In the Rock Harbour Cays, near Little Bahama Bank, the composition and microstructure of widespread, subtidal, sediment-binding, mat communities were examined and described. Mat-bound and unbound sediment surfaces were then subjected in situ to flume-created direct currents to test both resistance to erosion and breakdown behavior. The mat was removed by bleach treatment and the flume tests were repeated. The mats consist of algae, diatoms, and arenaceous tubes. Algae and/or tubes provide the resistant framework within which grains accumulate and are bound by mucilaginous secretions plus fine algal filaments. Three mat types were distinguished: a fibrous Cladophoropsis mat, a gelatinous Lyngbya mat, and an aggregated Schizothix mat. Each mat type eroded in a characteristi manner and sequence dependent on mat composition and microstructure. Areas of intact mat withstood erosion better than irregular or broken surfaces. Mat-bound sediment surfaces withstood current velocities of more than 100 cm/sec, more than five times those required to erode mat-free surfaces. These studies indicate that particle size, sorting, packing, structure, and bedding-plane morphology are influenced by the presence of mats. Thus, consideration of mat communities is important when examining depositional and erosional processes at the sediment-water interface, or when making interpretation from ancient rocks which are the products of these processes.

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