LOS COLORADOS, Limestone

See LAS MERCEDES,Formation

LAS MERCEDES, Formation

MIDDLE or UPPER MESOZOIC

State of Miranda, Venezuela

Authors of name: S. E. Aguerrevere and G. Zuloaga, 1937.

Original reference: S. E. Aguerrevere and G Zuloaga, 1937, p. 15.

Original description: ibid.

The name Las Mercedes schists was introduced by S. E. Aguerrevere and G. Zuloaga (1937, p. 15) to designate a "thick formation of schists, chiefly calcareous with sandy and granular phases, and also graphitic and micaceous zones, the latter show a characteristic pink color in its weathered outcrops". The significant occurrence of "white and brown calcite veins" was also mentioned. (According to Dengo 1951, p. 65, calcite and ankerite). On fresh surfaces, the schists are gray or light colored. The weathering pink tinge is caused by the oxidation of the pyrite, which is very abundant in the formation. Besides the high percentage of calcite, the occurrence of graphite, mica, quartz, and pyrite is mentioned.

The type-locality is the Hacienda "Las Mercedes" (today residencial zone), 3 km. to the east of Caracas, on the road to El Hatillo.

According to Aguerrevere and Zuloaga, within the formation there is "a zone composed mainly by thin, dark limestone beds interbedded with micaceous and argilaceous schists". "The whole has been very intensely folded in characteristic closed folds". The zone was named "Los Colorados Phase" after a site located 45 km from Caracas, on the road between Caracas and Valencia.

Within the Las Mercedes formation, Aguerrevere and Zuloaga (p. 15-17) also include "conspicuous lenticular masses of indurated and locally marmolized limestones, light and dark gray... that make salient topographic features such as the Morro in Valencia, and the Peñon de Lira and El Encantado limestone to the South-East of Caracas". Nevertheless, in their Stratigraphic Column, (front, p. 12) the authors include the El Encantado limestone in the Zenda phase of the Las Brisas formation, The stratigraphic position of said limestones, of a probable biohermal origin, is still today a matter of great discussion (see Las Brisas formation).

Aguerrevere and Zuloaga (p. 15) placed the Las Mercedes schist "above the Zenda Conglomeratic phase" (recent studies have modified this concept) and below the "Villa de Cura Series". This last name is obsolete.

Aguerrevere and Zuloaga (1938, p. 282) gave to the Las Mercedes unit the rank of a formation and pointed-out that the outcropping area is a "belt with an approximate N.75° E. trend from Tinaquillo (State of Cojedes), through Maracay and San Mateo (State of Aragua), Los Teques (State of Miranda), Caracas . . . and continues northeastward, towards Guarenas".

According to Dengo (1951, p. 65) the base of the Las Mercedes formation (in his nomenclature: Micaceous-Calcareous Schist) rests on top of the Antimano formation on the localities where such lenticular unit is present, or on top of the quartz-mica schist of the Las Brisas formation. Said schist overlies the Zenda phase and therefore there is no direct contact between such phase and Las Mercedes formation as it was originally suggested by Aguerrevere and Zuloaga. According to Dengo, the calcite percentage in the Las Mercedes formation varies between 25% and 50%; quartz is abundant. He also indicates the occurrence of chlorite, muscovite, graphite and pyrite. Graphite is more abundant towards the top of the formation, giving the rock a typically dark color. A complete section of the formation is exposed at Quebrada Tacagua.

According to Dengo: "at the lower part of the schist there are thin layers of black, fine-grained limestone, which become less conspicuous in its upper portion". Although he suggests that "the lower part of the formation characterized by thin layers or dark gray limestone, was named Los Colorados member" (by Aguerrevere and Zuloaga), no attempt was made to differentiate the facies within the Las Mercedes formation. The thickness of the formation, estimated by Dengo, is 500 m.

Although Smith (1952, p. 358) could not determine with precision the thickness of Las Mercedes formation in the Los Teques-Cúa area, he thinks that 1800 m. is a minimun. He intends (p. 358) to sudivide the formation into the Las Mercedes schist proper, the Charallave conglomerate (previously included in the overlying "Villa de Cura series" ) and the Los Colorados limestone facies. According to his Stratigraphic Column (p. 341), this latter facies corresponds to the upper part of the formation and not to the lower part, as it was previously. suggested.

Limestone lenses of a probable biohermal origin are included by Smith (1952, p. 362) in the Las Mercedes schist facies. He also says (p. 361) that the Antimano formation is probably one of these limestones. It would be useful to accept Dengo's proposition of making the unit a separate formation. Smith mentions lithological similarities (brecciation and crushing), between the El Encantado and the Antímano limestones. The same author (p. 359-360) points-out that the Charallave conglomerate is associated in its type-locality with a black recrystallized limestone of the Las Mercedes formation. He also redefined the unit. On the other hand, he mentions (p. 360) that "a similar fine-grained rock occurs some 10 m. below the Los Colorados limestone, at its type-locality, in the central outcropping area of the Las Mercedes formation". This suggests the equivalence with the Charallave conglomerate, placed by Smith (Stratigraphic Column, p. 341) in the upper part of the Las Mercedes formation, below the limestones of the "type Los Colorados". It should be noticed that Smith's idea of correlating the Charallave type-conglomerate with that of the Los Colorados is only a tentative proposal. On the other hand, there are some recent indications of similar conglomeratic phases occurring, perhaps, in different levels within thc section of the Caracas group. M. Nicklas (1953, p. 370), for instance, points out that a 60 m. thick quartziferous conglomerate located in the Quebrada Tapaima, southeast of Guatire, State of Miranda, is associated with overlying limestones which might represent the Antímano formation or the Zenda phase of the Las Brisas formation. The conglomeratic trend of the typical Zenda phase and the rocks stratigraphically associated with it, is also noticeable in other localities (see LAS BRISAS FORMATION).

As for Aguerrevere and Zuloaga's Los Colorados phase, Smith (1952, p. 361) thinks that its massive and lenticular limestones pertain, probably, to the upper part of the formation, but asserts that due to its lenticularity, none of the lenses might be considered as a key-bed, as it was originally suggested. As it was already said, Dengo believes that this type of limestone is predominant of the lower part of the formation. Until a systematic study would explain the problem of the vertical extension of such phase in the Las Mercedes section, it is here suggested to give up that name as a stratigraphic unit. According to Smith, the "Los Colorados phase" is closely associated with the inmediately underlying Charallave conglomerate.

A fossiliferous locality was found by Wolcott (1943, p. 1632) in Quebrada de Cara, north of Guarenas, in rocks originally considered as pertaining to the Las Mercedes formation, but later reassigned (Dusenbury and Wolcott, p. 22) to the Zenda phase of the Las Brisas formation. According to Bucher (1952, p. 49), the faunal collection is "distinctly Upper Jurassic". The age determination of the Las Mercedes formation as well as the age determination of the Caracas group in general, is still subject to great discussion, since so many authors admit S. E. Aguerrevere and G. Zuloaga's suggestion of assigning Las Mercedes formation to the Lower Cretaceous, while others think that it can be as old as Jurassic. The recent observation of gypsiferous facies associated with graphite lenses in the upper part of the Las Mercedes formation, would possibly constitute an interesting lithological information. Such lenses occur, for instance, at Guayas, in a new roadcutting on the Caracas-Maracay road, some 700 m. from the Hacienda Guayas. (Sellier de Civrieux's private information). It is well known that Kugler (1953, p. 30) noticed the importance of gypsum occurrence in Trinidad and Paria Peninsula, for the regional correlation in the Caribbean series. According to Kugler, the environmental conditions for the deposition of gypsum, suggest a Jurassic age rather than a Lower Cretaceous. (see MARAVAL BEDS). In every case, the primary origin of the gypsum should be proved on the basis of further studies, before a formal interpretation would be given.

According to Aguerrevere and Zuloaga, the Las Mercedes formation is the upper unit of the Caracas group. Later (Dengo, 1951), the overlying Tacagua formation was included in the same group. The Paracotos formation (Smith, 1952, p. 363) lies apparently in unconformity over the Las Mercedes formation, although the normal contact between both units has not been observed so far.

J. M. Sellier de Civrieux