FIGURE 6.6 Pleistocene sea-level fluctuations and origin of emergent Pleistocene strandlines. Emergent strandlines simultaneously record tectonic uplift and major sea-level highstands. The rising coastline is a moving strip chart on which sea-level highstands are recorded sequentially as strandlines whose ages increase with elevation. The slope (R) of the diagonal line connecting each highstand to the elevation of its strandline is the average uplift rate. If the uplift rate was constant, the uplift lines for all strandlines are parallel. Strandlines formed during lowstands are usually destroyed by subsequent sea-level fluctuations and rarely appear in the emergent geologic record. Strandlines younger than 60 ka appear above sea level only where the uplift rate is greater than 1 m/ka. The sea-level fluctuation curve was derived from a sequence of U-series dated coral-reef strandlines on the Huon Peninsula, Papua New Guinea (Figure 6.2B) by subtracting tectonic uplift from the relative strandline record (Figure 6.5A). Sea-level curve modified from Chappell (1983); oxygen-isotope stages (1–9) from deep-sea cores (Shackleton and Opdyke, 1973).
Quaternary sea-level history was characterized by periodic eustatic fluctuations of 100–150 m caused by the advance and retreat of continental glaciers. Apparent sea-level changes are not real but result from and are the inverse of vertical ground movements. Consequently, apparent sea-level changes are only local or regional in extent.
The primary task in coastal tectonics is separating the apparent component from the real component of a relative sea-level record. This is done graphically or algebraically by subtracting the real sea-level history from a relative sea-level record:
In effect, the real sea-level history is a fluctuating tectonic datum to which each strandline (tectonic marker) must be correlated by age (Figures 6.5, 6.6, and 6.7).
The real sea-level history is obtained by subtracting apparent sea-level changes (the inverse of vertical crustal movements) from a detailed and well-dated relative sea-level record (Figure 6.5):
In this case, the tectonic history is an absolute sea-level datum and can be either moving or stationary. A rapidly and steadily rising coastline is the best datum for measuring major, long-term sea-level fluctuations (Figures 6.5A and 6.6) (Chappell, 1983), whereas a stable coastline is the best datum for measuring minor, short-term sea-level changes (Figures 6.5B and 6.7) (Bloom, 1970; Scholl et al., 1970). Generally, the greatest uncertainties in most long-term sea-level histories derived from strandline data stem from the simplifying assumption of constant uplift. However, on a tilted coastline where Pleistocene strandlines converge, constant long-term uplift at a particular locality can be demonstrated empirically (Chappell, 1983). The greatest uncertainties in short-term sea-level histories stem from the assumption of coastal stability. Where short-term stability at a coastal locality cannot be demonstrated by independent geodetic data, subtle sea-level changes can be expressed in only relative terms. An additional complication is
FIGURE 6.7 Holocene sea-level changes and origin of emergent Holocene strandlines. In contrast to Pleistocene strandlines, emergent Holocene strandlines represent discrete uplift events or storm events, not sea-level fluctuations. The highest strandline correlates with the tangent point at the sea-level inflection between 7 ka and 5 ka BP. All lower strandlines represent two paleoshorelines, one occupied during the transgressive phase and one during the regressive phase (see Figure 6.5B for relative sea-level changes). The sea-level curve is general and may vary slightly from region to region owing to minor geoidal distortions.