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Bohio Formation, which contains marine and non-marine conglomerate, tuffaceous
sandstone and siltstone (Escalante, 1990; Woodring, 1957-1982). Stratigraphically
higher are the Oligocene Bas Obispo and Las Cascadas formations (L.C.F.), both of
which consist of agglomerate and tuff (Woodring, 1957-1982). Conformably overly-
ing the L.C.F. is the lower Miocene Culebra Formation (this study), which contains
marine mudstone, sandstone, limestone, conglomerate, and lignite. Portions of the La
Boca, Alhajuela and Caimito formations are correlative with the Culebra Formation,
as suggested by this study and previous lithostratigraphic and biostratigraphic stud-
ies (Jones, 1950; Van den Bold, 1972; Woodring, 1957-1982; Woodring and Thompson,
1949). The lower to middle Miocene Cucaracha Formation overlies the Culebra Forma-
tion and consists of subaerial claystone, sandstone, conglomerate, and lignite, all
showing paleosol development (Retallack and Kirby, 2007). The middle Miocene
Pedro Miguel Formation (P.M.F.) overlies conformably the Cucaracha Formation and
contains basalt and agglomerate. Stratigraphically higher is the upper Miocene Gatun
Formation (Coates, 1999), which contains marine siltstone, sandstone and conglomer-
ate (Collins et al., 1996; Escalante, 1990). East of the city of Colon, the Gatun Forma-
tion overlies nonconformably unnamed Cretaceous volcanic rocks; whereas, west of
Colon, the Gatun Formation overlies unconformably the Caimito Formation (Coates,
1999). The Gatun Formation is overlain disconformably by the upper Miocene Chagres
Formation (Coates, 1999), which consists of conglomeratic sandstone and a basal co-
quina of the Toro Member (Collins et al., 1996). Unconformably above the Tertiary
formations are unconsolidated Quaternary deposits, informally known as the “Pacifi c
muck” and “Atlantic muck” (Woodring, 1957-1982).
Stratigraphic Models for the Gaillard Cut
Hill (1898) was the first to systematically name and describe formations along the
Gaillard Cut (Figure 4). MacDonald (1913, 1919) later named and described several
formations in the Panama Canal Basin, including the Las Cascadas and Cucaracha
formations. Woodring and Thompson (1949) formally named and described the Pedro
Miguel and La Boca formations. They also placed the E.L. Member within the Cul-
ebra Formation. Based on field work by R. H. Stewart of the Panama Canal Company,
Woodring (1964) later restricted the Culebra Formation by placing sections containing
the E.L. (i.e., the lower two-thirds of the Culebra Formation) into the La Boca Forma-
tion, which he considered younger than the Cucaracha Formation. He kept the upper
one-third of the Culebra Formation stratigraphically below the Cucaracha Formation.
Woodring (1964) did not state explicitly the reasons or evidence for this revision in
the stratigraphy of the Gaillard Cut, only that the new evidence was derived from drill
cores made by R. H. Stewart. Writing in 1964, Woodring (1964) stated that (p. 244):
“After the drilling along the Empire Reach … got under way, R. H. Stewart, geologist
of the Panama Canal, soon realized that the geology of the northwestern part of the
Gaillard Cut area had been misinterpreted.” Stewart et al. (1980) later hypothesized
interfingering relationships between the Cucaracha and L.C.Fs., as well as between
the Pedro Miguel and La Boca formations, in order to justify placement of sections
containing the E.L. within the La Boca Formation (Figure 4) (Graham et al., 1985).
However, these interfingering relationships are not apparent in outcrop exposures or
