Highstands during Marine Isotope Stage 5: evidence from the Ironshore Formation of Grand Cayman, British West Indies

Abstract

The Ironshore Formation on Grand Cayman is formed of six unconformity-bounded packages (units A–F). Units A, B, C, and D, known from the subsurface in the northeastern part of Grand Cayman, formed during Marine Isotope Stages (MIS) 11(?), 9, 7, and 5e, respectively. Unconformities at the tops of units A, B, and C are highlighted by terra rossa and/or calcrete layers. Strata in core obtained from wells drilled in George Town Harbour and exposed on the west part of Grand Cayman belong to unit D, and the newly defined units E and F. Corals from unit E yielded Th/U ages of ∼104ka whereas conch shells from unit F gave ages of ∼84ka. Unit E equates to MIS 5c whereas unit F developed during MIS 5a.Th/U dating of corals and conchs from the Ironshore Formation on the western part of Grand Cayman shows that unit D formed during the MIS 5e highstand whereas units E and F developed in association with highstands at 95–110ka (MIS 5c) and 73–87ka (MIS 5a). Unit E, ∼5m thick in the offshore cores, is poorly represented in onshore exposures. Unit F, which unconformably overlies unit D at most localities, is formed largely of fossil-poor, cross-bedded ooid grainstones. The unconformity at the top of unit D, a marine erosional surface with up to 2.5m relief, is not characterized by terra rossa or calcrete in the offshore cores or onshore exposures. Unit D formed with a highstand of +6m asl, whereas units E and F developed when sea level was +2 to +5 asl and +3 to +6m asl, respectively. Thus, the highstands associated with MIS 5e, 5c, and 5a were at similar elevations.