LOS RANCHOS, Formation

TERTIARY (Upper Miocene ?)

State of Zulia, Venezuela

Author of name: Carl St. J. Bremner ( ? ).

Original reference: Liddle, 1928, pp. 308-312.

Original description: ibid.

The first published use of the term "Los Ranchos" is that of R. A. Liddle (1928), who identifies the geographical origin of the term with the "finca" Los Ranchos, which is located about 15 kilometers northeast of Machiques in the District of Perijá. Liddle does not acknowledge authorship, nor comment on origin of the name. Probably the formation was named by Carl St. J. Bremner around 1926; the name appears on a map in private files of the Richmond Exploration Company which was drafted late in 1926 or early in 1927. The original description of the Los Ranchos formation does not adequately define formation boundaries; also the references by Liddle (1928, 1946) and Sutton (1946), although recognizing a "type area" between La Villa del Rosario and Machiques, do not designate a type section. Quotations from the original description by Liddle (1928, p. 308) serve to briefly outline the physical character of the formation and its topographic expression: "Wellbedded, ferruginous, micaceous sandstones; a few thin beds of conglomeratic sandstone and conglomerates; grayish clay-shales, gray and buff sands and clays, and reddish-brown, thin, ferruginous seams and bands, comprise Los Ranchos beds. Toward the top, approaching La Villa beds, the sandstones are usually highly micaceous, and mottled red and white". - "The harder Los Ranchos beds form ridges or hogbacks which dip lakeward, whereas La Villa beds to the east, being much softer, form low, rolling plains on which stand occasional 'cerros' capped by more resistant or thicker sandstone beds". Prompted by deficiencies in existing literature, the writer has selected a type section, which extends along a road branching westward from the Perijá highway near kilometer 98, 12 kilometers southwest of La Villa del Rosario. The top of the formation is crossed on this road about 3.8 kilometers from the Perijá highway, and the rather arbitrary base of the formation is found about 5 kilometers farther west (speedometer distances). The base is about 200 or 300 meters eastward from the bridge over Cailo del Polvo, and 1,200 or 1,300 meters eastward from the Zulia 20D-1 well. Neither boundary of the formation is conspicuous on the road; however only the top boundary requires further definition. It lies at the east base of a prominent hill located about 400 meters north of the road, and 3.6 kilometers (airline distance) from the Perijá highway. A roughly measured thickness for the section is 4900 feet (1500 meters).

The Los Ranchos formation in the vicinity of the type section is more resistant to erosion than the underlying El Fausto group sediments or the overlying La Villa formation. Its outcrop area is typified by rugged topography and fine-grained drainage pattern, and differs in these features from the strike valley of the El Fausto unit sand rolling "sabana" topography of the La Villa. The topographic relief is about 100 meters.

About 50 or 60 percent of the formation at the type section is sandstone, and the remainder is mostly claystone. Minor amounts of siltstone and pebble conglomerate or conglomeratic sandstone are present. The rusty-brown color of the sandstones, the buff to reddish-colored soils, and the ferruginous, brown or grayish-brown talus, tend to impart a drab brown coloration to the general outcrop area. Sandstones in outcrops are variously stained by ferruginous pigments to hues of moderate-brown, light-brown, and yellowish-orange. Claystones are mostly very light-gray, yellowish-gray, light-greenish-gray, and may contain mottlings of grayish-purple, or moderate-and dark-red. In well samples, light-gray and olive colors appear, and the rusty-brown coloration in sandstones is less prevalent. Heavy mineral concentrates from the Los Ranchos include ilmenite, zircon, rutile, leucoxene, and tourmaline.

The formation apparently represents deposition in fresh or brackish water of lacustrine or lagoonal character, including shoreline and beach deposits and possibly some subaerial deposition on a piedmont slope. The formation is transitional with the underlying Cuiba formation, and with the overlying La Villa formation, in a stratigraphic sequence that represents a regression. The boundary with the Cuiba marks the change from claystones of the El Fausto to considerably sandier sediments of the Los Ranchos. The rather arbitrary boundary closely coincides with the base of the escarpment representing the change from the hilly area of the Los Ranchos to the valley or depression of the El Fausto.

Although the contact of the Los Ranchos with the overlying La Villa is also a transition, in the area from the Río Palmar to the Río Cogollo, it can be traced easily by different lihology of the two formations and the relatively sharp topographic break between them. The boundary becomes less distinct south of the Río Cogollo, although there is a physical change that can be traced as far south as the Río Negro and makes possible a rough delineation between the two formations. An unpublished report (Richmond Exploration Company, P. B. McGrath and J. B. Miller) describes the Los Ranchos south of the Río Cogollo: "Well over three-quarters of the exposed sediments are gray to buff-colored clays. The remainder is made up of thin, cross-bedded ferruginous sandstones. The Los Ranchos sediments are perhaps gradational with the beds of the La Villa sequence within this area. The two formations are similar, and the contact can only be mapped by noting the frequency of outcrops, and in a general way, a slow change in lihology from one formation to another. This gradation contrasts with the condition found farther north in the vicinity of the Macoa anticline, where there is a sharp break in lithology from brown to buff sandstones and gray clays (Los Ranchos) to white, friable sands (La Villa), and an accompanying topographic break from cuesta ridges to relatively flat 'cabana' lands."

Northward from the Río Palmar, the Los Ranchos-La Villa boundary cannot be readily traced for more than a few kilometers, and lateral change in the two formations again is responsible. Eastward in the subsurface, the physical character of the Los Ranchos changes in more pronounced manner so that there is no distinction from the La Villa; both formations are represented largely by claystones. This change in facies may also involve loss of section from the base of the Los Ranchos to the Cuiba formation.

The Los Ranchos evidently was deposited in a zone more or less parallel to the Perijá mountain front and reflects tectonic activity and erosion in the region to the west. Related tectonic events and approximately correlative time appear to be represented by the El Guayabo group of the Río de Oro area. Sutton (1946, p. 1704), aptly uses the term Los Ranchos in an equivalent sense to the term Los Melones for the Manuelote syncline area, Río Guasare, and in so doing has recognized the existing lithologic and sectional affinity of these two units. The problem of regionally separating the Los Ranchos from the La Villa justifies the use of the term "Arimpia group" for undifferentiatedd mapping of the two formations. Age for the Los Ranchos post-dates the Cuiba and Macoa units which evidently were deposited in early stages of the Miocene. Therefore middle Miocene or probable upper Miocene age might be reasonably assigned to the Los Ranchos. So far as is known, faunal content is restricted to rare occurrence of foraminifera of the genera Miliammina. Other faunal suites that sometimes have been assigned to the Los Ranchos (e.g. the suite listed for the Calentura-1 well, Liddle, 1946, p. 484), probably represent formations subjacent to the Los Ranchos and have been erroneously assigned.

John B. Miller