Sea-level highstands over the last 500,000 years; evidence from the Ironshore Formation on Grand Cayman, British West Indies

Abstract

ABSTRACT The Ironshore Formation (up to 19 m thick) in the Rogers Wreck Point area of Grand Cayman is divided into the unconformity-bounded units A, B, C, and D. The unconformities are highlighted by caliches and/or terra rossa. Th/U dates from aragonitic corals indicate that unit A formed at > 400 ka, unit B 346 ka, unit C 229 ka, and unit D 131 ka. These ages correlate with the highstands of the last four interglacial periods and probably represent marine isotope stages 5 (unit D), 7 (unit C), 9 (unit B), and 11 (unit A). The limestones in units A-D, which have similar biotic and lithologic characteristics, represent deposition in shallow water on a narrow coastal shelf. Unit A has a high faunal diversity and was deposited in open-marine conditions, whereas units B, C, and D were deposited in quieter-water lagoons that probably had fringing reefs along their sea-ward edges. Although Grand Cayman is located on the Cayman Ridge close to an active spreading center and transform fault, the island has undergone little or no vertical movement over the last 500 ky. Available evidence indicates that sea level at the time of deposition of each unit relative to present sea level was: unit A, -9.0 to -5.5 m; unit B, -3.0 to +0.5 m; unit C, -2.5 to +1.1 m; and unit D, +2.5 to +6.0 m. These data are good estimates of eustatic sea-level highstands over the last 500 ky.