QUEBRADON, Formation

TERTIARY (Oligo-Miocene)

State of Miranda, Venezuela

Author of name: J. Evanoff, 1951.

Original reference: J. Evanoff, 1951, p. 247.

Original description: ibid.

The name Quebradón formation was used for the first time by Evanoff (1951, p. 247) to define the strata situated between the top of Batatal and the base of Chaguaramas formations. This formation as well as the underlying Batatal and the overlying Chaguaramas constitute the beds known as Guarumen Group. The Quebradón formation is approximately equivalent to the Roblecito of the subsoil.

The Quebradón type section is exposed along the Quebrada (Creek) Quebradón, a tributary from the south of Quebrada Lele, in the vicinity of Batatal, State of Miranda. The section starts 630 m. upstream of the confluence of the two creeks and continues up for a distance of 2,650 m.

The formation is predominantly argillaceous, a characteristic which has served as a basis to separate it from the underlying Batatal formation. The shales are rather poorly bedded, soft and not resistent to erosion. They are blue-gray, reddish-brown, dark-gray and black and contain carbonaceous beds and thin beds of lignite. Gypsum crystals and jarosite are found in fragments. The inter-bedded sandstones are light-brown and gray in color and in fracture resemble the Batatal sandstones, but are impure and occur in thinner beds. They are fine to coarse-grained and locally conglomeratic. The sandstones are relatively more abundant in the type section than in the regions further west, but in the area south of the Galera de Guarumen, they become again rather prominent, forming long escarpments. Thick conglomerate beds are seen approximately in the middle of the section. The sandstones are frequently cross-bedded and ripple-marked. The thickness of individual beds varies from 20 to 80 cm. but some 10 m.

In the type section the thickness of the formation is 1,070 m.; in Quebrada El Baño, it oscilates between 750 and 1,000 meters and, further west, around 1,360 mts.

The formation is conformable at its base with the Batatal formation and at its top with the Chaguaramas. Its base is made of the first shales that overlay the clean Batatal sandstones; its top lies immediately below the first ferruginous conglomerates of the Chaguaramas formation. Quebradón exposures are observed immediately to the south of the Batatal escarpments in the foot hills of the States of Guárico and Aragua. They form the depression between the Galeras Guarumen and Uraya and the plains to the south of the Galera de Guarumen. To the east of the type reach as much as section the formation extends for a considerable distance reaching as far as the south of Sabana de Uchire.

The Quebradón formation in its type section contains a very poor fauna with little diagnostic value. Sellier de Civrieux (in Evanoff, 1951) identified Miliammina fusca (Brady), Ammobaculites nummus Garret and Trochammina sp., a faunal ensemble that suggests slightly brackish waters. He pointed out a marked change in the biofacies, together with a change in the lithofacies, with respect to the partly equivalent Roblecito formation. The same author also indicated that this last formation, which outcrops farther south in the basin shows numerous intervals containing a rich ensemble of marine foraminifera, interbeded in the faunal intervals of brackish ecology and pointed-out the fact that the change in bio and lithotopes reflects the relative positions of the Quebradón and Roblecito formations in the Guarumen depositional basin.

J. Evanoff