--> Abstract: Cyclicity in Carbonates: Simple Models Versus The World; #90063 (2007)

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Cyclicity in Carbonates: Simple Models Versus The World

 

Burgess, Peter M.1 (1) Shell Internation Exploration and Production, Rijswijk, Netherlands

 

Cyclical carbonate strata have been interpreted to take various forms (e.g. shallowing-upward, deepening-upward, symmetric, and asymmetric cyclothems) occurring as both parasequences and high-frequency sequences. Such cyclothems are typically defined by ordered facies stacking patterns and presence of suberial exposure and marine flooding surfaces. Interpreted causal mechanisms are often simple, invoking high-frequency allocyclic relative sea-level oscillations, often with an assumed element of orbitally-forced eustasy. Although this remains a frequently applied interpretation, various quantitative analyses of cyclothem thickness, bed thickness, and facies succession data suggest a more complicated situation. Various statistical analysis of carbonate strata show many successions are indistinguishable from random, and that bed thickness distributions are exponential, both suggestive of generation by random Poisson processes in a complex facies mosaic. Although additional information on significant surfaces may help distinguish order from disorder in carbonate strata, the evidence suggests that stratal order is relatively rare, apparently occurring only during quite specific intervals of geologic time. Various conceptual models, and several forward modeling studies have also demonstrated that autocyclic processes involving shoreline progradation driven by sediment transport are a plausible mechanism to explain origins of many carbonate cyclothems. Such autocycles may show periodic stacking patterns, or stacking that is indistinguishable from random. They may have high or low stratigraphic completeness, and simple or complex internal facies distributions. Adjusting the paradigm of carbonate cyclothem generation to include a broader spectrum of simple and complex processes, including both stochastic and ordered processes, with both allocyclic and autocyclic mechanisms operating, would be an important advance.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California