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Revision of Lower Cretaceous Stratigraphy of Venezuela

1954; American Association of Petroleum Geologists; Volume: 38; Linguagem: Inglês

10.1306/5ceadecc-16bb-11d7-8645000102c1865d

1558-9153

Emile Rod, Wolf Maync,

Evolution and Paleontology Studies

The understanding of the Cretaceous stratigraphy in Venezuela is hampered by some vaguely defined or unsuitable type localities. A revision of the formation of Western Venezuela is proposed as fresh exposures at the type locality on the Rio 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 Rio 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 Apon formations, are recognized in the group. The Apon formation was introduced by Sutton in 1946 for the lower part of the group. By a very conspicuous and widespread interval of black laminated limestones containing limestone concretions (La Luna lithotope), the Apon formation is again subdivided into three units: the lower, middle, and upper Apon members. The lithologic character, combined with a rich ammonite fauna, makes the middle Apon an outstanding key formation. The Apon 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 Barranquin formation below and the Guayuta group above. As type locality of a unit of such magnitude the section of El Cantil on Rio 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 Barranquin and Guayuta group into three formations: the Borracha, Chimana, and Boqueron formations which, in turn, are subdivided into members (lower Borracha, middle Borracha or Garcia, upper Borracha members; lower Chimana (Placeta), middle Chimana (Guacharo), and upper Chimana (Corral Viejo) members. The biostromal limestone facies of the shaly Boqueron formation is called the Majagual formation. All the unmetamorphosed Cretaceous sediments observed in Western Venezuela, the Andes of Merida, 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. End_Page 193------------------------------ 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 Guacharo limestone; and Globotruncana (Rotalipora) appenninica Renz is a key species of the Cenomanian Boqueron 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.