MONAGAS, Shale

TERTIARY (Oligocene)

State of Monagas, Venezuela

Author of name: H. D. Hedberg, 1936 (private report).

Original reference: J. H. Regan, 1938 p. 190-191.

Original description: ibid.

Regan (1938, p. 190-191) described the "Mio-Oligocene Monagas Shale" as the oldest formation encountered up to that date in the Quiriquire field of northern Monagas, Venezuela. It was said to lie unconformably below "the Green Series of the Miocene", a term now obsolete. The two deepest wells at that time, No. 73 and No. 127, each penetrated more than 3600' of the Monagas shale without reaching the base of the formation. The Monagas beds penetrated consisted chiefly of medium hard, black, foraminiferal clay shale with minor arenaceous beds. They were assigned to the Mio-Oligocene because of similarities in Ethology and foraminiferal content with Mio-Oligocene formations in other parts of eastern Venezuela and Trinidad. A list of fossils identified by G. E. Manger from a core taken in Well No. 71 in the western part of the Quiriquire field was appended. Most of these are genera of small foraminifera and indigenous, but two genera of large foraminifera, Lepidocyclina and "Camerina" (actually Operculinoides), are now known to have been derived from the upper Eocene Mundo Nuevo formation.

Hedberg (1950, p. 1206) mentioned "the so-called. Monagas shale (equivalent to Carapita shale) (Regan, 1938)".

Borger (1952, p. 2302-2304) did not directly refer to the Monagas shale but assigned its beds to the Oligocene and lower Miocene. At that time, as it is now, the name Monagas shale was virtually obsolete.

Present practice in Quiriquire field, the type locality of the Monagas shale, is to divide the strata formerly included in the Monagas shale between the upper Oligocene Carapita shale and the middle to lower Oligocene Buena Vista shale. An unconformable contact between the Carapita and Buena Vista is shown by the presence of a well-marked zone of reworked Cretaceous, upper Eocene and Buena Vista foraminifera at the base of the Carapita and by a shale pebble conglomerate cored at the base of the Carapita in one of the wells.

A. N. Duenbury, Jr.