PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY OF THE MIOCENE LOW LAYTON VOLCANICS, PORTLAND, JAMAICA

The Low Layton volcanics of northeastern Jamaica provide the only evidence of Neogene volcanism on the island. The Low Layton volcanics consist of pillow basalts, breccias, hyaloclastics, and disrupted pelagic limestones tentatively assigned to the Montpelier Group. The entire Low Layton complex occurs as an inlier surrounded by upper Miocene chalks of the Coastal Group. A late Miocene whole rock K/Ar age of 9.5 Ma was obtained from basalts of the Low Layton volcanics in 1982. This age creates an apparent inconsistency as the Montpelier Group at other localities provides biostratigraphic ages no younger than early Miocene and as old as late Oligocene. Resolving this inconsistency and determining the precise age of the Low Layton volcanics is essential to understanding the geologic history of the northern Caribbean region.

The limestones immediately surrounding the pillow basalts at Low Layton were studied in thin-section in order to identify the planktonic foraminiferal fauna present. Planktonic foraminifera were abundant and well preserved in the samples. The assemblage obtained from the Low Layton volcanics includes Orbulina universa?, Globorotalia menardii, Globorotalia lenguaensis, Globigerinoides trilobus, Neogloboquadrina mayeri, and Globoquadrina venezuelana. This assemblage is similar to that collected from the lower part of the well studied Buff Bay section 12 km to the west and suggests assignment to biozone M11 (late middle Miocene).

The biostratigraphic results from the limestones of the Low Layton volcanics indicate that the K/Ar age on the basalts is too young. Correlation of the Low Layton limestones to the global geochronologic scale on the basis of biostratigraphy places the age range between 11.4 and 11.8 Ma. This increases the age of the last episode of volcanism in Jamaica by approximately 2 million years.