Revision of Lower Cretaceous Stratigraphy of Venezuela

Abstract

ABSTRACT The understanding of the Cretaceous stratigraphy in Venezuela is hampered by some vaguely defined or unsuitable type localities. A revision of the Cogollo formation of Western Venezuela is proposed as fresh exposures at the type locality on the Río Cogollo revealed that the contact with the La Luna is a fault zone, and that a large part of the succession is repeated by a thrust. The name “Cogollo” is retained for a unit (group) between the underlying Río Negro formation and the overlying La Luna formation, as defined by Hedberg and Sass (1937). Three distinct rock units, namely, from top to bottom, the Maraca, Lisure, and Apón formations, are recognized in the Cogollo group. The Apón formation was introduced by Sutton in 1946 for the lower part of the Cogollo group. By a very conspicuous and widespread interval of black laminated limestones containing limestone concretions (La Luna lithotope), the Apón formation is again subdivided into three units: the lower, middle, and upper Apón members. The lithologic character, combined with a rich ammonite fauna, makes the middle Apón an outstanding key formation. The Apón formation of Sutton is unchanged but his overlying Aguardiente is just a small part of the new Lisure formation. The upper part of Sutton’s Capacho is an equivalent of the Maraca formation. For a long time it was felt that the El Cantil formation (Liddle, 1928) of northeastern Venezuela needed a revision. It comprises the interval between the Barranquín formation below and the Guayuta group above. As type locality of a unit of such magnitude the section of El Cantil on Río Punceres is so poor that it should be discarded. Hedberg and Pyre (1944) made the first step to correct the erroneous conception of the El Cantil when they recognized the Chimana formation. It is proposed to subdivide the interval between the Barranquín and Guayuta group into three formations: the Borracha, Chimana, and Boquerón formations which, in turn, are subdivided into members (lower Borracha, middle Borracha or García, upper Borracha members; lower Chimana (Placeta), middle Chimana (Guácharo), and upper Chimana (Corral Viejo) members. The biostromal limestone facies of the shaly Boquerón formation is called the Majagual formation. All the unmetamorphosed Cretaceous sediments observed in Western Venezuela, the Andes of Mérida, and northeastern Venezuela are shelf deposits. Beds transitional toward a geosynclinal facies (geosyncline used here in the restricted sense of Krumbein and Sloss) are exposed only in the Barquisimeto area or now form part of the metamorphics in the Caribbean Coast Ranges. New ammonite faunas, determined by Ralph W. Imlay and L. F. Spath, and some foraminiferal key species identified by the writer (W.M.) serve as a basis for a time-stratigraphic correlation of the Cretaceous sequence of Venezuela. A tentative biostratigraphic comparison with the Lower Cretaceous of Texas is presented. Certain foraminiferal species have a rather restricted stratigraphic range and thus lend themselves for correlative purposes. In Venezuela, Choffatella decipiens Schlumberger is confined to strata of Aptian age; Haplostiche texana (Conrad) and some species of Lituola occur in Albian beds; Dictyoconus walnutensis (Carsey) is indicative of the Middle Albian Guácharo limestone; and Globotruncana (Rotalipora) appenninica Renz is a key species of the Cenomanian Boquerón formation. In Venezuela, several genera and species appear earlier in the Cretaceous time scale than in the Texas column, which suggests a migration of these faunal elements from the Caribbean Province into the Gulf Coastal Plain of the United States.