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Department of Geological Sciences at the University of Florida. The Sr was loaded
onto oxidized tungsten single fi laments and run in triple collector dynamic mode. Data
were acquired at a beam intensity of about 1.5 V for 88Sr, with corrections for in-
strumental discrimination made assuming 86Sr/88Sr = 0.1194. Errors in measured
87Sr/86Sr are better than ±0.00002 (2σ), based on long-term reproducibility of NIST
987 (87Sr/86Sr = 0.71024). Age estimates were determined using the Miocene portion
of Look-Up Table Version 4:08/03 associated with the strontium isotopic age model
of McArthur et al. (2001).
Table 2.
The Sr chemostratigraphic analyses of the Cuelbra Formation, Panama.
Sample
Taxon
Latitude
Longitude
Member
87
Sr/
86
Sr
Std.
error%
Std. error
(external)
Age
estimate
(Ma)
Std. error
(Ma)
Pan8
Coral
9' 04.661 'N
79' 40.627'W
Emperador 0.708386
0.001
0.000023
20.99
0.71
Pan9
Coral
9' 04.661 'N
79' 40.627'W
Emperador 0.708371
0.0008
0.000023
21.24
0.44
Pan6
Pectinid
9' 04.661 'N
79' 40.627'W
Lower
0.708404
0.0008
0.000023
20.62
0.58
Pan7
Pectinid
9' 04.661 'N
79' 40.627'W
Lower
0.708386
0.0008
0.000023
20.99
0.46
Pan10
Bivalve
9' 04.468'N
79' 40.522'W
Lower
0.70825
0.0008
0.000023
23.07
0.53
Pan4
Ostrea sp.
9' 03.099'N
79°39.3SO'W
Upper
0.708502
0.0008
0.000023
19.12
0.42
Pan5
Pectinid
9' 03.099'N
79°39.3SO'W
Upper
0.70845
0.0007
0.000023
19.83
0.39
DISCUSSION
We infer that the Central American Peninsula was not short-lived in the early Miocene,
based on our revised stratigraphy for the Gaillard Cut (Figure 7). We instead find no
evidence for the disruption of this peninsula until 6 Ma, when there is evidence for a
short-lived strait across the Panama Canal Basin (Collins et al., 1996). This conclusion
is different from that of Whitmore and Stewart (1965), who was the first to present
evidence that Panama “was connected to North America by a land area of considerable
size and stability” in the middle Miocene, based on the land mammal fossils from the
Cucaracha Formation (p. 184). They also concluded, however, that the presence of
the Culebra and La Boca formations indicated inundation by the sea both before and
after, respectively, the time when the land mammals arrived to Panama by land from
North America (Whitmore and Stewart, 1965). Returning the La Boca Formation to
the lower part of the Culebra Formation removes the evidence for a transgression after
deposition of the Cucaracha Formation.
The earliest evidence for a terrestrial connection to North America is 19 Ma, based
on an estimated age of 19.12-19.83 Ma of the pectinid bivalves found 2 m below the
land mammal fossils in the upper Culebra Formation (Section 3 in Figure 6). Given
that the upper part of the Cucaracha Formation may be as young as 14 Ma, and that
there are at least 355 m of undated, terrestrial, volcanic rocks of the P.M.F. overlying
the Cucaracha Formation, we think it likely that the peninsula existed in this part of
Central America for much of the Miocene. Low precipitation and temperature esti-
mates derived from the paleosols of the Cucaracha Formation imply a rain shadow
