Caimito Fm

The Caimito Fm takes its name from a construction-period junction on the Panama Railroad near the Darien station (of 1958), 8 kilometers west of Gamboa. The formation was named by MacDonald (1913a, p. 569, 1913b). The name was proposed but not adequately defined. The age was shown as Oligocene on plate 69.

Though no type locality was specified in MacDonald (1913a, 1913b), he evidently intended Caimito and its vicinity to be the type region (in those days, Caimito, or Caimito Junction, was located on the present alignment of Panama Railroad and Darien.). It is not a good type region, but the characteristic lithology is shown there and, according to the regional relations near Darien, it is evident that the Caimito overlies the Bohío Fm (Woodring (1958)

Synonym: Caraba Facies

The Caimito Fm is a series of tuffs (here acidic tuff, the predominant constituent of this formation), tuff-agglomerates, tuff-conglomerates, and limestones, all thinly and thickly bedded and closely to moderately jointed. All are probably marine. Some beds contain sparse, poorly preserved, marine megascopic and microscopic fossils. The attitude of the beds is highly variable due to cross-lamination, faulting, and folding. Dips range from 5° to over 40°. The formation is divisible lithologically into three Members: lower, middle, and upper (Jones S.M. (1950))

Lower Member: Formerly designated "basal" phase (Jones (1947b), p. 24), is a tuffaceous sandstone-conglomerate of local extent containing pebbles, cobbles, and boulders of basalt and pebbles of tuff. The basalt fragments are reworked from the underlying Bohío Fm. All exposures are highly weathered indicating that this member of the Caimito Fm either weathers very deeply upon exposure or was completely weathered during its deposition. It is present only along the bottoms and side slopes of the Bohío surface where the pebbles, cobbles, boulders, and some of the sand weathered out of the Bohío Fm, were reworked and deposited with the first of the tuff which marked the new era of deposition. It is probably a marine facies of the Las Cascadas Fm.

Middle Member (also called “Quebrancha Member”): Formerly designated "lower phase" (Jones (1947b), p. 25), is a series of slightly fossiliferous tuffaceous sandstones and limestones. This member may be local and may intergrade in part with the lower member although the former is more widespread. But whether or not it was originally deposited throughout the Gatún Lake area has not been determined. The limestone beds within it are discontinuous. The zones of abundant Lepidocyclina candid, L. pancanalis, and L. vaughani appear in this member and are found on the peninsula jutting into Zetek Bay, Barro Colorado Island, in the Panama Railroad cut at the east side of Bohío Peninsula, and at other points.

Upper Member: (Jones (1947b, p. 25) It is a widely distributed series of tuffs and tuff-agglomerates, with sandy limestone beds interspersed throughout at irregular intervals. It is the thickest, most widely distributed part of the formation. When sufficiently detailed field mapping, laboratory study, or core drilling can be undertaken, it may be mappable as one or more other formations. The entire formation appears to be increasingly uniform and fine grained toward the northwest, away from Barro Colorado and Bohío Peninsula. In these areas it is essentially a tuff-siltstone series.

Thickness: At least 1,000 feet [300 m] (Woodring (1960))

References:

• Berry (1921) studies some plants from the Caimito Fm
• Olsson (1942b). As shown on correlation chart, comprises (ascending) Chilibrillo, Caimito, and Alajuela.
• Thompson (1943). MacDonald (1913a, 1913b) considered the Caimito to overlie immediately the Emperador Fm. Recent studies have indicated that this is incorrect, and that the formation bears closer affinity to the Panamá tuff. These two formations overlap many of the Oligocene beds within the Pacific area of the Canal Zone, locally lying upon the Emperador and thin only because of transgressive overlap. Gradational with overlying Panamá tuff.
• Woodring (1949). Formation name was introduced by MacDonald (1913a, 1913b) but was not properly defined then or later (MacDonald (1919), U.S. Natl. Mus. Bull. 103). Type region was not specified. Most of MacDonald’s brief description of formation is description of strata on Pacific side of Canal Zone now assigned to La Boca Fm. MacDonald’s statement that the Caimito overlies Emperador limestone—now assigned to Member rank in Culebra Fm—was based on misidentification of limestone in the La Boca Fm and in the Caimito itself. CaimitoFmand Emperador limestone Member of Culebra are nowhere in contact. On account of these confusing formation assignments, published data on fossils of Caimito Fm and Culebra Fm and Emperador limestone Member are misleading or erroneous. In Madden Basin, formation comprises (ascending) unnamed calcareous sandstone-siltstone, unnamed pyroclastic clay, Chilibrillo limestone, unnamed calcareous sandstone, and Alhajuela sandstone Members. In Quebrancha syncline, comprises (ascending) Quebrancha limestone Member and calcareous siltstone Member. In Gatún Lake area and Caribbean coast, Canal Zone, comprises three unnamed members. Thickness as much as 1,000 feet. Overlies Bohío Fm. Formation in Madden Basin includes considerable time span—late Oligocene to late early Miocene, or possibly early middle Miocene.
• Jones S.M. (1947); Jones S.M. (1950, pgs 900-901); Woodring & Thompson (1949); Woodring (1957, pgs 28-31, 32-34); Woodring (1958, 1958, 1960, 1973, 1982); Keroher et al. (1966); Montes et al. (2010);