The relative influence of ice-age sea-level change and tectonic displacement on the present elevation of Last Interglacial reefs exposed within the Falmouth Formation along the coastlines of Jamaica has been debated, casting doubt on the degree to which these outcrops can be used with other records to infer ancient ice volumes. To assess the potential for postdepositional tectonic displacement, we surveyed maximum elevations of fossil reef outcrops of the Falmouth Formation at nine sites along the northern and southern coastlines of Jamaica, and we found that its peak elevation varies by up to 8 m among study sites. Based on our analysis of reef facies and paleowater depth at each site, we conclude that the difference in elevations between outcrops cannot be explained by natural variation in reef topography or glacial isostatic adjustment, implying that postdepositional tectonic displacement has occurred. To estimate differences in the magnitude of vertical tectonic movement between sites, we reconstructed peak relative sea-level (RSL) position during the Last Interglacial sea-level highstand by combining survey measurements with paleowater depth interpretations derived from an analysis of the uppermost exposed reef facies. To estimate rates of postdepositional vertical tectonic displacement, we then compared our reconstructed RSL positions with predictions of RSL based on glacial isostatic modeling, including the fingerprint of meltwater signals. Estimated rates of vertical displacement decline from east to west along the north coast from >100 mm/1000 yr (k.y.) at Oracabessa to ∼1 mm/k.y. at Rio Bueno Harbor. In the south, subsidence is inferred, with estimates ranging from ∼−27 to −14 mm/k.y. The east-west gradient in uplift across the north coast is consistent with the proximity of the eastern sites to the inverted Wagwater belt and seismically active Blue Mountain restraining bend, and it indicates differential tectonic displacement of the Falmouth Formation since its deposition.
Evidence of tectonism based on differential uplift of the Falmouth Formation of Jamaica
Alexandra Skrivanek, Andrea Dutton, Thomas Stemann, Karen Vyverberg, Jerry X. Mitrovica; Evidence of tectonism based on differential uplift of the Falmouth Formation of Jamaica. GSA Bulletin doi: https://doi.org/10.1130/B31796.1
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