MORRO DEL FARO, Member

See: LOS MORROS DE SAN JUAN, Limestone and GUARICO, Formation.

LOS MORROS DE SAN JUAN, Limestone

TERTIARY (Paleocene)

State of Guárico, Venezuela

Author of name: R. A. Liddle, 1946.

Original reference: R. A. Liddle, 1946, p. 304.

Original description: R. A. Liddle, 1946, p. 304-306.

A. von Humboldt (1814-1825; Spanish edition, 1941-42, v. 3, p. 193, 201, 202) was perhaps the first author to describe the limestone which forms the "morros" of San Juan, but he did not distinguish it as a stratigraphic unit. Humboldt refers to it as a typical "transition limestone", closely associated with "trappean" rocks, green schists ("of transition"), serpentine and "greenstones". The same author (p. 198) describes the "morros" as "made up of limestone of crystalline texture... occasionally dense, cavernous, greenish gray". The author could not find "remains of organisms" in the rock, but he mentions that "other compact, white limestones containing some remains of shells, rests on the "morros" of San Juan". The author apparently refers to either the Paleocene fossiliferous beds which underlie the limestone or to the "Actaeonella beds" (Maestrichtian). Both groups outcrop in the eastern slope of a syncline, in a saddle located to the east of the Morro del Faro. (Caudri, 1943-1944, p. 26, sc. 2).

Karsten (1886, p. 12-14) also introduces some informatian about the rocks which outcrop in the "morros" of San Juan and its close vicinity. This information does not refer to the Los Morros limestone only, but to the underlying Paleocene and Maestrichtian beds as well.

Liddle (1928, p. 51, 138) describes in his turn, the limestone which forms the "morros" of San Juan, but he did not distinguish it as a stratigraphic unit. He includes it in El Cantil formation, based on "a fragment of a small pelecypod and a fragment of an Exogyra", that he says were found "in the marmolized limestone which forms the "morros"... at the thermal sulphur springs near San Juan". Liddle suggests a probable correlation with the limestone that outcrops 300 m. to the south of Villa de Cura, "evidently Cretaceous in age, a part of El Cantil formation".. (See "VILLA DE CURA Formation").

Later on, G. Zuloaga, found "an undoubted caprinid, in the "morro" at La Puerta, but the examination of the matrix, made by Mr. A. N. Dusenbury Jr., of the Creole's Paleontological Laboratory, revealed the abundance of "Ranikothalia", which is a conclusive evidence for the redeposited nature of the caprinid". (E. Mencher, unpublished letter to N. E. Weisbord, April 4, 1954). Therefore, S. E. Aguerrevere and Zuloaga (1937, Geological column, front p. 12) included the limestone which forms the "morros" in the Villa de Cura series of Mesozoic (?) age, which underlies the "San Juan de los Morros series", since at that time they did not have the information related with the microfauna associated with the Caprinid.

The first description of the Los Morros de San Juan limestone and the introduction of such a term, is due to Liddle (1946, p. 304).

The unit is described as a "chalky, reef limestone which forms a prominent, serrated ridge five kilometers northeast of the village of San Juan de los Morros in the District of Roscio, State of Guárico". The limestone (p. 305) is "chalky, white... fairly well indurated and compacted by compression and nearby igneous activity... Many of the massive beds form high, vertical cliffs which may be confused with bedding planes standing at high angles because of the sharp synclinal fold into which the formation has been squeezed". "The beds which form a compressed syncline, dip from 70° to 80° to the south". In spite of the difficulty in measuring the total thickness "there appears to be not less than 500 ft".

Liddle (p. 306) indicates that the limestone is found, towards the north, in fault contact with a large augite porphyry intrusion. He maintains (p. 307) that "the sedimentary rocks composing the greater part of the "morros" have no roots; their compressed base rests in a shallow syncline of igneous and metamorphic rocks". Liddle points out that the limestone also rests in discordance over a relatively small amount of shale and thinly bedded fossiliferous limestone, Maestrichtian in age. Such is the case in the western slope of the Morro del Faro (Caudri, 1943-1944, p. 26, sec. 2) but in the eastern slope (ibid) immediately on top of the Maestrichtian beds containing Ammonites and Actaeonella, there exists a sandy-marry section including limestone beds with Turritella ("Woodsalia") and Ostrea. Paleocene microfaunas have been identified, in said section, by Mme. de Cizancourt (1951, p. 11-14, fig. 3). Such facies (San Juan de los Morros "formation") constituted here the base of the Paleocene sedimentary cycle. The major bioherm of the Morro is not, in that locality, in direct contact with the Maestrichtian. The fact that Liddle did not mention such section, is because he did not suspect the existence of Paleocene facies other than the bioherms in the area of San Juan. It is evident that the reef facies of the Morros, are locally in contact below, above and laterally with marry, sandy and clayey facies of the same sedimentary cycle (San Juan de los Morros "formation") which Liddle (p. 306-307) assigns to the lower middle Eocene (p. 306) or lower to middle Eocene (p. 307).

On the other hand, Liddle assigns to the Los Morros de San Juan limestone a "lower Eocene" age (Midway), based on Miss Caudri's micropaleontological study (1943-44) which, by the way, refers to rock samples of the San Juan de Los Morros "formation" and not to the Los Morros de San Juan limestone. There is still some confusion in relation with age, in Liddle's text. For example, referring to thin limestone outcrops at the Cerro de Flores, to the south of San Juan, which contains Inoceramus and Ammonites, which he includes in the "Guayuta group", "probably Maestrichtian", Liddle (p. 246) comments that "these beds appear to be younger than those forming the basal part of Los Morros de San Juan". Although Maestrichtian beds (with Actaeonella) are associated with the Los Morros limestone, it is evident that they do not constitute "the basal part" of the bioherm but an underlying unit well differentiated lithologically and pertaining to another depositional cycle, which has not yet been named.

Other evident contradiction about the age of the reefs, was noticed by Mencher (1948, p. 384). Liddle (1946, p. 202-205) "repeats, almost word-for-word, his original description of the limestone "morros" in the area of San Juan de los Morros to San Francisco de Macaira, and reiterates his belief in an El Cantil (Early Cretaceous) age for them". Actually, and it is so stated by Mencher, Liddle suggests the coexistence of reefs, some Paleocene, others Lower Cretaceous, in the area. This way he assigns to the latter age (p. 305) those morros located "to the north of San Francisco de Macaira".

Among the real equivalents to the Los Morros de San Juan limestone, mentioned by Liddle (ibid., p. 305, 307) in the State of Aragua, minor morros located to the west of San Sebastián and 5 km. to the south of San Casimiro, could be mentioned.

Hedberg (1951, p. 1196) follows Liddle's interpretation but he includes also the large Morros de San Juan among the reefs, the age of which is "possibly as old as Middle Cretaceous": "At a number of points along the southern part of the Serranía del Interior from the Guárico-Anzoátegui boundary westward are isolated reef limestone, hills or "morros" (Batatal, San Francisco de Macaira, San Sebastián, San Juan de los Morros, etc.). Caudri (1944) has determined the fauna in association with some of these "morros" near San Juan and near San Sebastián as Paleocene and approximately equivalent to the Soldado formation of Soldado Rock and Trinidad. She has called attention, however, to certain rather anomalous data which might have suggested Cretaceous age. These limestones probably belong to the Santa Anita group and appear to be a reef development in sediments of Caratas age. Other "morros" such as those of Batatal, San Francisco de Macaira, and the large "morros" of San Juan are closely associated with rudistids and Exagyra or with shales carrying a Vidoño foram fauna and may be of quite a different age - possibly as old as Middle Cretaceous".

H. G. Kugler (1953, p. 41-43) suggests again the idea of the Lower Cretaceous age of the Morros de San Juan, supporting the theory that the "morros" are part of the Wildflysh and as such they represent huge masses of rocks resistent to weathering and are of older age than the surrounding formation".

Miss Caudri's (1943-44) or Mme. de Cizancourt's studies (1951) cannot be used directly in support of the Paleocene age of the Los Morros limestone. It has already been said that the former studies did not refer to that unit. Among Mme. de Cizancourt's localities (p. 10-11, figs. 2, 3, 4) only three (L. 735, "morro" to the north of El Faro; L-282 "morro" of El Baño; L-706 "morro" to the north of the Aqueduct) come from the Los Morros limestone. Actually, they do not contain Nummulites, but scarce corals, algae, small, foraminifera or undeterminable orbitoids (p. 11, 13, 14). L-282 and L-706 are referred to the "calcaire trouble", which is considered as level "a", the oldest in the sequence. L-735 "is probably somewhat younger and different in lithology", but does not contain among its clastic elements, fragments of "calcaire trouble", which is widely represented in the microbreccias (fore-reef and back-reef facies) of the upper levels of the San Juan de los Morros "formation".

On the other hand (J. Pfender and D. Schneegans, 1949, p. 91), samples of limestones collected by D. Schneegans, 1938, "in the immediate vicinity of La Puerta", "in the upper part of the Los Morros limestones" were sent to J. Pfender, for their paleontological study. Pfender identified the following algal flora, indicating its stratigraphical meaning also: Elianella elegantissima Pfender (lower Eocene of Madagascar); Pseudolithothamnium album Pfender (Eocene of the Caucasus, maximum development during lower Eocene) Archaeolithothamnium cf. gosaviense Rothpl. (mainly during the Santonian) Melobesia melobesoides Foslie; Distychoplax uniseriale Pfender (Madagascar's Danian: Gan and Arros beds). According to the same information, Pfender and Alloiteau identified also, among the corals, Actinacis sp. (Upper Cretaceous-lower Eocene). The importance of algal florae in the age determination of Lower Tertiary bioherms of Venezuela, should not be underestimated, since the algae were the main bioherm builders, specially during the Paleocene age. As it is seen, the floral assemblage described by Pfender, also suggest a "lower Eocene" age (Paleocene) for the Los Morros limestone.

The age of the Los Morros limestone is demonstrated in the field by the evident interdigitation of the limestone with the San Juan de Los Morros formation. In that respect, the lateral transition from the large Morro at La Puerta westward to small fossiliferous lenticular reefs ("Morritos") intercalated in the marls, shales and sandstones sequence (Mme. de Cizancourt, p. 15, fig. 5, L.708 to L-711 and L-93) is typical. Still further towards the east, the transition goes-on to well-bedded calcareous microbreccias (ibid., L-94). From this latter locality, towards the Guárico River to the south, the Paleocene sequence contains other various horizons of corded nummulites, interbedded with shales (De Civrieux's private information).

H. H. Renz (oral information) studied microfaunas rich in corded nummulites and Laffitteina sp. in the walls and top of the large Morro de San Juan, which dissipate all doubts about the Paleocene age of such Morros.

As for Hedberg's proposal (loc. cit.) of including the Paleocene reef limestones into the Santa Anita group, it meets with the following difficulty; the sedimentary cycle including the bioherms appears restricted to the Paleocene. On the other hand, it lies in discordance over the Maestrichtian (Liddle, loc. cit.). (See also ESCORZONERA and ORTIZ Formations).

When E. Mencher introduced (1950, p. 97) and described (Mencher et. al., 1951, p. 14) the Guarico formation, he included in it the formerly described San Juan de los Morros "formation" and the Los Morros de San Juan limestone. The San Juan de Los Morros "formation") should be lowered to members of the Guarico formation. As for Liddle's name, it is here suggested that it be changed to "Morro del Faro limestone member", in order to avoid a possible confusion with Garner's name and with the village of "Los Morros de San Juan", State of Monagas. The name "Morro del Faro" would have the advantage of indicating the place where the type-section of the unit is located.

The Morro del Faro limestone member of the Guarico formation is a coastal reef facies which indicates, approximately, the northern boundary of the Paleocene depositional basin in north-central Venezuela. This limestone unit constitutes a prominent and characteristic topographical feature of the region. It extends discontinuously from its type-locality, northwest of San Juan de los Morros, eastward into the State of Aragua. It is well exposed to the west, north and northeast of San Sebastian. It outcrops again further east, at Pardillal, a site located 5 Kms. south of San Casimiro. Still further northeast, it outcrops but occasionally. Morros - forming limestones at El Peñon, approximately 7 Kms. south-east of Ocumare del Tuy, on the trail to Democracia and Quiripital, State of Miranda, are worth mentioning. Samples were collected there by the writer during a recent field-trip. Algae (Griphoporella sp. and others), corals and small foraminifera were identified. The floral and faunal assemblage suggests a Paleocene age for those morros.

See: also GUARICO, Formation, SAN JUAN DE LOS MORROS, Formation and PALEOCE in Venezuela.

J. M. Sellier de Civrieux