of each layer yields different aquifer conditions. The variable lakebed and volcanic sediments act as confining units. AgeUnitDescriptionWater Bearing Properties HoloceneGravel depositsBoulders and gravels deposited by catastrophic flooding. 10 m. High permeability, unconfined, with low yield. HolocenePyroclastic depositsWidespread unconsolidated pumice and ash found mantling the topography. 10 m. High infiltration rate. Unconfined and generally above the water table. Pleistocene and Holocene AlluviumSands and gravels deposited by pre-Mazama eruption streams and rivers. Unconfined aquifer, recharge could move through these deposits off the Cascades. VariableVolcanic eruptive center facies Basaltic andesite, dacite, cinder, and volcanic breccia. Thickness is variable. Important source for recharge on the Cascade slopes Pleistocene and Older Younger Continental sediments Sandstone and diatomite sequences. Thickness is variable throughout the basin (up to 120 m thick in areas). Serves mainly as a confining bed. TertiaryHydro-volcanicsPalagonitic rocks found at the mouth of rivers throughout basin. Serves as a confining bed for the underlying older continental sediments. Older Continental SedimentsSandstone, diatomite, and clays Alternating low permeability and high permeability sediment layers. Produce high artesian yields. Volcanoes of the Upper Klamath Basin Basalt layers resulted from the building of the Western Cascades and other active volcanoes in the region. The structures and deformation of faults in the region has effected the extent and placement of volcanic centers. Major volcanoes in the area include Mt. McLoughlin, Crater Lake / Mt. Mazama, and Soloman Butte Figure 5. Crater Lake (Photo from DOGAMI) Conclusions The Upper Klamath Basin is comprised of far traveled terrains that where accreted to North American Plate and then underwent extension. Numerous faults, both active and inactive, are in the basin due to Basin and Range extension. The faults are striking in Northwest / Southeast direction. Basin region composed of terraces from Pleistocene and Holocene lakebeds. Volcanic activity in region comprises the large amount of basaltic bedrock.