The CwM team could not resist the early summer-like weather in the Pacific Northwest and took a “Geology-Friday” field trip into the Columbia River Gorge. Their trip took them east from Portland to the Sandy River, along the Historic Columbia River Highway through Corbett, with stops at the Latourell and Bridal Veil Falls, and back west along the banks of the Columbia River. The goal was to find outcrops of the Troutdale Formation, parts of which make up an important groundwater source in the Portland metro area and part of the City of Portland’s Columbia River Wellfield.
The Troutdale Aquifer is a sequence of silts, sands, gravels, and cobbles that were deposited starting about 3-4 million years ago and overlie much of the Miocene-age Columbia River Flood Basalts in the Portland area, filling the canyons and valleys that were cut into the basalts. The sediments of the Troutdale originated from ancient channels of the Columbia River, the many streams coming off the west slopes of the ancestral Cascades, as well as significant ashfall from the High Cascade volcanic activity. Potassium-argon dating of volcanic clasts in the Upper Troutdale suggest that deposition continued until at least 1.5 million years ago.
The Troutdale is generally divided into two subunits. The Lower Troutdale was formed primarily by deposition in ancestral channels of the Columbia River and varied from well sorted sandstone to gravel and cobble conglomerates with diverse lithologies. Both materials often exhibit strong characteristics of their depositional environment, such as cross-bedding in the sandstone and imbrication of cobbles. The Upper Troutdale exhibits greater influence from concurrent volcanic activity and primarily consists of vitric sandstone and basalt conglomerate. Both units are interlayered with much finer-grained silts and clays usually assigned to the Sandy River Mudstone Formation.
Of the eight hydrogeologic units that form the Portland Basin Aquifer System, the Troutdale Gravel Aquifer (TGA) and the Troutdale Sandstone Aquifer (TSA) are the most important for water supply. Both aquifers are common sources of water for municipal and irrigation wells in the Portland Basin. Some City supply wells in this aquifer have been tested at more than 2,500 gallons per minute.
Outcropping of the Troutdale Formation along the Historic Columbia River Highway
The CwM team found a good exposure of the Troutdale Formation along the Historical Columbia River Highway about 2.5 miles south of the Sandy River Bridge. This particular site exposes a relatively thin section of the Troutdale around the transition from Lower to Upper members. At the base of the outcrop, the upper 20 ft of grayish lithic sandstone is exposed. Cross-bedding features characteristic of river channel deposition can be seen. About 20-25 ft of coarse cobble conglomerate was visible above the sandstone. The cobbles were almost entirely of an olivine basalt composition, which is more representative of the Upper Troutdale. At the very top of the outcrop sit large boulders of the Boring Basalt, which formed in the early Pliocene.
Sources:
Evarts, Russell. (2021). Geologic Map of the Ridgefield Quadrangle, Clark and Cowlitz Counties, Washington.
Swanson, R.D., McFarland, J.B., Gonthier, J.B., & Wilkinson, J.M. (1993). A Description of Hydrogeologic Units in the Portland Basin, Oregon and Washington. USGS Water-Resources Investigations Report 90-4196.
Tolan, T.L. & Beeson, M.H. (1984). Exploring the Neogene History of the Columbia River: Discussion and Geologic Field Trip Guide to the Columbia River Gorge. DOGAMI. Oregon Geology, Vol 46 (9).