ZENDA, Facies

MESOZOIC (MIDDLE or UPPER)

Distrito Federal, Venezuela

Author of name: S. E. Aguerrevere y G. Zuloaga, 1937.

Original reference: S. E. Aguerrevere y G. Zuloaga, 1937, p. 14.

See LAS BRISAS, Formation

LAS BRISAS Formation

MIDDLE or UPPER MESOZOIC

Distrito Federal, Venezuela

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

Original reference: S. E. Aguerrevere and G. Zuloaga, 1937, p. 12.

Original description: ibid.

The "Las Brisas conglomerate" was named by S. E. Aguerrevere and G. Zuloaga (1937, p. 12-15). Its type locality is at the site Las Brisas, approximately 4 km. south of El Valle, on the road to Charallave. Such site is indicated on G. Dengo's Geological Map of the Caracas Region (1951).

The unit was described by Aguerrevere and Zuloaga (1937, Geological Column, front p. 12) as an arkosic "basal conglomerate" constituted by fragments of granite and quartz, up to 30 cm. in diameter. Its cement is mainly siliceous. It forms competent beds and includes schists "zones". The approximate thickness originally appreciated was 500 m. The unit unconformably overlies the Sebastopol "granite".

According to the above Geological Column, lenticular conglomeratic limestones, originally named "calcareous conglomerate or Zenda phase", are intercalated in the upper part of the formation. They are considered (p. 14) as a phase of the "Las Brisas conglomerate". Their type-locality is at km. 25 on the road from Caracas to Los Teques. The Zenda phase was described as made-up of fragments of feldspars up to 1,5 cm. in diameter, with calcareous cement. The limestone forms competent strata. It contains schists zones.

Aguerrevere and Zuloaga (1938, p. 288-284) referred again to the unit as the Las Brisas formation or Las Brisas basal conglomerate.

On the base of recent observations, for instance Dengo's (1951) and Smith's (1952) studies, the Las Brisas formation has been redefined. The present definition sudivides the unit into two main lithological members, well differentiated from each other:

Lower Member: This sequence starts with some metamorphosed sandstones and thin arkosic conglomerates. These conglomerates contain rounded fragments of granite and quartz derived from the Sebastopol Basal Complex. Towards the upper part of the section, they gradually pass into occasionally conglomeratic, quartz-sericitic schists which used to be argillaceous-sandy-rudaceous sediments. The maximum thickness (Smith, 1952, p. 348) is 200 m. According to the same author, further up in the section a change into quartzous schist with some carbonates, scarce microcline porphyroblasts and quartz fragments, is observed. It follows an interval of gneiss with abundant microcline porphyroblasts in a recrystallized calcareous matrix with interbeddings made-up of carbonates. The thickness observed at La Mariposa quarry is 70 m.

The microcline gneiss associatea with limestone is distinguished from Aguerrevere and Zuloaga's "Zenda phase" by Smith, and placed by the same author, further-up in the section. It seems to be the same one that Dengo (p. 56) placed in contact above or laterally with the Zenda limestone, since Dengo (p. 54) correlated La Mariposa limestone with that from Zenda. Therefore, Dengo places the gneiss further-up in the section and describes it as a coarse-grained, microcline-quartz-muscovite gneiss which grades upwards, into a fine "rained quartz-muscovite schists and locally, into the Antimano formation.

The microcline gneiss outcrops, typically, in the hills surrounding the Baruta Valley and extends, mainly, towards the west.

According to Smith, the microcline gneiss associated with La Mariposa limestones is not located above or laterally to the Zenda limestone, but some 100 m. further-down in the section.

The interval here discussed is designated "lower microcline limestone" by Smith (1952, p. 351). It follows-up a 100 m. thick noncalcareous micaschists with few microcline porphyroblasts. Such section is markedly more conglomeratic than the underlying section. It contains fragments of quartz and granitic rock, probably derived from the Sebastopol Basal Complex. In the upper part of the interval the rocks grade into a second major microcline-rich porphyroblasts bed (Dengo, 1951, p. 57) and clastic quartz grains (Smith, 1952, p. 352). According to Smith, this is Aguerrevere and Zuloaga's Zenda phase. This upper bed containing microcline is lenticular and varies in thickness from a maximum of 70 m. in the Guaire River (Smith, 1952, p. 352) to 0 m. in other localities. The thickness appreciated by Smith (p. 351) in the type-locality of the Zenda phase, is 60 m.

The type-locality of the Zenda phase was described all over again by Dengo (1951, p. 54) who mentions lenticular limestone beds intercalated with thin layers of microcline-muscovite schists and microcline gneiss (evidently Smith's "lower microcline bed"). The limestone is dark-gray, medium-grained and contains quartz and granite fragments. According to Dengo, the limestone lenses increase in size towards the east (neighborhood of Baruta). They underlie the microcline gneiss.

Actually, Dengo (1951, p. 57) correlates the El Encantado limestone (75 m. thick) outcrops (km. 13 of the Caracas-Santa Lucía railway) with the Zenda limestone outcrops, following in that way, Aguerrevere and Zuloaga's original criterion (Geological Column, 1937, front p. 12). The same authors (1938, p. 282) modified their first interpretation in order to include the Peñón de Lira and El Encantado limestones in the lower part of the overlying Las Mercedes formation. Smith (1952, p. 355) does not admit the correlation of such outcrops with the Zenda limestone outcrops.

According to Dengo (1951, p. 54) the part of the member between the Sebastopol gneiss and the Zenda limestone does not appear to be largely conglomeratic as it was originally thought. The difference of opinion between Dengo and Aguerrevere and Zuloaga, arises from the fact that Aguerrevere and Zuloaga considered the feldspars within the schists and in the Zenda limestone phase as detrital material, while Dengo considered them porphyroblastic.

Smith (1952, p. 354) considers that the interval immediately overlying the second major microcline bed ("Zenda phase") and the bed itself, is much more conglomeratic than the underlying section. The Zenda limestone, according to Dengo (1951, p. 55) is of a probable biohermal origin. Such idea arises from the lenticularity of the beds and the occurrence of questionable algal structures.

Actually, the lenticular limestone facies originally placed by Aguerrevere and Zuloaga on the top of the Las Brisas formation, constituted the top of the Las Brisas Lower Member.

Due to lateral variations and to the fact that the observations have been made on different localities, the thickness of the Lower Member has been difficult to measure. An approximate thickness of 400 m. is obtained from Smith's proposed maximum thicknesses.

Good outcrops of the Las Brisas formation Lower Member are found in the formation type-locality and in various road exposures between the km. 18-20 and 24-25 of the road from Caracas to Los Teques (quartz-mica schists with conglomerates). The calcareous facies of the Lower Member upper part is typically outcropping in the km. 25 on the Caracas Los Teques road, at La Mariposa, to the north and south of Baruta and at the Quebrada La Guairita. If the correlations proposed by Dengo are accepted, the most conspicuous outcrops of such phase are located in the km. 26 of the Caracas-Santa Lucía railway (Tusmare Station) and in the sites Los Naranjos, El Encantado and Peñón de Lira. The last three localities are located along the regional strike, but as it has been said, the stratigraphical position of the outcrops, is still a matter of discussion.

The Lower Member of the Las Brisas formation consists, essentially, of metamorphosed shales and scarce conglomerates with intercalations of limestones of a probable reefal origin. The conglomerates are partially derived from the Sebastopol Basal Complex.

Upper Member: If the section is accepted the way Smith describes it, the microcline quartz-muscovite gneiss, located by Dengo immediately on top of the Zenda limestone, will be located below such limestone.

According to Smith (p. 352) the section immediately overlying the conglomeratic microcline interval which ends with the Zenda limestone, is constituted by an interval of light-gray or brown graphite-sericitic schists which are more deformed and weathered than the rocks of the Lower Member of the Las Brisas formation. It ranges in thickness from 400 to 900 m., in the region surveyed by Smith. The deformation and the small-scale folding, although locally high, do not reach the intensity of those observed in the Las Mercedes formation. On weathering, it produces a red and yellow clay which is considered by Smith, typical of that section, in the Caracas group. The maximum thickness observed in the graphitic-argillaceous beds is 1,3 m. The Upper Member metasediments were originally made-up largely of shales.

According to Dengo (1951, p. 57) the estimated thickness of the entire formation is 800 m., sum which approximately coincides with the above Smith's partial thicknesses.

Lenticular limestones were distinguished by Dengo under the name Antímano formation; they are stratigraphically above the Zenda limestone, since they locally overlie the "quartz-mica schist", here named Las Brisas Upper Member, and underlie the Las Mercedes calcareous schist. Such limestones were originally included in the description of the "Zenda phase".

According to Dengo (1951, p. 56), the transitional upper contact of the Las Brisas formation, may be observed along the roads Caracas-Baruta and Caracas-El Valle, as well as in railway exposures south of Petare. The transition is into calcareous-quartz-muscovite schists pertaining to the Las Mercedes formation. López (1942, p. 7) described a similar relation in the Valencia region. Locally, the upper contact of the Las Brisas formation with the Antimano formation is well defined.

The age of the Las Brisas formation, as well as that of the entire Caracas group has been and continues to be a matter of great discussion. Aguerrevere and Zuloaga (1937, p. 22) proposed to correlate Las Brisas with the Lower Cretaceous non-metamorphic formations of western Venezuela. Wolcott (1945, p. 1632) published a list of Jurassic or Cretaceous fossil mollusks from a locality near Guatire, State of Miranda, in outcrops that he originally considered as pertaining to the Las Mercedes formation. Such assignment was modified by Dusenbury Jr. and Wolcott (1949, p. 22) in order to include the fossiliferous locality in the "Zenda phase" of the Las Brisas formation. Some authors believe the Las Brisas formation to be as old as Jurassic, since this age is the age of the lower part of the Caribbean "series" in Trinidad.

Correlations of the Las Brisas formation with schists outcropping north of Valencia, State of Carabobo, have been proposed by López (1942, p. 14) and Mac Lachlan (unpublished report). Hess and Maxwell (1949, p. 1863) proposed to correlate Las Brisas with non-calcareous schists exposed at the Island of Margarita, State of Nueva Esparta and Maxwell and Dengo (1951; p. 260-261) correlated it with metamorphic rocks of the Carúpano region, State of Sucre. Recent studies demonstrated the extension of Las Brisas formation towards the north of Maracay, State of Aragua. Manfred Nicklas (1953, Geological Map, front p. 376) correlates the Las Brisas with part of the metamorphic rocks outcropping in the Guarenas-Guatire area, State of Miranda. The same author mentions the occurrence, in that area (Quebrada Tapaima), of a quartziferous conglomerate facies below lime might be equivalents to the Antímano formation or the Zenda stones that limestone.

The Las Brisas formation constitutes the lower part of the Caracas group and therefore of the Caribbean "series" in north-central Venezuela. A local granitizated tectofacies of the Las Brisas formation has been distinguished under the name Peña de Mora formation.

See also PEÑA DE MORA Formation, CARACAS Group and CARIBBEAN "Series".

S. E. Aguerrevere and J. M. Sellier de Civrieux