Santiago Fm

Named by Hershey (1901) (Figure 1).

Type Locality is At Santiago and adjoining areas, Veraguas Province (Keroher et al. (1966)

[Figure 1. The earliest known geological sketch map of the Central Portion of the Isthmus of Panama zoomimg in on the Azuero Peninsula. The map displays the formations known at that time and described by Hershey (1901).]

Synonym: Sometimes referred to as the “Macaracas Fm” (Kolarsky et al. (1995a, 1995b); Buchs et al. (2011); Rodríguez-Reyes et al. (2020b)).

At the town of Santiago de Veraguas, where first discriminated, this is made up of thick layers of nonlaminated shale of a dull greenish gray color and a peculiar breccia and breccia-conglomerate. Although there are some thin layers of fine gravel and sand, the great mass of the formation is a shale, everywhere characterized by the same dull greenish or olive tint. The Santiago Fm forms the foundation of the entire Aguadulce-Santiago plain, over a large portion of which it is the surface formation. Here it always dips in some direction, but rarely at a high angle. On the lower two miles [3.2 kms] of the Torio River and at a promontory a short distance south of the mouth of the stream, it is exposed in beds of hard sandstone and shale over one thousand feet [300m] in thickness and dipping steeply in a general westerly direction.

Kolarsky et al. (1995a, 1995b) reported samples of pollen, foraminifera, and nanofossils from the Santiago Fm in central Azuero and concluded that they support a late Oligocene to early Miocene age. Stern and Eyde (1963) reported that the Ocú silicified woods are associated with tuffs, which may be the source of the silica (Murata (1940)) that were mapped as Eocene-Oligocene by Terry (1956). At the surface of the Tonosí Fm, large quantities of petrified wood/trees can be found on catle ranch and other private lands. The fossils, known locally as chumicos, are strewn about in pastures and piled up in gullies in such abundance that the townspeople of Ocú and Chitre use them as decorative building stones. Pieces are angular and range in size from small hand specimens to stumps a foot or more in diameter (Stern & Eyde (1963)) (Figure 2). However, even though they lay at the surface of the Tonosí Fm, no trunks have ever been found in-situ. This led Jud et al. (2017a) to describe several specimens of silicified woods from the area of Ocú, which in turn led them to suggest that they originated from the younger Santiago Fm (eroded away in this area). This was confirmed by Rodríguez-Reyes (2019, 2020a, 2020b) who found similar medium to large size tree trunks in-situ within the Santiago Fm more the the northwest. Rodríguez-Reyes et al. (2020a) describe a new fossil-genus and species of Anacardiaceae tree from the largest fossil trunk discovered in Panama and probably, Central America (up to 2020). The trunk was collected on a private farm in Llano de La Cruz (latitude 08° 09’ 4.7” N; longitude 80° 53’ 11.2” W) (Figure 3) and was lying within layers of sandy mudrock and sedimentary breccia, parallel to bedding. Fining upward sequences indicate fluvial environments as the main depositional system. The fossil silicified woods are interpreted as evidence for humid to perhumid megathermal climate in Panama during the late Paleogene-early Neogene (Jud et al. (2017a)). Rodríguez-Reyes et al. (2020b) collected fossil woods from two different locatities. Their sample STRI 44073 came from Los Cerritos, Los Pozos, Herrera (7° 46’ 43.5” N; 80° 35’ 53.3” W) and their sample STRI 45788 was collected on a private farm in Llano de La Cruz, Veraguas (08° 09’ 4.7” N; 80° 53’ 11.2” W). The two fossil wood types identified are new types for the “Ocú fossil forests” and adds Malvaceae to the list of families known from the Azuero Peninsula (e.g., Arecaceae, Humiriaceae, Salicaceae, Hernandiaceae, Lauraceae, Fabaceae, Moraceae, Euphorbiaceae, Anacardiaceae/Burseraceae, Simplocaceae, and Anacardiaceae). All of the genera identified from Azuero fossil assemblages grow in lowland rainforests of Central America, with the exception of Beilschmiedia, which occurs in tropical to temperate forests.

[Figure 2. A) Facade of "La Posada" in the town of Ocú, showing the abundance and quality of the fossil woods in the area. B) Close-up of some woods on the façade. C) Fossil wood used as ornaments in a home garden in Chitré. Rodríguez-Reyes (2019).]

[Figure 3. The large Anacardiaceae tree in its original position in the Santiago Fm. The preserved trunk is ~20m in length and 2.5m wide. The dimensions suggest a probable height of ~35m. Rodríguez-Reyes (2020a). (Material reproduced with the permission of the editor).]

The age of the Santiago Fm has not been clearly determined due to its stratigraphic complexity, lack of good outcrops, and absence of radiometric elements for dating (Rodríguez-Reyes et al. (2020a)). Note that Ministerio de Comercio e Industrias (1991) shows the Santiago Fm as older than the Gatún Fm while Instituto Geografico Nacional (IGN) "Tommy Guardia" (1996) shows it as younger.

The Fining upward sequences indicate fluvial environments as the main depositional system.