Processes of Recent Carbonate Sedimentation in Harrington Sound, Bermuda
Abstract:A shallow marine lagoon in the Bermuda Islands was chosen for the study of processes of carbonate production, deposition, and early diagenesis because its nearly enclosed situation provides an effective trap for products of local derivation. Regardless of its almost complete isolation, tidal exchange through a narrow inlet is sufficient to maintain near-normal oceanic temperature and salinity. Distinct stratification with consequent loss of oxygen occurs below 55 feet during the summer season.
In this study, bathymetric, hydrographic, and ecologic surveys provide environmental data. Textural and mineralogic (X-ray diffraction) studies of the surface sediments, skeletal materials, and surrounding rocks are supplemented by binocular and electron microscope examination.
The carbonate sediments collecting in this enclosed, low energy environment are largely silt and clay-sized derivatives of the indigenous skeletal biota and surrounding country rock. There is no evidence of physico-chemical precipitation. Mineral-textural analyses indicate that the fine-grained sediments which characterize the area are derived from: (1) the physical and biochemical disaggregation of calcareous algae, (2) the excavation products of boring and browsing organisms, (3) the physical attrition of fragile skeletal and rock-derived particles in the shallow turbulent zone.
Considering the abundant supply from calcitic algae, serpulids, and echinoderms, the paucity of fine-grained high Mg calcite is believed to be a result of the selective removal of this unstable phase by solution at or above the sediment-water interface. Comparison with other areas suggests that chemical solution is a more common modification of recent carbonate deposits than chemical precipitation. Furthermore, this comparison suggests that solutional early diagenetic modification on the sea floor follows the sequence of carbonate mineral stability, so that less stable phases (high Mg calcite, aragonite) are affected first and more stable low Mg calcite last. This process provides a framework for the classification of carbonate environments as well as an index of the degree to which solution in the environment of deposition alters the original mixture of mineral phases supplied. Associated with hydrographic regime and applied to buried carbonate sediments, this concept may provide a means to help reconstruct the oceanographic history of the basin of deposition.
Document Type: Research Article
Publication date: December 1, 1965
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