Holocene lagoonal development in the isolated carbonate platforms off Belize

Abstract

Thirty-one vibracores were taken in interior lagoons of Glovers Reef, Lighthouse Reef, and Turneffe Islands—three isolated carbonate platforms offshore Belize, Central America. Holocene facies successions overlying the Pleistocene limestone bedrock begin with soils, followed by mangrove peats, and marine carbonate sediments of lagoonal origin. The soils formed on top of subaerially exposed Pleistocene limestone before the Holocene transgression. Mangrove peats developed during initial flooding of the platforms (Glovers ca. 8.5 ky, Lighthouse ca. 7 ky, Turneffe ca. 6 ky BP). As water depths increased, reefs colonized platform margins, lagoonal circulation improved thereby promoting carbonate production. The basal lagoonal carbonate sediments are characterized by shell beds and/or Halimeda packstones–grainstones. Mollusk-dominated wackestones and packstones follow upsection in Glovers and Lighthouse Reefs. At present, open circulation prevails in Glovers and Lighthouse lagoons. In contrast, organic-rich Halimeda wackestones and packstones dominate the Turneffe Islands Holocene succession. The main lagoon area of Turneffe is enclosed by mangroves, and restricted circulation prevails. Factors that explain the differences in geomorphology, circulation, and facies are variations in depth of antecedent topography and degree of exposure to waves and currents. The thickness of Holocene lagoon sediments may exceed the maximum core length of 6 m in all atolls. Holocene sedimentation rates average 0.6 m/ky, with highest rates in Turneffe (0.82 m/ky), followed by Lighthouse (0.53 m/ky), and Glovers (0.46 m/ky). Like in many other isolated carbonate platforms and atolls, lagoon floor sedimentation did not keep pace with rising sea level, leading to unfilled accommodation space. At present, Glovers has an 18 m deep lagoon, while Lighthouse and the main Turneffe lagoon are 8 m deep. It is unlikely that the lagoons will be completely filled during the Holocene sea level highstand cycle. This observation should be kept in mind when using cycle thickness as a proxy for eustatic sea level change in fossil carbonate platforms.