Tonosí Fm

Type Locality is at Los Santos Province. Keroher et al. (1966)

[Figure 1. A zoom-in of the Azuero Accretionary Complex which forms the SW edge of the Azuero Peninsula. It represents an accretionary complex composed of Cretaceous to Eocene accreted seamounts and oceanic islands. Modified from Buchs et al. (2008, 2011). Also in Barat (2013). The Tonosí Fm is at the upper-left and lower-right cover.]

Synonym: Tonosi Fm, Pesé Fm, Pese Fm (included in the Tonosí Fm in Buchs et al. (2008, 2011)). See definition of Pesé Fm.

At Punta Bucaró, the Lower unit crops out as a 560 m-thick section of basal conglomerate and minor sandstones; with no limestones present (Figure 147; Figure 148). It is assumed to directly overlie basement, although at this locality the base of the section is not exposed, and the lowermost beds are assumed to be faulted against basaltic basement (Kolarsky et al. (1995a, 1995b)).

Conglomerate beds range from massively bedded to thinly bedded and exhibit no thinning or finning upward trends. Thinly bedded conglomerate is more common than massive conglomerate, with beds of sand and gravel commonly 4 to 5 cm thick. Beds define both planar parallel stratification and tabular foresets. In decreasing abundance, conglomerate clasts include basalt, sandstone and shale fragments similar in lithology to Ocú Fm, and shell fragments (Kolarsky et al. (1995a, 1995b)).

Minor sandstone and siltstone beds make up less than 5% of the section. They are commonly 5 to 200 cm thick, poorly sorted, fine to coarse grained, and rarely pebbly. Sedimentary structures in sandstone beds include normal grading and planar cross stratification and through cross-stratification (Kolarsky et al. (1995a, 1995b))

Dominance of stratified conglomerate over massive conglomerate and the complete absence of mudstone interbeds suggest deposition of the Lower Tonosí Fm by traction currents of sheet flow in a high-energy shallow water environment. The presence of rare fossil-rich beds and lenses in conglomerate and sandstone is interpreted as the product of storm reworking and supports a shallow-water environment of deposition (Kolarsky et al. (1995a, 1995b)).

Thickness: 1200m

[Figure 2. Stratigraphic sections of the Tonosí Fm on the Azuero Peninsula by Kolarsky (1992). A) Two measured sections at Punta Blanca on the southern coast of the Azuero Peninsula: the section to the left shows Middle Eocene reefal limestone unconformably overlying basaltic basement and the section to the right shows basal conglomerate and turbiditic section overlying a covered interval assumed to be basaltic basement similar to that seen at the section on the left; B) Measured section at Punta Búcaro on the southeast coast of the Azuero Peninsula; C) Measured section at Punta Malena on the west coast of the Azuero Peninsula.]

[Figure 3. Outcrop of the Lower Tonosí Fm at Bucaró beach. Basal conglomeratic sandstones and coarse sandstones in planar parallel stratification. Ramirez (2013).]

[Figure 4. Upper Tonosí - Gray, well-sorted, fossiliferous litharenites. Ramirez (2013).]

[Figure 5. Hemipelagic basal Tonosí Fm close to Cacao (511700/810125 UTM WGS84). Buchs (2008).]

The first specimen of Humiriaceous fossil wood was collected by John E. Ebinger in 1961 (Stern and Eyde (1963)), 3.2 kms northwest of the town of Ocú on the Azuero Penuinsula. Herrera et al. (2014b) revisited Ocú and collected abundant large, silicified wood fragments. The wood specimens are not found in situ but instead appeared scattered in pastures, without evidence of the parental rock (lat. 7°55.476’N, long. 80°47.959’W) (see “Note” below). Near Ocú they found siltstones from a small quarry (lat. 7°54.790’N, long. 80°47.085’W). This new locality yielded impressions of elliptical fruits resembling Humiriaceae and fragments of carbonized wood. Fossil invertebrates from the siltstones indicate a late Eocene age for these sediments. Recent mapping of the north-central part of the Azuero Peninsula suggests that sediments exposed near the town of Ocú and surrounding areas belong to the late Eocene–late Oligocene Tonosí Fm (Buchs et al. (2011)). Based on their observations near Ocú and the recent geological mapping, Herrera et al. (2014b) tentatively consider the fossil wood [of Humiriaceoxylon ocuensis] of Stern and Eyde (1963) to be Late Eocene (ca. 37.2–33.9 Ma) in age. Perez-Consuegra et al. (2018) describes permineralized and carbonized fossil endocarps and seeds of Humiriaceae from Oligocene and Early Miocene sections of the Tonosí Fm in western Azuero Peninsula, near the town of Torio, province of Veraguas. In the lower part of one of the sections, they also recognize vertebrate remains, echinoderms, mollusks, burrows, corals, algae, Talassinoids, shells and leaves. At the mouth of Río Pedregal (548410/800564 UTM WGS84) paralic facies including well-rounded basaltic conglomerates encroach unconformably on accreted seamount rocks and on deformed packages of the Covachón Fm. These conglomerates contain large oysters (Spondylus sp.) and a detrital matrix with abundant larger benthic Foraminifera (Buchs et al. (2011)).

Note: At the surface of the Tonosí Fm near the towns of Ocú and Chitre, 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 143). 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) who found similar medium to large size tree trunks in-situ within the Santiago Fm more the the northwest (See “Santiago Fm”).

References:

• Miller et al. (1939);
• Olsson (1942b). Name Tonosí limestone appears on correlation chart (p. 234). Occurs below upper Oligocene Santiago Fm and above unnamed lower Oligocene unit.
• Miller (1947); Terry (1956);
• Woodring (1960). An undefined name for strata of Oligocene age in Tonosí area. This age is defined by Larger foraminifera and corals.
• Stern & Eyde (1963); Keroher et al. (1966); Woodring (1973); Kolarsky (1992); Kolarsky et al. (1995a, 1995b); Krawinkel et al. (1999); Baumgartner-Mora et al. (2008); Buchs et al. (2010 & 2011); Corral et al. (2011); Herrera et al. (2012 & 2014b); Ramirez (2013); Vasquez & Pimiento (2014); Ramirez et al. (2016); Jud et al. (2017a); Perez-Consuegra et al. (2018).