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.

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