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good correlation between channel pattern and sediment load, such that the sandstone/
shale ratio provides a clear view of stream type, where meandering channels have
relatively low ratios and braided channels have high ratios (Pettijohn et al., 1987)).
Thick sequences of claystone represent fl ood-basin deposits on the coastal delta plain.
Most intervals of claystone show some evidence of soil development (Retallack and
Kirby, 2007). Evidence for paleosols include horizons of calcite and barite nodules,
rhizoconcretions, drab-haloed root traces, mottling, relict bedding, gradational con-
tacts between soil horizons C, B, and A, and abrupt contacts between soil horizon A
and overlying sediment (criteria of (Retallack, 2001)). Paleosols indicate periods of
stability in between fl uvial events of 1,000-10,000 years when soils developed on
fl ood-basin or channel deposits (Retallack, 2001). The four lignite interbeds represent
histosols of tidal or poorly drained distributaries that penetrated the coastal delta plain,
where thick vegetation resulted in the accumulation of much organic matter into layers
of peat within marshes (Reineck and Singh, 1975). The single interbed of welded tuff
represents a pyroclastic, ash-fl ow deposit (ignimbrite) produced by a nearby explo-
sive eruption. Conformably overlying the Cucaracha Formation is a basalt fl ow of the
P.M.F. (Section 8). Underlying claystone in the Cucaracha Formation shows baking
and the overlying basalt shows hydrothermal alteration.
Land Mammal Biostratigraphy
We found fossils of land mammals throughout the Cucaracha Formation. Land mam-
mal fossils of the peccary cf.
Cynora
sp., the artiodactyl
Paratoceras wardi
, the ore-
odont
Merycochoerus matthewi
and the rhinoceroses
Menoceras barbouri
and
Floridaceras whitei
were found in Sections 6, 7, and 8 (Figure 6). Taken together,
the age of these land mammals indicates a latest Arikareean to middle Barstovian age
(19.5-14 Ma), with a middle Hemingfordian age (18-17 Ma) likely (Figure 10).
CONCLUSIONS
Lithostratigraphic, biostratigraphic, and Sr chemostratigraphic analyses demonstrate
for the first time that the main axis of the volcanic arc in southern Central America
more than likely existed as a peninsula connected to northern Central America and
North America for much of the Miocene. The Culebra Formation dates from 23-19
Ma, with the E.L. dating from 21 Ma. The overlying Cucaracha Formation dates from
19 to possibly 14 Ma. What has been called the La Boca Formation underlies, not
overlies, the Cucaracha Formation. We, therefore, re-interpret the La Boca Formation
(with the E.L.) as the lower part of the Culebra Formation, as originally proposed by
Woodring and Thompson (1949).
Our revised stratigraphy for the Gaillard Cut shows that the Culebra Formation
represents a transgressive-regressive, marine sequence with environments that in-
clude, from lowermost to uppermost: lagoon, fringing reef, neritic, upper bathyal,
and prograding delta. Bathyal sediments in the upper member of the Culebra Forma-
tion suggest that a short-lived strait may have existed across the Panama Canal Basin
sometime between 21 and 19 Ma. The overlying Cucaracha Formation represents a
coastal delta plain with environments that include fl uvial channel, overbank, fl ood-
plain, and distributary channel marsh, all with extensive development of paleosols
