PARANGULA, Formation

MIDDLE TERTIARY

State of Barinas, Venezuela

Author of name: A. N. Mackenzie, 1937.

Original reference: A. N. Mackenzie, 1937. p. 264-265.

Original description: ibid.

The Parángula formation takes its name from Quebrada Parángula which flows southeastward and lies 800 meters west of the town of Barinitas in the State of Barinas (formerly Zamora). The formation is exposed in two places along this stream, one due west of Barinitas, and the other 1,600 meters southwest of Barinitas. Mackenzie does not state which of these is the type locality but the inference is that it is the latter.

The Parángula formation is described by Mackenzie as consisting of "soft, red and purplish mottled grey sandy shales and buff to brick red sandstones. The sandstones are coarse to finely conglomeratic and are usually soft to medium hard." The thickness of these rocks is given as 550 meters (1,805 feet) in the Quebrada Parángula region where they overlie upper Eocene strata with "probable but unconfirmed disconformity", and are themselves conformable and transitional with the overlying beds of the Río Yuca formation.

Type Section: The base of the Parángula formation in Quebrada Parángula is placed 1,340 meters due south of the main plaza of Barinitas and the top of the formation 2,000 meters south 3 degrees east of the same plaza.

Lithology: From a number of sections examined over a distance of 140 kilometers between the Río Acequia area on the southwest and the Río Portuguesa on the northeast, the Parángula formation has been observed to consist predominantly of nonmarine siltstones, mudstones and fine-grained sandstones. Interbedded with these are massive to cross-bedded friable quartz sandstones of varying grain size, and conglomerates. The conglomerates are relatively rare southwest of the Río Boconó, but northeastward therefrom, they may comprise as much as 25 per cent of the total section. They are of granule to boulder size and occur in streaks or in beds and lenticular masses of varying thickness up to as much as 100 meters. Often, the conglomerates are composed largely of quartz and quartzites in a matrix of sandstone. Locally, they contain granules and pebbles of crat, shale, siltstone, and igneous and metamorphic rock in streaks or bands, and, in the upper courses of the Ríos Pagüey and Bumbum, 6 to 15 kilometers southwest of Altamira, the thick conglomerates consist of pebbles and cobbles of tan quartzite and black chert in a matrix of quartz sandstone.

In general, the Parángula formation may be said to have a reddish cast as contrasted with the predominating but not exclusive greenish tone of the overlying Río Yuca formation. The finer-grained sediments of the Parángula formation are variegated to mottled in color with reds, tans, grays and purples prevailing, whereas the sandstones are usually brown, brick red and light gray in color and may be mottled purple and yellow. Colorations other than these are rare although bright green glauconitic sandstones which occur in thin beds at several localities northeast of the Río Masparro have been noted.

Other characteristics of the Parángula formation are the occasional small lime-cemented sand concretions of irregular shape, the occurrence of clay-balls in many of the sandstones, the general interlensing and crossbedding within the formation, the somewhat calcareous nature of the beds at the top of the formation near the contact with the Río Yuca, and the absence of large flakes of muscovite and arkosic material in the sandstones. The last-named constituents are abundant in the overlying Río Yuca formation which is also nonmarine.

Less widespread yet significant features of the Parángula formation are the basal conglomerates (in the upper reaches of Río Bumbum and in Quebrada Medero) containing fragments and blocks of sandstone resembling the sandstone of the underlying formation, and the occurrence, in Quebrada Negra, of thin shales containing foraminifera which are identical to certain species in the underlying marine strata of upper Eocene age.

Thickness: Between the Río Acequia and the Río Portuguesa the measured thickness of the Parángula formation ranges from 260 meters (853 feet) to 890 meters (2,920 feet), depending on the locality. The thickest section, constructed graphically from dip values, is in the Quebrada Arenosa, and although all surface measurements are suspect in continental type deposits, it would seem from the work done so far, that the Parángula formation thickens along the observed strike from the Río Guanare southwestward toward the Río Acequia.

Stratigraphic Relationships: The contact of the Parángula formation with the underlying "El Mene" formation of Mackenzie (1937, p. 264) is unconformable and usually sharp, representing a major change from shallow marine sedimentation to a nonmarine or continental type of deposition. In most places the angle of unconformity is imperceptible to perhaps a few degrees and, in Quebrada Ticoporo, one kilometer southwest of Río Acequia, it is extremely difficult in the field to differentiate the lowermost Paránguia from the uppermost "El Mene". This is one of the very few localities where the two formations appear to be transitional and concordant; elsewhere the change in Ethology is quite distinct even though the angle of disconformity is generally so small as to be undeterminable. A pronounced angular unconformity has been observed, however, in two localities. One of these is in the upper course of the Río Bumbum where there is a sharp irregular erosion surface upon which lies a granule to boulder-conglomerate of the Parángula formation. The other locality is 3,400 meters north of the town of Guanare in Quebrada Medero where a conglomerate at the base of the Parángula unconformably overlies weathered rocks of the underlying upper Eocene formation at an angle of 20 to 45 degrees. Sandstone blocks in the conglomerate are similar to sandstones found elsewhere in the earlier deposited "El Mene" formation and this, in conjunction with the angularity of superpositi on, implies considerable local uplift and erosión prior to the deposition of the Parángula. Nevertheless, in a considerable portion of the low foothill belt paralleling the edge of the present llanos and perhaps under the llanos themselves there seems to have been a relatively quiet period with but little erosion of the older formation prior to sedimentation of the Parángula.

The Parángula formation is conformably and transitionally overlain by the Río Yuca formation. The zone of gradation is generally 25 or more meters thick but the contact between the two formations is placed at the top of the læst, quartz sandstones which do not contain arkose and large flakes of muscovite. Arkosic sandstones, mica in large flakes, and the general greenish tone of the beds are field criteria for identifying the Río Yuca formation and distinguishing it from the Parángula formation.

Heavy Minerals: The heavy minerals of the Parángula formation include a simple zircon iron ore tourmaline suite with rare andalusite, muscovite, staurolite and titanite.

Fossils and Age: Although organic remains are rare in the Parángula formation, large pieces of lignitized wood have been observed in the conglomerates at the base of the formation in the Río Bumbum, and foraminifera have been collected from a horizon 170 meters below the top of the formation in Quebrada Negra, 24 kilometers southwest of Barinitas. The foraminifera are in a brownish gray silty to sandy shale with thin streaks Of white sandstone whose grains are faceted. Among the very sparse calcareous forms identified are two different species of Gümbelina, four species of Globigerina, and one species each of Anomalina, Bolivina, Bulimina and Uvigerina, and unless these foraminifera are reworked, they appear to indicate an upper Eocene age for the formation. The shale Ethology and fauna are very suggestive of the underlying formation of upper Eocene age and indicate, possibly, a brief recurrence during Parángula time of the shallow marine to brackish water conditions that prevailed in "El Mene" time. Although it is difficult to determine whether the fauna in the above-mentioned shale is reworked or not, the writer is given to understand that definitely reworked upper Eocene foraminifera have been encountered in similar shales in a number of places within the Parángula formation, and this latter interpretation suggests that the Parángula may have been deposited in postupper Eocene time, perhaps during the Oligocene and early Miocene.

Extent: The Parángula formation has been traced in outcrop along the southeast side of the Mérida Andes in the lower foothill belt adjoining the llanos for a distance of 140 kilometers from the Rio Acequia northeastward to the Río Portuguesa. The formation is probably exposed a short distance farther to the southwest to the Río Socopó and also a greater distance to the northeast perhaps as far as the latitude of Acarigua. From the western border of the llanos the formation has been identified back in the hills for a distance of 30 kilometers, and under the llanos themselves to the east and south the formation is believed to cover an extensive area, gradually thinning out, however, toward the Guayana Shield.

Correlation: The Parángula formation is probably correlative with the lower part of the Betijoque group on the northwest side of the Mérida Andes. The Betijoque group is also composed of continental deposits and E. Mencher, et al. (1953, p. 710-711) suggest that both the Parángula and Betijoque may have been deposited in an inland basin receiving sediments from the reelevated cordillera lying to the north and northeast near the present day coast range.

Norman E. Weisbord