Late Basalt Fm

Named by Woodring (1957). Some of the basalt in the southern part of the Canal Zone consists of remnants of flows and the undifferentiated volcanic rocks in the southwestern part of the map area include much basaltic lava (Figure 1&2).

[Figure 1. Geological map of part of Central Panama showing the location of the Late Basalt Fm. Modified from Montes et al. (2012b).]

[Figure 2. Geologic map of the southern Panama Canal Basin adapted from Stewart and Stewart. Symbols indicate the location of geochemical samples. Boxes denote the location of high-resolution mapping areas. Map from Farris et al. (2017).]

Synonym:

The Late Basalt Fm (Figure 3, 4&5) is composed of basaltic to basaltic andesite sills, dikes and flows. The sills and dikes intrude locally into the Culebra Fm, Cucaracha Fm and Pedro Miguel Fm, and where present, bedded lava flows occur conformably on top of the Pedro Miguel Fm pyroclastic units. In addition, the Late Basalt Fm occurs over a large area west of the Panama Canal. The unit can be differentiated from basaltic lava flows within the Pedro Miguel Fm both in terms of stratigraphic position, structural configuration, and texture / composition. The Late Basalt is most identifiable as larger sills / plugs that are several hundred meters thick and can be up to 500±1000 m in diameter. These sills often have well developed, near vertical columnar jointing in the upper parts of individual intrusions. The Late Basalt Fm is the youngest volcanic unit along the southern Panama Canal (Farris et al. (2017)).

Thickness: 135m

[Figure 3. Columnar basalts within a large sill.]

[Figure 4. Near vertical marginal contact between a sill of Late Basalt Fm and the Cucaracha Fm.]

[Figure 5. A stratigraphic contact between underlying Pedro Miguel Fm pyroclastic deposits and overlying Late Basalt Fm lava flows. Photos from Farris et al. (2017).]

None

Lower/Middle Miocene (Burdigalian-Langhian; >15 Ma) - Farris et al. (2017)

• The Late Basalt Fm is an informal name for the stratigraphically youngest volcanic unit exposed along the Culebra Cut.
• References: Woodring (1957); Stewart et al. (1980); Wörner et al. (2009); Montes et al. (2012b); Farris et al. (2017).