RIO GUASARE, Formation

GUASARE, Formation

GUASARE Formation

TERTIARY (Paleocene)

State of Zulia, Venezuela

Author of name: A. H. Garner, 1926.

Original reference: A. H. Garner, 1926, p. 679-680.

Original description: ibid.

The original name was "Río Guasare formation", by Garner (1926, p. 679-680). Because of locally abundant venericards, the formation has sometimes been called the "Venericardia limestone". Garner described the Río Guasare section as "alternating beds of hard, brown sandstone, black slaty shales and thin, hard, brown, fossiliferous limestones". Hedberg and Sass (1937, p. 87) modified the name to "Guasare formtion", and thus it is generally used at present.

E. R. Critchett (1948, Richmond Exploration Company private report) described the section on Río Guasare: "From the botton up it is composed of medium-bedded, smooth, dark-brown-weathering, fine-grained, buff, slightly argillaceous sandstone, grading upward into light-brown, angular-weathering, fine to medium-grained, somewhat calcareous sandstones. Minor dark grey shale is also present in this portion. Above this is a series of thin-bedded, shaly, calcareous, micaceous, grey, platy, dark-gray-weathering sandstones interbedded with minor dark grey, carbonaceous shales. Next above is a highly fossiliferous bed of sandy limestone, one meter in thickness, weathering brown and blocky. Overlying, is a thick series of interbedded and interlaminated, soft, fine to medium-grained, buff, silty, micaceous sandstones, very-fine, crystalline fossiliferous limestone occurs 12 meters below the upper dark-grey, carbonaceous, soft, often glauconitic shale. Above this are alternating sandstones and shales to within about 20 meters of the Guasare-Marcelina contact. This upper portion of the formation is largely composed of calcareous, fossiliferous, fine-grained, buff to grey, quartz sands, and sandy, fossiliferous, oyster limestones, all thin to medium-bedded. One bed of grey, very-fine-crystalline, fossiliferous limestone occurs 12 meters below the upper contact. This is very distinctive because of its color, texture, and bone content. A very small amount of tar and heavy oil was found in bugs and small solution chambers of the rock in a sandy, fossiliferous limestone immediately above this bed."

The Guasare formation is conformable with the underlying "Upper Cretaceous" (variously named "Mito Juan", "Upper Colon", or "La Paz shale"), and with the overlying Marcelina formation. The lower contact is generally placed at the top of the prominent shale below the lowermost, thick limestone unit. Below this point the limestone beds become rapidly thinner and fewer, and finally give way to the Upper Cretaceous shale of the Colon formation (or as otherwise designated). The upper limit is placed at the top of the previously mentioned ten-meter sand and limestone unit, above which lies a sequence of clastics similar to the Guasare, but containing coal beds and lacking the limestones of the Guasare.

Along the eastern flank of the Perijá mountains, 410 meters of Guasare formation have been measured on the Río Socuy in northwest Zulia, 390 meters are reported on the nearby Rio Cachirí, and 200 meters in Quebrada Caña Brava al Sur, near the Río Apón, about 85 kilometers to the south. Up to 425 meters are encountered by wells in the Mara Field, 100 meters in the Boscán Field, and about 120 meters in the Ensenada area, south of Maracaibo. An incomplete section crops out on Toas Island.

The Guasare formation represents the emergence or shallowing water facies of general Paleocene time, toward closing phase of the so-called "Cretaceous cycle" of deposition (a phase which actually extends to Eocene time). As such, it may be considered the stratigraphic equivalent of the Catatumbo formation of the Barco area, the limestone at San Juan de los Morros from which Caudri (1944) described a Paleocene fauna of larger foraminifera, and the deposists of a similar inner-neritic environment overlying the deeper-water, Upper Cretaceous deposits elsewhere in Venezuela and northern Colombia.

The Río Guasare section yields Venericardia which are probably conspecific with Venericardia (Venericord) toasensis Dusenbury, found on Toas Island, and have been related to V. parinensis from the Negritos formation of Perú. Several other tentative identities, including Turritella negritosensis Woods suggest correlation with the Negritos formation. The Venericardia are also found on Soldado Rock and in the Marac Quarry of Trinidad, in rocks of undisputed Paleocene age. From Toas Island, a foraminiferal fauna including Eponides lotus (Schwager) and Anomalina scrobiculata Schwager, has been described, and from a well in the Mara Field, Coleites reticulosus (Plummer), Vaginulinopsis wilcoxensis, Nodosaria latejugata Guembel, Discorbis midwayensis soldadoensis Cushman and Renz, and many others, have been identified.

The faunas denote time correlation with the Paleocene of Soldado Rock, the Marac Quarry, at least part of the Negritos, and the Upper Midway of the U. S. Gulf Coast.

K. L. Edwards