CALCITE CEMENTATION OF SIXTY-FIVE-YEAR-OLD ARAGONITE SAND SPOILS PILES

Dredging of the harbor at Stocking Island, Bahamas (23°31’45”N, 75°49’41”W) in 1942 produced four spoils piles of cross-bedded aragonite skeletal sand. The spoils piles are on the leeward (western) shore of the island, where they are subject to minimal wave energy. Collectively they are 350x50m in plan view and 2m high. The surface is very hard, having an estimated unconfined compression strength of 45–50 Mpa, which requires a hammer and chisel for sampling.

The skeletal sands are mostly spherical to oval shape, are reverse graded, and range in size 0.5–2.0mm over a depth interval 1.5m with gravel to cobble sized material dispersed intermittently throughout the interval. The surface of this deposit is a 4–6cm-thick crust of thoroughly micritized aragonite grains completely cemented with 4mm calcite spar. Beneath this crust micritized aragonite grains are cemented by sparry calcite cements with meniscus and isopachous textures. The amount of cement ranges from 34–6%. Cement volume decreases with depth; the deepest samples contain meniscus cements only. There are very subtle dissolution features.

When these micritized skeletal sands were moved from the marine to the meteoric vadose environment grainflow produced reverse grading. Calcite cementation proceeded from the surface downward; meniscus cement preceded isopachous cement. The source of carbonate is internal to the spoils piles. In this example of aragonite sand in the very early stages of mineralogical stabilization in a meteoric vadose environment, rapid cementation clearly occurred first. It is anticipated that mineralogical stabilization of the grains will follow.