Isla Desecheo is in Mona Passage between Puerto Rico and Hispaniola. The bedrock, 700-1,100 meters of middle Eocene volcaniclastic rocks, consists chiefly of dacitic volcanic sandstone, fine-grained volcanic breccia, and mudstone. Deposition in at least moderately deep water is indicated by the abundance of graded beds and by the fact that interbedded mudstone contains exclusively planktonic faunas. Sparse rounded grains and shallow-water fossil debris in the graded beds indicate that the volcaniclastic debris came from volcanoes in shallow seas. Steep submarine slopes are suggested by submarine-slide deposits and penecontemporaneous deformation structures. The rocks are extensively zeolitized. The southern two-thirds of the island, south of an eastwest vertical fault, has west-trending nearly isoclinal folds which are overturned to the north. North of the fault, the main structural feature is a gentle anticline which plunges south. The contrasting structural features are believed to represent different tectonic levels that are separated by a low-angle thrust fault. The strongly deformed beds may be the remnant of a gravity-glide sheet that was eroded from the northern part of the island. Isla Desecheo lies along the seaward projection of the great southern Puerto Rico fault zone, which is, perhaps, the major fracture zone of Puerto Rico. Large-scale transcurrent and vertical faults and associated gravity-glide structures seem closely related in age and geometry to similar features in Hispaniola and Cuba. Surficial deposits include marine terrace deposits, colluvium (in part phosphate cemented), and beach deposits. There is evidence of two stillstands of the sea, one at 12-13 meters and the other at about 2 meters higher than present sea level. The higher level is probably of Pleistocene age and the lower, of Holocene age, perhaps about 3,300 years before the present. Tectonic uplift is believed responsible for the 2-meter level and, therefore, at least in part for the 12-13-meter level.

Fossils

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Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/Desecheo_Island
Wikidata ID:	Q1200572
GeoNames ID:	7309526

References