Cay Sal Bank, Bahamas--A Partially Drowned Carbonate Platform: ABSTRACT

A. C. Hine, J. C. Steinmetz
1983 American Association of Petroleum Geologists Bulletin  
Association Round Table sents a transition between the bioturbation patterns of the channel and overbank deposits-that is, it results from a transition in the factors that dictate the preservation of biogenic sedimentary structures. HINE, A. C, Univ. South Florida, St. Petersburg, FL, and J. C. STEINMETZ, Marathon Oil Co., Littleton, CO Cay Sal Bank, Bahamas-A Partially Drowned Carbonate Platform Recent high resolution seismic profiling, sediment sampling, scuba observations, and Landsat
more » ... show that Cay Sal Bank (CSB) has very limited reef development, no active sand shoals (ooid or otherwise), few islands, a thin to nonexistent sedimentary cover, and a relatively deep margin (20 to 30 m [66 to 100 ft]) and shelf lagoon system (10 to 20 m [33 to 66 ft]). Windward and leeward margins can be discerned, but their health and general development are poor when compared to the shallower, more active margins of Little Bahama Bank (LBB) and Great Bahama Bank (GBB). Windward margins (facing north and east) along CSB are generally deep, rocky, sediment barren terraces supporting limited, low relief, relict(?) reefs. Leeward margins do have small sand bodies (maximum thickness 10 m [33 ft]) covering reef structures at the bank edge, indicating the offbank transport of sands has occurred. However, these marginal sand bodies are limited in extent, suggesting that this transport system was not ubiquitous along these south and west facing margins. Seismic and grab sample data from the deep (200 to 500 m [660 to 1,650 ft]) slopes seaward of the leeward margins show a thin, discontinuous unit of periplatform, shallow-water derived Halimeda, molluscan, nonskeletal sands. The limited extent (no deeper than 330 m [1,080 ft]) of this unit, which is easily recognized by its reflection-free seismic facies, also indicates that sand production and transport off the bank were never prolific. This is in stark contrast to new seismic data from the leeward margins of GBB which clearly show thick (20 m [66 ft]) sand bodies covering 12 to 15 m (40 to 50 ft) high reefs along the outer margin and multiple reflection-free units extending to great depths (600 m [2,000 ft]) all along the adjacent slope. The apparent immature development of normal bank-top processes and facies and the absence of key modern depositional environments on CSB may be related to the rate at which this platform was submerged. Due to its comparatively low elevation, the initial Holocene flooding occurred at approximately 8 to 10 ka when sea-level rise was rapid (6 m/ka [20 ft/ka]). By comparison, the higher LBB/GBB were flooded later at a much slower rate (1.5 m/ka [5 ft/ka]). The relatively rapid flooding of CSB provided little time for the shallow depositional environments to start up. The continued rapid rate of rise after drowning, plus offbank sediment transport and the export of chilled waters (formed during winter), prevented the resulting facies from catching up. Consequently, CSB appears to be partially drowned, particularly when compared to the other, healthier, rimmed Bahamian platforms. Other investigators have pointed out that drowned carbonate banks are very common in the ancient and that these features potentially provide excellent stratigraphic traps for hydrocarbons. CSB provides us with a modern example of a bank that may be in the very early stages of termination.
doi:10.1306/03b5b13c-16d1-11d7-8645000102c1865d fatcat:shpawjig6nh4bmf6nm7s4x7yau