Geoscience Reference
In-Depth Information
province into separate and distinct Pacifi c and Caribbean communities (Jackson et al.,
1993, 1996; Vermeij, 1978; Vermeij and Petuch, 1986; Woodring, 1965, 1966). The
formation of the Isthmus of Panama also ultimately led to profound changes in global
climate (Weyl, 1968) by strengthening the Gulf Stream and thermohaline downwelling
in the North Atlantic (Keigwin, 1982).
Although extensive study has constrained the timing of isthmian formation (Coates
et al., 1992, 2003, 2004; Collins and Coates, 1999; Duque-Caro, 1990; Emiliani et al.,
1972; Keigwin, 1978), the paleogeographic nature of southern Central America be-
fore the isthmus is still disputed. Paleobathymetric and other geologic evidence from
depositional basins suggests that southern Central America arose slowly from bathyal
depths during the Neogene as a result of the collision between the Panama microplate
and the South American plate (Coates et al., 2003, 2004; Collins et al., 1995), suggest-
ing that the volcanic arc during the Miocene consisted of an archipelago of volcanic
islands that was slowly uplifting through the Neogene until the ultimate formation of
the isthmus (Coates and Obando, 1996; Coates et al., 1992, 2003, 2004; Collins et al.,
1996). For example, Coates et al. (1992) stated that (p. 816): “It is likely that during
the late Neogene the Chorotega and Choco blocks formed an archipelago and there
were frequent marine connections between the Caribbean and the Pacifi c (Duque-
Caro, 1990b, his Figure 9). The topographic, tectonic, and regional geologic evidence
strongly suggests that the archipelago stretched from westernmost Costa Rica to the
Atrato Valley in Colombia …” However, most of the evidence suggesting slow uplift
of the volcanic arc from bathyal depths is derived from depositional basins that lie
peripheral to the main axis of the volcanic arc in southern Central America (Figure 1).
Figure 1.
Location of the Panama Canal Basin and other depositional basins in southern Central
America.
An alternative view is that the main axis of the volcanic arc had already arisen
above sea level by the early Neogene, which would effectively make Panama a peninsula
of Central America by this time (Whitmore and Stewart, 1965). Evidence supporting this
latter view comes from land mammal fossils found in the Miocene Cucaracha Forma-
tion exposed in the Gaillard Cut of the Panama Canal near the center of the Panama
Canal Basin (Table 1). Land mammals with only North American affi nities and
