Geoscience Reference
In-Depth Information
Figure 6.
Lithostratigraphic correlation of stratigraphic Sections 1 to 8 and drill cores logged by R. H.
Stewart of the Panama Canal Company (ECB-5, ECB-3, and GH-10) arranged northwest to southeast
along the Gaillard Cut portion of the Panama Canal. Location of stratigraphic sections and drill holes
in Figure 3. Absolute ages (Ma) are derived from Sr chemostratigraphic analyses (see text). C = cf.
Cynorca
sp. Mm =
Merycochoerus matthewi
. Pw =
Paratoceras wardi
. Mb =
Menoceras barbouri
.
Fw =
Floridaceras whitei
.
Many studies have shown that the Neogene is generally a time of rapidly increas-
ing 87Sr/86Sr in the global ocean and, hence, particularly amenable to dating and
correlating marine sediments using strontium isotopes (Jones et al., 1993; Hodell and
Woodruff, 1994; Hodell et al., 1991; Mallinson et al., 1994; Martin et al., 1999; McArthur
et al., 2001; Miller et al., 1991, 1995; Oslick et al., 1994). We analyzed seven fossil
specimens from the restricted, upper Culebra Formation and the La Boca Formation in
the Gaillard Cut, along the Panama Canal, in order to determine the ratio of 87Sr/86Sr
of the calcium carbonate composing the shell (Table 2). These data allow us to esti-
mate the geologic age for each fossil specimen. For isotopic analyses, we fi rst ground
off a portion of the surface layer of each shell specimen to reduce possible contamina-
tion. Areas showing chalkiness or other signs of diagenetic alteration were avoided.
Powdered aragonite (coral) or low-magnesium calcite (mollusc) samples were drilled
from the interior of each specimen using a hand-held Dremel tool with a carbide burr.
Approximately 0.01-0.03 g of powder was recovered from each fossil sample. The
powdered samples were dissolved in 100 μl of 3.5 N HNO
3
and then loaded onto
cation exchange columns packed with strontium-selective crown ether resin (Eichrom
Technologies, Inc.) to separate Sr from other ions (Pin and Bassin, 1992). The Sr
isotope analyses were performed on a Micromass Sector 54 Thermal Ionization Mass
Spectrometer equipped with seven Faraday collectors and one Daly detector in the
