ELECTRON SPIN RESONANCE AND GEOMORPHIC EVIDENCE OF HOLOCENE UPLIFT ALONG THE NORTH AMERICAN/CARIBBEAN PLATE BOUNDARY

Roatan Island, Honduras, is a continental fault block on the North American/Caribbean plate boundary along the southern margin of the Cayman Trough. The island is fringed by a coral reef, and beachrock is actively forming in the tidal zone. Subaerial fossil coral reef and beachrock is exposed on the west end of Roatan, suggesting recent uplift northwest of a fault crossing the island. As tectonic reference horizons, we mapped elevations of the fossil reef crest, a fossil beachrock terrace, and uplifted beach terraces. We collected cores and outcrop samples of fossil coral and beachrock for ESR dating.

Our geomorphic mapping revealed warping and secondary faulting of the reef and terraces. XRD analyses and thin section study show the uplifted coral has recrystallized to calcite. We interpret this recrystallization as a diagenetic response to fresh water flushing following uplift. None of the ESR spectra show levels of radiation damage above the resolvable limit for age analysis (as estimated by the radionuclide content of these rocks). This age resolution limit constrains the subaerial recrystallization of the reef and calcite cementation of the beachrock to Holocene. Radiocarbon ages for fossil beachrock CaCO3 cement and shell material from concordant beach terraces yield late Holocene ages of 1100 to 350 yr BP.

The south coast of the west end of Roatan is controlled by the fault margin of the uplifted block. Fossil reef rises to as much as 10.5 m above MSL along this coast, suggesting a minimum Holocene uplift rate of 1 m/ka. Taking Holocene sea level rise into consideration, the magnitude of uplift may be greater than 1m/ka. On this coast, a fossil beachrock terrace (ca.1100 ka) rises to 4.5 m above MSL coincident with the 10.5 m reef elevation, suggesting an uplift rate of 4.1 m/ka for late Holocene.