Rankey, Eugene C.1, Peter Burgess2
(1) RSMAS/University of Miami, Miami, FL
(2) SIEP (Shell),
ABSTRACT: The Role of System-Internal Dynamics on the Generation of Peritidal Carbonate Cycles: Insights from Modeling and Field Observations
Interpretations of meter-scale cyclicity in peritidal carbonate strata are dominated by assumptions of external forcing, even in the absence of unequivocal evidence, and autocyclic processes commonly are dismissed as insignificant. The purpose of this study is demonstrate via modeling and a modern analog the conceptual robustness and implications of autocyclic processes.
The fundamental concept is a modified 'Ginsburg autocycle' model, wherein tidal flats nucleate on topographic highs and accrete seaward. Accretion leads to seaward progradation, progressively moving the shore away from more landward areas. Continued relative rise in sea level, and reduced sedimentation due to increasing distance from source, causes these landward areas to deepen and 'bays' form, even as the shoreline continues to accrete and prograde. Eventually, water depth and circulation in the bays will increase such that normal carbonate production resumes. Subsequently, a new tidal flat system nucleates on the topographic high and renewed progradation will commence, even in the absence of external change.
Numerical forward modeling of carbonate systems clarifies the processes. The model suggests that rates of relative rise in sea-level, sediment production and sediment transport control rates of progradation, and therefore parasequence thickness. Spatial and temporal variations in production and transport can create complex parasequence stacking patterns. The southwest coast of Andros Island, Bahamas, illustrates a modern icehouse analog. On this coast, the shoreline has prograded seaward, and broad, deep bays are present between the supratidal ridge and exposed bedrock. As sea level continues to rise, this area likely will become open marine.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.