Paleoenvironmental significance of Holocene foraminiferal taphocoenoses in Guayaguayare Bay, Trinidad, West Indies: A coral reef near the Orinoco Delta

Abstract

A small suite of seafloor sediment samples (water depth less than 20 m) was collected in 2005 from the sheltered, logarithmic-spiral Guayaguayare Bay along the SE coast of Trinidad, West Indies. The easternmost shielded part of the bay was then occupied by a meadow of the seagrass Halodule wrightii, the closest seagrass bed to the Boca Grande, the main distributary of the Orinoco Delta. Four foraminiferal taphofacies were distinguished, ascribable to a relict coral reef system: (1) A shallow-water (approx. 5 m) taphofacies dominated by Pararotalia sarmientoi. (2) A back-reef taphofacies (approx. 5 m) with abundant Discorbis rosea, apparently associated with the H. wrightii. (3) A fore-reef taphofacies (water depth approx. 20 m) co-dominated by Amphistegina gibbosa and Quinqueloculina bicostata. (4) A reef-flat (approx. 5 m) taphofacies dominated by Q. bicostata. A ternary plot of specimen wall types (agglutinated, porcellaneous, calcareous hyaline) clearly distinguished Taphofacies 1 from Taphofacies 4, porcellaneous specimens being rare in the former. A similar plot of staining + test abrasion for A. gibbosa indicated that those on Taphofacies 3, assumed to be in situ, were more heavily stained than those elsewhere and were larger. Tiny but pristine, presumably allochthonous A. gibbosa were recovered from Taphofacies 1. Two formae of Asterigerina carinata, both widely illustrated in the literature, are distinguished for the first time and named A. carinata forma conica and A. carinata forma discoida. The bay is currently filled with turbid water. The coral reef system, postdating the early Holocene transgression, would have required clear water, perhaps when the Orinoco Delta had not prograded sufficiently to impact the bay greatly. The known best depth range for the growth of amphisteginids, which are symbiotic with algae, coupled with known Holocene sea level curves for the SE Caribbean, suggest an age of approx. 4000 YBP. Sedimentation in the protected bay has since the development of the reef been limited.