POST-PLIOCENE EVOLUTION OF THE COLUMBIA RIVER GORGE

The Columbia River valley was present prior to eruption of the Columbia River basalt (CRB) and channelized Grand Ronde and Wanapum lavas on the northern margin of a broad lowland plain. CRB lavas banked against older rocks on the north and back-filled several south-flowing tributaries to the ancestral Columbia River. Uplift of the CRB and development of the modern Columbia Gorge at 1-2 Ma was suggested by Tolan and Beeson (1984). Recent detailed mapping of the Bridal Veil and Multnomah Falls 7.5 minute quadrangles supports the hypothesis of recent uplift, but suggests the onset of uplift is 2-2.5 Ma, coincident with the decline of MORB-like low-K tholeiite basalt (LKT) eruptions. Throughout the Columbia Gorge from Bridal Veil to Hood River a thick section of LKT lavas is found atop and intercalated with LKT hyaloclastite. The 2.8-3.5 Ma LKT section marks the ancestral Pliocene Columbia River channel. LKT hyaloclastite was formed by eruption of LKT into the Pliocene river, and hyaloclastite debris downstream is spread across and beneath the Portland Basin where it is termed the upper Troutdale Formation. The hyaloclastite is 120-140 m thick on the eastern outskirts of Portland. Uplift of the LKT section gradually increases eastward through the Gorge at least as far as Cascade Locks, where the base of the hyaloclastite is at an elevation of 800 m. Uplifted LKT section extends farther east to Green Point Mtn where the section then declines eastward on the east dipping ramp of the Hood River half graben. The trace of the Pliocene Columbia is poorly known between Cascade Locks and Hood River, where Pliocene hyaloclastite is exposed along I-84. Emplacement of LKT clogged the Pliocene Columbia channel and pushed it northward to its modern location. Post-LKT uplift led to the formation of tributary valleys to the modern river, which were partially filled with lavas ranging from 1.9-0.24 Ma (the youngest ages are from lavas that filled the ancestral Multnomah Creek, thus accounting for the youthful Multnomah Falls). Uplift and creation of the Columbia Gorge thus was well underway by 1.9 Ma. Unconformities between 3.1 Ma LKT and 2.2-2.3 Ma basalts suggest uplift may have begun soon after LKT eruptions declined. Uplift is likely ongoing, but at a much reduced rate.

Tolan, TL & Beeson, MH, 1984: GSA Bull. v.95, p.463-477.