OVERWASH DEPOSITS FROM THE PAST 3,000 YEARS ON ST. THOMAS, U.S. VIRGIN ISLANDS

Tsunamis may account for sand layers composed of lithic fragments and either cerithiids or Halimeda within otherwise muddy salt pond sections on St.Thomas. The north shore faces a part of the Puerto Rico Trench that lacks a documented history of large earthquakes, while the south shore faces the source area of the 1867 earthquake and tsunami. We examined pond stratigraphy on St.Thomas and adjoining cays to test the southwestward extent of overwash that has been dated to AD 1650-1800 and AD 1200-1450 at Anegada, British Virgin Islands, which is located 172 km southwest of the Puerto Rico Trench. The coastal ponds that we cored are protected by 1.5 m high coral rubble beaches. In salt ponds on the north coast, the overwash deposits contain cerithiids, articulated and disarticulated shells, worm tubes, and coral fragments. In ponds on the southeast coast, on Saba Islet to the south, and on Salt Cay to the west, overwash layers contain Halimeda fragments; no cerithiids are present. At Saba Islet, a wood fragment provides a maximum age AD 1040-1100, AD 1120-1140, and AD 1150-1220 for a graded sand bed above it. At Salt Cay, a piece of wood about 20 cm below a sand layer gives a maximum age of AD 690-880 and a leaf fragment at the upper contact of a sand layer higher in the section yields a close minimum age of AD 1470-1650. At Magens Bay on the north-central coast, a piece of wood below a relatively thin sand layer provides a maximum age of AD 1440-1520. At Cabrita Pond on the northeast coast, a mangrove seed above a sand layer supplies a minimum age of AD 1650-1680, AD 1730-1810, and 1930 to post 1950. Twigs from mud above and below a sand layer provide minimum and maximum age constraints of AD 1440-1520, AD 1560-1630 and AD 990-1040, 1100-1120, 1140-1150, respectively. A twig within another sand layer lower in the section provides a contemporary age of 730-690, 660-650, and 540-400 BC. St.Thomas and nearby cays thus contain geologic evidence for at least three overwash events in the past 3,000 years, two of them similar in age to the inferred tsunami deposits in Anegada, and a prior event about 730-400 BC. Future field work in the U.S. Virgin Islands and Puerto Rico will help to define the geographical extent of tsunami deposits which will contribute to understanding of source areas, magnitudes, and recurrence rates of large earthquakes in the northeastern Caribbean.