Geology of the North Slope of the Puerto Rico Trench

Abstract

We use bathymetric and continuous seismic reflection profiles and rocks dredged in 1962 and 1964 to described topography, stratigraphy and structure of the North Slope of the Puerto Rico Trench between 65° 00′W and 66° 30′W. Abundant basalt, chert, pink and green claystone and minor limestone and soft mudstone were dredged from 3200–3700 fms at 20° 16′N, 65° 42′W. The basalt has porphyritic, amygdular or diabasic texture, is partly altered to montmorillonitic clay, contains plagioclase which is mainly labradorite or bytownite, is low in potassium and titanium, and is normally tholeiitic.The tentative stratigraphic succession of the North Slope is as follows: (1) Cenomanian basalt flows, possibly interbedded with chert, claystone and limestone, form the oceanic basement, and have compressional velocity near 5 km/sec. (2) Consolidated sediment, with compressional velocity of 3·0–4·5 km/sec, possibly not present everywhere, lies unconformably on the basement. Age is Late Cretaceous and Early Tertiary, and maximum thickness 300 m. (3) Acoustically transparent unconsolidated sediment mantles the basement and the consolidated sediment. Age is Late Tertiary and Quaternary, compressional velocity 1·5–2·6 km/sec, and maximum thickness 400 m. (4) Acoustically layered flat-lying unconsolidated sediment lies in east-west valleys in the North Slope, and forms the abyssal plain at the bottom of the Trench. Maximum thickness in the valleys is 500 m.Present evidence from echo-sounding and seismic reflection profiles is insufficient to indicate whether the scarps, ridges and valleys of the North Slope were formed by thrusting, normal faulting strike-slip faulting or gravity sliding or from a combination of these processes.