Abstract
Selective diagenesis of the Pleistocene Anastasia Formation produced moderately thick (up to 3 ft; 1 m) sheets of rock that were used to build the famous Castillo de San Marcos Fort in historic St. Augustine and other structures in eastern Florida. Petrographic examination of rock samples collected from selected sites near St. Augustine and in southern Florida revealed some textural evidence indicative of early marine and vadose diagenesis. In the House of Refuge Beach area, the petrographic evidence for early diagenesis as beachrock includes micrite cements and round coatings of finely crystalline calcite cement that apparently formed by the neomorphism of an aragonite precursor cement. Both micrite and aragonite rim cements are common in recent beachrocks. Meniscus cements present in some sampled indicate vadose zone, as opposed to sub-marine, cementation. Alternating beds of cemented and non-cemented predominantly shell material with a thickness ranging from 6 in to 3 ft (0.1 to 1 m) also suggest a beachrock origin for the deposit. Finer-grained Anastasia Formation deposits cropping out along the southeastern Florida shoreline contain pebbles, infilled "pipes", and organic debris indicative of a beachrock deposit.