Why Are Only Some Isolated Carbonate Buildups Grain-Dominated?

Rankey, Eugene C.¹, J. Rodrigo Garza-Perez¹, Stacy L. Reeder¹ (1) University of Miami, Miami, FL

Numerous studies have demonstrated the roles of eustasy, tectonics, age, and pre-existing topography on carbonate buildups and their geometric evolution, but there is currently no explanation for why a given shallow, tropical carbonate buildup might include more extensive grain-dominated (or mud-dominated) facies.

Study of Holocene carbonate systems provides a unique opportunity to explore this fundamental unknown. For this study, patterns of surface sediments on isolated carbonate buildups from around the globe were explicitly compared with regional-scale variables (including seasonal, and/or annual patterns in currents, waves, SST, dissolved oxygen, winds, as well as tidal amplitude, and storm frequency and intensity). Results reveal that tidal amplitude is the best predictor for estimating the aerial abundance of grainy facies (percent; ‘graininess') on isolated carbonate platforms (R²= 0.51). Wave height is a secondary factor influencing graininess (correlation between yearly average wind wave height and graininess alone shows no correlation (R² = 0.04); correlation between graininess and tidal amplitude + wind wave height has an R² = 0.80). Other variables (storms, temperature, platform size) are statistically insignificant.

The day-after-day persistence of tides exerts a pronounced influence on buildups. Beyond creating strong currents to transport sediments, tidal energy influences water circulation and quality and bank-top geochemistry, both of which can influence genesis of skeletal and non-skeletal grains. Future challenges center on understanding the details of how physical, biological, and chemical processes control these patterns. Nonetheless, this global perspective offers predictive insights into large-scale controls on graininess, a fundamental control on reservoir quality in many carbonate systems.