Coral-reef Geology: Puerto Rico and the US Virgin Islands

The distribution of Holocene coral reefs around Puerto Rico and the US Virgin Islands reflects the tectonic history of the region. By best estimates, the present-day Caribbean formed between 200 and 130 million years (MY) ago, when North and South America pushed apart and Pacific crust moved northeastward (Pindell 1994). By late Cretaceous time (80 MY: Fig. 7.1), Caribbean plate motion was starting to turn eastward, the Aves Ridge (presently southeast of St. Croix ) was forming, and rudistid molluscs were the dominant reef fauna throughout the region (Fig. 7.2). By Oligocene time, the geography of the Caribbean looked much like it is today, and the Greater Antilles lay along a major fault separating the Caribbean and Atlantic plates. Caribbean coral reefs were populated by a much more cosmopolitan fauna than what exists today (Frost 1977; Frost and Weiss 1979). Throughout the Miocene, the Greater Antilles were being torn apart and twisted by both tensional and compressive forces between the opposing Caribbean and Atlantic Plates (Pindell and Barrett 1990; Masson and Scanlon 1991). This has resulted in a leftlateral offset of similar formations on different islands (e.g., the Ponce/Aymamon Formations on Puerto Rico and the Kingshill on St. Croix), as well as counterclockwise rotation of many individual islands (Reid et al. 1991; Gill et al. 2002). On both St. Croix and Puerto Rico, transtensional basins received shallow-water carbonates derived from shallow-water reefs of unknown origins.