YUCALES, Formation

TERTIARY (upper Oligocenen to lower Miocene or younger)

States of Guárico and Aragua, Venezuela

Author of name: P. Leuzinger, 1944 (private report).

Original reference: R. F. Rutsch, 1951, p. 449.

Original description: none published.

P. Leuzinger (1944, private report) gave the name Yucales formation to a sequence of sandstones, shales and claystones exposed 5.5 kilometers southeast of Lezama in the Yucales area, State of Guárico. The sandstones are fine-grained, very micaceous with hard calcareous concretions, occasionally with lenses of yellow siltstones and sandstone breccias. Locally, the sandstones become conglomeratic, with pebbles of quartz and chert. Shales and claystones are carbonaceous, the latter being predominantly of a greenish variety. The thickness of the formation has not been accurately determined.

R. F. Rutsch (1951, p. 449) includes the Yucales formation into the Santa Inés group. However, E. Mencher et al. (1951, Correlation Chart) place the Yucales formation as unconformably overlying the Chaguaramas formation (equivalent to part of the Santa Inés group) and underlying unconformably Holocene alluvial deposits. At some localities the Yucales formation underlies unconformably, Quaternary boulder terraces containing large blocks of quartzitic sandstone and chert, which are comparable to the Pleistocene Mesa formation of Eastern Venezuela.

The Yucales formation appears to be of continental to brackish-water origin; it contains small foraminifera, particularly Streblus beccarii (Linné) and mollusks such as Pachydon, probably related to Pachydon cuneata.

As to the age of the Yucales formation, H. H. Renz points out that if it is in unconformable contact with the underlying Chaguaramas formation, an upper Miocene-Pliocene age can be assumed; but if it is transitional with the Chaguaramas formation and forms part of the Santa Inés group, an upper Oligocene-lower Miocene age should be assigned.

José Luis Padrón