Abstract

Carbonate beachrock is a common feature on many beaches of San Salvador Island, Bahamas. These well-indurated slabs of cemented beach sand are jointed normal and parallel to the shoreline and slope gently seaward. Constituent sediment grains are identical to those of adjacent, unlithified beach sands. Beachrock exposed along the northern, western, and southern coastline of the island is commonly characterized by aragonite-needle cement, micritized grains, and micritic envelopes, which are features of early marine-phreatic diagenesis. These beachrocks have whole-rock delta 13 C values of + 4.75 per thousand (PDB) and delta 18 O values of - 0.75 per thousand (PDB), which are identical to carbon and oxygen isotopic compositions of isolated aragonite-needle cement and unlithified beach sand. Inclusion of Recent artifacts in the rock indicates that these deposits are Holocene. Beachrock from one locality on the western side of the island is covered with a calcrete crust, indicating prolonged subaerial exposure and vadose weathering associated with the formation of organic-rich soils. Petrographic features include low-Mg calcite cement, evidence of neomorphism of aragonitic cement and micritic rims to low-Mg calcite, random needle-fiber calcite, calcified filaments, and micritic networks. Whole-rock delta 13 C values of these beachrocks range from + 3.66 per thousand to - 6.85 per thousand and delta 18 O from -2.16 per thousand to - 3.68 per thousand . The wide range of delta 13 C values in these samples is probably a function of the relative proportions of meteoric-vadose to meteoric and/or marine-phreatic cements in individual samples. Isolated meteoric-phreatic cements are only slightly enriched in the light isotopes relative to marine cements, whereas meteoric vadose cements are significantly lighter. The diagenetic features suggest the beachrock passed through marine-phreatic to meteoric-phreatic and meteoric-vadose diagenetic environments. This diagenetic sequence commonly forms in response to sea-level lowering. Evidence of deposition in a shoreline environment and prolonged residence in meteoric-phreatic and meteoric-vadose environments suggests that these beachrocks may be late Pleistocene.

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