AREO, Shale

TERTIARY (upper Eocene to Oligicene)

State of Monagas, Venezuela

Author of name: H. D. Hedberg, 1950.

Original reference: H. D. Hedberg, 1950, p. 1197-1198.

Original description: ibid.

Hedberg (1950, p. 1197-1198) describes the Areo shale as a provisional stratigraphic unit, but in the text it is occasionally referred to as formation. It consists dominantly of gray shales with seams of yellowish or reddish glauconitic ironstone concretions and occasional hard, writish-gray, quartzitic sandstones 5 to 30 feet thick. Fossils include rare specimens of Doscocyclina and Lepidocyclina.

The type section lies on Río Areo, west of the town of San Juan, from the mounth of Quebrada Juan Rosario upstream to the axis of the syncline just below El Salto. Here, a possible thickness of more than 1000 feet is indicated; its upper limit is not exposed.

Correlation is indicated in the original description as follows: "...The lower portion of the Naricual formation grades eastward into a shaly glauconitic marine fossiliferous facies which is exposed on Río Areo, Río de Oro, and a few other streans in northwesternmost Monagas an northeasternmost Anzoátegui. This unit has been variously called the Areo shale, the Juan Rosario shale, Buena Vista shale and La Moya formation, but in the absence of a formal definition it is herre referred to provisionally as the Areo shale from exposures on that river and its tributaries". The age is considered to be upper Eocene to Oligocene.

Some geologists refer to the lower and middle Oligicene shales found in the subsurface of Quiriquire fields as Buena Vista shale.

Wade H. Hadley Jr.