Introduction: Delineating and Quantifying Depositional Facies Patterns of Modern Carbonate Sand Deposits on Great Bahama Bank
Paul M. (Mitch) Harris, James Ellis, Sam Purkis, 2010. "Introduction: Delineating and Quantifying Depositional Facies Patterns of Modern Carbonate Sand Deposits on Great Bahama Bank", Delineating and Quantifying Depositional Facies Patterns of Modern Carbonate Sand Deposits on Great Bahama Bank, Paul M. (Mitch) Harris, James Ellis, Sam Purkis
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Processed satellite images, derived bathymetry (Digital Elevation Models), and sand body interpretation maps of three key areas of modern carbonate sand deposition on Great Bahama Bank (GBB) are organized into a GIS to develop morphometric data that hopefully will stimulate further studies of modern carbonates. Rimming the southern end of Tongue of the Ocean (TOTO) is the broadest expanse of “high-energy” sands found in the Bahamas characterized by narrow sandbars separated by wide, deep channels and a lack of islands. A variation of the tidal bar motif with broader and more irregular sandbars, relatively narrow channels, and few small islands occurs at the northern end of Exuma Sound (Schooners). Sands associated with tidal channels and the numerous islands of the Exumas chain along the western edge of Exuma Sound occur primarily as flood tidal deltas. Collectively, these three sand deposits occur along a belt paralleling the platform edge and show a range of depositional facies patterns.
Each carbonate sand body and subdivisions of it based on common sandbar patterns are analyzed for size, and then further differentiated by selecting different water depth intervals to define sandbars and bar crests. The TOTO sand body covers 3120 km2 with sandbars extending onto the shallow platform up to 27 km, whereas that of Schooners covers 716 km2 with sandbars up to 17 km long. The part of the Exumas examined here is a 450 km2 linear belt with flood tidal delta lobes extending up to 8 km onto the platform. The portion of each sand body that is occupied by sandbars varies from ∼ 15% - 20%. Sandbar percentage varies between domains in each of the three sand bodies, and can comprise as much as 25% of a particular domain.
Size (e.g., length, width, perimeter, and area), shape (using Form Factor, Sinuosity, Aspect, and Roundness), centerlines, and centroids are measured or calculated for the sandbars and analyzed spatially for interrelationships. Profiles and spatial analysis tools enable sand body and channel spacing, position relative to the platform margin, connectedness, and density to be characterized. Relationships are identified that link the size and shape of sandbars, their depths with regard to proximity to the platform edge, and mean separating distances between adjacent bars. For example, small sandbars tend to be rounded, whereas large ones (> 1 km2) are exclusively elongate. However, there is no pronounced change in complexity with increasing area; more complex sandbars do not have a significant likelihood of being large or small. The size of a sandbar has no bearing on the separating distance to the nearest neighbor; however, more separated sandbars have a distinctly lower variance in water depth. More rounded sandbars, as quantified by their Form Factor, are found within close proximity to one another, while those separated by great distance have a propensity to be elongate. There is also a clear trend such that the sandbars with the roughest surfaces (i.e., most thickness variation) tend to be of greatest area. TOTO and Schooners, the two sand bodies without islands, have very similar sandbar centerline density (32% and 35%, respectively), whereas the island-rich Exumas has a relatively lower centerline density (21%). Centerlines at Schooners and TOTO have almost identical complexity, but Exumas centerlines are considerably less complex. All three sand bodies show a similar pattern wherein simple centerlines are by far the most prevalent while each site displays a small number of highly complex/convoluted bar lines (< 50, or ∼ 25% of total). That is, the propensity for the formation of sinuous and intricate centerlines is similar between the sand bodies, suggesting similar formative mechanisms at play in each setting.
The results of the sand body and sandbar interrogation imply that certain architectural properties of high-energy sand deposits are generic. We think such results broaden our perspective of the types of information that can be derived from studies of the modern.
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Delineating and Quantifying Depositional Facies Patterns of Modern Carbonate Sand Deposits on Great Bahama Bank
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