CARRIZAL, Formation

PRE-CRETACEOUS (?Triassic-Jurassic)

States of Guárico and Anzoátegui

Author of name: J. A. Tong, 1940.

Original reference: H. D. Hedberg, 1942, p. 200-201.

Original description: J. A. Tong, 1940, in: Hedberg, 1950, p. 1184.

This formation is a subsurface unit that has been penetrated by wells in the States of Guárico and southwestern Anzoátegui. It was named from the well Carrizal N° 1, drilled by Socony-Vacuum Oil Company, in the District of Monagas, State of Anzoátegui.

Although published for the firsttime by Hedberg (1942, p. 200-201), the formation was named and originally described from the type section by J. A. Tong in 1940 (quoted by Hedberg, 1950, p. 1184) as follows: "The Carrizal formation is defined as those hard argillite or silty rocks (in the southern part of the Venezuelan Llanos area) underlying the Temblador or Oficina formations and overlying a sandstone formation designated as the Hato Viejo formation. In some cases it may lie directly on the basement complex. The upper part of the Carrizal formation is typical in SoconyVacuum Oil Companys Carrizal N° 1 well from 3654 to bottom at 5243. The lower part is typical in Socony-Vacuum Oil Companys Suata N° 1 well between 1503 and 2550. The upper part of the latter may be the equivalent of some of the lower part of the former, but because of the lack of markers the amount of overlap, if any, is not known. The upper contact is marked in some cases by oxidation coloring, possibly due to weathering on the old erosion surface. The lower part is marked by argillite beds containing pebbles and conglomerate beds".

Hedberg (1942, p. 200-201; 1950, p. 1182-1185) follows the original description by Tong, but makes the following observations: the formation is typically a tough massive dense greenish argillite, remarkably homogeneous, although variably silty and locally possessing intercalated beds of sandstones or even pebble conglomerate. It is generally noncalcareous but some thin siltstone laminae, as well as some vertical joints, are cemented with calcite. The formation is known to be at least 2,000 feet thick and is probably much thicker in full development. Fossils are largely limited to a single occurrence of poorly preserved fish remains (teeth, scales and bones), some poor indeterminate arenaceous foraminifera, and a questionable linguloid brachiopod. Mineralogically, the formation is characterized by garnet and biotite; feldspars are common. Glauconite in the fossil bed suggests a marine environment. Some reasons are given for supposing the Carrizal-Hato Viejo formations to be time equivalents of the Jurassic-Triassic (?) La Quinta formation of the Venezuelan Andes. However, the formation may bc as old as Paleozoic; such a point of view is also held by Liddle (1946, p. 144).

Liddle (1946, p. 100), Bucher (1952, p. 94) and Feo-Codecido (1953-4, p. 114) consider that, lithologically, the Carrizal-Hato Viejo formations are comparable to the outcropping metamorphic series (slate, argillite and quartzite) of the El Baúl area.

The formation is missing towards the east of southern Anzoátegui where the Cretaceous Temblador formation rests unconformably on rocks of the Guayana complex (González de Juana, 1947, p. 694). The section encountered in Guárico wells is in most places relatively thin (Patterson and Wilson, 1953, p. 2710).

No outcrops of the Carrizal formation have been identified in any portion of Eastern Venezuela (Hedberg, 1950, p. 1185).

Gustavo Feo Codecido