Date of this Version
In: Quaternary Geochronology Methods and Applications, AGU Reference Shelf 4 (American GeophysicaUl nion, 2000), pp. 434-446
Coastal landforms, and particularly emergent marine terraces, a re amongt he most importantg eomorphicfe aturesi n studieso f Quaternarys eal evel fluctuationsa nd neotectonics. A general model, first articulated by Alexander (1953) and borne out by detailedr ecent studieso n many coastlinesi,s that emergent flights of erosional terraces and coral reefs (Fig. 1) are the resulto f glacioeustaticallyc ontrolledf luctuations of global sea level superimposed on steady uplift. Similiarity of U-series ages of coral from both constructional reefsi n the tropicsa nd erosionalt erracesi n midlatitudesh as provided a rigorous test of this model (Mesolella and others, 1969; Bloom and others, 1974; Dodge and others, 1983; Muhs and others, 1994). Both erosional shore platforms and constructional reefs form when relative sea level is stable, i.e., during an unchanging sea level on a tectonically stable coast, or when the rate of sea level fall or rise matches the rate of tectonics ubsidenceo r uplift, respectivelyT. herefore,o n tectonically rising coasts, emergent marine terraces record interglacial or interstadial sea level high stands; terraces that formed during glacial-period sea-level low stands are usually submerged. On subsiding coasts submerged terraces may record both interglacial high stands and glacial-period low stands (Ludwig and others, 1991). Interglacial and interstadial periods correspond to odd-numbered stages and glacial periodsc orrespondto even-numbereds tagesi n the deep-sea foraminiferal oxygen isotope record. Although U-series dating has demonstrated that erosional marine terraces form during the same interglacial or interstadialh igh sea standsa s constructionarl eef terraces, there are often significantd ifferencesi n the completenesso f terrace flights on erosionala nd constructionacl oasts.L owlatitude coasts characterized by overlapping constructional coral reefs usually have fairly complete terrace sequences. On mid-latitude,h igh-energyc oastsh, owever,s eacliffr etreat rates are often high. For example, in California, most modem sea cliff retreat rates range from about 0.01-0.5 m/yr, but some are higher than 1.0 m/yr (Muhs, 1987). Thus, the potentiale xistsf or removalo f emergentt erraces,r esultingi n flights with variable numbers of terraces along a given coastline. Local correlation from one terrace flight to the next can be a problem, therefore, on the high-energy coastlines of New Zealand, Japan, Oregon, California, Mexico, Peru, and Chile. This paper reviews some of the methods by which firstorder approximationso f marine terrace ages can be determined, when material suitable for numerical dating is rare or absent. The methods described fall into the categories of "relative-age"a nd "correlated-age"te chniquesa, s described by Colman and others (1987). I consider first those methods which are useful for local correlation, and then review techniquesw hich havet he goal of correlatingt erracesw ith global sea level stands. The methods reviewed here are those that haveb eent estedo n a variety of coastlinesa, nd can therefore be evaluated for reliability.