Out of our depth: on the impossibility of fathoming eustasy from the stratigraphic record
The evolution of a sedimentary basin's gross morphology depends upon (1) basement movement, (2) sediment accumulation and compaction, and (3) variations of eustasy with time. Thus if an accurate history of eustasy could be derived, it would aid in our understanding of basin stratigraphy and help in the prediction of sediment geometry from described areas. Techniques which attempt to determine the magnitude of eustatic sea-level excursions include the measurement of (1) the amount of sedimentary onlap onto the continental margins with and without the use of hypsometric curves, (2) the thickness of marine sedimentary cycles and the elevation and distance between indicators of old strandlines, (3) the perturbations on individual thermo-tectonic subsidence curves and stacked crustal subsidence curves, (4) the variations in deep-ocean oxygen isotopes found in sediments, and (5) the size of variables used in graphical and numerical simulations of basin fill in terms of tectonic behavior, rates of sediment accumulation and eustasy which “invert” the problem. To date a combination of the use of relative sea-level charts derived from sediment onlap of the continental margin with dimensioning by oxygen isotopes responding to glacial events offers the best potential for relative (tectono/eustatic) sea-level curves, but even this method can not produce a unique solution for absolute eustatic variations. Mathematical modelling shows that, at best, it is possible to obtain only the sum of tectonic basement subsidence and sea-level variations from the above methods, and, at worst, not even that simple a combination. Thus, every proposed scheme to measure eustatic sea-level excursions assumes some behavior for two of the three underlying processes: tectonic movement of the basement, sedimentary accumulation and eustatic sea level. Each scheme then determines the third process relative to the assumed model behavior of the other two. It would seem that, like King Canute, we cannot command the sea though we can still use undifferentiated “relative” (tectono/eustatic) sea-level curves to generate a “family” of solutions which are the product of a variety of absolute sea-level curves and tectonic models. Each solution has then to be assessed in terms of geologic setting and the hypotheses it generates.