GUACHARO, Member

CRETACEOUS (middle Albian)

State of Monagas, Venezuela

Author of name: A. H. Garner, 1926.

Original reference: A. H. Garner, 1926, p.679.

See CHIMANA, Formation

CHIMANA, Formation

CRETACEOUS (Albian)

State of Anzoátegui, Venezuela

Author of name: H. D. Hedberg and A. Pyre, 1944.

Original reference: H. D. Hedberg and A. Pyre, 1944, p. 8 etc.

Original description: ibid.

The term Chimana formation was introduced by H. D. Hedberg and A. Pyre for the variegated predominantly shaly-sandy deposits that occupy the stratigraphic interval between the El Cantil formation of authors (see Borracha formation) and the Upper Cretaceous Querecual formation (Hedberg and Pyre, 1944, p. 8-10). The name is derived from Chimana Grande Island of Puerto La Cruz, Anzoátegui.

The Chimana formation is composed of varicolored fissile micaceous ironstone-bearing shales which contain some brownish fossiliferous limestone lenses in the lower part; glauconitic, often coarse-grained sandstones predominate in the lower portion (Hedberg and Pyre, 1944, p. 9). A rapid lateral transition into bioherm limestones takes place on which account the boundary between the El Cantil formation (Borracha formation) and the Chimana formation has to be interpreted mainly as a facies boundary (Hedberg and Pyre, 1944; Hedberg, 1950). Because of these facies changes, the thickness of the Chimana formation varies considerably (200 to 1,000 feet).

The stratigraphic boundary with the overlying Querecual formation (Transition Shale Zone) is gradational.

The Chimana formation as defined by H. D. Hedberg and A. Pyre replaces the previously used term Bergantín beds. It also includes other obsolete stratigraphic names, such as the Bergantín sandstones (pars) and the Punceres sandstone. The limestone bodies within the Chimana formation had formerly been given individual names like Cumanacoa limestone, Caripe limestone, Guácharo limestone, and El Cantil limestone (pars).

The field work carried out by E. Rod in 1952 resulted in a more detailed interpretation of the Lower Cretaceous sequence in the Serranía del Interior. E. Rod differentiates the reef limestone on top of the Barranquín formation as Borracha formation (lower El Cantil of authors). The overlying Chimana formation is subdivided into three members, viz. the Corral Viejo member (top), the Guácharo member (middle part), and the Placeta member (base) (Rod and Maync, 1954, p. 242 etc.).

3.- Corral Viejo member.

The name Corral Viejo formation, used by E. R. Stevenson in 1946 (private report), was accepted by E. Rod in 1952 and published in 1954 (Rod and Maync, 1954, p. 228 etc.).

A poorly exposed section is seen at the type locality, i. e., on Quebrada Corral Viejo, San Francisco-Caripe highway, near Periquito, Monagas. The Corral Viejo member is composed of basal greensands, glauconitic sandstones stained by hematite; local shell pavements; higher varicolored shales with ironstone concretions, buff marls with limes¨one nodules, and thin interbeds of glauconitic sandstone. Its thickness ranges between 50 and 225 meters. An unconformable contact with the overlying Boquerón formation is locally evidenced (folding and truncation).

Fauna: Beudanticeras cf. beudanti (Brongn )

Lyelliceras ? sp.

Oxytropidoceras cf. carbonarium Gabb (= O. acuticarinatum (Shum.)

Phylloceras seresitense (Perv.) Spath, etc.

2.- Guácharo member.

The name Guácharo limestone was introduced by A. H. Garner (1926, p. 679) for the limestone formation which harbors the Guácharo Cave (A. von Humboldt had used the name Caripe limestone for the same unit). E. Rod reinstated the name in 1952 and uses it for the middle member of the Chimana formation. The old type locality ("vicinity of Guácharo Caves, Monagas") is retained and Quebrada de la Cueva, downsteam to and beyond the Guácharo Cave, is chosen as type section (Rod and Maync, 1954, p. 249).

The Guácharo member consists of light-gray to cream-colored massive reef limestones, often oölitic, sometimes coquinoid, with some marry interbeds. Its thickness exhibits great changes (0-180 meters) as it fingers out the Caripe area from where the Guácharo member shows a thinning and wedging out in all directions.

The Guácharo member was correlated by A. H. Garner (1926) with the Río Cogollo limestone of Western Venezuela, by R. A. Liddle (1928) and H. D. Hedberg (1950) with the El Cantil formation of authors.

The Guácharo member is not exposed at the classic locality, El Cantil on Rio Punceres where the El Cantil section is faulted and incomplete.

The Guácharo member rests conformably on the Placeta member, and the contact with the Corral Viejo member above is generally marked by greensands; in areas of a complete pinchout of the Guácharo member, the boundaries above and below are unconformities.

Microfauna: Dictyoconus walnutensis (Carsey)

Orbitolina concava-texana (Roem.)

Patellina subcretacea Cush. & Alex.

Pseudocyclammina Hedbergi Maync, etc.

(see Rod and Maync, 1954, p. 274-275).

The biostratigraphic unit Dictyoconus zone of Middle Albian age, introduced by W. Maync (Rod and Maync, 1954, p. 277 etc.), is characterized by the occurrence of Dictyoconus walnutensis (Carsey) which was first recorded from Venezuela by W. Maync (1953, p. 101). This form appears to be confined to the Guácharo member of the Chimana formation.

1.- Placeta member.

Since 1952 the name Placeta member is used by E. Rod for the basal part of the Chimana formation. The term, derived from La Placeta on the road from San Francisco to Caripe, Monagas, was published in 1954 (Rod and Maync, 1954, p. 228 etc.).

The Placeta member is made up of greenish-gray and brown shales, sandy shales, buff marls, coarse grained sandstones; thin beds of glauconitic limestone and coquinoid limestone occur near the base and the top which develop laterally into thick limestone, bodies (nodular marly limestone, Orbitolina marls and limestones, sandstone interlayers, etc.); sandstone beds at the base (type locality), quartzitic sandstones and black shale in the Guariquén area, Sucre. A thickness of 100-180 meters is attributed to the Placeta member. The microfauna of the Placeta member is rich and contains many species of pre-Chimana beds (see faunal list in Rod and Maync, 1954, p. 273). Characteristic elements are

Haplostiche texana (Conrad)

Lituola aff. camerata Lozo

Lituola subgoodlandensis (Vanderpool)

Orbitolina concava-texana (Roem.)

Triplasia acutocarinata (Alex. & Smith) etc.

The biostratigraphic term Haplostiche zone for the Albian beds of Venezuela, characterized by the occurrence of Haplostiche texana (Conrad) was first published in 1954 (Rod and Maync, 1954 p. 277, etc.). This typical foraminifera had been observed in 1949 in rock samples from the Lisure formation of the Sierra de Perijá, Zulia, and later (1952) in the marls of the Placeta member (Chimana formation) of Monagas. It has hitherto not been found in other stratigraphic levels and is held to be a diagnostic microfossil of the Middle-Upper Albian (Rod and Maync, 1954).

Age of the Chimana formation:

The different levels with megafossils listed by R. A. Liddle et al. from the El Cantil formation cannot be precisely placed in the Chimana section.

The coral fauna of the formation does not differ from that of the Barranquín formation (Wells, 1944).

On account of the above-listed foraminifera a chiefly middle Albian age is assigned to the Placeta member; the basal Placeta may possibly represent lower Albian.

The ammonite genera Beudanticeras, Lyelliceras, and Oxytropidoceras strongly point toward a middle Albian age of the Corral Viejo member.

There is as yet no indication as to the presence of upper Albian beds in the Chimana sequence.

The overlying Boquerón formation is positively assigned to the Cenomanian which indicates a disconformable contact (stratigraphic gap representing the upper Albian); in places, an unconformity is even evidenced (truncation of the Chimana formation and direct overlap of the Turonian Querecual formation).

Wolf Maync