Application of sedimentologic principles to sequence stratigraphy allows one, among others, to better understand the various factors that cause depo-sitional systems to generate sequences. Recognition of direct and indirect controls is of the utmost importance here. For instance, relative sea level has direct control on the shifting strand line and the exposure and flooding of depositional surfaces; it has indirect control (namely via changes in accommodation) on progradation and retreat of clinoforms. Indirect controls can be very powerful but more often than not they must compete with other factors that have a similar effect. Control on depositional sequences is no exception.
Classical sequence stratigraphy postulates that systems tracts are essentially controlled by sea-level change. Sedimentologic analysis leads to the conclusion that direct control of sea level extends only to a small portion of the features in the sequence and systems tract model, such as exposure and flooding phenomena. For most other features, sea-level control is exerted indirectly, primarily by changing the accommodation. And here sea level faces competition from another factor—sediment supply. Progradation and retreat of shelf margins, occurrence of condensed intervals as well as development of turbidite accumulations are influenced at least as much by sediment supply as they are by changes in accommodation. Below we summarize the “double-control concept of systems tracts” first for siliciclastics, then for carbonates.
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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.