The report describes regional hydrogeologic units of the Willamette Lowland aquifer system in western Oregon and southwestern Washington. This investigation is one part of a comprehensive hydrogeologic investigation of the Puget-Willamette Lowland in Oregon, Washington, and British Columbia, Canada, conducted as part of the U.S. Geological Survey Regional Aquifer-System Analysis program.
The Willamette Lowland is a structural and erosional lowland between uplifted marine rocks of the Coast Range and volcanic rocks of the Cascade Range. The Willamette Lowland study area encompasses approximately 5,680 square miles, 3,700 square miles of which are underlain by basin-fill deposits. The Coast Range, to the west of the lowland, consists of several thousand feet of Tertiary marine sandstone, siltstone, shale, and associated volcanic and intrusive rocks. The Cascade Range, to the east of the lowland, consists of volcanic lava flows, ash-flow tuffs, and pyroclastic and epiclastic debris. Continental and marine strata interfinger beneath and adjacent to the Willamette Lowland. In the northern two-thirds of the lowland, the marine sedimentary rocks and Cascade Range volcanic rocks are overlain by up to a thousand feet of lava of the Columbia River Basalt Group. Folding and faulting during and after incursion of the Columbia River Basalt Group formed four major depositional basins. These basins, separated in most places by uplands capped by the Columbia River Basalt Group, have locally accumulated more than 1,600 feet of fluvial sediment derived from the Cascade and Coast Ranges or transported into the region by the Columbia River. During Pleistocene time, large-volume glacial-outburst floods, which originated in western Montana, periodically flowed down the Columbia River drainage and inundated the Willamette Lowland. These floods deposited up to 250 feet of silt, sand, and gravel in the Portland Basin, and up to 130 feet of silt, known as the Willamette Silt, elsewhere in the Willamette Lowland.
Five regional hydrogeologic units were delineated and mapped in the Willamette Lowland on the basis of lithologic information from field-located water wells, geotechnical boreholes, petroleum exploration wells, and published geologic and geophysical maps. These units are (1) the basement confining unit, (2) the Columbia River basalt aquifer, (3) the Willamette confining unit, (4) the Willamette aquifer, and (5) the Willamette Silt unit.
The basement confining unit consists of low-permeability marine sedimentary rocks and associated marine volcanic and intrusive rocks. This unit also includes low-permeability volcanic rocks of the western Cascade Range. The Columbia River basalt aquifer consists of accordantly layered basalt flows, which generally are characterized by low vertical permeability and high lateral permeability. The Columbia River basalt aquifer underlies the northern half of the Willamette Lowland and is locally capable of producing large amounts of water. The Willamette confining unit and the Willamette aquifer are contained within the basin-fill deposits of the Willamette Lowland. The Willamette aquifer, the principal water-bearing unit in the region, is composed predominantly of sand and gravel with lesser amounts of silt and clay, whereas the Willamette confining unit is dominated by silt and clay with substantially less sand and gravel. The Willamette aquifer includes regions of predominantly coarse-grained material up to 200 to 400 feet thick that are located where major drainages debouch into the Willamette Lowland from the Cascade Range. In most places, these thick deposits are hydraulically connected by thinner, but more widespread, gravel deposits near or at the top of the pre-flood, basin-fill section. The coarse-grained deposits that compose the Willamette aquifer are interpreted as the proximal facies of alluvial fans that existed for much of the depositional history of the lowland, and which prograded across much of the valley floor during Pleistocene time. Elsewhere in the section, the basin-fill deposits are dominated by fine-grained materials assigned chiefly to the Willamette confining unit. The fine-grained deposits formed primarily as distal fan facies and deposits of low-gradient streams on the valley floor. The Willamette Silt unit consists of silt and fine sand deposited in the central and southern Willamette Valley by late Pleistocene glacial-outburst floods. In the Portland Basin, the flood deposits are coarser grained and are considered part of the Willamette aquifer. In the Tualatin Basin, the fine-grained flood deposits directly overlie the lithologically similar Willamette confining unit and are considered part of that unit.
The lines that represent contacts between regional faults defined by Gannett and Caldwell (1998) were digitized and were checked for position by comparing plots of the original geologic maps. In addition, each segment of a fault trace was properly labelled.