Architecture of Carbonate Platforms: A Response to Hydrodynamics and Evolving Ecology
Published:January 01, 2008
Luis Pomar, Christopher G St.C. Kendall, 2008. "Architecture of Carbonate Platforms: A Response to Hydrodynamics and Evolving Ecology", Controls on Carbonate Platform and Reef Development, Jeff Lukasik, J.A. (Toni) Simo
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The spectrum of carbonate-platform types, their heterogeneities, and their architecture is complex. Each platform succession has a distinctive and unique character that is a response to the particular geotectonic context and the physical, chemical, and biological conditions to that specific Phanerozoic window. Each succession has a distinct depositional profile, facies-belt distribution, and platform architecture, and it is expressed by the order of the basic accretional units and their stacking patterns. Critical differences between platform types are often the result of differences in their ecological accommodation. End members include (A) low-relief carbonate ramps that match a shelf equilibrium profile and are composed of either loose, fine-grained sediments produced in shallow, well-illuminated areas but shed downdip, or sediment produced and accumulated (sometimes as a distal bulge) in the deeper part of the depositional profile (poor-light or no-light zones), (B) open-shelf platforms involving large-skeleton metazoans with a marked to moderate capacity to build a platform margin above the shelf equilibrium profile, (C) rigid rimmed platforms with biotic components capable of accumulating to sea level with a maximum ecological accommodation, and (D) platforms with steep, massive and thick marginal slopes characteristic of many Paleozoic and some Mesozoic settings.
Interpretation of carbonate platforms and prediction of their facies heterogeneities involves analyzing and integrating geometrically related data. Analysis involves iterative and successive backstripping of sediment accumulation from youngest to oldest. This is reassembled to determine the genetic character of the carbonate sequences, cycles, parasequences, and/or beds as products of changes in physical and ecological accommodation. This reassembly considers the evolution of the biota involved, and the resulting changes in ecological requirements, the hydrodynamic setting, the physical accommodation, and the ecological accommodation (capacity of building up above a certain hydrodynamic energy level). The limits to this analytical strategy are tied to the knowledge of the ecology of ancient biota, while its advantage is that it formulates new questions that lead to more realistic interpretations and enhanced predictions of lithofacies heterogeneities.
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Controls on Carbonate Platform and Reef Development
Carbonate platforms and reefs emerge, grow and die in response to intrinsic and extrinsic mechanisms forced primarily by tectonics, oceanography, climate, ecology and eustasy. These mechanisms, or controls, create the physical, biological and chemical signals accountable for the myriad of carbonate depositional responses that, together, form the complex depositional systems present in the modern and ancient settings. If we are to fully comprehend these systems, it is critical to ascertain which controls ultimately govern the “life cycle” of carbonate platforms and reefs and understand how these signals are recorded and preserved. Deciphering which signals produce a dominant sedimentological response from the plethora of physical and biological information generated from superimposed regional to global-scale controls is critical to achieving this goal. With this understanding, it may be possible to extract common time- and space-independent depositional responses to specific mechanisms that may, ultimately, be used in a productive sense. Extensive research on a wide variety of carbonate platform and reefal systems in the past few decades has provided the foundation and understanding necessary to take carbonate research to a new level. With assistance from rapidly advancing computer software and an increasing use of cross-disciplinary integration, carbonate research is shifting from description and morphological analysis towards a science that is more focused on the assessment of process and genetic relationships. The aim of this special publication is to present a cross section of recent research that shows this evolution from a variety of perspectives and scales using examples distributed throughout the Phanerozoic.