Institution

approximately equal to sediment thickness in kilometers if
an average sound velocity of 2.0 km/sec is assumed. Although
available data indicate that Datum A is of middle Eocene
age, the rei s the rea 1 p 0 s sib i 1 i ty t hat t ~ ere f 1 e c tor i s
not a time-line. If this is true, bias in the interpretation
of post-middle Eocene sediment thickness would be expected.
However, the pattern of sediment distribution that emerges is
well defined, and significant departures of Datum A from a
common time-line would be required to alter
following interpretations.
radically the
Except where basement peaks interrupt it, the surface
of the transparent 1 ayer forms the sea floor over the Greater
Antilles Outer Ridge. In general, this surface is not con-
formable to deeper reflectors, so it is unlikely that the
transparent layer represents deposition by normal pelagic
drapi ng of sedi ment over an i rregul ar sea floor.
The thickness of the transparent layer along the north
slope of the Puerto Rico Trench is highly variable, but
virtually everywhere it is thinner than 0.2 sec refleètion
time (Fig. 3.3). Its irregular distribution probably is due
to slumping and faulting along the slope.
The maximum thickness of the transparent layer over the
eastern sector of the outer ridge is about 0.5 sec reflection
time. The axis of thickest sediments runs east-west near
21°151N lat., and it is displaced about 60 km north of the
topographi c crest of the eastern outer ri dge. The trans-
52
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