REPORT:
Geology of Portland, Oregon, and Adjacent Areas

Geologic History

The region then was uplifted and deformed, probably in late Oligocene or early Miocene time, and the seas retreated from the area. Probably contemporaneous with this orogeny, the volcanic rocks in the eastern part of the mapped area were intruded by the Silver Star granodiorite, which silicified and metamorphosed the surrounding rocks. This was followed by a period of stability and erosion. The deformed marine beds were truncated and partly eroded, but the volcanic rocks in the northeastern part of the area were more resistant and formed a highland area north of the Columbia River. some later volcanism is represented by the upper flows of the Skamania volcanic series.

Fissure eruptions of flood basalt during middle Miocene produced great plains of Columbia River basalt to the east of the Portland area. Lavas of this stage of volcanism flowed into the map area through a wide gap between highlands to the north and south of the site of present Columbia River, and covered the eroded surface at lower altitudes. In late Miocene time the present Cascade Range to the east began to be uplifted as a result of deformation of the Columbia River basalt. Volcanism was renewed along the western flank of this newly formed range, and flows of the upper Miocene Rhododendron formation were extruded. Mudflows, probably originating as saturated pyroclastic debris, ran beyond the flow rocks and built up a mudflow deposit several hundred feet thick, overlying Columbia River basalt.

During later Miocene and very early Pliocene time, weathering processes attacked the exposed rocks and partly changed them to clay. An extensive laterite crust was formed on the exposed Columbia River basalt and Rhododendron formation.

Continuing diastrophism in early Pliocene time further developed the structural features and in some cases closed basins apparently were formed. The Sandy River mudstone was deposited in the quiet water of avast lake in at least one of these basins, locally burying the laterized surface. Probably there was contemporaneous deposition in similar adjacent structural basins.

As the basins were filled, or when the outlets had been breached to the level of the surface of the sedimentary accumulations, conditions were changed suddenly to an environment of stream deposition. The course of the Columbia River, through the Cascade Mountains was practically in its present position. the river brought in its load of coarse gravel and sand of foreign origin and deposited it in the gradually subsiding basin. These gravel deposits of the early Pliocene Columbia River from an important part of the Troutdale formation. The Cascade Mountains continued to be a center of intermittent volcanism, and basaltic flows poured into the Columbia River at intervals. Glassy particles were borne downstream and deposited as the vitric or sideromelane sandstone phases of the Troutdale formation. perhaps as a result of accelerated folding, conditions reverted to a closed-basin environment for a short period during Troutdale time when fine-grained materials were again deposited. later fluvial deposition was resumed. The sediments of the Troutdale formation continued to accumulate while folding progressed, but after folding ceased in early Pliocene time the surface of the Troutdale formation was extensively eroded.

The post-Troutdale Pliocene was a time of renewed small-scale discontinuous volcanic activity. Boring volcanoes dotted the land surface in the vicinity of Portland. The lavas were viscous and in most cases did not flow far from their source vent, but a few vents were explosive. In some areas, notably along the foothills of the Cascade Mountains east of the Sandy River and in the area southeast of Oregon City, several vents furnished enough lava to form a lava plain. The lavas southeast of Oregon City displaced the ancestral Willamette River to the west, where it cut its present gorge. Most of the volcanoes that produced Boring lava were in or near the map area, except for their equivalents to the east in the Cascade Mountains. this fourth episode of Tertiary volcanism continued into the early or middle part of the Quaternary.

Piedmont alluviation began in the latter part of the Pliocene, and continued perhaps into the Pleistocene. Coarse gravel and mudflow deposits accumulated in considerable thickness and partly buried some of the Boring volcanoes. This may have been caused by volcanism in the mountains supplying increased load to the streams.

The land surface in the lowland areas was extensively eroded during the early part of the Quaternary period and the piedmont plain had little relief except for isolated erosional buttes and the Boring volcanoes. Again increased volcanism, or perhaps interglacial rise in sea level, caused extensive alluviation of the piedmont plain. Vast mudflows poured down the mountain valley of the Sandy River from the vicinity of Mount Hood and spread out on the plain at the foot of the mountains. These mudflows may represent,in part, the early eruptive activity of Mount Hood. They were buried by more gravel or successive mudflows. The Clackamas River, in the Cascade Mountains farther south, contributed its load of coarse gravel to the wide plain of alluviation.

In the early to middle part of the Pleistocene epoch the Sandy and Clackamas Rivers cut valleys locally more than 200 feet deep into the piedmont plain. The rivers continued to occupy and modify these valleys.

The Columbia River by this time had cut a wide valley and the winds whipped silt from its flood plain and deposited it to the south and west of the flood plain. The silt thinned away from the flood plain to a thin edge less than 10 miles away. This silt mantled the weathered and laterized surface of the Columbia River basalt in the Tualatin Mountains.

The valleys of the Sandy and Clackamas Rivers were partly filled with fluvial gravel and mudflow deposits, probably in middle Pleistocene time. These fills were incised and the inner valley was partly filled with fluvial deposits probably in late Pleistocene time. The deposition and renewed cutting may have been controlled in part by eustatic changes in sea level, or may have been in response to changes in load owing to changes in climate of volcanism.

Sometime after the third period of Pleistocene fluvial deposition, the area was suddenly flooded by glacial melt water that backed up as a vast lake in the lower valley of the Columbia River and in the Willamette and the Tualatin Valleys.

A lobe of ice in the Purcell Trench in northern Idaho had acted as a dam shutting of the drainage of the Clark Fork River and forming a large lake in western Montana. Upon recession of the ice lobe, the ice dam suddenly failed, releasing 500 cubic miles of glacial melt water and draining the entire lake in a matter of days. This tremendous flood of water was diverted across the Columbia Plateau, by the narrows of Wallula Gap. Only one-sixth of the water arriving at this point could pass through the gap, and as a result much of the flood backed up to form a lake in the Pasco Basin and the Yakima Valley in southeastern Washington. Wallula Gap, however, was discharging about 40 cubic miles of water daily, part of which was again impounded by the Umatilla Basin. The amount of flood water that finally arrived in the lower Columbia River valley, however, was too great to pass through the relatively constructed channel downstream, and the excess water ponded. The maximum lake level was between 350 and 400 feet above sea level. The deposits of this lake are evidence of the tremendous volume and energy of the flood water responsible for the lake. The floors of the valley of the lower Columbia River and Tualatin and Willamette Valleys were mantled with extensive lacustrine deposits of gravel, sand, silt, and clay.

Reduction in the amount of load carried to the Portland area by the flood waters allowed erosion of channels in the lacustrine deposits and the creation of great scours on the down-current (west) side of obstacles that caused turbulence. With slackening of the flood, these scours were filled with bedded sand and silt. As the flood waters diminished, the level of the lake declined rapidly, and drainage was re-established in the area. The rivers then began to cut down through the unconsolidated deposits left by the flood and locally deposited slack-water sediments.

In late Pleistocene time and early Recent time there was further downcutting and intermittent alluviation. The slight degree of weathering of the surface of the late deposits indicates their youth. The Clackamas River flowed into the Willamette River through a channel between Clackamas and Milwaukie until the channel was abandoned owing to capture.

Recent entrenchment of about 150 feet in the valley of the Clackamas River, and locally of more than 300 feet int he valley of the Sandy River, may represent normal lowering of the stream profile along its lower course, or it may be in part the result of regional uplift.

The climate of the region throughout late Tertiary and Quaternary time has been a temperate one characterized by at least moderately heavy rainfall, perhaps decreasing amounts of summer rainfall. Continuous exposure of the surfaces of successively younger deposits to climatic conditions probably much like those of the present has produced a series of profiles of weathering that are successively thinner and less severely weathered with decreasing age of the deposits.

Landslides have occurred probably throughout the late Tertiary and all of Quaternary time, and have modified slopes over large areas in some places.

Recent alluviation of the modern flood plains, and locally the accumulation of bogs of high organic content have modified the surface only slightly during the late history of the area.