Marine terraces in mid-latitude settings: the case of the Oregon USA coast

Marine terraces are terraced landforms that occur as elevated flats inboard of, and parallel to, coastlines. Outstanding examples of a stair-step landscape of marine terraces can be found in many places in the world. In this discussion, we are concerned with mid-latitude marine terraces that owe their origin to erosional planation in the surf zone where wave action, in combination with moving sediment, sculpts flat marine platforms (Figure 1). Be forewarned, however, that in tropical regions, marine terraces are formed by constructional, not erosional, processes. Corals thrive in the tropics at sea level and some coral species will grow up to a low tide level and then spread laterally. Therefore tropical marine platforms are coralline flats (Chappell, 1974). But in mid-latitude settings where colder ocean water precludes growth of corals, marine platforms are cut in rock or sediment.

Preservation of marine platforms as subaerial sand-covered marine terraces requires an orchestration of several independently operating Earth surface processes. Only those marine platforms that are formed in the surf zone at or near times of climatically-induced global sea level highstands (during interglacial periods) will ultimately be preserved (Bradley and Griggs, 1976) (Figure 2). When sea level is rapidly rising or falling, the rate of relative sea level change is too fast to form a platform. Furthermore, marine platforms that are formed at times of sea level low stands (during ice ages) will be submerged and thus not subaerially preserved when sea level rises during melting of glaciers. Only those marine platforms that are on coasts subject to land surface uplift will be preserved (Chappell, 1974; Bradley and Griggs, 1976). If the marine platform, formed at a global ocean highstand, is not uplifted "out of harm's way" before the next sea level highstand, then that platform will be occupied by the next sea level highstand and the platform will be re-planated and not preserved. A succession of marine terraces will be preserved as a succession of stair steps ascending from an active coast only if surface uplift is persistent over multiple highstands and intervening lowstands. In the stair-step morphology, the risers are the paleo sea cliffs and the treads are the beach-covered former wave-cut platforms. Finally, the marine platform is formed in the surf zone but the marine terrace is preserved with a sandy cover over the platform. This veneer of beach and dune sand is deposited on the marine platform as sea level drops from a highstand.

The coast of Oregon USA is one of many localities globally that preserve stair-step flights of marine terraces. The stair steps are not continuous along the coast because the unique set of conditions to preserve marine terraces are not everywhere present. On the Oregon coast, the main factor that determines why there are marine terraces on some stretches of coast and not on others is local tectonics. In some places the long term uplift rates are too low, and no marine terraces are preserved. But where the land surface uplift rate is a bit higher than the long-term rate of relative sea level rise, then flights of marine terraces--as many as five--will be preserved.

Along-coast variation in marine terrace elevation provides insight to coastal tectonic deformation. The uplifted flights of marine terraces on the Oregon coast are often preserved at headlands, because headlands are in the axial zone of active anticlines or within uplifted blocks bounded by an active fault. The Oregon coast is on the leading edge of the upper plate of the Cascadia subduction zone, and the active faults and folds on the Oregon coast (often found on these coastal headlands) are a consequence of strain associated with the subduction zone. The correlation of similar-aged marine terraces along the coast allows a view of the along-coast deformation above the subduction zone (Kelsey et al., 1994; 1996) (Figure 3). Therefore, in the late Quaternary (the time period of terrace preservation), marine terraces are the most useful means to envision along-coast deformation because any one marine terrace was formed at a singular elevation datum (sea level) and the variable modern elevations of that datum record the along-coast, variable tectonic uplift of the marine terrace since it was formed.

• Bradley and Griggs, 1976, Form, genesis, and deformation of central California wave-cut platforms, Geological Society of America Bulletin, 87, 433-449.
• Chappell, J., 1974, Geology of coral terraces, Huon Peninsula, New Guinea: A study of Quaternary tectonic movements and sea level changes, 85, 553-570.
• Kelsey, H. M. and Bockheim, J. G., 1994, Coastal landscape evolution as a function of eustasy and surface uplift rate, Cascadia margin, southern Oregon, Geological Society of America Bulletin, 106, 840-854.
• Kelsey, H. M., Ticknor, R. L., Bockheim, J. G. and Mitchell, C. E., 1996, Quaternary upper plate deformation in coastal Oregon, Geological Society of America Bulletin, 108, 843-860.