10. Sites 143 and 144 - Deep Sea Drilling Project

SHIPBOARD SCIENTIFIC PARTY
I0"
8°
56° 54 C
200"
53 C 52° 51 c
' CIRCE 10
GLOMAR CHALLENGER 14
KANE 9
KONINKLIJKE SHELL
51"
Figure 1. Location map for Sites 143-144. Contours in nominal fathoms, modified from a map supplied by Koninklijke Shell
(pers. comm.j. Letters key seismic profiles shown in Figures 2, 3 and 4.
samples all indicate that progressively older strata outcrop
along increasing depths of the northeast flank of the
Demerara Rise. Resistant layers are suggested by small
topographic terraces (Figure 4) along the flank. Care was
taken to locate the sites slightly above such terraces to help
ensure the presence of sufficient soft material to spud
into.
Privileged seismic data provided by Shell (Figure 2)
indicate that a prominent angular unconformity exists at a
depth of about 400 to 500 meters below the flat surface of
the Demerara Plateau. The surface defining the unconformity
is widespread and now lies at a depth of about 3000
meters below sea level. The considerable relief on the north
flank and our knowledge of the precise water depth at the
drill site allows us to accurately locate Site 144 in relation
to the Shell seismic profile and the surface of the
284
unconformity. A detailed interpretation of the Shell seismic
profile is shown as part of the composite Figure 5.
Seismic Reflection Data: Koninklijke/Shell Challenger
Major reflector 0.32 sec None
(top of unconformity)
Reflector 0.80 sec
The Demerara Rise was approached from the southeast
to a point about 5 km from proposed Site 143. The ship's
course was then altered to 225 to duplicate one of the
SHELL geophysical lines. It was intended to make a
continuous traverse from the flat basin floor onto the
northern slope of the Demerara Rise, however, the best
available topographic base map for this area was in error
and the Challenger turned at the base of the slope instead
of within the flat basin floor.
10°
6°