Relationships Between the Distribution of Amphistegina and the Submerged Pleistocene Reefs Off Western Puerto Rico

Abstract

The study area lies off the central part of the western coast of Puerto Rico. It has a maximum north-south extent of 12 kilometers and is 11 kilometers in the east-west direction. The sample net consists of over one hundred samples. The remnants of an incipient Pleistocene reef at 80 to 85 meters of depth and a wave cut terrace at 55 meters of depth were found with five echo-sounder profiles. Three main foraminiferal reef assemblages exist in the Antillean Caribbean region. Two are Amphistegina faunas (with no Archaias present): one in the Gulf of Mexico and the other one off eastern Venezuela. The third, an Amphistegina-Archaias fauna, occurs in the warmer waters of the Antilles. Three shallow-water modern reefs were sampled bordering the eastern side of the area where Amphistegina-Archaias assemblage occurs. The highest percentage of Amphistegina gibbosa (up to 61%) is present in these reefs from 8 meters to 15 meters of depth. The percentage decreases down to approximately 80 meters where it increases to a maximum of 12% in front of Los Negros reefs. A relict foraminiferal fauna occurs at this depth. It consists mainly of Amphistegina gibbosa and Quinqueloculina lamarckiana indicating waters probably colder than present. Amphistegina gibbosa was found living off Puerto Rico in the shallow reefs at 6 meters of depth down to 70 meters of depth. However, in the submerged Pleistocene reefs most of the specimens of this foraminifer are glauconitized. Its percentages are many times higher in the glauconitized assemblage than in the total assemblage of foraminifera. The submerged reefs off western Puerto Rico correlate in depth with the calcareous prominences of the Gulf of Mexico and the submerged reefs off the western coast of Barbados.