Dolomite, Phosphorite, and Carbonate Diagenesis on a Caribbean Seamount

Abstract

Carbonate rocks consisting of micritic phosphorite, reefoid conglomerate, and calcarenite were dredged from a seamount on Ayes Swell at 13°30 ' N, 63~I0 ' 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.

Genetic Relations Between the Aves Ridge and the Grenada Back‐Arc Basin, East Caribbean Sea

“…We calibrated the seismic units using offshore well data and onshore geological observations (Figure 3). We also used dredge results from previous studies (Bouysse et al, 1985;Fox et al, 1971;Marlowe, 1971) and six new dredge hauls carried out during the GARANTI cruise on bathymetric highs on the eastern flank of the Aves Ridge ( Figure 2). Thin sections were made from 12 collected carbonate rock samples to identify their microfacies and fossil content (Table S1).…”

“…Our results show that subsidence remains relatively moderate from the Eocene to the middle Miocene (Figure 9). This allowed the development of carbonate platforms on bathymetric highs throughout the Aves Ridge (Bouysse et al, 1985;Fox et al, 1971;Marlowe, 1971;Münch et al, 2019).…”

“…Most of these sites were dredged during the GARANTI cruise, except the Pelicano seamount. For this one, we used dredge results from previous studies (Bouysse et al, 1985;Marlowe, 1971) to constrain the age of the carbonate sequences. Alcatraz seamount is the only site without MCS data.…”

Genetic Relations Between the Aves Ridge and the Grenada Back-Arc Basin, East Caribbean Sea 

“…Seismic lines covering the Aves Ridge display parallel, low-frequency, strong reflectors atop bathymetric highs (Fs15 in Table A2). This seismic facies characterizes middle Eocene to middle Miocene (U1b-U2b) carbonate platforms that were dredged during GARANTI (Table A1) and previous cruises (Bouysse et al, 1985;Fox et al, 1971;Marlowe, 1971). The GARANTI samples described in Table A1 were dredged from 670 to 2385 m deep.…”

“…We calibrated the seismic units using offshore well data and onshore geological observations ( Figure 3). We also used dredge results from previous studies (Bouysse et al, 1985;Fox et al, 1971;Marlowe, 1971) and six new dredge hauls carried out during the GARANTI cruise on bathymetric highs on the eastern flank of the Aves Ridge ( Figure 2). Thin sections were made from twelve collected carbonate rock samples to identify their microfacies and fossil content (Table A1).…”