RIO YUCA, Formation
UPPER TERTIARY (Mio-Pliocene?)
State of Barinas, Venezuela
Author of name: A. N. Mackenzie, 1937.
Original reference: A. N. Mackenzie, 1937, p. 265.
Original description: ibid.
The Río Yuca formation is named from the Río Yuca which is in the western part of the State of Barinas (formerly known as Zamora). Although the limits of the formation are not given by Mackenzie, it is believed that the formation as exposed in this river extends across the regional trend of the foothills from their boundary with the llanos to a point 21.45 kilometers north 37 degrees west of the new bridge crossing the river at the edge of the hills. Upstream, the base of the Río Yuca formation is in conformable contact with the underlying Parángula formation whereas the upper unconformable contact with Quaternary sediments is placed at the physiographic contact of the llanos and the immediately adjacent hills. The Río Yuca formation extends for a long distance northeast and southwest of the Río Yuca and thicker and more complete sections are exposed in other streams such as the Ríos Pagüey-Escagüey, Curbatí and Canaguá some 30 to 55 kilometers to the southwest.
Mackenzie describes the Río Yuca formation as consisting of 3,000 meters (9,845 feet) or more of "highly lenticular beds containing all gradations of sandstones, clay-shales and conglomerates, irregularly disposed. The sandstones vary from soft to hard and are highly micaceous and argillaceous. The shales are invariably soft, clayey and micaceous. The predominatiny color is mottled limonitic-yellow, light grey with small amounts of hematitic red stain. Occasional strata of light blue-green and dark grey color occur in the middle and lower parts. In the outcrops, the sandstones have a typical dull greenish-gray color. The formation is undoubtedly of continental origin and nonfossiliferous."
Type Section: Despite the rapid changes in Ethology from locality to locality, Mackenzie's type section is fairly representative. It should be pointed out, however, that in the Río Yuca, the formation is involved in folding, that midway between the upper and lower field contacts the Parángula formation is exposed, and that the measured thickness is only about 1,800 meters as compared with 4,100 meters in the Río Canaguá about 55 kilometers to the southwest.
Lithology: Southwest of the Río Yuca, and notably in the Ríos Pagüey-Escagüey, Curbatí, Canaguá and Acequia, the Río Yuca formation is composed essentially of conglomerates and gravels, sandstones, siltstones and claystones. These are distributed irregularly and in varying abundance throughout the section, though in general the coarser clastics increase in abundance upward. The conglomerates contain quartz, quartzite and metamorphic and igneous rocks of granule to boulder size, and vary in thickness from streaks of a few centimeters to 15 or 20 meters. The sandstones are fine to coarse, loosely cemented to moderately compact and are generally arkosic and highly micaceous. In the lower part of section, the sandstones are occasionally calcareous, and a few sandy limestones 3-4 centimeters thick have been observed. The siltstones and claystones are soft to medium-hard and are usually micaceous. Leaf impressions, pyritized lignite and woody material are haphazardly distributed throughout, and in one plane a log was noted to be so little altered as to retain the flexibility of its fibers. The lower part of the formation is mottled red, tan, gray and green but upward there is less mottling and the beds are predominantly gray-green, blue-gray, brown and gray in color. The strata often intergrade, and lenticularity and cross-bedding are characteristic throughout.
From the Río Yuca to the northeast, the Río Yuca formation exhibits the same characteristics as it does to the southwest. However, thin beds of nodular fresh-water limestone and associated calcareous claystones have been observed more frequently and, from the Río Portuguesa northeastward, toward Acarigua, irregular lenses of fine-grained, soft, highly calcareous sandstones are common. Another interesting type of rock that makes its appearance between the Río Calderitas and the town of Guanare is a paperthin, laminar shale often associated with micaceous and arkosic sandstone. This shale which has been observed in a number of localities southwest of Guanare occurs near the base of the Río Yuca formation whereas in one outcrod in the Río Calderitas, 1,500 meters south of the village of Carmelo, a similar shale lies 800 meters stratigraphically above the top of the Parángula formation.
Heavy Minerals: The following mineral suite obtained from a sample at the base of the Río Yuca formation in Quebrada Parángula has been identified: abundant chlorite and muscovite; common staurolite, tourmaline, zircon, iron ore; rare biotite, corundum, colorless garnet and rutile.
From the upper part of the Río Yuca formation in Quebrada Parángula the following suite has also been identified: flood of epidote; abundant colorless garnet, muscovite, staurolite, zircon, and iron ores; common andalusite; rare chlorite, pink garnet, glaucophane, titanite, and tourmaline. The differences in the heavy mineral assemblage of the Río Yuca and Parángula formations plus the abundance of mica and arkose in the Rio Yuca and their absence or rarity in the Parángula indicate that the two formations had different sources even though both were deposited in a dominantly continental environment.
Thickness: The measured thickness of the Río Yuca formation along the southeast side of the Mérida Andes ranges from 1,150 meters (3,773 feet) near Guanare to 4,100 meters (13,452 feet) in the Río Canaguá approximately 117 kilometers to the southwest. The maximum thickness is greater than 4,100 meters and is believed to occur in the syncline which lies at varying distances from, and is generally parallel with, the foothills between the Río Pagüey and the Río Caparo. In the foothill belt adjoining the llanos, the Río Yuca formation, like the older Parángula, thickens along the regional strike from northeast to southwest, and also thickens from the foothills toward the syncline on the southeast.
Stratigraphic Relationships: The Río Yuca formation is transitional and conformable with the underlying Parángula formation from which it is differentiated in the field by its arkosic sandstones, large flakes of mica and general greenish color. The transitional zone is 25 or more meters thick and the contact between the Parángula and Río Yuca is placed at the top of the highest quartz sandstone of the Parángula formation. The upper contact of the Rio Yuca formation is angularly unconformable with the Guanapa formation or, if this is absent, with the Llanos formation of Recent age.
Fossils and Age: Aside from the slightly altered woody material encountered sporadically in the Río Yuca formation, organic remains are scarce in the Río Yuca formation. However, the bones of a ground sloth, Prepotherium venezuelanum Collins, have been collected by W. Wray Love in the Río Tucupido about 7 kilometers northwest of the village of Tucupido in the State of Portuguesa. R. Lee Collins (1934, p. 243) who named and described the sloth, states that Prepotherium venezuelanum is somewhat akin to Prepotherium potens Ameghino from the Santa Cruz beds of Patagonia, Argentina, which are quite generally regarded by North American and European geologists as lower Miocene in age, (see also ZARAZA GROUP). Collins (1934, p. 238) further states that the sloth remains occur, according to W. H. Heroy, in a basal sandstone within a formation consisting of "red, purple, gray and brown clays usually mottled, containing red, brown, and gray sandstone and some conglomeratic layers." In the Río Tucupido at the locality from which the sloth remains were collected, the upper Parángula and lower Río Yuca formations are exposed and it is inferred by us that Prepotherium venezuelanum Collins comes from a horizon close to the contact of the two formations, probably from the base of the Río Yuca formation. Collins provisionally assigns an upper Miocene age to the sloth beds, but in the absence of definite paleontologic evidence, the time span of the Río Yuca formation cannot be bracketed accurately. However, from its considerable thickness and its position between the Middle Tertiary Parángula formation and the Pleistocene Guanapa formation, it is surmised that deposition started in the Miocene and continued into Pliocene time.
Extent: The Río Yuca formation has been traced along the east side of the Mérida Andes from the Socopó River on the southwest to the latitude of Acarigua on the northeast, for a distance of approximately 240 kilometers. Beds similar to those in the Río Yuca formation re-appear in the "Vicinity of the Río Burgua and continue southwestward therefrom for a Considerable distance into Colombia along the edge of the llanos of that country. The Río Yuca formation is believed to be widespread under the llanos of Venezuela where the formation is postulated to thin out as it approaches the Guayana Shield.
Origin and Correlation: Inasmuch as Río Yuca conglomerates contain boulders of igneous and metamorphic rocks found in the present day Mérida Andes, it is probable that these mountains which had already been uplifted and stripped down to their crystalline core, were the source of much of the material comprising the Río Yuca formation. E. Mencher, et al., (1953, p. 715-716) believe that rejuvenation of the Mérida Andes continued throughout the deposition of the Río Yuca formation with the culmination of uplift occuring in the Pliocene. The continental deposits of the Río Yuca formatìon were laid down to the southeast while the correlative Isnotú and Sanalejos formations of the Betijoque group were laid down to the northwest of the then existing Andes.
Norman E. Weisbord
TERTIARY (middle Miocene)
State of Guárico, Venezuela
Author of name: P. Christ (?) et al.
Original reference: H. G. Stehlin, 1938, p. 227.
Original description: ibid.
According to G. G. Simpson (1943, p. 55) the name Zaraza series was introduced in private reports by Christ and other geologists to designate Tertiary strata outcropping in the Zaraza region, State of Guárico. A mammalian jaw was found by P. Christ in a good outcropping area and was originally referred to as Astrapotherium christi Stehlin and later changed (Kraglievich, 1928) to Xenastrapotherium genus.
Stehlin (1928, p. 227-232) first published the name "Zaraza series" pointing-out as type-locality the Pozo Rendivú fossiliferous outcrops in the Quebrada Honda, affluent of the Tamanaco River, approximately 10 kilometers to the northeast of the town of Zaraza. The fossiliferous beds are made up of brownish concretioned sandstone. According to Stehlin (p. 231-232) the Astrapotherium indicates that the outcrops are more or less equivalent in age to those of the Santa Cruz formation of Patagonia; probably not older and perhaps slightly younger, but never post-Miocene.
H. D. Hedberg (1937, p. 2013) refers to Stehlin's "Zaraza series" indicating its equivalence with some part of the Santa Inés "formation" (today group).
In discussing the stratigraphy of the State of Guárico, M. Kamen-Kaye (1942, p. 33) mentions a Zaraza group of possible middle Miocene age which includes "strongly fossiliferous beds", but he did not refer to Stehlin's report.
New vertebrate fossils in a horizon probably almost equal to the Astrapotherium horizon were found by G. G. Simpson (1943, p. 54) who visited Stehlin's type-locality in Pozo Rendivú in 1939. The fossiliferous occurrence was described by the author as a brownish sandstone bed containing "numerous fragments of turtle shells, less frequent crocodile remains and relatively few Astrapotherium remains". he identified a turtle (Podocnemis) pertaining to the Pleurodia group, of scarce diagnostic value. Nevertheless, basing himself on the above mentioned presence of Xenastrapotherium, Simpson (p. 55) verifies Stehlin's opinion according to which the Zaraza horizon is "almost certainly, not older than Santacruzian which... (Stehlin) . . . considered lower Miocene and it is most probably younger. The Friasan is more or less lower Miocene". Simpson ends-up by assigning a middle Miocene age to the Rendivú bone horizon, saying that it might be lower or upper Miocene but it is very improbable that it would be as old as Oligocene or as young as Pliocene. According to some information received from Dr. R. Hoffstetter, the Santacruzian of Patagonia (Argentina) is considered of yower Miocene age by all modern paleontologists and geologists like Feruglio.
On the other hand, a glyptodont (Asterostemma venezolensis) was described by Simpson (1947, p. 1-10) who based himself on J. H. Todd's field information. The fossiliferous outcrop is found in the valley of the Güere River, in the near vicinity of the village of San Francisco, located to the southeast of Clarines, State of Anzoátegui. Simpson points out its probable correlation with the section exposed near Zaraza. According to the same author, the glyptodont found in the Güere River could be middle Miocene (Chasicoanian); there is the possibility that it would be much younger than middle Miocene.
It would be very interesting to determine with more precision the age of the Zaraza beds, a part of the sequence in Guárico where there are scarce faunal guides; it would be desirable to make a careful comparison between their saurian, chelonian and mammalian skeletons and those of the Urumaco bone bed, Distrito Democracia, State of Falcón, which is located almost at the top of the Urumaco formation and is of proved upper middle Miocene age, (molluscan assemblage). The material containing Xenastrapotherium in the Pozo Rendivú might perhaps be compared also with the Prepotherium level, Tucupido River, State of Portuguesa, located in the lower part of the Río Yuca formation.
The author of the present article believes it possible that the beds studied by Stehlin represent a continental facies included somewhere in the Cucharo formation, (see) considered of approximate middle Miocene age and equivalent to the Yucales formation; another lateral facies generally considered as a northern equivalent of the Cucharo formation is the lower part of the Quiamare formation (see); the Zaraza bone beds could also be compared with beds pertaining to the middle or upper part of the Chaguaramas formation (sensu lato) (see) as it was defined by Patterson and Wilson (1953). If, on the contrary, the restricted definition of the Chaguaramas formation, (proposed by Evanoff (1951) and used by the author of the present article in the redefinition of the Guarumen group (see), is accepted, the Zaraza "group" represents, or is a part of, a post-Chaguaramas unit, and therefore post-Guarumen. On the other hand, the Zaraza "group" is older than the Freites formation of the subsurface, the Punche formation of the surface, the "Aragua", Santa Lucía, Zuata, and middle and upper Quiamare formations (see). According to this, it seems rather improbable that the latter formations include Oligocene beds, as it was suggested by some geologists.
The formational nomenclature of what is considered as probable Miocene in eastern Guárico is not yet firmly established; on the other hand, the published descriptions of the type-sections are somewhat unprecise and the understanding of the stratigraphic relations between the brackish water, paludal, fluvial, deltaic and marshy formations with frequent and rapid lateral variations in the facies, present problems not yet solved. The scarcity, or lack, of fossils, increases the confusion. The disputable redefinition of the Santa Inés group, by Hedberg (1950, p. 1199-1205) suffers from those confusions. For instance, Hedberg places the Peña de Mota conglomerate above the Caño Dulce formation and below the Bruzual formation, and correlates this last one with the Roblecito formation. Today (see GUARUMEN, Group) it seems evident that the Caño Dulce formation is equivalent to the Batatal formation and that the Peña Mota facies constitutes a transgressive marginal conglomerate in the base of the Chaguaramas. Therefore, the normal sequence, from base to top, should be: Caño Dulce, Bruzual, Peña Mota. The latter represents the lowermost Miocene.
In synthesis, it is here believed that only future investigations on this complex of facies that Hedberg proposed to include in his Santa Inés group, considerably enlarged in 1950, would permit to decide the best name for the small part of the section discussed by Stehlin, which certainly does not deserve the rank of a formation and much less of a group, and the Miocene age of which, (probably middle Miocene), has been suggested by Stehlin and by Simpson.
(See also CUCHARO and CHAGUARAMAS, Formations).
J. M. Sellier de Civrieux