Institution

(Table 8.1). The interbedded silts and silty lutites below
Datum A are enriched in carbonate, montmorillonite, and
clinoptilolite, and thus form an assemblage which may have
been derived 1 argely from the vol cani c ashes and carbonate
banks of the Greater Antilles. In the overlying transparent
layer, however, these components are rare, and they are
replaced by a northern assemblage of minerals (quartz, mica,
chlorite and plagioclase), which suggests current transpor-
tation of sediment. The low carbonate content of the
homogeneous 1 uti tes in the transparent 1 ayer may al so
reflect increased dissolution by the cold abyssal currents.
As the juvenile Western Boundary Undercurrent
encountered the elevated basement ridge structure which is
presently under the eastern Greater Antilles Outer Ridge,
and as it interacted with the AABW entering from the south,
fine-grained sediments probably were deposited to form the.
earliest unit of the transparent layer (Fig. 8.2). This
type of deposition was ubiquitous over the eastern sector of
the Greater Antilles Outer Ridge, the north slope of the
i nci pi ent Puerto Ri co Trench, and the northern conti nental
slope of Puerto Rico (south slope of the trench). It is
proposed here that the sediment cover deposited by currents
on the north slope of the Puerto Rico Trench during middle
to 1 ate Eocene ti me represents the thi n transparent 1 ayer
which presently extends beneath the sediments of the Puerto
Trench Abyssal Plain (Fig. 8.2) (Bunce and Hersey, 1966;
227
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