Bohío Fm

Type Locality is at Bohío Peninsula. Bohío or Bohío Soldado, was a station on original line of Panama Railroad, near southwest end of ridge forming present Bohío Peninsula. Many localities described by early writers, including French quarries at Bohío and excavation at near-by lock site, are covered by Gatún Lake.

Synonym: Bohio Fm; Bohio Conglomerate; Bohío Conglomerate (Vaughan (1918); Wilmarth (1938)); Bujio

The Bohío Fm is characterized by siltstones, sandstones, and sandstone-conglomerates, very hard and massively bedded and jointed. It contains angular to rounded pebbles, cobbles, and boulders, locally up to as much as 6 feet in diameter, in a dark-gray, generally coarse-grained, angular-grained sandstone matrix. Both the sandstone matrix and conglomeratic fragments are notably basaltic. Some tuff-siltstone interbeds as much as 90 feet [28m] thick are present. Exposures of the Bohío Fm show dips of 15° to 20°. The formation is characteristically transected by basalt intrusions ranging in width from a few inches to an observed maximum of 200 feet [61m]. These intrusions are more or less localized, being very numerous, for example, in the vicinities of Gamboa and Darien. They are so numerous and discontinuous that only the large ones are mappable units. The Bohío Fm in localities containing numerous faults and basalt dikes has been highly indurated over broad areas by incomplete fusion and by other igneous effects. From Darien to Gamboa the overall outcrop picture is one of uneven increase in both coarseness of matrix and angularity of fragments of all sizes. At Darien some fused sandstone-agglomerate is present; at the Obispo High locality, some hard, friable sandstone conglomerate is exposed. But eastward from Gamboa, angularity of fragments and induration of the Bohío Fm by basalt intrusives are progressively greater to the extent that at Obispo Point, across the Chagres River Bridge from Gamboa, and at Bas Obispo and eastward therefrom, the rock is considered to be the Bas Obispo Fm (Bas Obispo agglomerate Fm). That the Bas Obispo Fm and Bohío Fm are different facies of the same stratigraphic sequence has become apparent through field mapping along the west bank of the present canal in the vicinity of and opposite from the town of Gamboa. Here numerous rock exposures have demonstrated a gradual change in character from that of the typical Obispo agglomerate to that of the Bohío sandstone-conglomerate. (Jones S.M. (1950)).

Thickness: The thickness of the formation is about 75 to 450 metres (246 to 1,476 ft).

References:

• Vaughan (1918) correlating the “Bohío Conglomerate”.
• Ross and Reeves (1931) gave the name Bohío (originally Bohío Soldado), for a village on the Panama Railroad, located on a bluff overlooking Río Chagres, to the entire group of sediments in the Chagres Valley above Madden Dam, and referred them to the Oligocene.
• The samples of Foraminifera which were collected from Ross & Reeves (1931)’s "Bohío Fm” by Coryell et al. (1937b), however, showed that it included both an upper Eocene and an Oligocene fauna. The Oligocene beds are well exposed near Bohío Switch; the Eocene beds appear farther east along the Río Chagres, near the town of Tranquilla. Therefore, Coryell et al. (1937b) reduced the thickness of the Bohío Fm and named the shale unit the “Tranquilla Shale Fm”. The Bohío Fm is since restricted to include only the Oligocene sediments which overlie the Tranquilla shale (obsolete name; now included in the Gatuncillo Fm) and which outcrop farther downstream the Chagres River.
• Woodring (1949). Formation consists of massive or poorly bedded conglomerate, tuffaceous sandstone, and tuffaceous siltstone. Estimated thickness as much as 1,000 feet. Overlies Gatuncillo Fm; base of the Bohío not exposed along Panama Canal. Underlies Caimito Fm. Grades laterally into Bas Obispo Fm. Upper Eocene and lower Oligocene.
• Jones S.M. (1950);
• Woodring (1957). Basal part of Bohío in Quebrancha syncline contains smaller Foraminifera of early Oligocene age and upper part of formation in Pacific coastal and Gatún Lake areas, respectively, contains late Oligocene larger Foraminifera and mollusks. Whether formation represents so great a time span in each of areas where it crops out is not known at present. It represents, however, more than the early Oligocene age previously suggested (Woodring and Thompson, 1949). That it does not include all of the Oligocene is shown by late Oligocene age of overlying Caimito Fm.
• Woodring (1958, 1959);
• Woodring (1960); The Bohío Fm, consisting chiefly of boulder-conglomerate and greywacke grit, for the most part evidently is nonmarine. In the Quebrancha syncline, a few kilometers east of the Canal Zone, some 80 species of early Oligocene smaller foraminifera have been found in sandy siltstone in the basal part of the Bohío. These fossils include Bolivina alazanensis, Bulimina alazanensis, Globigerina ciperoensis, and Gümbelina cubensis [Chiloguembelina cubensis]. On Barro Colorado Island, in Gatún Lake, subgraywacke in the upper part of the Bohío contains late Oligocene marine fossils: larger foraminifera, including Lepidocyclina canellei, L. vaughani, and Miogypsina antillea, and about 70 species of mollusks, including Globularia aff. G. fischeri, Orthaulax cf. O. pugnax, and Chione cf. C. spenceri. The widely distributed late Oligocene (formerly middle Oligocene) eulepidine orbitoid fauna — Lepidocyclina favosa, L. gigas and associated species — is found in thin lenses of algal limestone in conglomerate of the Bohío at the continental divide a few kilometers east of the Canal Zone.
• Woodring (1964); Keroher (1966); Woodring (1973, 1982);