Last interglacial reef limestones, northeastern St. Croix, US Virgin Islands-evidence of tectonic tilting and subsidence since MIS 5.5
Most last interglacial (MIS 5.5) coral reef deposits in the Caribbean are emergent. However, in St. Croix, these are found mainly at depth, underneath Holocene material and confirmed by TIMS U-Th dated corals from eight cores through Holocene reefs. The only emergent MIS 5.5 marine deposit peaks at +1.5 m MSL at the northwestern end of the island. The Late Pleistocene surface decreases at least 9.25 m (based on reef crest elevations) in elevation over 15 km along a 0.62 m/km eastward (alongshore) slope. Neither differential erosion nor a naturally sloping deposit is likely, thus the directional elevation decrease requires the influence of tectonic processes. Platform tilting or differential subsidence increasing in rate to the east probably operated both during and since the last interglacial and created progressively greater accommodation space for increasingly thicker overlying Holocene reefs in an eastward direction. Rates of subsidence since MIS 5.5 increase from west to east, from 0.02 mm/year to 0.1 mm/year, assuming a MIS 5.5 +6-m sea level and +4 m initial reef elevation. St. Croix's association with extensional shelf faulting from the northern part of the Virgin Islands Basin, the Anegada Fault to the east and the Puerto Rico Trench to the north may be significant in terms of identifying mechanisms for, or past events resulting in, directional tilting. Identification of differential elevations of MIS 5.5 reefs adds substantially to the information on Late Quaternary tectonism of the area.
|Keywords||Fossil coral reefs, MIS 5.5, Platform tilting, St. Croix-USVI, U-Th dating|
Toscano, M.A., Macintyre, I.G., & Lundberg, J. (2012). Last interglacial reef limestones, northeastern St. Croix, US Virgin Islands-evidence of tectonic tilting and subsidence since MIS 5.5. Coral Reefs, 31(1), 27–38. doi:10.1007/s00338-011-0822-7