SANTA INES Formation

TERTIARY (Oligo-Miocene)

States of Anzoátegui, Monagas and Guárico, Venezuela

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

Original reference: A. H. Garner, 1926, p. 681.

Original description: ibid.

The name Santa Inés formation was created and first published by Garner (1926, p. 681) who applied it to a series of "friable brown sandstones, brown sandy shales, calcareous sandstone and dark gray to brown shales", with type locality in the small hills north of the town of Santa Inés. Hedberg (1937, p. 2008-2009) extended the type locality of the formation 20 kilometers southward of Santa Inés to include the Dividive ridge and described it as 10,000 to 20000 feet of sediments as follows: the base of the formation in Quebrada Carapita consists of yellow and brown or brown and gray mottled gritty sandstones and conglomerates; in Río Querecual, it consists of hard, gray, thick-bedded, fine-grained sandstones and gray fissile shales with clay-ironstone concretions; bluish and greenish-gray foraminiferal shale, fine-grained sandstones with black chert conglomerate seams and gray to greenish-gray, brownish-green and "pepper and salt" sandstones and gritty sandstones, with black chert conglomerates. The middle part of the formation is composed of yellowish to brownish-weathering sandstones, grits and conglomerates and thin sandy fossiliferous limestones, with some intercalated lignites and carbonaceous shales. Its upper part is made up of gray fossiliferous shales with clay-ironstone concretions, mottled claystones, brownish sandstones and sandy limestones; thin, hard, sandy and conglomeratic limestones and calcareous sandstones constitute the uppermost part of the formation which forms the Dividive ridge. A probable lower to middle Miocene age is given, based on mollusks and foraminifera. Hedberg also indicates the probable existence of an angular unconformity between the Santa Inés and the underlying Carapita formations.

Hedberg and Pyre (1944, p. 21) in a more extensive study, broaden the scope of the Santa Inés formation (or group) to include within it the underlying Carapita formation. For this purpose, the original Carapita was subdivided into two separate members. The type section of the revised group extends from Quebrada Carapita, at a point some 5 kilometers north of Santa Inés, southward including the Cerro Dividive ridge east of San Mateo. In this type section Hedberg and Pyre recognized several members as follows (from bottom to top): the Capaya (sandy) tongue or member, previously described (Hedberg, 1937, p. 2004) as the lower member of the Carapita formation; the Carapita shale (formerly known as the upper shale member of the Carapita formation); the Uchirito (sandstone-conglomerate) member; the Revoltijo (carbonaceous shale) member; the Salomón (claystone) member and the Dividive (sandstone-conglomerate) member, now known as the San Mateo member. Towards the west, on the Puerto La Cruz-Oficina highway, the Santa Inés group is subdivided into the lower Capiricual (shale-sandstone-conglomerate) member, lateral equivalent of the combined Capaya-Carapita-Uchirito members, and the upper Quiamare (claystone) member, lateral equivalent of the Revoltijo-Salomón-Dividive (San Mateo) members. The total thickness of the Santa Inés formation or group is given as some 24,000 feet.

Hedberg (1950, p. 1199) revised the subdivision of the Santa Inés group, and raised most of the formerly described units to formation rank. On this occasion, Hedberg states: "...This group includes many laterally and vertically transitional subordinate stratigraphic units, more or less clearly defined by lithologic or lithogenetic characters, which are here ranked as formations. Boundaries between these units are naturally rather indefinite and subjective. . . " In accordance with this concept, Hedberg also mentions several other units as forming part of the Santa Inés group as follows: (1) "Orégano formation", representing the shallow-water marine facies into which the Uchirito and Quiamare formations apparently grade in an easterly direction; (2) the El Pilar conglomerate, a thick series developed in the lower part of the Quiamare formation in northern Anzoátegui; (3) the Bruzual claystone, laterally equivalent to some part of the Capiricual formation in northwestern Anzoátegui and northeastern Guárico; (4) the Guanape conglomerate, probably approximately equivalent to the El Pilar conglomerate, developed in the vicinity of the Valle de Guanape area; (5) the Caño Dulce formation, outcropping in northeastern Guárico, which probably includes isome sediments equivalent to the basal Capiricual formation; (6) the Peña Mota conglomerate, which occurs at the top of the Caño Dulce formation and underlies the Bruzual claystone at Altagracia de Orituco in Guárico; (7) the Chaguaramas formation, applied to the far west equivalent of the Oficina and Quiamare formations in the State of Guárico; (8) the "Roblecitos" shale, a subsurface unit underlying the Chaguaramas formation in Guárico, which is probably of the same age as the Bruzual claystone, and possibly a western time-equivalent of part of the Carapita formation; (9) the Guarumen group, probably equivalent in age to the Merecure formation and lower part of the Santa Inés group, which is developed in Guárico; (10) the Zuata formation, outcropping in western Anzoátegui and southeastern Guárico, which grades northward into the upper middle part of the Quiamare formation; (11) the Santa Lucia formation, underlying the Zuata in southeastern Guárico; (12) the Cucharo formation, which underlies the Santa Lucía formation but is closely related to it, and grades northward into the Quiamare formation; (13) the Oficina formation, applied to the major part of the predominantly Oligocene subsurface section in south-central Anzoátegui, which has been traced throughout southern Monagas, southern and central Anzoátegui and far northward and eastward within the Eastern Venezuela Basin, and is equivalent in time to the outcropping Zuata formation and older Santa Inés group units farther west; (14) the Miocene Freites formation, extending throughout south and southcentral Anzoátegui and Monagas, and northern and central part of the Eastern Venezuela Basin, and constitutes the uppermost exposed unit of the Santa Inés group in central and southern Anzoátegui, and towards the north grades laterally into the upper member of the Quiamare formation.

Hedberg's Santa Inés group (1950, p. 1199-1205) represents an extremely complex lithostratigraphic unit, including almost all the formations in the Eastern Venezuela Basin overlying the Merecure group and underlying the Sacacual group. There is still some disagreement among the geologists as to whether Hedberg's latest definition of the group should be followed, or whether the term Santa Inés group should be used in its former and more limited sense.

The Santa Inés group is essentially conformable and transitional with the Merecure group wherever it overlies this unit. According to Hedberg (1950, p. 1200), it overlaps the Merecure on the southern limb of the basin and rests unconformably on Cretaceous formations of basement rock. It is unconformably overlain by the Mio-Pliocene strata of the Sacacual group.

Paleontological evidence indicates that the group includes sediments from middle Oligocene to middle Miocene in age, and may include some lower Oligocene and some upper Miocene.

Typical deposits of the Santa Inés group outcrop over a distance of about 60 kilometers in a north-south direction in the northeastern part of Anzoategui, and have an east-west extension of approximately 100 kilometers. If the different lateral equivalents of the units of the Santa Inés group mentioned by Hedberg (1950, p. 1202-1205) are included, the group would extend over most of the States of Monagas, Anzoátegui and Guárico

Cecily Petzall