Geology of Isla Desecheo, Puerto Rico, with notes on the great southern Puerto Rico fault zone and Quaternary stillstands of the sea

Abstract

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.

Geomorphology of Lake Lisan terraces along the eastern coast of the Dead Sea, Jordan

“…This could be due to a gradual drop of the lake level in the gently-sloping areas, which allows the lake water to remain over the terraces for a longer time and therefore sort the surface deposits better. This is consistent with the conclusion of Seiders et al (1970), who reported that the rapid retreat of sea level allowed the surface deposits of the terraces to remain, while the slow retreat allowed removal and sorting of the terrace deposits by wave-reworking and surface runoff. The existence of a thick layer of fine deposits covering a layer of colluvium in some terraces may indicate a temporal transgression of the lake.…”

Extension in Mona Passage, Northeast Caribbean

“…Pre-Oligocene rocks are known to crop out along the crest and south wall of Desecheo Ridge (Nagle et al, 1978) and form the bulk of Isla Desecheo . Seiders et al (1972) assigned the rocks on Desecheo, which are chiefly deformed dacitic volcanoclastics rocks (volcanic sandstones, mudstones, and conglomerates) with secondary interbedded calcareous foraminiferal mudstones and sandstone, to the Eocene Río Culebrinas Formation. The Río Culebrinas Formation is a unit of the Eocene Cerrillos Belt (Dolan et al, 1991), a sequence of turbidite facies that crops out along the northwest coast of Puerto Rico along the Rincon Peninsula.…”

A workshop on Reducing potential losses from earthquake hazards in Puerto Rico

“…During this time period, and the following 20 million years, two major fault systems, the Great Northern and Southern Puerto Rico fault zones were active, displacing rocks on either side in a left-lateral sense (Briggs, 1968, Seiders et al, 1972 Expected Long-term Seismic Activity…”