LOS CUERVOS Formation

TERTIARY (lower Eocene)

Department of Santander del Norte, Colombia

Author of name: F. B. Notestein et al., 1944.

Original reference: F. B. Notestein et al., 1944, p. 1192.

Original description: ibid.

The Los Cuervos formation is named by Notestein et al. (1944, p. 1192) from Quebrada Los Cuervos, a tributary of the Río Catatumbo above Puerto Barco, in the Barco concession, Colombia, and the type section is in the same stream. However, the formation is better known from sections measured farther north on the Río de Oro anticline.

The formation is composed mainly of claystones and shales, with coal beds in its lower part and some sandstone beds throughout. The lower 75 meters consist of dark-gray carbonaceous shales and claystones interbedded with micaceous-carbonaceous siltstones, fine-grained sandstones, and coal. Rarely thin limestones showing cone-in-cone structure have been noted. The coal beds are commonly 8 to 10 in number and range in thickness from 0.1 to 2,5 meters. The usual thickness is 0,5 to 1 meter. The coals are lignitic to bituminous; the fixed carbon ratio in 22 samples ranges from 49.0 to 58.4 and average 54.5. They constitute the major coal resources of the region.

Above the coal series, the Los Cuervos is made up of gray and greenish-gray in part silty and commonly sideritic claystones, the siderite occurring as spherules generally less than 1 millimeter in diameter. There are some greenish-gray argillaceous sandstones, commonly in beds less than 6 meters thick. In the lower part of the claystone sequence there is considerable shale, generally dark gray and carbonaceous, and the claystones are only slightly mottled. However, the overlying claystones, which compose the bulk of the division, are characterized by locally abundant red, yellow, and purple mottling. The thin but hard sandstones of the Los Cuervos at many places form strike ridges.

In 16 measured surface sections, the formation ranges in thickness from a minimum of 245 meters in the northwestern part of the Esperanza dome to a maximum of approximately 490 meters on the Río de Oro anticline near Puerto Barco. Along the west side of the Concession it appears to thicken both to the north and south of the thinner sections on the Esperanza dome. Well sections range from a minimum of 249 meters in Carbonera N° 1 to a maximum of 246 meters in an incomplete section in Oro N° 3, which began drilling a little below the top of the formation. Thicknesses ranging from 282 to 316 meters were logged on the Sardinata, Tibú, and Socuavó anticlines. The well sections show a northward thickening.

The base of the Los Cuervos is drawn on top of the first fairly prominent sandstone immediately below the coal series, and the contact is apparently conformable. The top is drawn where the claystones or argillaceous siltstones of the formation give way to the distinctive cleaner and coarser-grained sandstones of the overlying Mirador formation. This contact is at least locally unconformable.

No conclusive evidence concerning the age of the Los Cuervos has been obtained, the formation being largely barren of fossils other than plant remains, but the equivalent Third Coal formation of western Venezuela is generally considered to be lower Eocene. Although the formation is thought to be mainly non-marine, there was some salt water invasion when the basal beds were being deposited. A thin fossiliferous black shale occurs very close to the base of the Los Cuervos in the Puerto Salado area, and in a sample of this shale A. A. Olsson (personal communication to Notestein) recognized what was probably a species of Diplodonta or an allied genus, the possible presence of a species of Anomia, and fragments of an oyster. He considered the fauna to be of "marine to slightly brackish-water" origin and that it could be Cretaceous or Tertiary. The Los Cuervos formation crops out in the southern half and along the western side of the Concession. It is a recognizable unit throughout the Concession and adjacent areas.

Sutton (1946, p. 1666) assumes a middle Eocene age for the Los Cuervos formation, which he correlates with the part of the Third Coal formation above the Tabla Sands in the Colón district of the State of Zulia. The combined Los Cuervos and Barco formations are considered by Sutton (1946, p. 1663) to be correlative with the Angostura, Marcelina and Trujillo formations.

W. A. Mohler