EL FAUSTO, Group

TERTIARY (upper Eocene? to middle Miocene?)

State of Zulia, Venezuela

Authors of name: Geologists of the Richmond Exploration Company.

Original reference: H. D. Hedberg and L. C. Sass, 1937, p. 101.

Original description: ibid.

The term El Fausto, applied first as a formational name later as a group name, originates with the "finca" El Fausto, located about 18 kilometers northeast of Machiques and 9 kilometers west of the Perijá highway, District of Perijá. The El Fausto sequence was named about 1927, the first evident use of the name appearing in an unpublished report by R. P. Miller (Richmond Exploration Company). In its early application (private reports of the Richmond Exploration Company and Creole Petroleum Corporation), the name was erroneously spelled, and was written commonly "El Fauto". Of the formation, R. P. Miller writes as follows: "The El Fausto formation comprises mottled and variously colored clays, sandy clays, and sandstones, with a few shales... All of the formation between the (Los Ranchos) sandstone and the Frontal sandstone is mapped as the El Fausto." (Unpublished report dated 1927.) He furnishes a complete description of the sediments as observed in pits and surface exposures.

The earliest published description (Hedberg and Sass, 1937, p. 95) recognized the formation status. Of particular interest are the following comments: "The type locality may be considered as the belt of low relief some 7 kilometers wide between the prominent Arimpia-El Rodeo sandstone ridges on the east and the Perijá mountain front on the west... The El Fausto formation consists dominantly of claystones and siltstones distinctively mottled in dull shades of red, purple, gray, green, and brown. There are minor intervals of gray green and dark gray shale or claystone and greenish fine-grained sandstone. The upper portion of the so-called 'Frontal sandstone' along the Perijá mountain front is frequently the basal part of the El Fausto formation". The later change of the El Fausto from formation to group rank is recorded in published literature by Liddle (1946), who does not, however, explain basis for the change.

Because of unsatisfactory exposures, no subdivision of the El Fausto sequence may be justified from surface geologic studies. Subsurface studies, however, outline three distinctive units, which in 1946 were given formation rank and assigned the names Perijá, Macoa, and Cuiba. The name Perijá was later changed to Peroc, when it was realized that the term "Perijá" had already been used, in published literature, with reference to a Paleozoic or pre-Cambrian sedimentary sequence exposed within the Serranía de Perijá. Subdivision of the El Fausto into formations is restricted on basis of available data, to an area about 60 kilometers long, extending approximately from the Rio Palmar to Machiques. As an undivided group the El Fausto is more widely distributed.

Unrelated to the change of the El Fausto sedimentary sequence from formation to group rank, there is some difference of opinion as to proper placement of the lower boundary of the unit. Descriptions by Hedberg and Sass (1937) and by Liddle (1946) include, within the El Fausto, part of the "Frontal sandstone" sequence that is currently and preferably included in the La Sierra formation. Exclusion of these sandstones from the El Fausto agrees with general practice by Richmond geologists, and with the original (unpublished) definition. The boundary is placed in a narrow transitional zone between the predominantly sandstone lithology of the La Sierra, and the predominantly claystone sequence of the El Fausto.

The El Fausto group consists almost wholly of claystone, and relatively minor amounts of sandstone and siltstone. Its outcrop area is characterized by a strike valley confined between the rugged hills and ridges eroded in the superjacent Los Ranchos formation on the east, and the La Sierra strike ridge and foothills of the Perijá range on the west. Much of the valley is mantled by stream alluvium and terrace deposits. The Peroc and Cuiba formations consist of mottled claystones, probably deposited in coastal swamps and lagoons, and the Macoa is typified by olive-colored claystone probably representing deposition in bays, or shallow, brackish marine waters. Separate discussions on these formations provide further descriptive detail.

The El Fausto unit is recognized throughout an area bordering the Perijá mountain front from the Rio Palmar southward beyond Machiques and eastward into and beyond the general vicinity of La Villa del Rosario. Outside of these limits, use of the term must be determined arbitrarily by considering factors like available nomenclature, stratigraphic associations, and changing facies. Photogeology provides firm evidence, southward from Machiques, for continuity between the strike valleys of the E:l Fausto, and of the Leon shale. This condition and general lithologic similarity attest to validity of stratigraphic correlation, and probably to approximate time correlation as well, between the two units. It is suggested that the term El Fausto be used preferably in Venezuelan territory, except in the immediate border area where the name Leon is established by precedent. Northward from the Rio Palmar the El Fausto can be traced, possibly with less assurance, into the Orumo formation which is recognized in the general region of the Rios Guasare and Socuy. Eastward the El Fausto group changes toward greater marine affinity, recognized by transition into the Icotea claystones, the Lower La Rosa, and Upper La Rosa units of the Boscán oilfield area. It is likely that time lines tend to transact the upper and lower boundaries of the El Fausto group; exact age correlations are not implied between El Fausto group sediments and corresponding sequences like the Leon, Orumo, and Icotea-La Rosa. The upper E:ocene to middle Miocene age is assigned on basis of the apparent stratigraphic relationships and facies change separately discussed for the Peroc, Macoa, and Cuiba formational units. Faunal data is provided in descriptions of these three units.

John B. Miller