Sea-level highstands over the last 500,000 years: Evidence from the Ironshore formation on Grand Cayman, British West Indies

The Ironshore Formation (up to 19 m thick) in the Rogers Wreck Point area o f 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 ha, unit B similar to 346 ha, unit C similar to 229 ka, and unit D similar to 131 ka , These ages correlate with the highstands of the last four interglacial pe riods and probably represent marine isotope stages 5 (unit D), 7 (unit C), 9 (unit B), and II (unit A). The Limestones in units A-D, which have similar biotic and lithologic chara cteristics, represent deposition in shallow water on a narrow coastal shelf Unit A has a high faunal diversity and was deposited in open-marine condit ions, whereas units B, C, and D were deposited in quieter-water lagoons tha t probably had fringing reefs along their seaward edges. Although Grand Cayman is located on the Cayman Ridge close to an active spr eading center and transform fault, the island has undergone little or no ve rtical 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 se a-level highstands over the last 500 ky.