The Role of Downhole Measurements in Marine Geology and ...

35, 3 /REVIEWS OF GEOPHYSICS Goldberg: DOWNHOLE MEASUREMENTS ß 329
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175.3 -
175.4 -
175.5 -
CORE
PHOTOGRAPH
FMS
ORIENTATION
0 60 120 180 240 300 360
- 174
- 175
- 176
Figure 12. Example of the correlation between (left) a core photograph and (right) an FMS image of a
l-cm-thick ash layer in ODP Hole 798B in the Sea of Japan [from deMenocal e! al., 1992]. The FMS images
of four orthogonal traces are oriented to north at 0 ø magnetic declination and represent the ash as a
high-resistivity (white) fiat layer at 174.7 m bsf. The expansion-corrected core depth was shifted by +0.70 m
to match the downhole log. .
example, downhole logs were used to determine zones of logic changes observed from core sections. At most of
abundant ash separated by 150,000- to 200,000-year pe- these locations, however, incomplete core recovery has
riods of marine deposition [Kastens et al., 1987]. made the exact location and thickness of the boundary
Using downhole logs, the boundary between the Cre- difficult to determine. Although the downhole signatures
taceous and Tertiary periods has been studied at eight of the Cretaceous-Tertiary (K-T) boundary are not alsites,
in the south Atlantic, Caribbean, and eastern In- ways distinctive, these data can be critical in determining
dian Oceans. At each site the extinction boundary was the contact and thickness of sedimentary deposits near
determined using biostratigraphic markers and litho- the K-T event. The boundary is often marked by abrupt
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