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:
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
SANTA RITA, Conglomerate
TERTIARY (upper Eocene)
State of Lara, Venezuela
Author of name: A. Senn, 1932 (private report).
Original reference: A. Senn, 1935, p. 60.
Original description: ibid.
The name Santa Rita conglomerate was published by A. Senn (1935, p. 60) who described it as the basal member of his Agua Negra formation (now group) which he described in the same publication. The other two members (in ascending order) were called Jarillal shales and Churuguara beds. Senn's description reads (translated):
"Santa Rita conglomerate (400-450 meters), coarse-grained conglomerates, sandstones and sandy marls with intercalated orbitoidal limestones. The conglomerate consists principally of cobbles up to 10 cms in diameter, of white quartz, black Cretaceous cherts, and coarse quartzites of the Misoa-Trujillo and other sandstones." (Senn, 1935, p. 60.)
According to geologists who have worked in the region, pebbles of green jasper are especially characteristic of these conglomerates.
The name was doubtless given in allusion to the hamlet of Santa Rita, about 12 kms. NE of Baragua, in the state of Lara, on the old road between Baragua and Piedra Grande. Since Senn designates the type section of the Agua Negra "formation" as a whole as being located on the flanks of the Buena Vista anticline, in the Agua Negra range, along the old Spanish highway, the type section of the Santa Rita is included in this designation.
Senn interprets the basal contact of his Agua Negra "formation" (hence that of the Santa Rita conglomerate) as an angular unconformity above the underlying middle or upper Eocene formations. To the north (Seen states), the beds lie on the Paraiso formation or on the Paují shales; to the south on the lower Misoa-Trujillo beds. Senn states that it is difficult, however, to observe this lower contact, since it is generally hidden by later foldings. From geologists who have worked in the region we may add the following data: in the type locality of the Santa Rita, the conglomerate is overturned and the base is not visible: but N of Carora, a good section of Santa Rita is exposed, Iying on Paují shales. The Paraiso formation which Senn considered Eocene, is now believed to correspond to a higher horizon (Oligocene), so that the Santa Rita conglomerate could not overlie it in a normal sequence, as Senn believed. The upper contact of the Santa Rita conglomerate with the Jarillal shales, is transitional, as Senn states. He also observes that south of the town of Churuguara, the Jarillal shales thin out due to facies changes, so that the Santa Rita conglomerate comes to lie directly below the Churuguara beds.
Senn cites species of foraminifera and mollusks which determine the upper Eocene age of the Santa Rita. These same molluscan species are also characteristic of the lower 50 meters of the overlying Jarillal shales, which Senn distinguished as the "Raetomya shales"; hence Senn proposes to recognize a "Santa Rita stage", comprising the Santa Rita conglomerate plus the Raetomya shales, which would correspond in age to youngest upper Eocene. (The lower part of the upper Eocene, Senn would call the "San Pedro stage".) These proposed stages of Senn have not been recognized by later workers however.
Senn correlated the Santa Rita conglomerate with the Cerro Venado limestone (see article).
The Santa Rita conglomerate has been mentioned in various publications, without any additional details or modifications. As noted in the article on the Agua Negra, it seems convenient to treat that unit as a group, and hence raise the Santa Rita to the rank of formation.
Paleontology.- Senn recapitulates the identifications of foraminiferal species effected by Gorter and Van Der Vlerk (1932) and by Gravell (1933). (In the first named-paper, the species cited from the localities numbered 1142, 1149, 1152, and 1162, which were described as "Guayaval series, Cerro Campaha (sic) limestone", also belong to the Santa Rita beds) - Senn's list includes the following species: Discocyclina flintensis Cushman, D. (Asterocyclina) georgiana Cushman, Lepidocyclina trinitatis H. Douvillé, Nummulites cf. parvulus Cushman, Operculina cf. cookei Cushman, and Bulimina jacksonensis Cushman. Rutsch (1930, 1937) had identified the characteristic Eocene gastropod Rimella (Ectinochilus) gaudichaudi alauda Olsson, and Seitz and Rutsch (1930) identified a pelecypod, Mya (Raetomya) schweinfurthi falconensis Rutsch. In addition, Senn mentions several genera of mollusks. A. K. Miller (1947, p. 105) has identified the nautiloid Aturia peruviana Olsson from two localities near Santa Rita, which provides still further confirmation of the upper Eocene age.
Frances De Rivero