Sedimentary Structures in Modern Carbonate Sands of the Bahamas: ABSTRACT

Abstract

Sedimentary structures of carbonate sand bottoms near Bimini, Sandy Cay, and the Berry Islands have been examined in depths ranging to 10 ft. below MLW. Box cores with a vertical face 18 in. high and 12 in. wide were impregnated in the field with epoxy resin to preserve and accentuate the structures. Four distinct types of internal structures occur in contrasting depositional environments. Type I, characterized by 3-10 cm.-thick sets of cross-strata and either horizontal or curving truncation surfaces, is found in intertidal, rippled, oolite shoals. Type II, with 20-40 cm.-thick sets of tabular cross-strata and horizontal truncation surfaces, is found in intertidal-subtidal, unrippled, grapestone bars. Type III, with even, parallel laminae inclined at low angles to the horizontal, is found in the backshore and foreshore of beaches. Type IV is mottled, lacks stratification, and is found in unrippled level bottoms. Bahamian field observations confirm pertinent hydrodynamic conclusions of J. R. L. Allen, H. W. Menard, and D. B. Simons. Type I is observed to form by small ripple trains induced at low flow intensities by tidal currents. Type II is judged to form by a large-scale ripple induced at high flow intensities by rare storms. According to M. M. Ball, deposits with similar structure were formed by Hurricane Donna in Florida Bay. Type III is observed to form from swash and backwash currents on beaches. Type IV reflects an environment in which organisms destroy stratification as fast as it is produced. These four types should be considered as end-members representing hydrodynamically pure situations. Many actual sites represent alternating conditions (beach-shoal, beach-bar, etc.), as is clear from preserved structures. End_of_Article - Last_Page 533------------