NEOGENE EVOLUTION OF THE PORTLAND BASIN

The Portland Basin is a 30x60 km, NW-elongate, topographic and structural depression within the Puget-Willamette forearc basin of Cascadia. The shallow (<0.5 km), asymmetric basin is floored by Eocene–Miocene volcanic and sedimentary rocks and filled with unconsolidated to weakly lithified Neogene–Quaternary fluvial deposits, largely extrabasinal material transported by the Columbia River. The basin fill records alternate episodes of aggradation and incision driven by a complex interplay of local and regional tectonics, volcanism, and climate. The proto Portland Basin originated as a broad syncline related to uplift of the Oregon Coast Range after 20 Ma and influenced the distribution of Columbia River Basalt (CRB) flows that entered from the east at 16-12 Ma. The basin continued to subside, partly along NW-striking right-lateral oblique-slip faults during post-CRB time, becoming narrower and accumulating several hundred meters of predominantly fine-grained Columbia River deposits (Sandy River Mudstone). Although the northern Cascade volcanic arc was relatively quiesent at this time, a large fan of andesitic debris (Rhododendron Fm.) encroached the basin from the southeast at 13–8 Ma. In latest Miocene–early Pliocene time, a thick wedge of cobbly gravel (older part of Troutdale Fm.) prograded across the basin; this unit consists of CRB and exotic quartzite clasts and micaceous sand, with little contribution from adjacent highlands. There followed an episode of uplift and incision, which narrowed the basin to approximately its present width; the uplifted Troutdale gravels are preserved as basinward-dipping benches and other erosional remnants now as high as 300m elevation. A pulse of basaltic volcanism in the arc to the east at 4–3 Ma had a dramatic impact on the basin. Interaction of lava with the ancestral Columbia River generated an enormous quantity of hyaloclastite, which was flushed downstream and constructed a synvolcanic alluvial fan (younger part of Troutdale Fm.) in the eastern Portland Basin. Subsequent uplift of the Cascade Range and coincident basin subsidence resulted in westward tilt of region east of the basin, causing the river to entrench this fan, and creating the modern Columbia River Gorge.