Gatún Fm

• The Gatún Fm (Figure 1) was first defined and named as such by Howe (1907a, 1907b) after the village of Gatún which lies at the northern margin of Gatún Lake, 12 km southwest of Colon. He excluded oldest strata near Gatún from Gatún Fm and grouped them into Bohío Fm. Before it was named, however, the formation was already known. When William Phipps Blake traveled across Panama in 1853 on his way to California to join one of the transcontinental railroad surveying parties, he collected a few Gatún Fm invertebrate fossils. Vertebrates (shark teeth) fossils were unknown in the Gatún fauna until the discovery and excavation that led to Gillette (1984)’s study.
• The type area is from Gatún to Mount Hope (Monkey Hill of Howe’s time), Canal Zone (Keroher, 1966).
• The stratotype of the Gatún Fm was defined by Coates et al. (1992). It runs from Sabanita on the main transisthmian highway 12 km east of Colon, to 0.7 km west of the junction with Route 77 (the turnoff for Portobelo).
• Four reference sections were measured by Coates (1999) that include both the Gatún Fm and the Chagres Fm and reflect the lateral changes that the formations undergo from Gobea, 40 km west of Colon, to Sabanita, 12 kms east of Colon. The formation can be observed in roadside construction sites, and quarries from Colon to Sabanitas (north-south), and from Maria Chiquita to Gobea (east-west).
  

[Figure 1. Geological map and stratigraphic section showing the 15 fossil shark teeth localities where Pimiento et al. (2013a) (also partly in Pimiento C. (2010)) collected 800 specimens from the Gatún Fm : 1) 9°21'15.66"N, 79°50'11.34"W; 2) Isla Payardi: 9°22'57.18"N, 79°49'16.50"W; 3) Cuatro Altos: 9°20'7.08"N, 79°52'58.80"; 4) Banco EE: 9°17'59.11"N, 79°55'5.36"W; 5) San Judas: 9°2115.66"N, 79°5011.3274"W; 6) Sand Dollar Hill: 9°21'2.628"N, 79°48’35.532”W; 7) Alborada: 9°20'27.024"N, 79°49'0.804"W; 8) Texaco: 9°20'58.8474"N, 79°48'47.304"W; 9) I.D.A.A.N.: 9°20’35.5194”N, 79°48'45"W; 10) Third Set of Locks: 9°16'46.92"N, 79°54'48.24"W; 11) Highway Extension 9°20'37.5"N, 79°49'0.99"W; 12) Gatún Dam: 9°15'46.08"N, 79°56'26.16"W; 13) Gatún Dam: 9°15'53.2794"N, 79°55"54.4794"W; 14) Panama: 9°22'56.64"N, 79°49'16.32"W; 15) Sabanitas (Gillette, 1984) 9°20'59.99"N, 79°47'59.9994"W. Lower Gatún localities include 1, 2, 5, 6, 7, 8, 9, 11, 14, 15; middle Gatún localities include 3, 10, 12, 13; and the upper Gatún locality is 4.]

Synonym: Gatun Fm; Monkey Hill; Mindi Beds; Mindi Hill Beds; Sabanitas Fm.

The name of Gatún has come to supersede other earlier informal names for this unit, such as Monkey Hill, Mindi Beds, Mindi Hill Beds (Hill, 1898). The “Sabanitas Fm” of Thompson (1947) and Jones (1950) is now the Lower Member of the Gatún Fm.

The middle Gatún Fm is described in Section 2 of Coates (1999) and covers the composite section from Gatún to Margarita. The middle Gatún Fm is about 350 m thick and consists of alternating siltstone and sandstone with occasional 4 to 5m units of interbedded sandstone and conglomerate. Concretion zones like those of the lower Gatún Fm are largely absent. Shell hash and diverse molluscan assemblages are somewhat less abundant than in the lower Gatún Fm, but pervasive bioturbation is still very extensive. Bentonitic horizons and a higher wood fragment content are also typical of the middle Gatún Fm.

The upper Gatún Fm is exposed around Mount Hope (upper part of Section 2 in Coates (1999)) and more extensively on the western side of the canal, along and adjacent to the road to Pina, and is about 40m thick. The lithology is more consistently volcaniclastic sandstone or fine conglomerate, with minor mudstone and siltstone. Thin bentonite horizons and shell hash are common but diverse, whole mollusks are relatively rare. A distinctive horizon is exposed below the overflow dam on the Chagres River west of Gatún Locks and has conglomeratic, tuffaceous sandstone beds with extensive thalassinoid burrows, wood, and scattered coral colonies up to 50 cm in diameter. Armored mudballs, 6-10 cm in diameter, are also abundant at one horizon that has numerous pockets filled with conglomerate.

Thickness: The thickness of the Gatún Fm varies between localities and authors. Woodring (1957) states that the total thickness of the Gatún Fm, recorded in a borehole near Colon City, is about 500m. Hendy (2013) notes a total thickness of at least 600 m of which the basal half belongs to the lower Gatún Fm.

According to O'Dea et al. (2007), the best outcrops of the Gatún Fm for the purpose of fossil collecting can be found in the area of Sabanitas and near the community of Gatún in the extreme north of the Panama Canal. For instance, on a small hill about 50m before the gates of the "Isla Payardi" refinery (9° 22.957’N, 79° 49.288’W) can be found an extremely rich mollusks bed. Also, when new commercial or residential constructions take place along the highway between the cities of Panama and Colon new sites become exposed.

[Figure 2. Geographic map of Landau et al. (2012a)'s mollusks collecting area of Cativa-Sabanita area (Left), Panama. 1) Cativa-Las Lomas 09°21'20.74"N, 79°50'20.69"W; 2) Cativa-Sabanitas 09°21'36.87"N, 79°49'52.65"W; 3) Refineria Panama 09°22'57.17"N, 79°49'16.42"W and the one of Gatún area (right) 09°17'40.40"N 79°55'07.66"W.]

References:

• Toula (1908, 1911); Brown and Pilsbry (1911, 1912); Dall (1912); Vaughan (1918); Rathbun (1918); Pilsbry (1918); Jackson (1917 & 1919); Cushman (1919); Pilsbry (1919); Vaughan (1919); Olsson (1922).
• Coryell et al. (1937a). At Cativa, includes Cativa marl in lower part.
• Thompson (1943); Jones S.M. (1950).
• Woodring (1957). Chiefly massive medium to very line grained sandstone and siltstone. Estimated thickness at least 500 meters. On faunal basis, divided into lower, middle, and upper parts; lower part not represented at type region; middle part includes best known strata. Overlies Caimito Fm with contact covered by waters of Gatún Lake; farther east, overlaps Caimito and lies directly on Cretaceous(?) basement: underlies Chagres sandstone. Lower and middle Miocene. It is now known that oldest outcropping part of formation is not represented in type region.
• Woodring (1959).
• Woodring (1960). Siltstone, sandstone, conglomerate, and tuff, extending from Gatún, Canal Zone, northward to Monkey Hill [Mount Hope] were described as the Gatún Fm. According to Dall (1912), the age is Oligocene. Most of the sandstone is fine-grained. The sandstone and siltstone are more or less calcareous. The formation crops out on the Caribbean side of the isthmus of Panama in an area that has a length of 40 miles [68 kms]. A thickness of 1,400 feet [426m] was penetrated near Mount Hope, and the base was not reached. Fossils, particularly mollusks, are abundant. These mollusks include Turritella altilira, T. gatunensis, Conus molis, Anadara dariensis, Clementia dariena, and Lirophora mactropsis. On a faunal basis the formation is divided into lower, middle, and upper parts. The lower part, not represented in the type region, overlaps onto the pre-Eocene basement east of the Canal Zone. In the Canal Zone the three parts are assigned to the middle Miocene, but the upper part west of the Zone is considered late Miocene.
• Woodring (1964, 1970, 1973, 1982); Keroher (1966); van den Bold (1967); Gertman (1969); Petit (1976); Roth (1981); Vokes (1969, 1970, 1983a, 1983b, 1989a, 1989b); Gillette (1984); Graham et al. (1985); Jung (1989); Graham (1991); Collins (1999); Collins et al. (1996 & 1999); Aguilera et al. (1999); Coates A.G (1999); Fierstine (1999); Todd & Collins (2005); O’Dea et al. (2007); Beu (2010); Montes et al. (2010); Pimiento (2010); Uhen et al. (2010); Pimiento et al. (2010 & 2013a); Cadena et al. (2012); Landau et al. (2012a); Hastings et al. (2013); Hendy (2013); Todd et al. (2013); Jaramillo et al. (2014); Herrera et al. (2014b); Aguilera et al. (2017); Anderson et al. (2017); Alberti et al. (2018); Redwood Stewart D. (2019);