THE EARLY TO MIDDLE EOCENE TRANSITION IN JAMAICA: THE RIO SAMBRE SECTION

A well exposed series of outcrops of the early-middle Eocene Richmond and Font Hill Formations in the valley of the Rio Sambre contains a diverse assemblage of planktonic and benthic foraminifera . This fauna has received only limited study to date although the Richmond and Font Hill have been placed within a biostratigraphic framework using calcareous nannofossils and larger foraminifera.

The most abundant planktonic foraminifera in the lower 4 m of the Richmond Formation in the Rio Sambre Valley are Subbotina inaequaspira, S. triangularis, Acarinina gravelli, Igorina broedermanni, Morozovella formosa, M. subbotinae, and Pseudohastergerina wilcoxensis. This assemblage suggests assignment to the Morozovella formosa Lowest-occurrence Zone (Zone E4). The appearance of Morozovella aragonensis at 5 m above the base of the section suggests assignment to the Morozovella aragonensis/Morozovella subbotinae Concurrent-range Zone (Zone E5). Common planktonic foraminifera within this 32 m interval of the Richmond include Acarinina bullbrooki, A. esnaensis, A. pentacamerata, Morozovella aragonensis, M. formosa, M. subbotinae, and Pseudohastergerina wilcoxensis. The highest occurrence of Morozovella subbotinae in the Rio Sambre section is at 47 m above the base. This is within the transition from the Richmond to the Font Hill Formation and indicates assignment to the Acarinina pentacamerata Partial-range Zone (Zone E6). The upper bluffs of the Rio Sambre Valley are within the Font Hill Formation and contain Morzovella aragonensis, M. spinulosa, M. caucasica, Acarinina pentacamerata, A. aspensis, A. inaequispira, Subbotina senni, and Astrorotalia palmerae among others. This assemblage is characteristic of the Acarinina cuneicamerata Lowest-occurrence Zone (Zone E7). Turborotalia frontosa was not identified in the section thus the subzones of Zone E7 could not be delineated. The uppermost Font Hill Formation contains planktonic foraminifera of the Guembelitriodes nuttalli Lowest-occurrence Zone (Zone E8) including Guembelitriodes nuttalli, Subbotina crociapertura, and Hantkenina mexicana.

The Rio Sambre section in Jamaica spans the same interval as the Lutetian GSSP at Gorrandaxte, Spain. The Rio Sambre section thus provides a tropical example of this important horizon.