Diagenesis in the Upper Miocene Cayman Formation of Grand Cayman in Response to Rapid Sea Level Rise Over the Last 16,000 Years

Abstract

Today the surface of Grand Cayman is mostly less than 10 m above sea level. On the eastern part of the island, there is well-developed freshwater lens that is developed in the limestones and dolostones of the Cayman Formation. Since deposition during the Miocene these deposits have endured repeated cycles of exposed and submergence as sea level has oscillated. For example, about 16,000 year ago, sea level was probably about 120 m below present day sea level. Like all other isolated oceanic islands, Grand Cayman stood well above sea level with coastlines formed of steep coastal cliffs. Sea level rose rapidly to about -10 m (∼ 7,000 years ago) and then rose more slowly to its present position. As sea level rose, so the diagenetic regimes in the Cayman Formation progressively changed. This ultimately led to the present day situation where a well-developed freshwater lens, an underlying brackish water lens, and a deep saline zone exists in the Cayman Formation on the eastern part of Grand Cayman. A continuous core obtained from well GFN-2, which is located in the central part of the eastern end of island is 95 m long and therefore covers the interval that would have been affected by the rise in sea-level over the last 16,000 years. The limestones found in this core, which belong to the Cayman Formation, are characterized by high porosity (up to 50%), minor dolomitization, and at least two generations of calcite cement. The porosity includes original intraskeletal pores and intergranular pores along with numerous fossil-mouldic pores that formed as the aragonite skeletons of the corals, gastropods, and bivalves were dissolved. This study examines the possibility that many of these diagenetic features may have formed over the last 16,000 years in response to the rapid rise in sea level. This possibility is supported by the distribution of the diagenetic features throughout the succession. In addition, analysis of the ground waters in the modern groundwater system shows that they are, from a geochemical perspective, capable of precipitating calcite cements. Thus, on isolated oceanic islands like Grand Cayman it is readily apparent that transgressive cycles, like the one that has affected it over the last 16,000 years, can impose a distinct diagenetic overprint on the older carbonate bedrock.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016