Sand Scripts Of Past Tsunamis In Coastal Ponds Of St. Thomas, Us Virgin Islands

Five unusual overwash deposits within four coastal ponds and a mangrove site on St. Thomas, U.S. Virgin Islands, may be the telltale signs of tsunamis. The estimated ages and locations of the deposits suggest that the tsunamis originated from different offshore sources. These deposits were found during a reconnaissance- level study on St. Thomas and adjacent cays located near the eastern end of the Greater Antilles volcanic arc, 60 km east of Puerto Rico and 150 km south of the Puerto Rico trench. The study sites occur within the narrow coastal zone in an otherwise high-relief setting. The unusual deposits, distinct from the pond sediment, including six other overwash deposits similar to other known tsunami deposits, are composed of mixed lithic and carbonate sands, and hypersaline and marine shells, contain coral boulders and fragments, exhibit sharp or erosive lower contacts, and, in some cases, are capped by mud. Beach berms that front the sites are 1-2 m above mean sea level, composed of coral and lithic cobble-size fragments as well as coarse, mixed-composition sand, appear to be a source of the likely tsunami deposits. On the basis of sediment stratigraphy and radiocarbon dating, age estimates of the deposits suggest five tsunamis in the past 3000 years, two of which fall within date ranges of likely tsunami deposits identified on Anegada, British Virgin Islands, located 70 km northeast of St. Thomas. Findings of this reconnaissance-level study suggest that the unusual overwash deposits on St. Thomas may be related to the 1867 tsunami triggered by an M similar to 7: 3 earthquake in the Anegada Passage, the 1755 trans-Atlantic tsunami triggered by the M 8.5 Lisbon earthquake, a tsunami between A. D. 1200 and 1450 first recognized on Anegada, extending its known area of impact farther to the southwest, and possibly two earlier events between 2000 and 3000+ years ago.