The extent and patterns of sediment filling of accommodation space on Great Bahama Bank
Abstract
Great Bahama Bank (GBB) is the modern example of a flat-topped, isolated carbonate platform. It is a major modern location of carbonate deposition that stands behind much of the understanding of modern processes of carbonate sedimentation, serves as a training venue for academia and industry, is the basis for numerous geological models, and is commonly used as a reservoir analog. GBB also provides valuable insight into the extent and patterns of sediment fill of accommodation space atop an isolated carbonate platform. Satellite imagery (Landsat TM and ETM+) and an extensive set of water-depth measurements (n=5,723) were used to map bathymetry across GBB and derive a digital terrain model (DTM). Analyzing the extent, patterns, and nature of sediment fill of accommodation space was facilitated by partitioning a depositional facies map on the basis of the DTM to show that 18% of accommodation on the GBB is "overfilled and filled" (emergent to-1.5 m water depth), 52% is "underfilled" (-1.5 to-6.0 m), and 30% is "unfilled" (<-6.0 m). Considering the bathymetric variation, the DTM shows that only 10% (10,800 km2) of the awash platform has aggraded to sea level in the form of sand shoals or mud flats (>-1.5 m water depth) and therefore the greater part of available accommodation (88,000 km2) remains incompletely filled with sediment. Areas of filled accommodation mostly extend platformward from the western coastlines of islands, which in turn are preferentially distributed along the eastern (windward) margin of the GBB. Seventy percent of sediment in the areas of filled accommodation is rudstone, high-energy grainstone, grainstone, and mud-poor packstone. Although dominated by grainstones (45%), since it occupies so much space (55,000 km2), the underfilled sector also contains the most heterogeneous facies mosaic and by definition, therefore, the greatest and most complex lateral facies variations. Although islands are numerous (n=1,430) they occupy only 8%, or 8,700 km2, of the platformtop. Despite their small proportional occupancy of the platform, islands play a direct role in the accumulation of muddy fabrics and exert a sphere of influence over the character of sedimentation formany tens of kilometers fromtheir coastlines. Further, since islands represent the portion of the bank where accommodation has been overfilled, their rarity emphasizes the challenges that such a large platform faces in filling accommodation space, even given the diverse grain factories producing carbonate sediment (mud precipitation throughwhitings, ooids and reefs along the platformmargins, skeletal sediments and nonskeletal grains such as fecal pellets, peloids, and pelletoids). Two factors can be evoked to explain the inability of GBB to fill accommodation space. First, falling sea level during the Pleistocene appears to have repeatedly aborted the filling process. By analogy, it will not be able to do so in the Holocene either, a situation exasperated by the second factor, which is that most of the Holocene GBB, particularly areas away from islands, presently lack any appreciable filling. Instead, facies analysis suggest reduced sedimentation and enhanced cementation. Through regional mapping, the work delivers a platform-wide assessment of how accommodation is filled and the facies responsible for the filling. Amongst other trends, by highlighting how the thickest accumulations of Holocene sediment are skewed to the windward margin of the GBB and related to the presence of islands, whereas the thinner accumulations, which dominate the platforminterior and are more grainy but alsomore heterogeneous than the thicker deposits, these datamight contain useful guidelines as to how depositional cyclesmight vary laterally in ancient systems.