Lower Miocene Stratigraphy Along the Panama Canal - Earth Science: New Methods and Studies - page 172

similar body sizes to conspecifi c taxa in North America suggest a terrestrial connec-
tion with North America by the early Miocene (Kirby and MacFadden, 2005; MacFadden,
2006; MacFadden and Higgins, 2004; Whitmore and Stewart, 1965). The purpose of
the present study is to further resolve the Neogene paleogeography of southern Cen-
tral America by placing the Cucaracha land mammals into a stratigraphic framework
through lithostratigraphic, biostratigraphic, and strontium chemostratigraphic analy-
ses that test long-standing hypotheses concerning the stratigraphy of the Gaillard Cut.

Table 1. Land mammal taxa from the Gaillard Cut Local Fauna, Cucaracha Formation, Panama.

Order Family Genus & species Common name Biogeographic affi nity

Rodentia − Texomys steworti Geomyoid rodent North America

Carnivora Canidae Tomarctus brevirostris Dog North America

Carnivora Amphicyonidae or

Hemicyonidae

− Bear dog North America

Artiodactyla Tayassuidae cf. Cynorca sp. Peccary North America

Artiodactyla Oreodontidae Merycochoerus matthewi Oreodont North America

Artiodactyla Protoceratidae Paratoceras wardi Protoceratid North America

Perissodactyla Equidae Anchitherium clarencei Horse North America

Perissodactyla Equidae Archaeohippus sp. Horse North America

Perissodactyla Rhinocerotidae Menoceros barbouri Rhinoceros North America

Perissodactyla Rhinocerotidae Floridaceras whitei Rhinoceros North America

Excavation of the Gaillard Cut during the original construction of the Panama
Canal exposed lower Neogene sediments of the Panama Canal Basin (Figures 2 and
3). As upper Neogene volcanic rocks cover much of the Panama Canal Basin, the
Gaillard Cut offers a window into the underlying Oligocene-Miocene rocks beneath
this volcanic cover (Figure 2). Although this excavation exposed hundreds of meters
of section, the structural complexity caused by extensive faulting has obscured the
stratigraphic relationships between the various formations, such that only a portion
of one, or at most two, formations are present in any given fault-bounded block (Figure
3). The most recently published stratigraphy and geologic map for the Panama Canal
Basin indicates that the Cucaracha Formation lies stratigraphically above the shallow-
marine Culebra Formation and below the shallow-to-upper-bathyal La Boca Forma-
tion (Stewart et al., 1980). If this stratigraphic arrangement is correct, then we may
conclude that the peninsula containing North American land mammals was short-lived
in the early Neogene (1-2 m.y., based on the temporal duration of paleosols in the
Cucaracha Formation (Retallack and Kirby, 2007)), having been submerged in part
by the marine transgression represented by the overlying La Boca Formation (Whit-
more and Stewart, 1965). Earlier stratigraphic arrangements, however, placed strata
presently in the La Boca Formation not above the Cucaracha Formation, but below
it (MacDonald, 1919; Van den Bold, 1972; Woodring and Thompson, 1949)). Given
this stratigraphic arrangement, the marine transgression represented by the La Boca
Formation occurred before deposition of the Cucaracha Formation, which would indi-
cate that there is no evidence for submergence of the Central American Peninsula until