CUIBA Formation

TERTIARY (middle Miocene?)

State of Zulia, Venezuela

Authors of name: Geologists of the Richmond Exploration Company.

Original reference: E. Mencher, et al. (Correlation chart), 1951.

Original description: none published.

Uppermost of the El Fausto group formations is the Cuiba. The name is derived from the "finca" Cuiba, and vicinity of the Río Cuiba, about 19 kilometers north of Machiques, District of Perijá.

The formation status and definition of the Cuiba, and its separation from the subjacent Macoa, is based on subsurface geologic work. The Cuiba, however, is considerably better represented in surface outcrop than either the Macoa or Peroc units. It is predominantly a mottled claystone formation, although it includes minor and variable amounts of sandstone. The best surface outcrops of the formation are found near the "finca" Cuiba, and in the vicinity of the "finca" El Fausto located several kilometers farther east near the Río Cogollo. In neither locality is the lower boundary of the formation apparent, or the exposures good enough, without augering or pitting, for accurate lithologic study.

Originally, two type sections, of the Cuiba were recommended, one in the well Neopig-1 and the other in the well Zulia 26D-1. These two sections are located 16 kilometers northeast of Machiques in the District of Perijá, and are separated by a distance of only 700 meters. The Neopig-1 well is better represented in samples, including 91 feet of core recovery from 27 cored intervals within the formation. The present reserve of sample material is small. The Zulia 26D-1 well has electric log data, not available at Neopig-l, but there are no cores. Since the formation is defined on basis of lithology and not on electric log characteristics, it is suggested that the Neopig-1 section, represented by the interval 1547-3730, be retained as the type section. The interval 1910-3735 in the well Zulia 26D-1, and the interval 70-1890 in the well Zulia 20D-1, are proposed for supplementary sections. (Footage intervals). The Zulia 20D-1 section is incomplete however; the well was spudded within the outcrop area for the Cuiba unit so that probably erosion has removed about three hundred feet of the uppermost Cuiba beds at the wellsite. The thickness of the Cuiba typically ranges from about 1.650 to 3,000 feet (500 to 900-plus meters).

The Cuiba formation consists largely of mottled, silty and sandy claystone, some olive or light olive gray claystone, and lesser amounts of fineand medium-grained sandstone. There are minor, variable amounts of lignitic coal and lignitic or carbonaceous sediments. Minor amounts of calcareous matter, mostly in the sandstones, are also reported. The unit resembles sediments of the Peroc formation, in the basal part of the El Fausto group, except that there appears to be no anhydrite in the Cuiba. Sandstone beds are thicker and somewhat more numerous in the Cuiba than in the Peroc.

Claystones in the Cuiba are mottled olive-gray to pale-olive and moderate greenish yellow, with further mottlings and attainings of very dark red, very dusky red, grayish red purple, and other hues in the olive, gray, red, brown, and purple range. Olive-gray to light-olive and dark and dusky yellowish brown claystone beds, without mottlings, are less common in the formation. Sandstones tend to light olive gray and pale-olive colors. Some shale is reported in surface outcrops and pits. Brownish-gray to nearly black lignitic beds occur in the formation, and beds of coal are present locally. The mineral suite includes ilmenite and zircon, and small amounts of rutile, titanite, and tourmaline. Tan siderite pellets are present in variable amounts.

The Cuiba is underlain by the Macoa formation, which is distinguished by its general darker, olive gray coloration, and drastically reduced amounts or virtual absence of mottled claystones. The boundary is transitional and apparently involves from 20 to 100 feet or more of section. Since the boundary has not been sampled by cores, and its definition is entirely dependent on "ditch" samples from soft, sloughing formations, it is reasonable to suppose that the actual Macoa-Cuiba boundary may tend to be somewhat sharper than is generally recorded and observed.

The selection of a boundary between the Cuiba and overlying Los Ranchos formation is no less difficult, and it appears that the transition may involve a greater range. This boundary is largely based, in the well samples and electric logs, on gradually increasing amounts of sandstone upward into the Los Ranchos. On basis of surface geology, the Cuiba-Los Ranchos contact is generally marked by a topographic break between the relatively flat strike valley representing the Cuiba and other formations, within the El Fausto group, and the higher, rugged terrain representing the Los Ranchos. In places, this topographic break involves an escarpment and difference in relief of about 100 meters. The transition from Los Ranchos to the Cuiba begins below the general rim of the escarpment, and the boundary is usually placed near the base of the escarpment, where it is based on a gradual change lacking in characteristics that lend themselves to specific definition.

The Cuiba is recognized throughout the area from the Totumo structural arch, about 15 kilometers northwest of La Villa del Rosario in the District of Perijá, eastward to the wells Zulia 14D-1 and 15G-1, southward through out the area of the Macoa structural nose, and to the well Zulia 36E-1, 20 kilometers east of Machiques. Eastward, toward the Boscán field, the unit evidently grades into a unit known, at Boscán, as the Upper La Rosa, which is considered to be a very approximate equivalent to the Lagunillas formation of the Lake Maracaibo area farther east. The change from Cuiba to Upper La Rosa is accompanied by marked decrease in the amount of mottlings, increase in green and olive coloration, and appearance of sporadic occurrences of brackish-water faunas, including foraminifera and thin shelled ostracods, that are apparently not present in the Cuiba. The marine affinity of the La Rosa increases eastward into the Lake Maracaibo area. The correlation from Cuiba to Upper La Rosa in time sense is considered to be only approximate. In facies sense, there is gradation from presumed subaerial to lagoonal conditions into marginal, sub-saline marine conditions.

No faunas have been found in the Cuiba formation in any of the area bounding the Perijá mountain front, except for occasional reworked foraminifera from the Cretaceous.

John B. Miller