LA GUAIRA, Beds

See CABO BLANCO, Group

CABO BLANCO, Group

TERTIARY (Mio-Pliocene) to QUATERNARY (Pleistocene)

Distrito Federal, Venezuela

Author of name: A. von Humboldt, 1801.

Original reference: A. von Humboldt, 1801.

Original description: A. von Humboldt.

The first allusion to the beds exposed in Cabo Blanco and its vicinity appears to have been that by Humboldt in a geologic map (Humboldt, 1801), in which the beds were referred to the "Floetzgebirge", an inclusive term then used for relatively unconsolidated formations. In 1823, Humboldt compared the formations of Cabo Blanco, Cumaná, etc., with the "calcaire grossier" of Paris (middle Eocene). Apparently the first description appears in Humboldt's famous work on his travels in the New World, the first edition of which appeared in Paris between 1814-1825. This is not available to us, but the second edition (13 volumenes, published in Paris, between 1816 and 1831; Spanish translation by L. Alvarado, 1941), we may quote:

" . . . On the west slope of the Cabo Blanco hill the gneiss is covered by a very recent formation of sandstone or agglomerate. This sandstone contains angular fragments of gneiss, quartz and calcite, magnetic sand, corals, and petrified bivalve shells. Is this formation perhaps the same age as that of Punta Araya and Cumaná?" (Humboldt, trans. Alvarado, 1941, vol. II, p. 282-283).

The next important mention appears in Karsten (1886), who in connection with the presence of geologically young sediments along the coast mentions that:

" . . . only locally is there found a marginal zone of more modern beds, for instance at Cabo Blanco near La Guaira, which is formed by Tertiary sediments of marls with shells (Pecten gigas), sands, gravels and shellbearing breccias, which together dip 45 degrees S. In addition, very recent sediments here and there covers the metamorphic crystalline schists, up to an elevation of 20 meters, for example in Punta Araya and La Guaira, evidencing a slight uplift of the coast, which must have taken place at a very recent date." (Karsten, 1886, p. 8). Karsten compares the Cabo Blanco beds with a "Tertiary... or perhaps Quaternary" limestone, which outcrops in the San Antonio hills, near Cumaná. He gives a list of species found at Cumaná, but does not list any from Cabo Blanco.

K. Martin visited Cabo Blanco prior to 1887, and collected fossils which were studied by M. M. Schepmann; a list of Schepmann's identifications appeared in Martin's publication (1887). Meantime, it appears that J. Lorié independently studied the collections and published an article (Lorié, 1887). According to Rutsch, the identifications made by Schepmann and by Lorié were considerably different. Martin referred the beds to the Pleistocene. A. Jahn (1921) quotes extensively from Martin's work. Martin distinguished in his collections two horizons, both in beds said to be flat-lying.

A Pleistocene age for the Gabo Blanco beds was accepted by Jahn (1921), Maury (1925), and Liddle (1928, p. 351), who gives a rather vague description of the beds. Rutsch (1930; 1934, p. 144-145; 1937, p. ?) and Senn (1935, p. 84) also accept the Pleistocene correlation; Senn adds that the beds are "strongly dislocated".

Aguerrevere and Zuloaga (1937, p. 21) state that "in Cabo Blanco there is a series (probably two series) of marls, clays and calcareous sands, highly fossiliferous, which have been correlated as Miocene or Pliocene" and which dip southward.

Kamen-Kaye, (1938, p. 1230), although he does not mention the Cabo Blanco beds by name, refers to "sharply upturned Quaternary molluskbearing beds, on the Caribbean coast." L. Kehrer (1939, p. 699) states that the Cabo Blanco beds are middle Miocene, on the base of studies of the mollusks and foraminifera. Kamen-Kaye (1939, p. 704) was not convinced, and Kehrer countered with a second note on the age of the Cabo Blanco beds (1939b). In this paper, Kehrer notes that a distinction should be made between the 'typical Cabo Blanco beds", which dip steeply southward, and other beds almost horizontal, which overlie the former. The former (Kehrer states), contain mollusks (identified by Woodring) and foraminifera (identified by Tash) as middle Miocene; whereas the horizontal beds, which contain the fauna described by Schepmann in Martin, represent a Quaternary marine terrace.

Liddle (1946, p. 582), following Kehrer, changed the age assignment of the Cabo Blanco beds to middle Miocene, although he stated that he himself would prefer to consider them as no older than upper Miocene, and suggested a correlation with the "Capadare horizon" (which he did not define). For the beds from which Martin collected his fossils, he proposed the name "La Guaira beds". His description of the "La Guaira beds" is identical with the description he gave in 1928 for the "Cabo Blanco beds".

Dengo (1951) was the first to publish a map of the Cabo Blanco region on a sufficiently large scale (1:50,000) to show any detail. He refers the beds, here called "Cabo Blanco group", to the Miocene-Pleiocene (following Aguerrevere and Zuloaga) and considers that there are at least three divisions represented within the group: "The lower (beds) are cross-bedded sandstones which dip 50 degrees S., above this there is another sandstone, cross-bedded and with conglomerate beds, which also dips south, but at angles around 20 degrees. In the upper part, there is a conglomerate with large boulders". (Dengo, 1951, p. 68). In the English version of his paper Dengo, (1954, p. 18) he remarks that there are least three divisions, possibly a fourth, but does not discuss them.

R. J. Smith (1952, p. 385) suggested a correlation between the Cabo Blanco beds and formations in the Tuy valley, but without details.

Bucher (1952, p. 48) accepts the interpretation of the structure of Cabo Blanco as indicated by Kehrer (1939), that is, that there is a thrust fault between the metamorphics and the Cabo Blanco beds. He adds that Kehrer now believes the beds to be upper Miocene, and that Mencher considers that at least a part is not older than the Punta Gavilán formation of Falcón, that is, Pliocene.

In the correlation table of Mencher et al. (1953), "Cabo Blanco" appears as Pliocene.

Woodring (1954, p. 729) refers the "Cabo Blanco formation", to the lower Pliocene, assigning to the same age the Cumaná beds and the Matura formation of Trinidad.

Studies in 1954 by professors and students of the Department of Geology and Mines of the Central University indicate that in the region of Cabo Blanco it is possible to distinguish three distinct formations, two certainly Tertiary, and the third possibly Quaternary. In addition, three raised beaches or marine terraces are recognizable, two Pleistocene (at elevations of 6070 and of about 20 meters respectively) and a still lower, very recent one (Holocene). (See also the article, QUATERNARY).

We believe that it is convenient to retain the term Cabo Blanco, with the rank of a group, to designate the complex of geologically young, relatively unconsolidated sediments which, in that region, contrast so notably with the metamorphic of the Caracas group. Such a group name (though it includes beds of different origin and probably of different ages) is convenient, since the "Cabo Blanco beds" or "formation", as used by different authors, may refer to one or more of these divisions, and an attempt to restrict the name to any one of them would lend itself to further confusion. For example, the references to strongly southward-dipping beds, are true only of the lowermost formation, which is not fossiliferous, and which has not so far been distinguished from the upper horizons in the literature. Again, citations of fossils may refer in some cases exclusively to the uppermost formation (for example, Martin's lists, the allusions by Senn and Rutsch), or fossils from the two upper horizons (probably so for the mentions by Kehrer and Woodring.) Karsten, Kehrer and Liddle have recognized the upper fossiliferous horizon as a distinct entity, but the name of "La Guaira beds" proposed for it by Liddle is so inexact, geographically, as well as so poorly defined, that we have thought better to suppress it. We proposed to define the Cabo Blanco group as composed of three formations (separated by unconformities), in ascending order: Las Pailas, Playa Grande and Mare. (The name "Mare" takes the place of the "La Guaira beds" of Liddle, 1946).

Las Pailas formation. -This, the lowermost formation, consists of nonmarine, unfossiliferous conglomerates and sandstones, with conspicuous crossbedding, which dip strongly to the south (around 45 degrees). Locally it contains some carbonaceous beds. In the exposures, the generally whitish color of the beds contrasts strongly with the yellowish-brown color of the Playa Grande outcrops. The name Las Pailas is proposed since the beds are well exposed on the north side of the Quebrada Las Pailas near its mouth, a little way back from the beach. The Las Pailas beds are overlain discordantly by younger beds; the base of the section and hence the lower eontaet, are not exposed, but the formation may be presumed to lie uneonformably on the metamorphies of the Caraeas group. The thickness of the exposed section is of the order of 430 meters. The Las Pailas formation outcrops only between the Las Pailas quebrada westward to the vicinity of the large quebrada east of the Playa Grande real-estate development, where the Las Pailas beds disappear beneath a steep west-dipping fold in the overlying Playa Grande formation (possibly there may also be a fault at greater depth). Due to its unfossiliferous character, the age of the Las Pailas formation could not be directly determined; obviously it is older than the Playa Grande formation below described. Possibly it' is Miocene or even Oligocene.

Playa Grande formation. -This name is proposed for the marine fossiliferous formation, sandy, calcareous and conglomeratic, which forms the upland on which the Playa Grande real-estate development is located; although the beds are not well exposed on the surface of the upland, they are well exposed in the cliffs just back of the beach and in the large gullies which limit the upland on the east side. Here the lower contact is not visible, but on the east side of the large gully just mentioned, the Playa Grande beds form the upper part of the cliffs along the coast as far as the Las Pailas quebrada, lying unconformably on the Las Pailas formation. In the region of the Las Pailas quebrada, there is a fault striking about 75 E., with the southern side downthrown, so that to the south and east of the quebrada the Las Pailas formation is not visible, and the Playa Grande is represented only by a reduced section, overlain unconformably by the third and youngest formation, the Mare formation. In the large quebrada east of Playa Grande, a section of 78 meters of the Playa Grande formation was measured. The beds are more conglomeratic towards the western part of their extent, where they contain banks of conglomerate and of fossiliferous limestone, containing Pecten arnoldi Aguerrevere and other species of pectens, Ostrea cf. O. haitensis Sowerby, and various other fossils. A considerable microfauna of foraminifers and ostracods has also been collected from these beds. The age is thought to be probably upper Miocene.

Mare formation. -This name is proposed for the youngest formation, which so far has been identified only in the small area between the Las Pailas quebrada and the next large quebrada to the east, which is just back of the small settlement called Mare Abajo. (We have called this quebrada the Quebrada Mare.) These beds are fine micaceous sands crowded with mollusk shells. They are about 10 meters thick, and terminate above in a conglomerate, which occurs at about 20 meters above sea level (the level of the Maiquetía international airport); this level corresponds to a raised beach. The most abundant species in the bed is Macrocallista maculata (Linné), followed by Oliva reticularis Lam., but species of Architectonica, Conus, Terebra, Glycymeris, Strombina, Venericardia, Trigoniocardia, Pinna and various other genera are abundant. These are evidently the beds from which Martin made his collections. The age is possibly Pleistocene, but there are various species which suggest a possibly slightly older age. The writer is at present engaged in an intensive study of the fauna.

At the extreme eastern edge of the Playa Grande upland, there is a very small exposure of marine sands, believed to date from the formation of the Pleistocene raised beach which now is found at 60-70 meters above sea level. These sands are crowded with shells of small gastropods of very recent appearance. The thickness of this bed is negligible.

Frances de Rivero