Alternatives to Eustasy in Sequence Control
Classical sequence stratigraphy holds that succession and geometry of sequences are controlled by relative changes of sea level (e.g. Vail et al., 1977; Haq et al., 1987). The theory postulates furthermore that a global stack of relative sea-level curves from sequence stratigraphy reveals a strong eustatic signal that dominates the record even in tectonically mobile settings such as convergent ocean margins. Consequent application of the above concepts has produced the most widely cited eustatic sea level curve to date (Haq et al., 1987). This curve has proved very useful in many instances, particularly in periods with independent evidence for strong glacio-eustasy. However, the curve has also generated a growing wave of concern and reports of serious discrepancies. We refer to Miall (1986), Hubbard (1988) and Underhill (1991) for critique of the general concept. This chapter restricts itself to questions specifically related to sequence stratigraphy in reefs and carbonate platforms.
We contend that, particularly in carbonates, eustasy is less dominant than postulated by classical sequence stratigraphy and that other processes contribute significantly to the formation of stratigraphic sequences. The alternatives are of two kinds: First, many sea-level fluctuations reflected in sequence stratigraphy are related to local or regional tectonics.
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Sedimentology and Sequence Stratigraphy of Reefs and Carbonate Platforms: A Short Course
Classical sequence stratigraphy has been developed primarily from siliciclastic systems. Application of the concept to carbonates has not been as straightforward as expected even though the basic tenets of sequence stratigraphy are supposed to be applicable to all depositional systems. Rather than force carbonate platforms into the straightjacket of a concept derived from another sediment family, this publication takes a different tack, starting out from the premise that sequence stratigraphy is a modern and sophisticated version of lithostratigraphy. It reviews sedimentologic principles governing the large-scale anatomy of reefs and platforms; looks at sequences and systems tracts from a sedimentologic point of view; assesses the differences between siliciclastics and carbonates in their response to sea level; evaluates processes that compete with sea level for control on carbonate sequences; and presents a set of guidelines for application of sequence stratigraphy to reefs and carbonate platforms.