Fig 1.
Geologic map of the southern Panama Canal basin adapted from Stewart and Stewart [30].
Symbols indicate the location of geochemical samples. Boxes denote the location of high resolution mapping areas.
Fig 2.
Stratigraphic column of volcanic and sedimentary rocks exposed along the southern Panama Canal.
Depicted total thickness of section is 1218 m. Unit thicknesses are from this study, Montes et al. [33], Kirby et al. [31], and Lutton and Banks [29]. Most units have lateral variations in thickness and structural complications that make absolute thickness determinations difficult. Therefore minimum well-constrained unit thicknesses are shown.
Fig 3.
Photos and photomicrographs of the Bas Obispo Formation.
A) Field photo of the welded basaltic pyroclastic rocks characteristic of the Bas Obispo Formation. B) Photomicrograph of an amphibole phenocryst surrounded by basaltic glass. C) Photomicrograph of plagioclase phenocrysts surrounded by pyroclastic fragments and pockets of basaltic glass.
Fig 4.
Photos and photomicrographs of the Las Cascadas Formation.
A) Field photo of the contact between a welded silicic pyroclastic bed and an ash fall tuff layer typical of the Las Cascadas Formation. B) Ropey andesitic lava C. Photomicrograph of dacitic obsidian layer. Note glass fragment with flow lines surrounded by amorphous glass.
Fig 5.
Field photo of a fossilized tree trunk within the Cucaracha Formation ash fall tuff.
Fig 6.
Field photos and photomicrographs of the Pedro Miguel Formation.
A) Field photograph of inward dipping basaltic lava flows and welded pyroclastic deposits. B) Photomicrograph of a Pedro Miguel lava flow. Note the equal sized sub-to-euhedral clinopyroxene and plagioclase phenocrysts surrounded by plagioclase microlites. C) Photomicrograph of welded pyroclastic deposit. Note stretched glass bands. D) Photomicrograph of fragmented pyroclastic deposit. Note angular welded pyroclastic fragments surrounded by darker fine-grained matrix.
Fig 7.
Field photographs and photomicrographs of the Late Basalt Formation.
A) Columnar basalts within a large sill. B) Near vertical marginal contact between a Late Basalt sill and the Cucaracha Formation. C) A stratigraphic contact between underlying Pedro Miguel Formation pyroclastic deposits and overlying Late Basalt Formation lava flows. D) Photomicrograph from a Late Basalt Formation sill. The strong plagioclase alignment and trachytic texture is characteristic. Also note the interstitial anhedral clinopyroxene.
Fig 8.
Field photographs and photomicrographs of the Panama City Formation.
A) Field photograph of the intermingling of dacitic and andesitic magmas within a shallow intrusive body. B) Euhedral plagioclase phenocrysts surrounded by microlites from the Ancon Hill dacitic plug.
Fig 9.
Geologic map of the Culebra Cut along the Panama Canal.
The map depicts Miocene pyroclastic pipes of the Pedro Miguel Formation, and large basaltic sills and flows of the Late Basalt Formation both of which intrude and are deposited atop the sedimentary Cucaracha Formation. See Fig 1 for map location.
Fig 10.
Geologic cross-sections A-A’, B-B’, and C-C’ across the Culebra Cut.
See Fig 9. for line locations. Cross-sections are derived from structural field observations and constrained by drill core data from Lutton and Banks [29]. These cross-sections clearly show the subsurface thickness of basaltic sills and pyroclastic bodies and provide vertical offset constraints for normal faults. The cross-sections indicate that the Panama Canal is roughly centered on a small structural graben.
Fig 11.
Geologic map and cross-section of Hodges Hill.
Hodges Hill is the “type” Pedro Miguel Formation locality. It is composed of inward dipping pyroclastic deposits and lava flows deposited over multiple episodes of explosive eruptions. The 3-D internal geometry as shown in cross-sections A-A’ and B-B’ are derived from structural measurements and Lutton and Banks [29] drill core data. See Fig 9 for unit key.
Fig 12.
Cartagena Hill geologic map and cross-section.
At this locality, inward dipping Pedro Miguel Formation pyroclastic strata are observed, but the volcanic edifice is partially dismembered by subsequent normal and strike slip faulting. Cross-section A-A’ shows pre-excavation topography. The horizontal dashed line on the cross-section indicates the excavation level on which geologic mapping was conducted.
Fig 13.
This map contains the northernmost exposure of the Pedro Miguel Formation along the Panama Canal. At this location, the Pedro Miguel volcanic edifice has been crushed by faulting to a degree that it is not stratigraphically coherent. A large normal fault separates the Pedro Miguel and underlying Cucaracha and Culebra Formations from the Las Cascadas Formation to the northwest. A large left-lateral strike-slip fault also bounds the Pedro Miguel body to the southeast.
Fig 14.
Comparison of major and trace element chemistry to standards and replicate analyses using multiple labs and techniques.
A) P2O5 vs. SiO2 wt. % standard and replicate analyses, B) MgO vs SiO2 wt. % replicate analyses, C) Replicate analysis of Pedro Miguel Fm. rocks using INAA and ICP-MS, D) Replicate analysis of Bas Obispo Fm. rocks using INAA and ICP-MS, E) Ba vs. La plot of INAA and ICP-MS analyses from the Pedro Miguel and Bas Obispo Fm.’s, F) Ta vs. Yb plot of INAA and ICP-MS analyses from the Pedro Miguel and Bas Obispo Fm.’s,.
Fig 15.
Major element chemistry of the Canal volcanic rocks.
The Wegner et al. [19] Chagres and Miocene arc data are plotted for comparison. A) MgO vs SiO2 wt. % B) FeO* vs SiO2 wt. % C) FeO*/MgO vs SiO2 wt. % D) TiO2 vs SiO2 wt. % E) K2O vs SiO2 wt. % F) Na2O vs SiO2 wt. % G) CaO vs SiO2 wt. % H) Al2O3 vs SiO2 wt. %.
Fig 16.
Trace element chemistry of the Canal volcanic rocks presented via MORB normalized spider diagrams.
A) All Canal volcanic rock analyses. Wegner et al. [19] data also shown for comparison. The Canal rocks fall roughly within the Miocene arc field. B) Trace element data averaged by unit. Note that the units separated into three main groups: Bas Obispo, Pedro Miguel/Late Basalt, and Las Cascadas/Panama City Formations. C) Pedro Miguel Formation data averaged by sub-unit. Note oscillation between high and low trace element concentrations with unit eruptive order.
Fig 17.
Tectonic discrimination diagrams.
A) V vs. Ti diagram of Shervais [38]. This plot sharply divides the Bas Obispo and Pedro Miguel Formations into arc tholeiite and back arc/MORB fields, respectively. B) Hf/Th/Ta diagram of Wood [39]. Pedro Miguel Formation rocks show a continuous trend from arc tholeiite to MORB fields. C) Ta vs Yb diagram of Pearce et al. [40]. This diagram is for rocks of granitic composition. Note Panama City and Las Cascadas Formation rocks trend from the volcanic arc to the ocean ridge granite fields.
Fig 18.
Farris et al. [16] data from Paleocene-Oligocene arc and Miocene and younger arc are plotted in addition to the Canal region analyses (all are INAA data). Wegner et al. [19] ICP-MS data is also shown for comparison. Note the Canal volcanic rocks are located at a transition between the early depleted arc and the younger enriched arc. Also, Canal volcanic rocks show a distinct negative anomaly in fluid mobile elements. A) Hf/Yb vs. Ta/Yb. B) La/Yb vs. Ta/Yb. C) Th/Yb vs. Ta/Yb. D) Ba/Yb vs. Ta/Yb.
Fig 19.
MELTS major element model of Miocene Canal volcanic rocks.
Model runs at 0.1, 0.5 and 1.0 kbar are depicted. Liquidous temperatures are between 1190 and 1100°C. Most Pedro Miguel analyses can be reproduced with melt fractions greater than 0.5, however the most silicic Pedro Miguel rocks require melt fractions of 0.3 and the Panama City Formation rocks require melt fractions less than 0.15, which we consider to be physically unrealistic. A) MgO vs SiO2 wt. %, B) TiO2 vs SiO2 wt. %, C) Na2O vs SiO2.
Fig 20.
A) Fractional crystallization model. This model best reproduces the observed range of Pedro Miguel Formation compositions with the most silicic compositions reached at melt fractions of 0.3. B) Equilibrium crystallization model. This model also reproduces Pedro Miguel compositions, but require lower melt fractions. C) Assimilation fractional crystallization model. This model can reproduce trace element compositions from the most silicic Canal volcanic units (e.g. Las Cascadas Formation) using a Late Basalt Formation assimilant.
Fig 21.
FeO* wt. % vs MgO wt. % used to calculate the tholeiitic index (THI) after Zimmer et al. [48].
The THI can be used to calculate the amount of water in a melt of a given composition. The Bas Obispo Fm. has a THI of 0.88 and a calculated H2O wt. % of 3.31, whereas the Late Basalt has a THI of 1.45 and a calculated H2O wt. % of 0.55, indicating that the Canal volcanic rocks became much dryer and more tholeiitic in the Miocene.
Fig 22.
Schematic model of Canal region magmatic variation due to changing tectonic conditions.
A. Map view tectonic model of Panama in the Oligocene (modified from Farris et al. [16]). This is previous to the collision with South America. B. Map view tectonic model of the Panama arc in the Miocene, after collision with South America. Note the existence of the Canal extensional zone. C. Oligocene arc. Magmatism at this time is defined by deeper hydrous amphibole bearing magmas that form standard arc composite volcanoes underlain by plutonic systems. D. Miocene arc. Volcanism is heavily influenced by the onset of arc perpendicular extension in the Canal region. Magmatism is hot, shallow, anhydrous and occurs dominantly as basaltic sills and inward dipping maar pyroclastic pipes that intrude the Canal Basin.
Fig 23.
Summary of Panama arc evolution from initiation through Pliocene.
The extensional Canal magmatism is unique throughout this period. Note this figure does not contain the modern adakitic magmatism that today dominates western Panama.