SAN JUAN, "Formation"

See SAN JUAN DE LOS MORROS, "Formation"

SAN JUAN DE LOS MORROS, "Formation"

State of Guárico, Venezuela

Author of name: A. H. Garner, 1926.

Original reference: A. H. Garner, 1926, p. 680.

Original description: ibid., p. 678, fig. 1.

A. H. Garner (1926, p. 680) first named the San Juan de los Morros formation and designated as type-locality "small hills in the vicinity and south of the town of San Juan de los Morros", at that time located in the State of Aragua. The original description is missing in the text, apparently due to a typographical omission. Under such circumstances, his stratigraphical column of central Venezuela (p. 678, fig. 1) must be taken as original description. In this column, Garner assigns to the formation, a thickness of 2,500 ft. and an Eocene age (correlated with La Rosa and Naricual Coal Measures) and shows an upper sedimentary contact with "El Pao sandstone". Actually, this does not correspond to the more recent observations which have failed to found any upper contact, due to the erosion and the structural complications. The section is a sequence of interbedded shales and sandstones in which no major limestone body that might represent the Los Morros de San Juan limestone, may be observed.

Some geologists have recently proposed to give up Garner's term, basing themselves on the fact that some Cretaceous section (Arrayanes formation), along with Tertiary sediments, could very well be exposed among the "small hills in the vicinity and south of the San Juan de los Morros". Nevertheless, various authors that for many years have been using the name originally proposed by Garner, have restricted the definition to the "Eocene" (Paleocene) sequence which outcrops in the region of the Guárico and San Juan River Valleys. Provided that such restrictions could be formally explained and that the reef facies named by Liddle (1946), Los Morros de San Juan limestone could be excluded from the San Juan de los Morros "formation", the name San Juan de los Morros would be evidently adequate to designate the unit. Such unit, according to suggestions made at the end of the present article, could be included as member of the recently named Guárico formation.

S. E. Aguerrevere and G. Zuloaga, in their Geological Column (1937, front p. 12), raised Garner's term to San Juan de los Morros "series" to describe an "Eocene" sequence of "conglomerates, sandstones, laminar clays and fossiliferous marls". Such description is completed by the following information: (p. 19) "in the Eocene sediments of the neighborhood of San Juan de los Morros", we have found volcanic ashes"; (p. 21): "At La Puerta, between Villa de Cura and San Juan de los Morros", thick sandstones, marls, conglomerates and limestones containing fossils which age was determined by Woodring as Eocene, basing himself on the presence of Turritella aff. mortoni, are Iying directly on top of the Los Morros limestone". In such description, as in Garner's original description, the Los Morros limestone, then considered as underlying and probably Cretaceous in age, was excluded. Such limestone (ibid. Column, front p. 112) was then included in the socalled Villa de Cura "series" which is assigned by the authors to the Mesozoic (?). The same authors (1938, p. 281) were of the opinion that the Villa de Cura "unit", then considered "Cretaceous or younger", was "apparently in fault contact" with "the rocks" (shales, sandstones, conglomerates and limestones) whose age is Eocene, that outcrop from La Puerta (to the north of San Juan de los Morros) towards the south.

Another reference about the "Eocene" fauna found at La Puerta is due to L. Kehrer (1937, p. 66) who makes clear that the Turritella mentioned before is "similar to a type Midway of Alabama", and refers for the first time to an associated orbitoidal and nummulitic fauna. Kehrer, who was quoted with no comments by Liddle (1946, p. 308-309), referred to "Lepidocyclina", the orbitoids found at La Puerta and at the foot of Los Morros, which are today referred (Mme de Cizancourt, 1951) to various species of Actinosiphon, Discocyclina, Bontourina and Pseudophragwina and he referred to "Operculinella" some nummulitic forms, probably Nummulites s. s. ó N. (Operculinoides). It is not strange that under such conditions, Kehrer included in his list of "correlative units", along with Paleocene (Midway) beds, others of Jacksonian age, such as the La Pedrera limestone to the north of Clarines (Tinajitas formation) and small bioherms on the road from Altagracia de Orituco to San Francisco de Macaira, just to the east of the El Morrito Farm, where the writer identified Lepidocyclina (Pliolepidina) pustulosa and Discocyclina (Asterocyclina) asterisca (see GUARUMEN GROUP). These correlations appear again in Liddle (1946, loc. cit.).

A new reference to the unit here discussed, the name of which is abbreviated to "San Juan formation" is due to V. López (1942, p. 49) who indicates that such rocks "not metamorphosed appear from the region of Los Morros de San Juan towards the east and south. The contact between the sediments and the metamorphics is found along the east-west fault that goes through La Puerta". V. López (p. 56, 57 and 73) assigns to the form ation a Cretaceous and lower Eocene age.

C. M. B. Caudri (1943-44) published a study of a nummulitic and orbitoidal fauna that was based on samples collected by S. E. Aguerrevere in different levels of the San Juan de los Morros "formation". These samples were coming from limestones, marls and sandy marls interbedded as beds, lenses and occasionally (p. ex. G. 125c) small biostroms which are included in the mainly clayey-sandy, microconglomeratic sequence. Such microfauna was assigned to the Paleocene and the sediments correlated with the Soldado formation of Trinidad, by Miss Caudri.

Liddle (1946, p. 304) and some other authors restricted the correlation made by Miss Caudri, not to the San Juan de los Morros meinber as it seemed logical, but to biohermal facies of the large reefs (San Juan de los Morros limestone). He does not even mention the occurrence of occasional, lenticular, thin limestones containing large foraminifera that are intercalated in the facies of the type fore -reef and back- reef from which almost all samples investigated by Miss Caudri, came from. On the contrary, the sediments intercalated with said limestones do not contain foraminifera. Such clayey-marry-sandy facies (San Juan de los Morros member) was assigned (p. 305) to the lower middle Eocene: "hills formed by shales and sandstones of Lower Middle Eocene age are believed to be the ones referred to by Garner as the type locality for his San Juan de los Morros formation". The beds which form such hills (p. 304) lie in conformity on top of orbitoidal limestones, a few kilometers to the southeast of the Los Morros de San Juan. The age proposed by Liddle for the San Juan de los Morros "formation" is probably based on A. A. Olsson's opinion on the Campanile, Turritella and Morgania fauna which was collected around the area east of La Puerta ("Morritos") (see GUARICO FORMATION for this fauna now referred to as Paleocene which is associated in the outcrops with "corded nummulites").

This fauna is obviously similar to that outcropping to the south of La Puerta (Aguerrevere's locality G-91, Caudri 1943-44, p. 7, 25) in marls where "Woodsalia" (probably Turritella cf. mortoni) is associated with corded Nummulites and Actinosiphon fauna. It is also similar to another Aguerrevere of fossiliferous localities which underlies eastward the Morro del Faro limestone. This locality is called "Woodsalia" and Ostrea bed (Caudri, 1943-44, p. 25-26 Sect. 2) and it is also part of the San Juan de los Morros "formation". Karsten described such "clays and conglomerates" Iying under the Los Morros limestone, when he studied (1886) the rocks that outcrop at the large morros. The Turritella (Caudri's Woodsalia) and Ostrea collections were probably coming from these beds. Other fossils mentioned by Karsten (p. ex. "Hiprurites") suggested a Lower Cretaceous age, and in his section, Karsten mentions "caprinids" in the foot of the Los Morros limestone.

Liddle (p. 305) mentions, without further explanation, an upper contact of the San Juan de los Morros 'formation" with "Upper Eocene shales containing mud flows". Actually, there is no evidence today for the occurrence of upper Eocene sediments in the region, and the upper contact of the San Juan de los Morros "formation" is not seen.

The most recent study on the microfauna from the thin limestone intercalated in that member is due to Mme. de Cizancourt (1951), who introduced some taxonomical changes in respect of Miss Caudri's study without changing the age determination. Mme. de Cizancourt distinguishes three "levels" in the section here studied which she called "a", "b" and "c". The "b" and "c" levels are included in the "corded Nummulites zone" and are definitely referred to the Paleocene. The oldest, "a" level, constituted by a "calcaire trouble", does not contain diagnostic fauna, and it is identified in clastic fragments which constitute calcareous microbreccias in the upper levels. The "d" level, the uppermost in the "corded Nummulites zone", considered as Eocene, does not seem to be represented in the neighborhood of San Juan de los Morros.

Excellent outcrops of the San Juan de los Morros "formation" containing microfauna typical of levels "a" ("calcaire trouble") "b" and "c", are exposed immediately to the west-northwest of San Sebastián (Quebrada Honda; cerro El Pilón and between the Pao and Caramacate Rivers) and further east, at Pardillal (level b) (Mme. de Cizancourt, p. 18-22; 23).

Mme. de Cizancourt (p. 34) establishes a close correlation between the faunas from the region of San Juan and the faunas from the Quebrada Roncona (affluent of the Guanare River, State of Portuguesa) of Guarico (State of Lara) and the Carache River (State of Trujillo). The Quebrada Roncona beds are restricted to the Paleocene as those in the neighborhood of San Juan, while the same section in Guarico and Carache River, comprises lower and middle Eocene beds also.

E. Mencher (1950, p. 97) introduced the name Guárico formation, unit subsequently described by Mencher et al. (1951, p. 14). The new name obviously includes, besides Garner's San Juan de los Morros "formation" (1926), bioherms named by Liddle (1946) Los Morros de San Juan limestone. Considering the advantage of grouping..the "formations" which constitute interdigitated facies of the same sedimentary cycle under a single unit, the Executive Committee of the present Lexicon suggested that the units described by Garner and Liddle, would continue to be distinguished as members of the Guárico formation. The name proposed by Liddle for the limestone, should be modified in the future to "member of Morros del Faro limestone" and Garner's name, which is used for the sequence of conglomerates, marls, tuffs, flows, shales and sandstones, changed into "San Juan de los Morros member".

Hedberg, H. D. (1950, p. 1196) tried to include the reef facies and associated sediments in Anzoátegui and Guárico in his Santa Anita group, enumerating with what is called Morro del Faro member, bioherms of apparently older age, as those of San Francisco de Macaira. He said that the "morros" represent reefs in sediments of Caratas age. Said inclusion, purely chronostratigraphical, of different lithotops deserving formational differentiation, has not been generally accepted. Hedberg's proposition is comparable to that of including the Guarumen group (see) in the Santa Inés group.

(see also LOS MORROS DE SAN JUAN LIMESTONE, GUARICO FORMATION, "ORTIZ FLYSCH SERIES" and SAN FRANCISCO DE CARA BEDS.)

J. M. Sellier de Civrieux