GSA Connects 2021 in Portland, Oregon

Paper No. 161-18
Presentation Time: 9:00 AM-1:00 PM


RAHAMUT, Matthew and STEMANN, Thomas, Department of Geography and Geology, The University of the West Indies, Mona Campus, Kingston, 7, Jamaica

Subaerial exposures of Late Pleistocene coral reefs of the Falmouth Formation provide a unique opportunity to study long term changes in reef growth and accretion patterns across Jamaica during the last interglacial. Exceptionally well-preserved exposures up to 3m high are found in the Treasure Beach area in St. Elizabeth, Jamaica.

Field observations show two distinct reef units characterized by changes in coral species and reef fabric; a lower unit associated with relatively high turbulence and an upper unit associated with higher sedimentation and less turbulent conditions. The line-intercept transect method was used to document six representative sections along the coastline of the study area. Using ImageJ software, the taphonomy of 22 coral samples (100 slabs) collected along the transects was quantified based on encrustation, bioerosion and sediment occlusion. The relative abundances of macroborer and encrusting communities were also found for each transect locality. Cluster analysis of encrustation, bioerosion and sediment occlusion along with field observations were used to describe four taphofacies. These were associated with four respective paleoecological zones along the upper unit of the ancient reef tract- the reef crest, recolonized reef flat, patch reef to back reef, and rear zone of the reef crest.

Presence of the two reef units does not necessary indicate an intervening lowstand due to the lack of substantial evidence of subaerial exposure such as paleosols or rhizocretions at the top of the lower unit. However, vertical changes in coral species and instances of erosion and recolonization of massive coral heads imply two distinct stages of accretion possibly due to a rapid sea level jump leading to the growth of the upper unit. Additionally, taphofacies characterization of the upper unit allowed for turbidity, instance of partial mortality, and relative time of corals in the taphonomically active zone to be inferred for each of the paleoecological zones. These findings form part of a larger project to reconstruct the accretionary history of Late Pleistocene reef exposures at various sites in Jamaica. This will contribute to an understanding of the interaction between local paleoenvironmental influences, tectonics, and global sea level changes on Late Pleistocene reef growth and geometries.