Coastal landscape evolution as a function of eustasy and surface uplift rate, Cascadia margin, southern Oregon

Abstract

We explore the relative importance of eustatic versus tectonic factors in sculpting, preserving, and modifying coastal landforms. The study area is a 30-km coastal reach in southern Oregon. Using degree of soil development as a means of correlating wave-cut platforms along the coast, we document that surface uplift rate is variable in a shore-parallel sense, variability being a function of differential vertical displacement of crustal blocks along faults and flexures in the upper plate of the Cascadia subduction zone. A consequence of this variability is that in areas of moderate uplift rate (0.7-0.9 m/k.y.), flights of up to seven emergent wave-cut platforms are preserved along the interfluves of coastal drainages, whereas in areas of low uplift rate (0.05-0.2 m/k.y.), only one wave-cut platform commonly exists, within a few meters of present sea level. Preservation of shore platforms in both areas is the result of the inter-action of tectonic uplift and eustatic sea-level changes operating on a high-energy coast subject to platform planation. Platform reoccupation can be a consequence of late Pleistocene eustatic sea-level changes operating on low-up-lift-rate coasts. Particularly in the cases of the ∼200 ka and ∼125 ka high sea stands, reoccupation by the later highstand is probable when coastal uplift rates are about 0.2 m/k.y. The same eustatic history, operating on a higher-uplift-rate section of the coast, results in multiple emergent platforms and no instances of reoccupation. Coastal drainage basins in the size range of 5-10 km2 have different hypsometries, reflecting whether platforms in coastal basins are preserved over a range of altitudes or only near sea level. The hypsometric differences are thus a product of differences in uplift rate. The dominant mechanism of base-level fall varies between the two areas: coastal retreat during sea-level highstands drives baselevel fall in lower- uplift-rate basins, whereas tectonic uplift mainly drives base-level fall in moderate-uplift-rate basins, even though the magnitude of coastal retreat may be about the same in the two areas. Where coastal retreat largely provides the mechanism for base-level fall, steep coastal drainages discharge their debris onto coastal piedmonts (benches), and the resultant alluvial fans prograde over marine deposits. In southern Oregon, the alluvial fan-building episode was relatively short, about 2-20 k.y. in duration. Fan aggradation ceased as a consequence of fan entrenchment, most likely brought on by buildup and steepening of the fan over time.