AGUANEGRA, Formation

See AGUA NEGRA, Group

AGUA NEGRA, Group

TERTIARY (upper Eocene to middle Oligocene)

States of Lara and Falcón, Venezuela

Author of name: A. Senn, 1932 (private report).

Original reference: A. Senn, 1935, p. 59.

Original description: ibid.

The Agua Negra unit (Aguanegra-formation) was proposed by A. Senn as a designation for a complex of sediments exposed in the Serranía de Agua Negra (sometimes written, Aguanegra) in the boundary region of the states of Falcón and Lara. The unit comprises principally clastic sediments, from conglomerates to shales, but includes some intercalated reef limestones. Lateral facies variation is conspicuous.

Senn lays stress on what he considers the complex as a single geologic formation:

"Although this formation includes two stages, upper Eocene and lower Oligocene, we are obliged to introduce this new formation name, since only under favorable conditions is a division of the Eocene and Oligocene parts possible. In other words, frequently a large part of the Aguanegra formation consists of thick unfossiliferous mottled clays, which do not permit any positive delimitation between the Eocene and the Oligocene... The Aguanegra formation, from the stratigraphic point of view, forms a unit, since it is bounded above and below by marked unconformities." (Senn, 1935, P. 59).

Senn observes, that the unconformity in the base, is rarely observable directly, but it is deduced from the distribution of the facies within the formation, and from the presence of pebbles of older rocks (e. g. of the Misoa-Trujillo) in the conglomerates. Above, the Agua Negra formation is overlain with unconformity by reef limestones of the San Luis.

Senn presents a restored geologic section (1935, p. 61), which represents his interpretation of the different facies represented within the formation, which he considers was laid down in a basin whose axis was directed N 75 E. From north to south, he distinguishes seven different facies zones, characterized respectively by: clays, varying proportions of clays, sands and silts, basal conglomerates, reef limestones, and (towards the south) a coal horizon. In general, the coarse clastics are more prominent towards the south, whereas the reef limestones are more conspicuous in the Oligocene in the center of the basin (the "Churuguara facies").

The best subdivision of the formation (the word "Gliederung" suggests a division in members, but Senn does not use the term member in any place with reference to the subdivisions of the formation") is observed, according to Senn, in the Buena Vista anticline, on its north and south flanks, along the old "Camino real" between Piedra Grande and Baragua. The divisions here, in ascending order are: a) Santa Rita conglomerate, 400-500 meters thick; b) Jarillal shales (about 375 meters), of which the basal 50 meters are called the Raetomya shales, and referred to the upper Eocene, the rest to the lower Oligocene; and c) the Churuguara beds (800-1,000.) referred by Senn to the lower Oligocene. All the contacts are transitional. The total thickness of the Agua Negra varies between 1.575 and 1825 meters. Senn discusses at length the paleontology of the divisions and correlations with other faunas of tropical America. (For detailed descriptions, see SANTA RITA Conglomerate; JARILLAL Shales and CHURUGUARA Formation).

In the restored section and in the correlation table (Seen, 1935 taf. VIII) Senn indicates that towards the south and east, part of the Santa Rita conglomerates passes into the Cerro Campana beds; the Jarillal shales (including the Raetomya beds) into the Tacamire shales and Guayaval marls; and part of the Churuguara beds, passes into the Tacamire sandstone. (See articles on these units).

Hedberg, in 1938, pointed out that the Agua Negra might better be considered a group than a formation, but later authors have not made it clear whether they consider it as a group or formation; however, no one has used the term "member" with reference to any of the subdivisions and Liddle (1946) describes the Churuguara as "formation". (For other mentions, see Senn, 1940; Renz, 1942; Renz, 1948; Bucher, 1952).

Renz (1948) has given arguments for referring the Churuguara beds to the middle Oligocene, instead of lower Oligocene as Senn considered them. (Renz, 1948, p. 43).

The undersigned considers that the Agua Negra stratigraphic unit should be formally recognized as a group, composed (in the type section) of three formations, Santa Rita conglomerate, Jarillal shales, and Churuguara formation. Assuming that the variations of facies are as Senn describes them, there would be no objection to using the same term, Agua Negra, as a formation name, in regions where the equivalent of the group consists of barren shales impossible to subdivide. On the other hand, it does not seem desirable to follow Senn in extending the use of the name Agua Negra in eastern Falcón, where the equivalent beds do not form a natural group. (See articles TACAMIRE Shales, TACAMIRE Sands, GUAYAVAL Marls, CERRO CAMPANA Limestone). The Agua Negra as interpreted by Senn (1935, 1940) should therefore be restricted to exclude the above-named beds eastern Falcón. With this restriction, the original description by Senn is adequate for the recognition of the group, but cannot be considered entirely satisfactory; it would be very desirable to have a more complete study, with type sections precisely located, detailed measurements and descriptions, geographic distribution of the formations and detailed paleontologic studies.

Frances de Rivero