JUAN GRIEGO, Group

CRETACEOUS or older

State of Nueva Esparta, Venezuela

Author of name: unknown.

Original reference: Hess and Maxwell, 1949, p. 1858, 1860.

Original description: ibid.

Hess and Maxwell (1949, p. 1858, 1860) published the term Juan Griego group, to designate the metamorphic rocks of higher-grade facies, characterized by the presence of garnet in Margarita Island. The metamorphics of lower-grade facies, characterized by the presence of sericite, were distinguished by them as the Los Robles group, and were equivalent to the Juan Griego group, although the outcrops of the two groups are separated by an abrupt change which suggests a fault. The metamorphics of the Juan Griego group make up the greater part of eastern Margarita where they crop out to the N and W of the Los Robles group.

The name is derived from the town of Juan Griego on the northwest coast, north and northeast of which the group is well exposed. The authors did not designate a type-section, nor give figures on the thickness of the group.

According to Hess and Maxwell, the Juan Griego group is made up of two divisions: a lower division, composed of quarzose rock (metamorphosed to schists and gneisses); and an upper division, composed of greenstones and amphibolites, that is, metavolcanics. The lower division or quartzose rocks comprises various types of gneisses and schists (Hess and Maxwell describe ten different varieties). The quartz-muscovite-graphite-hematite schist, which outcrops north of Juan Griego and in the María Guevara Peaks, are believed to be the oldest beds with which possibly may be included a schist which outcrops in a valley east of Barrancas. Above these rocks there is a thick sequence of graphitic quartz-mica schist and albite-mica gneisses which outcrop on the coast north of Juan Griego, in the hills northeast of it, and in the valleys of the north and southeast flanks of the Cerros de San Juan. Near the top of this lower division of quartzose rocks, there appears to be a thick section of graphitic quartz schists and quartzites which contains beds of greenstone.

The metavolcanic rocks of the upper or "greenstone division" were observed only in the northern part of eastern Venezuela, in a triangular area bounded by Otrabanda, Altagracia and Manzanillo. A thick section is exposed along the road between Tacarigua and Otrabanda. The rocks of this division are principally greenstone schists and amphibolites, but in the section just mentioned they contain lighter colored masses of feldspar-hornblende gneiss which appear to be sheets intruded parallel to the foliation. Other good exposures may be seen on the coast, at La Playa, and along the trail about a kilometer east of Altagracia. Hess and Maxwell describe six different petrographic types of schists, gneisses and amphibolite, noting that most of these rocks belong to the albite-epidote-amphibolite metamorphic facies, but a small proportion belong to the higher-grade amphibolite facies.

The base of the Juan Griego group has not been seen. The contact with the metamorphic rocks of low grade facies (Los Robles group), which outcrop to the east and south, is believed to be a fault. In general, the rocks strike northeast and dip steeply to the southeast. Hess and Maxwell interpret the regional structure as follows: the schists have been isoclinally folded and overturned to the northwest, the folds plunging to the northeast. One major anticlinal access apparently passes through the Cerros de San Juan, possibly extending westward through the peaks of María Guevara; another access extends northeaswards from the vicinity of Santa Ana, and a third brings the graphitic hematite schists to the surface northwest of Juan Griego. The masses of ultramaphic rocks which are intrusives in the schists in various areas (for instance the cerros de San Juan), appear to be emplaced along or near the crests of the anticlines, while the areas of metavolcanic rocks found at Tacarigua and the La Playa-Pedro González area, are believed to occupy major synclinal axis. In the neighborhood of Juan Griego, there is a small area of Pleistocene sediments Iying on the metamorphics (Richards, 1943).

Hess and Maxwell discuss the age of the Margarita metamorphics, and conclude that they are probably Cretaceous, and that the metamorphism was brought about before the end of the Cretaceous. This conclusion is based on their studies of regional geology and tectonics. They compare the metamorphics with those of the Caracas group, which have generally been considered Cretaceous. They consider that all these metamorphics of Margarita and the Cordillera de la Costa correspond to the geotectoclinal zone. They compare the metavolcanic rocks of Margarita (which have no equivalent in the Caracas group) with similar rocks in Tobago, the Greater Antilles and the Dutch West Indies, where such rocks have generally been considered as Upper Cretaceous. The downbuckling and the greater part of the metamorphism of the rocks of the Senonian (Hess and Maxwell, 1949, p. 1866). R. J. Smith, however, in his study of the Los Teques-Cua region in the Coast Range (Smith 1952-1953) believes that the first deformation of the rocks of the Caracas group took place at about the end of the Cenomanian, followed by another period of less intense metamorphism after the deposition of the Paracotos formation, that is, during the lower Senonian (Smith, 1952, p. 393). This evidence would have to be taken into account in an interpretation of the history of Margarita.

On the other hand, Rutten (1940) and González de Juana (1947), have considered more probable a Pre-Cretaceous age for the Margarita metamorphics.

An excellent resumé of earlier studies on Margarita has been given by Rutten (1940), from whom we have taken many of the following references. According to Rutten, the first author to mention metamorphic rocks on Margarita was Dauxion de Lavaysse (1813), who mentions micaceous schists in the hills of Macanao. Wall (1860) indicated the presence of his "Caribbean System" (metamorphics) and of younger sandstones (=Eocene Punta Carnero group in the modern classification). Sievers (1898) mentioned gneissic micaceous schists, graphitic schists and phyllites, all referred to the "Arquean". Liddle (1928, fig. 4, op. pág. 72) gives a geologic section of the island and a rather confused discussion of the geology (p. 73-74). In brief, Liddle thought that part of the Margarita schists represented very old rocks (Silurian or older), part lower and upper Cretaceous, but he gives no grounds for distinguishing the differences. In 1946 (fig. 3, op. p. 70, p. 77-78), he repeats almost word for word his earlier description. Although he states (p. 72) that he is limiting the term "Caribbean series" to supposedly Paleozoic schists, he does not make a distinction between Cretaceous and supposedly pre-Cretaceous schists in Margarita (p. 77), and indeed admits that a separation is practically impossible (p. 78) (see CARIBBEAN, Series).

P. I. Aguerrevere (1936) published a short note on Margarita accompanied by a geologic sketch-map, in which he indicates outcrops of gneiss and mica-schist.

Rutten (1940) gives petrographic descriptions of numerous rock samples collected in Margarita, Cubagua, Coche and other small islands by P. W. Hummelinck in 1936, together with a locality map. Rutten sets forth briefly his conclusions on the geology: he distinguishes in Margarita (1) a metamorphic basement: (2) Eocene sediments: (3) Pleistocene beds. As regards (1), he further distinguishes a group of older rocks (paraschists) and a somewhat younger group of intrusive "ortho-rocks", both acid and basic. He finds the samples from Coche and from Lobos, Caribes and Chacopata islets quite different from those of Margarita; these samples are sericitic quartzites. In Rutten's opinions, these quartzites of Coche, Chacopata and Lobos may well be metamorphosed Cretaceous, and the Margarita metamorphics would be older.

González de Juana (1947) gives the following interpretation; considering the notable difference between the non-metamorphic facies of the Cretaceous in the north of Anzoátegui and Sucre as compared with the intensely metamorphosed character of the rocks in Araya-Paria, together with the fact that Jurassic has been identified in the northern range of Trinidad, by evidence of an ammonite, he believes that the Araya-Paria metamorphic group is pre-Cretaceous and that its metamorphism is due to a post-Tithonian, pre-Urgonian orogeny (perhaps even to one or several earlier orogenies). A similar interpretation had been suggested by Hedberg (1942, p. 202). González de Juana considers the Margarita metamorphics as forming part of this Araya-Paria group.

Maxwell and Dengo (1950, p. 152-153) consider that while the metamorphics which are found at Carúpano are very similar to those of the Caracas group, both of these are different from the Margarita metamorphics, which lack the calcareous phyllites found in the former.

In any event, there is no doubt that the metamorphics of Margarita are at the least pre-Eocene, since pebbles of the ultrabasic intrusives, as well as chromite and enstatite grains derived from them, are found in the Punta Carnero group (Eocene).

The only objection to the name Juan Griego group is, that the selection of this locality name for a metamorphic group leaves us with materially no geographic name available to designate the Pleistocene beds which outcrop on the coast near Juan Griego.

Frances de Rivero

CARIBBEAN, "Series"

predominantly MIDDLE and UPPER MESOZOIC

Northcentral and northeastern Venezuela

Author of name: G. P. Wall.

Original reference: G. P. Wall, 1860, p. 465.

Original description: ibid.

The name Caribbean "system" was used by G. P. Wall (1860, p. 465) to designate a unit of metamorphic rocks outcropping in northeastern Venezuela (Paria and Araya Peninsulas) and northern Trinidad. The name was changed to Caribbean "series" by Wall and Sawkins (1860). They intended to subdivide the "series" into an old group and a young group. Nevertheless, due to the complexity of the units which constitute the Caribbean "series", the establishment of a type-section has not been attempted so far.

The name Caribbean "series" has been frequently used in Venezuela and Trinidad.

The sediments of the Caribbean "series", today metamorphosed, were affected by the formation of a great downbuckle or tectogene. Crushed rocks of said tectogene are now forming a geotectoclinal (Caribbean arch).

Liddle (1946, p. 46) intended to restrict the use of the name Caribbean "series" to a pretended Paleozoic schists sequence ("lower schists series") and to differentiate it from what he considered to be another sequence, of Mesozoic age.

Today it is well known (see Dusenbury and Wolcott, 1949, p. 17-19, and Bucher, 1952, p. 39-40) that such differentiation was based on an identification of Silurian fossils made by Dreverman. As it was demonstrated by Schuchert (1928, D. 951952) the fossils did not come from Venezuela (see VALENCIA, Limestone). The only "proof" for the occurrence of Paleozoic schists in the Caribbean "series" was thus gone and consequently the name "Lower Schists" had no value.

According to Kugler (1953, p. 35) the Caribbean series in the Island of Trinidad includes the following formations: a) (pre-Triassic or ?Jurassic) Dragón formation; b) (Jurassic) Maracas beds, Maraval beds, Laventille formation, Río Seco Formation, Grande Riviere formation; c) (Lower Cretaceous) Cumana formation, Galera formation, Toco formation, Sans Souci formation. According to the author, the first four formations are also represented in northeastern Venezuela. According to Kugler (ibid., p. 33) the series total thickness in northern Trinidad is of approximately 18,770 meters, number which according to some geologists seems exaggerated.

In the regions of north-central Venezuela studied so far, the Caribbean "series", locally unconformable over the ancient igneous complex (Sebastopol complex), begins with the Caracas group (Las Brisas formation with the tecto-phase of the Peña de Mora augengneiss; Antímano formation, Las Mercedes formation, and-according to Dengo, 1951-Tacagua formation). The Paracotos formation (of a lower metamorphic grade) unconformably overlies the Caracas group (fide Smith, 1952). The Paracotos is locally overlain by the volcanic rocks of Tiara or sediments of a very low metamorphic grade, the study of which has not been published yet.

Although it would be precocious to indicate the total thickness of the Caribbean "series" in northern-central Venezuela, the information available on the average thickness of the formations that constitute it, suggests a much lower thickness than that given by Kugler in Trinidad. Maxwell and Dengo (1950, p. 152) indicate a thickness of 1,500 to 3,000 meters for the Caracas group. The metamorphic section which outcrops in Carúpano, State of Sucre, and considered by said authors as probably correlative with the Caracas group, is 3,100 meters thick. The Paracotos formation, overlying the Caracas group, is 2,300 meters thick as appreciated by Smith (1952, p 363).

The age of the rocks constituting the Caribbean "series" has been and continues to be a matter of great discussion. According to Kugler (1936, p. 1441) Cunningham Craig was probably the first author to suggest the occurrence of Cretaceous rocks in the Northern Range of Trinidad; he based himself on lithologic similarities with Cretaceous rocks of the Central Range and mentioned the presence of limestone lenses containing caprinids, in calcareous schists of the Northern Range. He considered that the fauna is "Cenomanian in age and identical to the Cogollo limestone of Venezuela". Jurassic fossils were found by Hutchison (1939, p. 1243) in the Northern Range. Said age is today accepted for the lower part of the Caribbean "series" in the Island of Trinidad (see MARACAS, Beds and MARAVAL, Beds).

Hedberg (1942, p. 202) and Kugler (1953, p. 30-31) believe that in the Paria-Araya region (Venezuela) there are rocks probably equivalent to the Jurassic limestone of the Northern Range of Trinidad. González de Juana (1947, p. 694) states that the "Araya-Paria metamorphic group" includes "not only Jurassic... but some older rocks as well". He further states that the metamorphic rocks of the region of Paria and Araya are unconformably below the Lower Cretaceous (Barranquín formation) and therefore the main metamorphic period in the geotectoclinal was pre-Cretaceous.

In northern-central Venezuela, S. E. Aguerrevere and G. Zuloaga (1937, p. 22) proposed to correlate the lower part of the Caribbean series (Caracas group) with Cretaceous sediments of western Venezuela; they based themselves on lithologic similarities only. They also correlated the limestones near Villa de Cura (post-Caracas in age) with those of the vicinity of San Juan de los Morros; the Las Mercedes formation with the Colón formation; the "Los Colorados phase" with the La Luna formation; the Zenda phase with the Cogollo "formation" and Las Brisas formation with the Río Negro conglomerate. Kehrer (1937, p. 62) admits the possibility of such correlations saying that the transition from metamorphosed Cretaceous to normal Cretaceous can be seen in the Quebrada Auro, between Acarigua and Sarare, and in the Duaca-Aroa section, in western Venezuela.

The possibility of correlating the lower part of the Caribbean "series" in northern-central Venezuela (Caracas group) with the nonmetamorphosed Cretaceous section has been accepted by a number of geologists, for instance W, H. Bucher (1952). Bucher (ibid., p. 49) as well as other authors, indicates that equivalents of the metamorphic Jurassic of Trinidad and western Venezuela might occur in the Caracas group. When discussing the same problem, Hedberg (1942, p. 203) states that the possibility of a metamorphosed Triassic-Jurassic Caracas "series" should be also considered. He also says that it is interesting to notice that at the base of both Cretaceous and Triassic-Jurassic groups there is a sequence of sandstones and conglomerates which pass gradually into limestones and shales, towards the upper part. According to Hedberg, the character of the Caracas metamorphic rocks might be compared with the Triassic-Jurassic sequence as much as to the Cretaceous sequence.

Kugler (1953, p. 30) when discussing the gypsiferous facies in the Jurassic metasediments of the Paria Peninsula and Trinidad, emphasizes their importance in the correlation of the Caribbean "series" (see MARAVAL, Beds). On the other hand (see LAS MERCEDES, Formation) gypsiferous facies have been recently observed by the writer of the present article in the upper part of the Caracas group. Nevertheless, it should be found out whether the gypsum contained in said formations is of a primary origin or not, in order to be able to use the datum for correlation purposes. It should be mentioned here that gypsum veins of a secondary origin have been locally observed in the Lower Cretaceous of Venezuela (Chimana formation) (c. Francken, oral information).

Paleontological evidences in the Caribbean "series" of northern-central Venezuela are scarce. The available data consists of a microfauna collected by Wolcott (1943, p. 1632) in rocks of the Caracas group. It was originally considered Cretaceous or Jurassic, and "distinctly Upper Jurassic" by Bucher (1952, p. 42).

Other fossiliferous samples were collected by Wolcott in 1940, in a place located 5 kilometers to the south-southwest of Capaya, upstream in the Quebrada Yaguapa, from the crossing of the Quebrada Yaguapa with the old road to Caucagua (State of Miranda), a place which is located about 70 kilometers to the east-southeast of Caracas (Dusenbury and Wolcott, 1949, p. 22). The age of said microfauna is Turonian "because all the foraminifera species have been previously found in the Turonian section of the Querecual and La Luna formations and the radiolaria species in the Turonian section of the Querecual formation in Guárico" (ibid., p. 23).

The Paracotos formation, of a lower metamorphic grade than the underlying Caracas group and according to Smith in unconformity over said group, has provided Upper Cretaceous microfaunas in some localities, probably Cenomanian. The first mention in this respect is due to R. J. Smith (1952, p. 367) and it is based on a sample collected by H. H. Hess and studied by H. H. Renz. Other localities containing a similar fauna were studied by the writer of the present article. Recent studies by H. H. Renz indicate the Maestrichtian age of a part of the above mentioned formation. (see PARACOTOS, Formation).

Further information on the igneous rocks intimately associated with the formations of the Caribbean "series" could be found in the article "CORDILLERA DE LA COSTA" (IGNEOUS IN).

J. M. Sellier de Civrieux