RE-CORRELATION OF THE EARLY - MIDDLE EOCENE MAGENTOSTRATIGRAPHY FOR LEG 165 SITE #998B ON THE CAYMAN RISE, CARIBBEAN SEA

Ocean Drilling Program (ODP) Leg 165 site #998b, located in the Cayman Rise, consists of pelagic limestone with interbedded volcaniclastic sedimentary rocks and turbidity volcanic ash flows which were deposited during the early to middle Eocene transition. Planktonic foraminifera were studied from this site using thin section analysis (cores 28R through 37R) in order to correlate the section to the global chronostratigraphic scale. A high-resolution biostratigraphic framework was also developed for site #998b. In the course of this study, it became apparent that the polarity chronology for the lower portion (798.9-904.8 mbsf) of the site needed to be revised to better fit the biostratigraphic data.

Cores 37R and through the lower part of 36R were correlated to E5 and contained the lowest occurrence (LO) of Morozovella aragonensis and the highest occurrence (HO) of Morozovella subbotinae. The upper part of core 36R through the lower half of 35R was correlated to E6 and contained the partial appearance of the HO of Morozovella subbotinae and the LO of Acarinina cuneicamerata. The use of the HO of Acarinina quetra and HO of Morozovella formosa helped in defining the upper boundary of E6 and the bottom of E7a. The upper part of 35R through the lower half of 32R was correlated to E7. E7 was subdivided into E7a and E7b using the LO of Turborotalia frontosa and the LO of Acarinina cuneicamerata. The upper part of core 32R through the top of core 29R was correlated to E8 and contains the LO of Guembelitrioides nuttalli and Globigerinatheka kugleri. In the upper part of site #998b contained the HO of Morozovella aragonensis and the LO of Globigerinatheka kugleri and was correlated to biozone E9.

Previously published polarity chronology for site #998b stated the chronology was constrained and highly dependent on the nannofossils and that the correlations were tentatively placed. New polarity chronology shifts down from C20r as noted by the Shipboard Scientific Party at core 28R to C19r and continues shifting down core. Core 37R has been divided in to the chronology subgroup C23.n.1n. The new chronology also indicates C22r is missing from the site as C23n in the lower section of site #998b jumps to C22n. This missing chron may be indicative of an unconformity.