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Active Tectonics: Studies in Geophysics
that different measurement techniques (e.g., geodetic, tide gauge) occasionally yield conflicting results for short-term sea-level changes (Brown, 1978).
Along steep coastal slopes in uplifting areas Pleistocene strandlines occur most commonly as narrow (<1 km), steplike terraces within a few hundred meters above present sea level. Consequently, a vertical sequence of uplifted strandline terraces resembles a flight of stairs (Figures 6.1 and 6.2). Each strandline terrace consists of a virtually horizontal erosional or depositional platform backed by a steep sea cliff along its landward margin. The shoreline angle, the intersection of the relict platform and sea cliff, closely approximates the location and elevation of the abandoned marine shoreline and is the linear structural marker depicted on most longitudinal profiles of deformed strandlines (for examples see Figures 6.8A, 6.13, 6.17, 6.20, and 6.21). The shoreline platform, also referred to as the terrace platform, is the planar tectonic marker depicted on most cross-sectional profiles and detailed isobase maps of deformed strandlines (Figures 6.2 and 6.3A; also see Figures 6.19 and 6.21 below).
Commonly, a thin (1–3 m) veneer of shallow-water sand, gravel, and cobbles, which locally contains fossil marine shells used for dating, overlies the terrace platform. Where subaerial slope degradation is rapid, a seaward-thinning prism of alluvium derived from upland streams and abandoned sea cliffs overlies the platform and its associated marine sediments and buries the shoreline angle (Figure 6.2A). In these areas the geomorphic expression of successively higher (older) Pleistocene strandline terraces is progressively subdued, and the position and configuration of both the shoreline angle and the terrace platform must be derived from borehole or shallow seismic data (see Figure 6.21 below) (Bradley and Griggs, 1976; Lajoie et al., 1979b). However, in some areas the progressively greater degradation of successively older relict sea cliffs provides a means of dating emergent strandlines (Figure 6.2A) (Hanks et al., 1984). Along gentle coastal slopes in slowly uplifting areas emergent strandlines commonly consist of broad (1–10 km) terrace platforms backed by low sea cliffs obscured by relict beach ridges and dune fields (Hoyt and Hails, 1974; Lajoie et al., 1979a).
A general consensus has developed over the past two decades that a flight of emergent Pleistocene strandlines is the geologic record of periodic glacio-eustatic sea-level highstands superimposed on a rising coastline (Figure 6.6) (Broecker et al., 1968; Mesolella et al., 1969; Matthews, 1973). In this model, a rising coastal land-mass is a moving strip chart on which brief sea-level highstands were successively recorded as depositional or erosional strandlines. Strandlines also formed during sea-level lowstands (Emery, 1958; Lewis, 1971a,b, 1974; Ridlon, 1972), but along uplifting coastlines most of these strandlines were destroyed by wave erosion during subsequent sea-level fluctuations and, therefore, rarely appear in the emergent marine record (Figure 6.6). An important exception is found in the deeply incised sequence of emergent strandline terraces on the Huon Peninsula of Papua New Guinea, where sea-level lowstands are recorded as deltaic accumulations preserved beneath a protective cover of coral reefs deposited during subsequent sea-level highstands (Chappell, 1974a, 1983). Along many subsiding coastlines, strandlines formed during sea-level lowstands probably dominate the submergent geomorphic record but are difficult to distinguish from strandlines formed during highstands (Moore and Fornari, 1984).
The most detailed tectonic datum for deriving uplift from emergent Pleistocene strandlines on coastlines throughout the world is the paleosea-level curve obtained by subtracting well-documented constant tectonic uplift from the relative sea-level record of emergent coral-reef strandlines on the Huon Peninsula of Papua New Guinea (Figures 6.2B and 6.6) (Veeh and Chappell, 1970; Bloom et al., 1974; Chappell, 1983). There, uranium-series dates on fossil corals yield a paleosea-level curve back to about 340 ka before present (BP) that agrees with longer, but less detailed, sea-level curves derived from emergent coral-reef strandlines on the island of Barbados in the West Indies (Broecker et al., 1968; Mesolella et al., 1969; Matthews, 1973; Bloom et al., 1974; Bender et al., 1979) and from oxygen-isotope data from deep-sea cores (Shackleton and Opdyke, 1973). The uplift rate (maximum 4.0 m/ka) of the Huon Peninsula was derived from the elevation of the prominent 120-ka strandline that formed 2–10 m above present sea level during the last major interglacial sea-level highstand (Thurber et al., 1965; Veeh, 1966; Ku et al., 1974; Neumann and Moore, 1975; Marshall and Thom, 1976; Stearns, 1976; Harmon et al., 1978; Shubert and Szabo, 1978; Szabo et al., 1978; Szabo, 1979). The most important features of the New Guinea paleosea-level curve as a tectonic datum are the periodic interglacial highstands at approximately 100 ka intervals and the successively lower interstadial highstands at approximately 20 ka intervals over the past 120 ka. Virtually all Pleistocene strandlines along emergent coastlines throughout the world were formed during these brief paleosea-level highstands. The longer paleosea-level records on Barbados and in the deep-sea cores indicate that the main 100-ka cycle of interglacial high-