Calcite cementation of sixty-five-year-old aragonite sand dredge pile.

摘要

Dredging of the harbor at Stocking Island, Bahamas (23°31'45"N, 75°49'41"W) in 1942 produced four dredge 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 350 x 50 m in plan view and 2 m high. The surface is very well cemented, which requires a hammer and chisel for sampling.;Samples were collected from six sites at various locations of the dredge pile. Samples were analyzed for both chemical and physical properties using thin-section examination, X-ray diffraction, X-ray fluorescence, bulk density measurements, isotopic analyses, and scanning electron microscopy.;The skeletal sands are mostly spherical to prolate shape, are reverse graded, and range in size from 0.5--2.0 mm over a depth interval of 1.5 m, with gravel to cobble-sized material dispersed intermittently throughout the interval. This deposit is capped with a 4--6 cm-thick crust of thoroughly micritized aragonite grains completely cemented with 4 micron 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 7--16%. Cement volume decreases with depth; the deepest samples contain meniscus cements only. There are very subtle dissolution features on the surface of individual grains.;Reverse grading caused by grainflow occurred when these micritized skeletal sands were moved from their marine setting and debauched into the meteoric vadose environment. Calcite cementation proceeded from the surface downward; meniscus cement preceded isopachous cement. The source of carbonate for the cements 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 from aragonite-to-calcite will follow.