Cucaracha Fm

Named by MacDonald (1913a, 1913b). According to Graham et al. (1985), the formation is named after a railroad station and a small town that existed prior to Canal construction.

Along Culebra Cut south of Continental Divide where village of Cucaracha was located near site of Cucaracha slide. Cucaracha was a canal-construction period village, now abandoned, on the east side of Culebra Cut. For the location see Woodring (1957, p. 39), cited under the name.

[Figure 1. Geology of the southern part of the Panama Canal (Culebra Cut and new Pacific locks area). (A) Revised geological map. (B) Revised chronostratigraphic chart with previous and new geochronological constraints (new data in dark circles and bold text) (Buchs et al. (2019)).]

Synonym: Cucuracha (a misspelling)

The Cucaracha Fm is the continental volcaniclastic upper part of the Gaillard Gr (See “Gaillard Gr”). It is about 180 m thick (Head et al. (2012)) and consists mostly of terrestrial tan, reddish and green mudstones and claystones interbedded with fluvial conglomerates, sandstones, lignite and welded tuff (Figure 2). The “Cucaracha Ash” is a distinctive silicic welded tuff located in the upper portion of the Cucaracha Fm (Figure 1). Lenticular beds of conglomerate and sandstone are more common in the lower half of the formation (progradational deltaic and nearshore wetland deposits) below a distinctive welded tuff bed of volcanic origin, whereas tabular beds of claystone and lignite are more common in the upper half (fluvial and associated floodplain deposits) above the welded tuff bed [A 1-m-thick tuff, here referred to as the Cucaracha tuff (or ignimbrite), is a prominent marker horizon that allows unambiguous correlation between the measured sections at Centenario Bridge and Hodges Hill]. This ignimbrite (5–8 m thick) is locally found on top of fallout tuff and/or charcoaled tree stumps (Figure 3). The ignimbrite is an important stratigraphic marker in the Culebra Cut, and has been dated at 17.86 ± 0.05 to 17.98 ± 0.05 Ma at one locality by Buchs et al. (2019). The base of the Cucaracha Fm is marked by a distinctive pebble conglomerate bed that lies unconformably over the Culebra Fm. This conglomerate bed is widely distributed and contains volcanic pebble clasts with rare fragments of carbonized wood (without teredinid borings) and oysters. This and other pebble conglomerate beds higher up in the Cucaracha Fm commonly become finer upsection, grading into lithic wacke, siltstone and claystone. Medium to coarse-grained, lithic wacke beds are commonly cross-bedded, which show an average paleocurrent direction to the east (N87°E+/-6.4°). These interbedded channel deposits contain permineralized logs of up to 1 m in length and 30 cm in diameter oriented parallel to bedding. Pebble conglomerate and lithic wacke also contain rare fossils of land mammals. Olive-gray to blackish red claystone is the most common lithology in the Cucaracha Fm. This claystone is commonly structureless to slickensided, but may contain mottling and drab-haloed root traces. Horizons of calcite nodules and rhizoconcretions are common throughout the claystone. Two horizons contain spherical to platy barite nodules (~2 cm in diameter) in olivegray claystone. Fossils of land mammals, turtles, fish, crocodiles and gastropods (Hemisinus (Longiverena) oeciscus) are present locally in claystone, as noted by Whitmore and Stewart (1965), Woodring WP (1957–1982) and MacFadden BJ (2006). Four lignite beds are present in the upper half of the Cucaracha Fm. The Cucaracha Fm contains a distinctive bed of welded tuff 4.3 to 7.7 m thick (also known colloquially as the ‘‘ash flow’’), which is broadly distributed and serves as a useful marker bed (Kirby et al. 2008).

Thickness: 180 m (Head et al. (2012))

[Figure 2. Stratigraphic correlations among MacFadden et al. (2014)’s measured sections containing the Centenario Fauna from the localities depicted in Figure 031. The Centenario Fauna extends from the lowest stratigraphic locality in the Culebra Fm, at 20m above the base of the measured section northwest of Hodges Hill to several localities at ∼75 m above the base of the measured section at Centenario Bridge (see also Kirby et al. (2008)). PmF = Pedro Miguel Fm; Fm = Formation .]

[Figure 3. Ignimbrite of the Cucaracha Fm. Insets show the eutaxitic texture of the ignimbrite and charcoaled tree stump below the ignimbrite. Buchs et al. (2019).]

Terrestrial animal and plant fossils such as trees are abundant (Figure 4). The fauna recovered to date from the Cucaracha Fm is entirely continental and includes an assemblage of land mammals that indicate an early Miocene, early Hemingfordian North American land mammal age (APP-A) (Whitmore & Stewart (1965) and MacFadden et al. (2014)). After Whitmore & Stewart (1965) reported the discovery of Miocene land mammals from outcrops along the banks of the Panama Canal, the layers have since been called the Gaillard [Culebra] Cut Local Fauna. All other fossil evidence indicates terrestrial and freshwater environments; there are no marine units known from the Cucaracha Fm (MacFadden et al. (2015b)). Fossil vertebrates are usually concentrated within the coarser intervals (conglomeratic sandstones and conglomerates) of the upper part of the Cucaracha Fm (Figure 5), above the prominent Cucaracha tuff, and are dominated by freshwater turtles represented exclusively by cryptodires inhabiting delta plain environments (Cadena (2009); Cadena et al. (2012)), crocodilians (Hastings et al. (2013)), and freshwater ichthyofauna. Mammals, although rare, are represented by dental and postcranial elements that exhibit evidence of moderate to intense hydrodynamic transport. Among these fossils, diagnostic dental elements include small artiodactyls (protoceratids, moschids, and floridatraguline camels), followed in abundance by rodents (Slaughter (1981): Texomys stewarti), rhinoceroses, anchitherine horses, tayassuids, carnivores, oreodonts, and bats. Snake specimens (Boa) from the Curacacha Fm were found in terrestrial facies (upper part of the formation) within a variably thick (~20-85 m) section of predominantly terrestrial volcaniclastic sediments, lignite, and paleosols at the Cartagena Hill and Hodges Hill localities (Figure 076). Steadman et al. (2016) discuss the discovery of the remains of a bird (large eagle) along the west side of the Panama Canal in the Centenario fauna. Kirby et al. (2008) found fossils of land mammals throughout the Cucaracha Fm. Land mammal fossils of the peccary cf. Cynora sp., the artiodactyl Paratoceras wardi, the oreodont Merycochoerus matthewi and the rhinoceroses Menoceras barbouri and Floridaceras whitei. These mammals of the Cucaracha Fm are the same as those found in Nebraska, Kansas, and Florida, where climate was drier and cooler and vegetation more open (Retallack & Kirby (2007)). The fossil assemblage at the Lirio East locality includes at least 61 morphotypes of fossil fruits, seeds, flowers, and fungi (Herrera (2010, 2014a, 2014b)), 52 pollen types (Jaramillo et al. (2014)), and calcareous fossil wood (Jud et al. (2017a, 2017b) & Rodríguez-Reyes et al. (2014, 2017a, 2017b)). The fruits are preserved as calcareous three-dimensional permineralizations in poorly sorted volcaniclastic sandstones and conglomerates. The best age indicators for the Cucaracha Fm are its fossil mammals. Taken together, the age of these land mammals indicates a Burdigalian-Langhian age (19 to 14 Ma), with a middle Burdigalian age (18 to 17 Ma) likely.

[Figure 4. Fossilized tree trunk within the Cucaracha Fm. Barat (2013).]

[Figure 5. Conglomerate in the uppermost part of the Cucaracha Fm, with scoriaceous clasts (arrow) at Cerro Hodges. Buchs et al. (2019).]

The Cucaracha Fm represents a coastal delta plain that consists of channel, levee, flood plain and marsh deposits. Abundant paleosols indicate that soils commonly developed on these deposits. Retallack and Kirby (2007) recognized 12 different pedotypes that represent as many vegetation types, including mangrove, freshwater swamp, marine-influenced swamp, early successional riparian woodland, colonizing forest, dry tropical forest and woodland (Figure 6). The pebble conglomerate bed at the base of the Cucaracha Fm represents a fluvial-channel deposit that is broadly distributed (based on its geometry and sedimentology, which are typical of fluvial-channel deposits). Incision of this channel into underlying marine mudstone and sandstone of the Culebra Fm indicates that part of the underlying section has been eroded by the channel. The pebble conglomerate contains fragments of wood that show no evidence of teredinid borings (unlike the wood found in the underlying Culebra Fm), suggesting that this basal conglomerate was deposited above sea level. The presence of oyster fragments probably represents reworking of the underlying marine Culebra Fm. Interbedded lenses of pebble conglomerate and lithic wacke further upsection represent small fluvial channels, based on their lenticular geometry and sedimentology. The small ratio of channel deposits to claystone (the sandstone/claystone ratio for the entire formation is 18.4%) suggests that these were small meandering channels (there is generally a good correlation between channel pattern and sediment load, such that the sandstone/shale ratio provides a clear view of stream type, where meandering channels have relatively low ratios and braided channels have high ratios). Thick sequences of claystone represent flood-basin deposits on the coastal delta plain. Most intervals of claystone show some evidence of soil development. Evidence for paleosols include horizons of calcite and barite nodules, rhizoconcretions, drab-haloed root traces, mottling and relict bedding. Paleosols indicate periods of stability in between fluvial events of thousands to tens of thousands of years when soils developed on flood-basin or channel deposits. The four lignite interbeds represent histosols of tidal or poorly drained distributaries that penetrated the coastal delta plain, where thick vegetation resulted in the accumulation of much organic matter into layers of peat within marshes. The single interbed of welded tuff represents a pyroclastic, ash-flow deposit (ignimbrite) produced by a nearby explosive eruption. Conformably overlying the Cucaracha Fm is a basalt flow of the Pedro Miguel Fm. Underlying claystone in the Cucaracha Fm shows baking and the overlying basalt shows hydrothermal alteration.

[Figure 6. Reconstruction of soils and vegetation in the coastal delta plain of the middle Miocene Cucaracha Fm. Retallack & Kirby (2007).]