Cores from a Modern Carbonate Sand Body; the Joulters Ooid Shoal, Great Bahama Bank
Coring of Holocene sediments in the Joulters Cays area of Great Bahama Bank has revealed five facies. Three facies — ooid grainstone, ooid packstone and fine-peloid packstone — are responsible for the buildup of the shoal. Ooid grainstone occurs in a narrow band along the windward-facing borders of the shoal, an occurrence that coincides with the formation of clean ooid sands today. Bankward of these grainstones, ooid packstone overlies fine-peloid packstone with both forming a wedge of sediment that thins in all directions away from the grainstones. Interpretation of the packstone sequence suggests the shoal was more active in the past over a larger portion of its area than it is today and the packstones represent reworking of grainstones through burrowing during development of a vast sand flat.
In addition to providing modern counterparts for facies, grain types, structures and diagenetic overprint that are common in ancient ooid sands, the Joulters example graphically shows the rapidity with which complex facies relations can form. The shoal, covering over 400 km2 and averaging 4 m in thickness, formed during the last 5000 years. Nevertheless, the facies record of shoal growth, largely a response to changes in relative sea level, indicates significant changes such as rapid expansion of ooid sands, island formation and major reworking of sands by burrowing. In some facies, the depositional texture, sedimentary structures and grain types have all been modified since deposition. These changes point out the difficulty of interpretation of cores from ancient examples.
Figures & Tables
Carbonate Sands-A Core Workshop
Carbonate sands, both skeletal and non-skeletal, have been studied by geologists as intensely as carbonate buildups. The underlying reason for the studies is the importance of those sands as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record of carbonate sands with emphasis on lithofacies, stratigraphy of the sands and surrounding deposits, geometry of the sand deposits, diagenesis and porosity evolution, and wireline log data.