RIO NEGRO, Formation

CRETACEOUS (basal Aptian-Neocomian)

State of Zulia, Venezuela

Author of name: H. D. Hedberg, 1931.

Original reference: H. D. Hedberg, 1931, p. 230.

Original description: ibid.

The name Río Negro Conglomerate was applied by H. D. Hedberg to the clastic deposits of variable thickness which initiate the Lower Cretaceous sedimentation in the Sierra de Perijá, Zulia (Hedberg, 1931, p. 230). These transgressive beds had previously been mentioned in the geologic literature on Venezuela as Basal Cretaceous Conglomerate (Liddle, 1928, p. 118); the term Basal Sandstone has lately been used in the Maracaibo Basin (Smith, 1951, p. 64).

R. A. Liddle characterized his Basal Cretaceous Conglomerate below the Cogollo limestone to consist of a variety of red, yellow, and grayishwhite sandstones, shales, and conglomerates, with massive, white to yellow coarse-grained quartzose and saccharoidal sandstones. Gray-brown moderately coarse conglomerates and coarse-grained sandstones are mentioned from the Sierra de Perijá. Furthermore, gray medium-grained sandstones and carbonaceous-calcareous, gray to purplish shales are mentioned. Rare black shales and a few limestone stringers are reported to oceur in the upper part of the clastics (Liddle, 1931, in Hedberg, 1931, Discussion, p. 245).

The name Río Negro formation was introduced by H. D. Hedberg and L. C. Sass (1937, p. 79). The formation was characterized as being composed of light-colored whitish to pink or greenish sandstones and coarse arkosic sandstones, crossbedded, with layers and pockets of quartz conglomerate, as a rule loosely cemented but with some very hard quartzitic beds. A few thin interbeds of fine-grained sandstone and siltstone occur, and the formation becomes slightly calcareous toward the top. lt is transitional into the overlying Cogollo limestone (see Apón Formation).

The type locality is on Río Negro, Sierra de Perijá, Zulia.

The thickness of the Río Negro formation in the Sierra de Perijá is very variable and attains a maximum of about 5000 feet or more in the Machiques trough (Río Negro area). It exhibits a rapid thinning toward north and south. On Toas Island, the basal Cretaceous clastics that correspond to the Río Negro formation still reach up to 100 meters in thickness. The correlative, medium to coarse-grained sandstones with shale partings, grits, and conglomerates at the base of the Apón formation ("Cogollo" of authors) in the La Paz-Mara Field display a thickness of 10-60 feet (Staff of Caribbean Petroleum Company, 1948, p. 599; Smith, 1951, p. 64, Fig. 4), and the corresponding pre-Apón quartz sandstones drilled near El Tablazo Bay show a reduced thickness of about 20 feet.

Basal sands, grits, and conglomerates appear to occur everywhere below the Apón formation, though often in a greatly reduced thickness. There is, accordingly, every reason to include all the pre-Apón Cretaceous clastics of the Sierra de Perijá and of the Maracaibo Basin in the Río Negro formation. In the writer's opinion, also the merely 5-12 meters thick conglomerates and coarse-grained sandstones (grading upward into fine-grained calcareous sandstones), which occur at the base of the Tibú Member of the Uribante formation in the Barco Concession (Notestein et al., 1944, p. 1174), represent a reduced remnant of the Río Negro formation.

The Río Negro formation, representing the initial clastics of the Lower Cretaceous in a very variable thickness, is thus known te underlie the Apón formation in the Sierra de Perijá and in the Maracaibo Basin.

F. A. Sutton (1946, p. 1641) favors the idea of restricting the term Río Negro formation to the Sierra de Perijá instead of applying it to other clastic basal Cretaceous deposits. In his opinion, the Río Negro formation is rather a facies that transacts time boundaries and steps upward in the time scale. Such a concept is, however, not supported by the paleontological evidence hitherto at hand. The "probable Barremian" age of the overlying basal Apón formation on Río Negro, or the late Aptian age of the lowermost Apón formation on Río Cachirí and Toas Island is hypothetical and may not be used as proof of a different age of the Río Negro formation here and there. The assumed correlation of the Río Negro formation with the Uribante ("Tomón") formation does not stand as the latter represents not only the lowermost conglomeratic and sandy deposits but the entire Lower Cretaceous sequence up to the Capacho formation in a chiefly clastic facies; the so-called "Cogollo limestone" that surmounts the Uribante formation actually should be referred to as Capacho formation. The latter may not be correlated with the Apón (formerly Cogollo) formation that overlies the Río Negro formation, but corresponds only to the uppermost formation of the Cogollo Group.

Rather than assuming that the Río Negro formation in places might reach higher up in the stratigraphic sequence than elsewhere, in other words, that the boundary Río Negro-Apón is a facies line, the writer favors another alternative to explain the great thickness of the Río Negro formation near the type locality: It is conceivable that the deposition of the conglomeratic deposits set in earlier in the Machiques trough and progressively spread over larger areas. This interpretation makes the gradational boundary Río Negro-Apón formation a time line, instead of an oblique facies boundary.

In the Andes, on the other hand, the sedimentation of clastic material continued during the better part of Aptian-Albian time (Uribante formation).

No fossils have as yet been found in the Río Negro formation, and the assigned basal Aptian and/or Barremian-Neocomian age is only based on stratigraphic deductions (transition with the overlying lower Apón formation of early Aptian age; unconformable contact with the subjacent preCretaceous).

Wolf Maync