Chucunaque Fm

Period:

Neogene

Age Interval:

Upper Miocene (Tortonian-Messinian; 10-5.6 Ma) - Coates et al. (2004)

Province:
Eastern Panama

Type Locality and Naming

Sapper (1937) named it informay on a correlation chart for strata in the Darien area. The formation was formally named for the Chucunaque River by Shelton (1952) without defining a stratotype.(See location of Lago Bayano in figure 1)
Coates et al. (2004) propose as stratotype the sequence on the Chucunaque River from south of El Salto to above Caleta on the Tuquesa River (CT on Figure 2 & Figure 3). A reference section is located on the Chico River (CR on Figure 2).

[Figure 1. Map of Lago Bayano, Panama with collecting sites denoted by black circles. Numbers labelling each collecting site refer to the last three digits of the Smithsonian Tropical Research Institute (STRI) field number. Perez et al. (2017).]

Synonym: Chuqunaque Fm (misspelling); Chucunaque River Fm

Lithology and Thickness

The Chucunaque Fm consists of gray weathering, greenish blue to black, blocky to massive, silty claystone and siltstone, with minor thin horizons and stringers of volcanic sandstone. Slabby to oval calcareous concretions are common, and the formation contains abundant gypsum crystals at some horizons. In the north, along the Membrillo River, the lower part of the formation is dominated by cobble conglomerate and cross-bedded sandstone.

The Chucunaque Fm was deposited at an inner neritic depth around the Chico River valley region, and at upper bathyal depths in deeper portions of the Basin (Membrillo River section). It commonly contains oxygen-deficient assemblages. In the Membrillo River and Chucunaque River sections, samples contain characteristic upper bathyal taxa.

In the southeastern-most portion of Lago Bayano (Figure 1), exposed uneroded blocks comprise strongly weathered and sparsely fossiliferous orange mudstone. The islands in the northern portion of southern Lago Bayano contain a more varied range of facies, including fossiliferous, gritty orange sandstone and a fine-grained tuffaceous white sandstone.

Thickness: The thickness of the Chucunaque Fm is not less than 400m in the northwest and not less than about 1200m in the center of the Chucunaque-Tuira Basin.

[Figure 2. Stratotype (CT marked by blue arrow) of the Chucunaque Fm (see continuation on Figure 067). Coates et al. (2004).]

[Figure 3. Reference Section (CR) and continuation of the Chucunaque stratotype (CT) (Figure 066). Coates et al. (2004).]

Lithology Pattern:

Sandy_claystone

Relationships and Distribution

Lower contact

Yaviza Fm or Membrillo Fm. The lower contact of the Chucunaque Fm is nowhere exposed in the area in Coates et al. (2004)’s studied area.

Upper contact

None. The upper contact of the Chucunaque Fm is nowhere exposed in the area in Coates et al. (2004)’s studied area.

Regional extent

Eastern Panama

GeoJSON

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Fossils

Calcified thalassinoid burrows are typical and many levels are packed with clearly visible foraminifera, scattered small mollusks, including cancellarriids, naticids, Tellina, and turrids. Crabs, pteropods and the deepwater Pectén and Palliolum have also been observed (Coates et al. (2004). Perez et al. (2017) describe the chondrichthyan fauna of Lago Bayano as the first such Miocene fauna from the Pacific shelf of Panama. The authors confirm that shark teeth material originates from a sandstone within the formation but that the chondrichthyan remains are left as a residue among reworked gravel, sand, and mud grains along island shorelines on Lago Bayano. The chondrichthyan fauna is correlative with the other important chondrichthyan-bearing units in Central and South America, including the Gatún Fm and Alajuela Fm of central Panama.

Age

The age span of the Chucunaque Fm in most of the Chucunaque-Tuira Basin can be broadly estimated as ca. 7.1 Ma to ca. 5.6 Ma. To the north, where the Chucunaque Basin changes over to the Bayano Basin, the base of the Chucunaque, with a calcareous nannofossil assemblage, appears to be older than 9.4 Ma. Perez et al. (2017) confirms an age of 10–9.5 Ma (Tortonian) for the chonrichthyan-bearing sandstone strata in Lago Bayano area. Therefore, from the Bayano Basin to the Chucunaque-Tuira Basins, the Chucunaque Fm spans an age of 4.4 Ma from 10 to 5.6 Ma.

Age Span:

Beginning stage:

Messinian

Fraction up in beginning stage:

Beginning date (Ma):

7.25

Ending stage:

Zanclean

Fraction up in the ending stage:

Ending date (Ma):

5.34

Depositional setting

On the basis of evidence from fossil sharks, the paleobathymetry of the Chucunaque Fm in the Bayano Basin is from the littoral zone, with a mean depth of about 100 m (Perez et al. (2015)).

Depositional pattern:

Additional Information

Terry (1956) used the name again without defining it.
Wilson et al. (1957), Woodring (1960), Coates et al. (2004); Perez et al. (2015 & 2017).

Compiler:

Jacques LeBlanc (2021), Stratigraphic Lexicon: The Onshore Cenozoic Sedimentary Formations of The Republic of Panama. Biosis: Biological Systems, 2(1), 173 pp. https://doi.org/10.37819/biosis.002.01.0095 (or via https://sites.google.com/site/leblancjacques)