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Infilling of coastal lagoons along the southern coast of Vieques, Puerto Rico, during the Holocene transgression is the result of contributions from both terrigenous siliciclastic and marine carbonate sources. Four lagoons displaying varying infilling and interaction with open marine waters were chosen for detailed stratigraphic study. Cores taken perpendicular to modern lithotope trends show a well-defined sequence that is similar throughout the lagoons. The stratigraphy also defines two distinct origins of the lagoonal basins: those resulting from sheltering provided by Oligocene-Miocene limestones and those developed by accretion of beach ridges seaward of shallow embayments.
Sedimentation in both types of lagoons began with deposition of terrigenous colluvium. Rising sea level was accompanied by storm-generated marine derived gravels that accumulated above the colluvium. Intertidal mud-flat facies and subsequent Diplanthera peat deposits denote the existence of restricted intertidal and subtidal environments respectively. These facies were overlain by molluscan gravel and Halimeda sand indicating increased water depth and improved circulation with the open marine environment. Mangrove peats are prominent in cores from the lagoonal margins. They show seaward migration of this environment as terrigenous sediment continued to prograde into the lagoons.
Lagoonal margins display a terrigenous, siliciclastic-dominated progradational sequence, whereas the central and seaward portions display a mixed siliciclastic and carbonate transgressive sequence. These sequences occur in close geographic proximity and could provide problems of interpretation for the geologist concerned with the ancient record if detailed stratigraphic data were not available.
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