The archive of change on the Earth's surface is contained in its stratigraphic record. Deposits of ooids (spherical concentrically laminated grains) in particular have provided key insights into physical and chemical variability in the ocean over billions of years. On-going research has illustrated how variable bedrock configurations, tides and waves lead to different geometries (tidal deltas, tidal sand ridges, parabolic bars, sigmoidal bars) in tidally dominated oolitic sand shoals that are active today. This study will expand on those insights, exploring the hypothesis that these variable processes impart a distinct and unique stratigraphic record to each shoal geometry. To address these questions and test the hypothesis, this study will utilize high-resolution subbottom geophysical profiling, coring, and age dating from a suite of Holocene Bahamian shoals characterized by different processes and landforms. In comparing the thicknesses, the elevation of the bedrock beneath the shoals, and sedimentologic trends within the shoals, the results of this study will lead to enhanced understanding of evolution of these oolitic systems, and to tidal sand shoals in general. Furthermore, precise age dating of the oolitic sediments will reveal the rates of geomorphic change, information difficult to ascertain in siliciclastic systems. Collectively, these data will provide key insights to better constrain interpretation of the planet's history in response to global change, to predict the distribution and variability of oolitic sedimentary rocks in the subsurface (e.g., in water or hydrocarbon reservoirs), and to understand possible rates of recovery from sand mining. This proposal will fund development of mix of lecture, hands-on lab, and field experiences on ooid shoals for Bahamian and international students, offered by the PI and student for several semesters, at the Island School on Eleuthera Island in The Bahamas.
Ooids are sand-sized ovoid to circular carbonate particles less than 2 mm in diameter that include one or more concentric laminations around a nucleus. These non-skeletal grains can be found in sedimentary rocks of almost any age, from Archean to recent. Because of their broad temporal distribution and because their precipitation is controlled by ambient conditions, hydrodynamics and seawater, ooids in the rock record have proven valuable sources of information regarding changes in paleoceanography and circulation patterns, oceanic Mg/Ca ratios, atmospheric pCO2 levels, paleodepositional setting and even plate tectonics. Stratigraphy represents the preserved depositional record of Earth-surface processes, and to better understand and constrain the paleo-records preserved in ancient oolitic strata, numerous studies have examined Holocene analogs, focusing on their sedimentology, hydrodynamics or regional setting. Through these studies, sedimentologic criteria indicative of shoals, the hydrodynamic factors influencing them, and the general factors that control the distribution of shoals on platforms have been well documented. What remain unclear, however, are the details of shoal evolution - how did shoals with different geometries form? How rapidly do they change and evolve? And, for the geologist, how might these processes and depositional geometries be recognized from the stratigraphic record of ancient shoals? Whereas the patterns and the causative processes in shoal systems are not random, there have been few systematic efforts to directly and explicitly link the details of Holocene shoal geometries to their potential stratigraphic record. Yet, this information is central for accurate and meaningful interpretation of the Earth surface processes (see above) that generated, and can be reflected in, ancient analogs. The most important results of this study focus on documenting how sedimentologic processes create geomorphic bodies, which in turn build the stratigraphic record of ooid shoals. These linked dynamics came from observations of Holocene systems, and how sedimentologic processes vary with geomorphic character (bar form geometry), and the means by which the signature of these processes are preserved in the stratigraphic record of the shoals. Our efforts included four distinct Holocene Bahamian oolitic sand systems: Lily Bank (Little Bahama Bank), Schooner Cays (Great Bahama Bank), Fish Cays (Crooked-Acklins Platform), and French Wells (Crooked-Acklins Platform). In each, field efforts included examining 1) physical hydrodynamic processes; 2) surface sediment observations; 3) a suite of cores and (for Lily Bank and Schooner Cays) shallow sub-bottom profiles. The project included results such as: Describing the geomorphology and thickness of the ooid shoals. Characterizing stratigraphic-sedimentologic trends and internal complexity of shoals. Comparing and contrasting among shoals, developing predictive models of internal variability within and among shoals. Comparing these tide-dominated shoals with wave-dominated systems. Incorporating data into numerical models of variability. The importance of understanding the mechanisms responsible for the genesis, evolution, and stratigraphic record of carbonate geomorphic bodies extends beyond just filling a conceptual gap. Different interpretations can lead to significantly different conclusions concerning both dynamics of Holocene coastal systems and in interpretation of Earth surface processes from ancient analogs. This study – focused on understanding how different formative processes and shoal morphologies might be recognized in the geologic-stratigraphic record - represent fundamental insights that are required to take the ‘next step’ in linking process-based studies of Holocene carbonate sedimentary systems with studies of ancient analogs.
