LEON Formation

TERTIARY (upper Oligocene)

Department of Santander del Norte, Colombia

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

Original reference: F. B. Notestein et al, 1944, p. 1201.

Original description: ibid.

The León formation (also referred to as León Shale or León Shale formation, was named by Hedberg from Quebrada León, a tributary of the Río Zulia on the east flank of the Petrolea anticline in the Barco concession, Colombia. The type section is in the same stream, about 9 kilometers northwest of Puerto Villamizar on the Río Zulia.

The formation is composed almost entirely of gray and greenish gray tough shales. There are rare thin beds of gray and buff sandstones with carbonaceous laminae, and the shales tend to become silty toward both the base and the top. In the Tibú-Socuavó area a thin series of buff sandstones and interbedded shales occurs near the middle of the formation. This series of sandstones is only locally developed and attains a maximum thickness of 28 meters. It has been called the Machete sandstone member.

The León formation ranges from 510 to 785 meters in thickness on the east flank of the Petrolea anticline, and 663 meters was found on the east flank of the Leoncito anticline. In the Tibú-Socuavó area it has been estimated to be from 350 to 475 meters thick. The formation appears to thicken and become sandy in the Río de Oro area.

The upper and lower contacts are drawn where the essentially continuous shale section of the formation gives way to sandier sediments. Both are normally distinct boundaries and appear conformable.

According to Sutton (1946, p. 1703) the León formation lies with probable disconformity on the Carbonera and with probable conformity beneath the Guayabo.

The formation is poor in fossils on the Barco concession except for fish remains and a few foraminifera belonging to the following genera: Ammobaculites, Haplophragmeides, Flabellammina, Spiroplectammina, Quingueloculina, and Trochammina. Sutton (1946, p. 1703) believes that this microfauna is probably reworked. The section between Uracá and Estación Táchira on the Río Lobaterita (Kehrer, 1938, p. 45-46) is included by Notestein et al. in the León formation and has been discussed by Sutton (1946, p. 1701-1702) as part of the Palmar formation. The León of the Barco concession is thought to correlate with the Palmar formation. For fossil lists, see Sutton (1946, p. 1702).

The age of the León formation is believed to be upper Oligocene by Notestein et al. (1944, p. 1201) and lower and middle Miocene by Sutton (1946, p. 1703).

The León formation is identical to the "Upper Shales" formation of western Venezuela, which term, according to Notestein et al., is unsatisfactory.

According to Schaub (1948, p. 221), the Lobaterita formation of Sutton (1946, p. 1677) represents the "Upper Shales" formation which is the equivalent of the León formation. Dusenbury (1949, p. 146) reports that the Lobaterita formation has been traced across the Venezuelan-Colombian border into the León formation.

Part of the Ledn formation probably correlates with part of the Peroc formation.

W. A. Mohler