ANGOSTURA, Formation

TERTIARY (lower to middle Eocene)

State of Táchira, Venezuela

Author of name: F. A. Sutton, 1946.

Original reference: F. A. Sutton, 1946, p. 1662-1663.

Original description: ibid.

Sutton (1946, p. 1662-1663) used the name Angostura, which is derived from the Angostura bridge across the Río Lobaterita on the road from La Cortada to Colón in western Táchira, to designate a coal-bearing formation which crops out at this locality. The base of the f ormation, where it is in contact with the Catatumbo formation, is a short distance downstream from the bridge. Its upper contact with the Misoa formation is about 600 meters upstream from the bridge. Sutton pointed out that the coal measures of this area were originally referred to by Liddle (1928, p. 181) and Kehrer (1937, p. 48) as the "Third Coal horizon". Sutton described the Angostura formation as mainly composed of interbedded sandstone and shale with a few beds of coal. The sandstones are well bedded, laminated, and cross-laminated, gray to dark greenish gray, fine-grained, locally carbonaceous, and plant-fossiliferous. The shales are gray to black, hard, arenaceous, and locally sandy, carbonaceous, and plant-fossiliferous. The coal beds are sub-bituminous to bituminous, commonly shaly, and vary in thickness from a few centimeters to one meter. Sutton cited a thickness of about 400 meters at the type locality of the formation. It thins northward to 200 meters on the Río de Oro in northern Táchira and 75 meters on the Río Buena Vista in southwestern Trujillo. In Táchira and Mérida the Angostura formation rests disconformably on the underlying Mito Juan formation. In southwestern Trujillo, the Mito Juan formation is missing and the Angostura lie's unconformably on the Colón formation. An unconformable relationship is believed to exist between the Angostura and the overlying Misoa formation. The Angostura formation is present as a recognizable unit throughout the States of Táchira and Mérida, and northward to the southwestern part of the State of Trujillo. Beyond there it appears to wedge out. Sutton stated that macrofossils were never found and that foraminifera are extremely rare in the formation. Those reported consist mainly of brackish water species. He considered the formation to be lower to middle Eocene in age. Sutton pointed out that the Angostura formation is the equivalent of the Marcelina formation and that it can be correlated with the combined Barco and Los Cuervos formations.

At present, a number of geologists prefer to consider the Angostura as a group including (from top to bottom) the Los Cuervos and Barco formations. This usage they base on the fact that the Los Cuervos and Barco formations can be traced into the Angostura. The "Third Coal horizon" of Táchira is equivalent to the combined Angostura and Catatumbo formations. This latter f ormation appears to be absent in Mérida where the "Third Coal horizon" is equivalent to the Angostura only.

Leo Weingeist