Carbonate rocks consisting of micritic phosphorite, reefoid conglomerate, and calcarenite were dredged from a seamount on Aves Swell at 13 degrees 30' N, 63 degrees 10' W. The petrology of these rocks was investigated by optical, chemical, X-ray diffraction, and electron microprobe techniques in order to study the effects of diagenesis. Samples of micritic phosphorite were dense and hard, contained Oligocene-Miocene foraminiferal remains, and were composed dominantly of microcrystalline carbonate-apatite. The phosphorite was obtained from the slopes of the seamount at 620 to 730 m. It is considered to have formed as a result of replacement of an original calcareous ooze by phosphorous from sea water, according to mechanisms proposed by Kazakov (1937) and Ames (1959). Conglomerate composed of highly altered, reefoid detritus and containing fossils of probable Pleistocene to early Holocene age was collected in the depth range of 340 to 455 m. Etched and embayed calcareous structures show that the rock has been partially replaced by a microcrystalline matrix of carbonate-apatite and iron/manganese oxides, in which dolomite euhedra occur abundantly. Epitaxial rims of dolomite crystals around algal detritus suggest that fragments of high-magnesium calcite may have served as local nuclei for the development of dolomite within the rock. Because of its intimate association with Fe/Mn oxides, it does not appear likely that the dolomite formed at shallow depths in this open-ocean, unrestricted environment. Very coarse, porous calcarenite composed dominantly of unabraded foraminiferal tests and molluscan fragments and cemented by fibrous, high-magnesium calcite were collected from the top of the seamount, at 340 m. Abundant pelagic forms, uninverted aragonite grains, and the scarcity of shallow-water forms suggest that this rock has not been exposed to the effects of atmospheric water. Cementation has therefore probably taken place below sea level. A paragenesis involving changes in water depth over the seamount is proposed to explain the origin of these different rock types.

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