CACHIRI, Group

See RIO CACHIRI, Group

RIO CACHIRI, Group

DEVONIAN

State of Zulia, Venezuela

Author of name: R. A. Liddle, 1928.

Original reference: R. A. Liddle, 1928, p. 97-99.

Original description: ibid.

This group was originally named the Río Cachirí series by Liddle (1928, p. 97) to designate an assemblage of black, gray and shales, gray micaceous sandstones, gray quartzitic sandstones and red, blue-gray and blackish limestones outcropping in the upper course of the Rio Cachirí about 17 kilometers southwest of Dibujado and 89 kilometers west of Maracaibo, in the District of Maracaibo, State of Zulia. Field data relating to the group was first obtained early in 1924 by C. W. Yeakel, P. W. MacFarland and R. A. Liddle, and again in 1926 by Joe Netick, G. A. Weaver and Malcolm Madera. Fossils collected during the 1924 expedition were described by N. E. Weisbord (1926) who assigned an upper lower to lower middle Devonian age to the faura.

In 1942, the Río Cachirí group was examined in more detail at its type locality by R. A. Liddle and, in the following year, the results of this study were published by Liddle, Harris and Wells (1943). The strata comprising the group were again referred to as the Río Cachirí series but, as has been done by Sutton (1946, p. 1,634) and González de Juana (1951, p. 131), it is preferable to use the word group to signify a geographically named rock sequence consisting of two or more formations. The Río Cachirí group is not be confused with A. H. Garner's Cachirí limestone (1926, p. 3) which is Cretaceous in age.

The Río Cachiri group, as now defined, is composed of three formations: Caño Grande, occupying the lower subdivision of the group, Caño del Oeste, the middle, and Campo Chico, the upper. The type section for the group is in Caño Grande, a tributary of the Río Cachirí. The lowest exposed bed occurs in Caño Grande 1,130 meters upstream from the mouth of Caño del Sur, where it is discordant and probably in fault contact with schists of preDevonian age. The top of the group is just under a basal conglomerate of the overlying Palmarito formation in the Caño Grande at a point 2,065 meters downstream from the mouth of the Caño del Sur. The upper contact is presumed to be one of unconformity.

The thickness of the Río Cachirí group is calculated to be at least 2,438 meters (8,000 feet). The lowermost 762 meters (2,500 feet) constitute the Caño Grande formation which consists largely of gray micaceous and calcareous shales except at the base where gray quartzitic sandstones and argillaceous sandstones are present. The shales in the upper half of the formation are richly fossiliferous containing brachiopods, corals, bryozoa, mollusks, crinoids and an occasional trilobite. In view of the fault at the base of the Caño Grande formation in the type section, it is possible that the lowermost beds of the Río Cachiri group have not been observed and that the true thickness of the unit in question is greater than has been measured on the surface.

Transitionally above the shales of the Caño Grande formation, and forming the middle unit of the Rio Cachirí group in the type section, are 1,067 meters (3,500 feet) of bluish-gray, fine-grained quartzitic sandstones and black micaceous unfossiliferous shales which constitute the Caño del Oeste formation. Near the base of the formation is a mine-meter sill of basalt on either side of which are indurated slaty shales. A more complete section of the middle unit of the group is exposed in Caño del Oeste 3 kilometers northeast of Caño Grande and here there are a number of gray sandy shale beds and gray calcareous shale beds with limestones that carry fossils identical to some of those occurring in the lower unit in Caño Grande.

The uppermost 609 meters (2,000 feet) of the Río Cachirí group consist of dark gray quartzitic sandstones and dark gray shales interbedded with occasional thin black limestones. This unit, which constitutes the Campo Chico formation, is conformable with the underlying Caño del Oeste formation but is believed to be unconformable with the overlying Palmarito formation.

The fifty or more fossils identified by Weisbord (1926) and Liddle, Harris and Wells (1943) from the lower and middle parts of the Rio Cachirí group, indicate that the Caño Grande and Caño del Oeste formations were deposited between Oriskany and Hamilton times as represented in North American chronology, and that the age of these strata is Middle Devonian. The upper part of the Río Cachirí group is said by Liddle to be unfossiliferous but the transitional nature of the subdivisions and the recurrences of similar sediments throughout the whole of the group are thought to indicate that deposition continued into Upper Devonian time.

From the occurrence north of the Río Cachirí of fossiliferous float resembling lower beds of the Río Cachirí group, and from outcrops south of the Río Cachirí (in the Ríos Tinacoa, Cuiba and Macoita) resembling rocks in the upper part of the group, it is inferred that the Río Cachirí group or equivalents thereof, occupy a belt all along the east side of the Sierra de Perijá extending southward from the north end of the range in the District of Páez, into the Districts of Mara, Maracaibo and Perijá for a distance of approximately 110 kilometers. How much farther south this Mesodevonian belt on the east flank of the Perijá Mountains continues is not known although it is not unlikely that it continues into northernmost Colombia.

Elsewhere in Venezuela, Middle Devonian rocks are reported by Liddle (1928, p. 99; p. 114) in the valley of the Río Momboy near Mendoza in the State of Trujillo, and by Christ (1927, p. 403-406) in his Mucupati series in the State of Mérida. Both of these localities are in the Cordillera de Mérida, a spur of the Andes Mountains some 250 kilometers southeast of the Sierra de Perijá. Although Liddle correlates the Momboy Devonian with the Caño Grande formation of the Cachirí group, details concerning the stratigraphy and paleontology of the former are so meagre that the correlation must be considered tentative. As for the Mucupati series, fossils collected therein by Kehrer (1938, p. 509) have been determined by Woodring to be Cretaceous in age.

In the neighbooring Republic of Colombia, Middle Devonian rocks are reported in the Guajira Peninsula by Schuchert (1935, p. 672), at Floresta in the Cordillera Oriental by Caster (1937, p. 269; 1942, p. 27), in the Curumaní-Santa Isabel region some 30 kilometers or so southeast of Chiriguaná, and in an area lying 1.5 kilometers east of Manaure, by Trumpy (1943, p. 1,291). The last two localities are both on the west side of the Sierra de Perijá although Manaure is approximately 135 kilometers northeast of the Curumaní-Santa Isabel region.

Little is known about the Guajira occurrence except Schuchert's statement that fossils collected by Scholl and Remington were identified as Middle Devonian by Galloway. Considerably more is known about the Floresta Devonian from the work of Olsson and Caster (1937, p. 269; 1939) who believe that the Floresta beds were deposited about the same time as those of the Río Cachirí group. Fossils collected from the Curumaní-Santa Isabel locality have been identified by Emeis as Middle Devonian and the rich fauna from Manaure also has been so dated by James Steele Williams. Trumpy (1943, p. 1291) is of the opinion that the Curumaní-Santa Isabel and Manaure faunas are more closely allied to Floresta than to Río Cachirí even though they occur nearer (190 and 70 kilometers, respectively) to the Venezuelan locality than to Floresta which is 375 kilometers southeast of Curumaní-Santa Isabel and 500 kilometers south of Manaure. However, it may well be that the Mesodevonian beds at Floresta, Curumaní-Santa Isabel, Manaure and the Guajira peninsula in Colombia were all coetaneously deposited with the Río Cachirí group, or portions thereof, despite differences in lithology and faunal composition which may be due to environmental factors at the time of deposition. Caster (1942, p. 58) visualizes a Mesodevonian seaway extending from North America southward across the Caribbean Sea into Venezuela, Colombia, Peru, Brazil and Bolivia, and it would seem from the data available that it was in these waters that the Venezuelan and Colombian deposits were laid down.

For further information on the divisions of the Río Cachirí group see also the CAÑO DEL OESTE, and CAMPO CHICO Formations.

Norman E. Weisbord