MEDERO, Formation

TERTIARY

State of Portuguesa, Venezuela

Author of name: E. Mencher et al., 1951.

Original reference: E. Mencher et al., 1951, p. 20.

Original description: ibid.

The Medero formation was named by Mencher, et al. (1951, p. 20) after the Quebrada Medero, three kilometers northeast and north of the town of Guanare, State of Portuguesa. As described by Mencher et al. (ibid.), the formation consists of ripple-marked and crosslaminated, carbonaceous sandstones, oyster reefs and orbitoidal limestones which occur in the southeastern foothill area of the Venezuelan Andes and correlate with the upper Eocene Mene Grande formation of the northwestern foothill area. From Mencher's et al. (ibid.) lithologic description and upper Eocene age assignment, the Medero formation would be identical to the marine Altamira formation of the Barinitas area as named by Sinclair geologists (see Sutton, 1946, p. 1686-87). However, on the Quebrada Medero no such marine beds occur. Instead of oyxster reefs and orbitoidal limestones (Mencher et al., 1951, p. 20) and specimens of Raetomya (Sutton, ibid. p. 1686), there are variegated, thinly-laminated claystones interbedded with fine to medium-grained, grey sandstones and shales with an absence of the marine fossils so characteristic of the upper Eocene Altamira and Mene Grande formations. Consequently, if Mencher et al. (ibid.) had in mind the upper Eocene marine sandstones and shales of the Altamira area when they used the term Medero formation, they should not have chosen a type locality 87 kilometers away at the Quebrada Medero where non-marine beds are exposed that may not even be upper Eocene in age. Furthermore, the Medero formation on the Quebrada Medero is of limited extent and has been folded and beveled and covered with reworked blocks of its own sandstone beds. The base of the formation is not exposed.

Because of this, it is recommended that the term Medero formation shoul not be used as it was by Mencher et al. (ibid.) to apply to the upper Eocene marine sandstones and shales of the southeastern Andean foothill belt. As mentioned by Sutton (ibid., p. 1686-87), the name Altamira formation in reference to these beds would be much more appropriate.

H. Alberding