Geoscience Reference
In-Depth Information
Chapter 12
Canal
Michael Xavier Kirby, Douglas S. Jones, and Bruce J. MacFadden
INTRODUCTION
Before the formation of the Central American Isthmus, there was a Central American
Peninsula. Here we show that southern Central America existed as a peninsula as early
as 19 Ma, based on new lithostratigraphic, biostratigraphic, and strontium chemostrati-
graphic analyses of the formations exposed along the Gaillard Cut of the Panama Canal.
Land mammals found in the Miocene Cucaracha Formation have similar body sizes
to conspecific taxa in North America, indicating that there existed a terrestrial con-
nection with North America that allowed gene flow between populations during this
time. How long did this peninsula last? The answer hinges on the outcome of a strati-
graphic dispute: To wit, is the terrestrial Cucaracha Formation older or younger than
the marine La Boca Formation? Previous stratigraphic studies of the Panama Canal
Basin have suggested that the Cucaracha Formation lies stratigraphically between the
shallow-marine Culebra Formation and the shallow-to-upper-bathyal La Boca Forma-
tion, the latter containing the Emperador Limestone (E.L.). If the La Boca Formation
is younger than the Cucaracha Formation, as many think, then the peninsula was short-
lived (1-2 million years (m.y.)), having been submerged in part by the transgression
represented by the overlying La Boca Formation. On the other hand, our data support
the view that the La Boca Formation is older than the Cucaracha Formation. Stron-
tium dating shows that the La Boca Formation is older (23.07-20.62 Ma) than both
the Culebra (19.83-19.12 Ma) and Cucaracha (Hemingfordian to Barstovian North
American Land Mammal Ages; 19-14 Ma) formations. The E.L. is also older (21.24-
20.99 Ma) than the Culebra and Cucaracha formations. What has been called the “La
Boca Formation” (with the E.L.), is re-interpreted here as being the lower part of the
Culebra Formation. Our new data sets demonstrate 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, which has
profound implications for our understanding of the tectonic, climatic, oceanographic
and biogeographic history related to the formation of the Isthmus of Panama.
The paleogeography of Central America has changed profoundly over the past
30 m.y., from a volcanic arc separated from South America by a wide seaway, to an
isthmus that connected North and South America by 3 Ma (Coates and Obando, 1996;
Coates et al., 1992, 2003, 2004; Duque-Caro, 1990). The formation of the Isthmus of
Panama was important because it allowed the mixing of terrestrial faunas between the
two continents (Webb, 1985), as well as physically separating a once continuous marine
