Pedro Miguel Fm
Type Locality and Naming
The type region is the Pedro Miguel area. Recognized along and near Panama Canal from Pedro Miguel area to Pacific terminus. The Hodges Hill location contains what is considered to be the "type" volcanic stratigraphy for the Pedro Miguel Fm (Figure 1).
[Figure 1. Geologic map and cross-section of Hodges Hill. Hodges Hill is the "type" Pedro Miguel Fm 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 BB' are derived from structural measurements and Lutton and Banks (1970) drill core data. See Figure 034 for unit key. See Figure 033 for location. Map and Cross-Sections from Farris et al. (2017).]
Synonym: Pedro Miguel Agglomerate Member (of Panamá Fm)
Lithology and Thickness
The Pedro Miguel Fm also has a definite sequence of volcanic sub-units. These sub-units were initially defined based on field mapping and observations at Hodges Hill (Farris et al. (2017)). The stratigraphy observed at Hodges Hill contains the following five sub-units from oldest to youngest: 1) Initial silicic layered pyroclastic deposits, 2) Basaltic lava flows, 3) Steeply dipping welded tan pyroclastic deposits with well developed bedding planes, 4) Massive pyroclastic deposits composed of angular fragments and large remobilized blocks (1+ m), and 5) Shallowly bedded black to gray pyroclastic deposits composed of angular fragments. These five units are found at Hodges Hill, and are present to a lesser degree at other locations. The most common sequence is the transition from stratigraphically lower layered and highly welded tan pyroclastic deposits to the overlying massive and highly fragmented black pyroclastic deposits (Farris et al. (2017)).
The volcanic agglomerate and the basalt flows indicate that this area began to experience volcanism before the formation of the delta (Kirby (2006)). It was deposited after the accumulation of the continental paleosols of the upper part of the Cucaracha Fm, and represents the youngest lithostratigraphic unit in the Culebra Cut area (Montes et al. (2012b); Kirby et al. (2008); MacFadden et al. (2014); Rincón et al. (2015a).
Several areas of the Pedro Miguel Fm were mapped by Farris et al. (2017) along the Culebra Cut, where they were interpreted as maar-diatreme pyroclastic pipes with large basaltic sills and bedded tuffs. Buch et al. (2019)'s detailed lithostratigraphic observations of these areas and the Pacific locks exposures provide additional constraints on volcanic processes associated with this unit, showing that it is mostly composed of tuff cones and large mafic intrusions, with only limited evidence for diatremes. Volcanic edifices of the Pedro Miguel Fm are predominantly composed of basaltic to basaltic andesitic tuffs and large mafic intrusions with olivine ± pyroxenes ± plagioclase phenocrysts.
Due to geochemical and age similarities and apparent parental linkage of the tuffs and mafic intrusions, the Pedro Miguel unit is subdivided into Tuff and Basalt Members. Volcaniclastic deposits of the Pedro Miguel Tuff Member are predominantly composed of fine tuff to primary volcanic breccias with a distinctive layered structure. Low angle cross-stratification is common in fine tuff to coarse lapilli-tuff whereas the primary volcanic breccias are generally matrix-supported and massive (no grading).
Accidental clasts in the tuff include larger benthic foraminifera and red algae in the Pacific locks area, suggesting reworking from an underlying shallow-marine (La Boca?) unit. Very rare lava flows are locally interbedded with the tuffs. The occurrence of mafic ash, small accretionary lapilli and low angle cross-stratification in the tuffs, with only very rare lavas, are compelling evidence for common phreatomagmatic to rarer strombolian eruption deposits in the Pedro Miguel Fm.
Peperitic textures (a range of ductile to brittle structures typical of interaction of wet material (here tuff) with magma) were observed in some of the Pedro Miguel dykes that crosscut older units of the Culebra Cut At Cerro Hodges (Culebra Cut) and Cerro Fabiana (Pacific locks area) new excavations reveal large scale (100–500 m wide) volcanic structures with mega lenses of tuff embedded in peperitic sills (Buchs et al. (2019)).
Thickness: 355 m. The top of the Pedro Miguel Fm is not present, as it has been removed by erosion in the Panama Canal area. (Kirby (2006)). The thickness of the unit has significant spatial variability, but is locally greater than 200 m. The thickest portions of the unit occur in locations that exhibit inward dipping bowl-shaped stratigraphy and are interpreted as individual volcanic breccia pipes or maars. Away from these locations, the unit thins considerably (Farris et al. (2017)).
[Figure 2. Inward dipping basaltic lava flows and welded pyroclastic deposits in the Pedro Miguel Fm. Photo from Farris et al. (2017).]
Relationships and Distribution
Lower contact
Upper contact
Regional extent
GeoJSON
Fossils
Age
Depositional setting
Additional Information
References:
- Thompson (1943).
- Woodring & Thompson (1949). Lens of pyroclastics formerly referred to Las Cascadas agglomerate. In type region, pyroclastics overlie Cucaracha Fm, but lower part apparently equivalent to upper part of Cucaracha sections. Farther south, pyroclastics occur as tongue in basal part of La Boca Fm. Thickness varies; maximum averages about 300 feet.
- Woodring (1955). Rank reduced to Member status in Panamá Fm.
- Woodring (1960). The Pedro Miguel agglomerate is now interpreted as a relatively thick lenticular agglomerate in the Panamá Fm. It is recognized along and near the Panama Canal from the Pedro Miguel area (the type region) to the Pacific terminus. The average maximum thickness is 300 feet [92m]. According to the stratigraphic relations, it is of the same age as the upper part of the Cucaracha Fm and the lower part of the La Boca marine Member of the Panamá Fm.
- Stewart RH, Stewart JL, Woodring WP (1980). It is unclear when the Pedro Miguel was termed as a “Formation”, however on this 1980 map, it is now named “Pedro Miguel Fm”.
- Kirby (2006 & 2008); Montes et al. (2010); Head et al. (2012); Montes et al. (2012b); MacFadden et al. (2014); Rincon et al. (2015a); Farris et al. (2017); Buchs et al. (2019).