GUARICO, Formation

TERTIARY (Paleocene)

States of Guárico and Aragua, Venezuela

Author of name: E. Mencher, 1950.

Original reference: E. Mencher, 1950, p. 97.

Original description: E. Mencher, et al., 1951, p. 14.

The Guárico formation derives its name from the upper reaches of the Río Guárico in the northernmost part of the State of Guárico and the adjoining State of Aragua. The formation was first named by E. Mencher (1950, p. 97) and then briefly described by Mencher, et al., the following year (1951, p. 14). In a personal letter to N. E. Weisbord dated April 4th, 1954, Mencher discussed the formation in greater detail, and the remarks in quotations which follow are taken directly from this letter. Certain other statements in the letter, however, have been modified to accord with the recent work of P. Leuzinger (private report) and with interpretations of the Venezuelan Lexicon Committee.

The type section of the Guárico formation designated by Mencher is along the Guárico River from La Puerta de San Juan to the large northpointing meander between the Garrapata and Totumo haciendas. Here, the Guárico formation consists of Paleocene gray limestones, conglomerates, calcareous sandstones and sandstones, and dark calcareous to non-calcareous shales interbedded with sandstones. Associated with the foregoing are volcanic rocks-agglomerates, tufas, and flows - and "these seem to be limited to the lower part of the formation." Similar volcanics are abundant in the underlying Arrayanes group of Cretaceous age, but Mencher states there is no doubt about the Paleocene age of some of them since interfingering with fossiliferous Paleocene strata can be observed at some places. Although the type section is difficult to measure because of stratigraphic coinplexities, Leuzinger calculates that the thickness of the Guárico formation is 450-500 meters. The base of the formation is exposed in many localities and is unconformable on the earlier geosynclinal deposits of the Arrayanes group, and on metamorphic rocks of presumed pre-Cretaceous age. The top of the formation has not been observed but is inferred to be one of unconformity and to mark the termination of geosynclinal activity which began in Upper Cretaceous time.

Prominent within the Guárico formation are reef limestones which are white to gray in color, are sometimes characterized by a distinctive pseudooölitic structure, and occasionally form large, isolated hills called morros. One such limestone at San Juan de Los Morros has been named the Los Morros de San Juan limestone by Liddle (1946, p. 304-309) and it is proposed that this unit be included within the Guárico formation. Smaller reefs and lenses of similar limestone are abundant and these are often highly fossiliferous, containing "Ranikothalia" and other foraminifera, corals, Lithothamnium, and mollusks. "Many of the limestones consist of re-cemented angular blocks of limestone. These are interpreted as the broken edges of reefs and may grade into conglomerates bearing large limestone blocks in a sandy or shaly matrix. It is probable that there are a number of places in the Paleocene section where such reefs occur, although it would seem that in general they are limited to the lower part."

Although the limestones of the Guárico formation are the more striking, the bulk of the formation consists of calcareous and noncalcareous sandstones, dark calcareous to non-calcareous shales interbedded vith sandstones, and dark, olive-weathering calcareous shales. "Good sections of the sandstones-shale sequence can be seen north of San Juan de Los Morros, where the lower sandy beds sometimes carry an abundant molluscan fauna (with) Turritella, Cerithium, etc. The... shale section can also be seen best north of San Juan."

The type section of the Guárico formation lies in a syncline. "On the eastern flank of the syncline between the Garrapata and Totumo haciendas the formation begins with a dark gray, brown-tinged, massive, reefal, typically 'Morros' limestone cut by white calcite veinlets, and containing in places, angular boulders and pebbles of limestone, sandstone, chert, quartz, etc." This limestone lies unconformably upon hard, dense, black, well-bedded limestone of the Arrayanes group and contains pebbles of the latter. "On the western flank, the Guárico formation begins with 'Morros' limestones at La Puerta which change laterally and upwards into coarse calcareous sandstones and brown-weathering shales, some of which are very rich in molluscan remains. These are followed by soft, dark, calcareous shales with interbeds of gray, very calcareous sandstones. There are occasional beds and lenses of black, clastic limestone (calcarenite), limestone breccia, and fine-grained limestone. Towards the upper part of the exposed section, thinly-bedded dark calcareous shales become more abundant."

One or the other of the limestones of the Guárico formation has, at one time or another, been dated as Cretaceous by R. A. Liddle (1928, p. 138-139), as "Eocene", possibly Midway by L. Kehrer (1937, p. 64), and as Lower Eocene (Midwayan) by R. A. Liddle (1946, p. 306). However, studies by C. M. Bramine Caudri (1944) and M. de Cizancourt (1951) indicate that Liddle's Los Morros de San Juan limestone is Paleocene in age, and this determination is now the generally accepted one by geologists in Venezuela. The Los Morros de San Juan limestone (now referred to as the Morro del Faro member) occurs as a reef development at various levels throughout the Guárico formation.

From collections made by E. Mencher in the sandstone facies of the Guárico formation at Morritos just north of the town of San Juan de Los Morros, A. A. Olsson identified Campanile cf. giganteum, Turritella mortoni, Turritella restinensis and Morgania, which he considered indicative of the middle Eocene (Lutetian). Mencher believes, however, that since these mollusks occur together with Paleocene foraminifera described by Caudri, the evidence of the microfauna is the more conclusive. This opinion is supported by R. Rutsch, who, in a personal communication to H. H. Renz, states that the molluscan fauna appears to him to be Paleocene rather than Cretaceous or Eocene. Thus, both the microfauna and macrofauna suggest that the whole of the Guárico formation was deposited during Paleocene time, with the sandstone and shale facies (now known as the San Juan de Los Morros member) being more or less contemporaneous with the limestone facies of the Morro del Faro member.

The Guárico formation in its type lithofacies has been traced from the vicinity of La Puerta eastward to beyond San Sebastián for a distance of at least 25 kilometers. Limestones resembling the Morro del Faro member in Ethology and physiographic expression have been observed sporadically in an east-trending belt as far east as the Unare River, or 260 kilometers east of the type locality, but some of these morro limestones pertain perhaps to formations older and younger than the Guárico. East of the Paleocene morros of La Puerta, San Juan and San Sebastián, reef limestones have been observed, among other localities, near San Francisco de Macaira about 120 kilometers east of La Puerta, near Batatal 138 kilometers from La Puerta, and at the Unare River a short distance south of the coast, some 270 kilometers east of La Puerta. Two kilometers southeast of San Francisco de Macaira there are several prominent morros composed of reef limestone associated with stratified gypsum deposits. These limestones occur as lenticular, structureless masses with gray and brown laminated shales interbedded with white sandstones tinged brown and gray. This assemblage is described by J. Evanoff (1951, p. 244-245) under his Macaira group to which he assigns a Paleocene age. The Macaira group is presumed to rest concordantly on the underlying Guayuta group and is in fault contact above with beds of upper Eocene age. Evanoff notes a strong resemblance between the limestones of the Macaira group and the limestones at San Juan de Los Morros and infers, therefore, that they are of the same Paleocene age. Since Mencher's Guárico formation includes the morro limestones at San Juan de Los Morros, Evanoff's Macaira group may, in part at least, be equivalent to Mencher's prior-described Guárico formation. Hedberg (1950, p. 1196), however, cautions against assuming the age equivalency of all morros since some of them may be as old as Middle Cretaceous (and as young as upper Eocene) and, until correlation is more firmly established, it seems best to retain local terminologies. To recapitulate, the Guárico formation at its type locality is now defined as a Paleocene assemblage of sandstones and shales with reef limestones of varying magnitude intergrown throughout them. The sandstone-shale facies is named the San Juan de Los Morros member, and the reefal limestone facies the Morro del Faro member.

See also SAN JUAN DE LOS MORROS "FORMATION" and LOS MORROS DE SAN JUAN LIMESTONE.

Norman E. Weisborc

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