Cyclicity in
Carbonates: Simple Models Versus The World
Burgess, Peter M.1 (1) Shell
Internation Exploration and Production,
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