LOS ROBLES, Group

CRETACEOUS or older

State of Nueva Esparta, Venezuela

Author of name: unknown.

Original reference: H. H. Hess y J. C. Maxwell, 1949, p. 1858, 1860.

Original descripcion: ibid.

Hess and Maxwell (1949, p. 1858, 1860) published the name, Los Robles group, for a group of rocks, principally seritic-graphic schists and marbles, exposed in eastern Margarita. The name is derived from the town of Los Robles west of Pampatar. These rocks, according to the authors, represent a metamorphic facies of lower grade than the garnet-bearing schists of the Juan Griego group, and probably are the equivalent of at least a part of it.

The Los Robles group is exposed in the southern and eastern parts of eastern Margarita, to the south and east of the Juan Griego group, the out crops being divided into two areas by the Las Maritas lagoon. West of the lagoon, the Los Robles schists make-up the region north of the coast as far as Punta de Piedras, being covered locally by Miocene sediments (not formally named) and by Eocene beds of the Punta Carnero group near that locality. Northeast of the lagoon the rocks of the Los Robles group outcrop in a belt which includes Porlamar, El Piache and La Asunción, and extends as far as the Matasiete hills in the north; the authors show an outcrop of marble (the El Piache marble) included in the Los Robles group, on the western border of the Matasiete hills. They believe that an unconformity exists between the Matasiete porphyry and the Los Robles group. The nature of the contact with the Juan Griego group on the west was not determined, but is thought to be a fault. This contact is locally parallel to the regional strike, for example at El Piache, but in the north it cuts across the strike. Towards the east, along the coast between Punta Moreno and Punta Ballena, the Los Robles group is overlain unconformably by the Eocene beds of the Punta Carnero group, but the schists reappear southeast of the Eocene syncline in Punta Moreno, where the Los Robles schists are intruded by a coarse-grained igneous rock, described as a pyroxene diorite or gabbro. The strike of the Los Robles beds varies from almost N.-S. to N.-NE. and even N.-E. and the dips vary from 60° E. (west of Cerro Matasiete and in the El Piache marble) to 30° E.

The rocks of the Los Robles group comprised sericite-chlorite schist, quartz-sericite-mica schist, calcite-sericite schists and marbles. They represent a very low grade of metamorphism corresponding approximately to the chlorite zone of Harker (1939). Graphite is frecuent, as: also in the Juan Griego group.

Hess and Maxwell (1949, p. 1858) used the term, Piache marble (should be: El Piache marble) for a marble which forms the El Piache hill, 3 kilometers northwest of Porlamar, and also forms a ridge which paralells the road from Porlamar to La Asunción. This rock is described as a uniform fine-grained marble, light colored (white, gray, pink and green), which occurs in beds 1 to 3 ft thick, separated by thin beds of bright green chlorite which correspond to original thin shaly partings. Hess and Maxwell also mentioned a more sandy facies of the Los Robles group which is exposed below the Eocene, 3.5 kilometers southwest of Los Bagres.

On the basis of regional tectonics and geologic history, these authors believe that the Los Robles schists are probably Cretaceous.

For further discussion of the age, as well as for references to earlier publications, see: JUAN GRIEGO, Group. In the geologic map by Liddle (1946), part of the area which corresponds to the Los Robles group is indicated as Cretaceous. "The highly metamorphosed limestone of El Cantil formation" mentioned by Liddle (1946, p. 518), must be the El Piache marble.

Frances de Rivero

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