Sedimentary Structures in Modern Carbonate Sands of the Bahamas
Sedimentary structures in carbonate sands of portions of the Great Bahama Bank are described and interpreted using data from air photographs, visual inspection of the bottom, echo-sounding traverses, and 50 box-cores impregnated with epoxy resin. Surface forms described include: avalanche ripples, with angle-of-repose lee slopes; accretion ripples, with lee slopes less than the angle of repose; flat, swash surfaces; large-scale current lineations on the order of 1000 feet long; bars; and irregular shoals. Strata are classed as avalanche deposits if they are cross-strata dipping at the angle of repose with coarser particles concentrated down their dip; accretion deposits if they are cross-strata less than the angle of repose without down-dip assortment; and sheet deposits if they are flat or gently dipping strata approximately parallel to underlying truncation surfaces From hydrodynamic theory and field observations the conclusion is reached that avalanche deposits form where the current separates from the bed on the lee side of avalanche ripples and other migrating embankments; that accretion deposits form where a traction carpet is moving down the lee side of an accretion ripple or other embankment with a significant component of tangential fluid flow; and that sheet deposits are formed in the upper flow regime on beaches, and on level-bottom areas by rare, high velocity currents. Meta-ripples and cross-ripples are described. Biogenic structures, including biolaminites and bioturbites, dominate deposits in intertidal and sheltered subtidal areas of the shelf-lagoon where bottom traction is small compared to the rate of organic activity.