LA SIERRA, Formation
TERTIARY (upper ? Eocene)
State of Zulia, Venezuela
Author of name: H. D. Hedberg (private reports).
Original reference: H. D. Hedberg and L. C. Sass, 1937, p. 89.
Original description: ibid.
The La Sierra formation, otherwise called the "La Sierra Sandstone" and "Frontal Sandstone", is (by chance or otherwise) one of the most descriptively named formations in Venezuela. The name "La Sierra originates with the "finca" La Sierra, located about 10 kilometers west of the town of Machiques, District of Perijá. Yet selection of this name may have been prompted in part by the narrow, saw-like profile of the ridge itself. The synonymous name "Frontal Sandstone" was suggested by frontal position of the La Sierra strike ridge to the Perijá range.
Although the terms, "La Sierra" and "Frontal Sandstone" were first published by Hedberg and Sass (1937), previous uses of these terms are found in various private reports. It appears that the term "Frontal Sandstone" is probably the older, and may have been first used by R. P. Miller of the Richmond Exploration Company. Definition of the terms is confused by disagreement on whether to divide the sequence into two formations, or leave it as one. Most writers have preferred to restrict the term "La Sierra" to the middle and basal parts of the sequence, and have included the upper sandstone member in the overlying El Fausto group, or assigned it as a separate "Basal Sandstone formation".
In the decade 1920-1930, the La Sierra sequence was most frequently called "Third Coal horizon", "Second Coal horizon", or simply "Lower Coal series". History of the name "Frontal Sandstone" so far as available literature is concerned, begins in 1927 with an unpublished private report (R. P. Miller, Richmond Exploration Company): "The Frontal Sandstone is about 296 feet thick on the Río Tinacoa. It is poorly sorted, and fine to coarse-grained. Cross-bedding is common, and the formation is often made up of poorly cemented, rounded and angular quartz grains presenting a white, sugary appearance. A few members are fine-grained conglomerate. Pebbles of chert are sometimes several centimeters in diameter. Oil residue in the rocks frequently cause them to appear dark, or even black".
First available reference to the formation name "La Sierra" is found in an unpublished memorandum written by H. D. Hedberg and dated October 21, 1931. It is interesting to note that Hedberg, at this early date, included rocks of the Guasare-Marcelina sequence in the La Sierra. From this memorandum are the following interesting comments: "The term 'Frontal Sandstone' has been replaced in four more reports by 'La Sierra sandstone' or 'La Sierra formation'." He describes two units in the formation. The upper unit he calls the "sandstone and shale member", and specifies a thickness of about 375 feet. This is the lower part of the present La Sierra formation. He writes, "sandstones in this unit constitute the Eocene part of the Frontal Sandstone exposed along the mountain front". The lower unit he calls the "shale, limestone, and calcareous sandstone member", and specifies a thickness of 300 feet. He comments: "This unit is also characterized by poorly preserved mollusks and a scanty foraminiferal fauna. Some coal has been noted. The member probably corresponds approximately to the Guasare limestone and the lower part of the Paso Diablo formation of Mara, and the 'third coal' of Colón".
The definition was later modified to exclude the Guasare (Guasare-Marcelina) interval. The publication by Hedberg and Sass (1937) contains the following pertinent comments: "The type section is that exposed on the trail along the north bank of the Río Negro where the river cuts through the frontal ridge about one kilometer east of (the ranch) La Sierra. The formation is about 110 meters thick in the type section. (It) consists of thick to medium-bedded, massive, hard, brown-weathering sandstone with some interbedded shale and sandy shale... The formation appears to lie conformably on the Río de Oro formation in the type section; the contact between the two is placed arbitrarily at the top of the first limestone below the main ridge-forming sandstone. The formation grades upward into interbedded sandstones and shales which are equivalent to the El Mene formation of the Maracaibo Lake area".
Hedberg and Sass make one other significant comment (ibid., p. 95): "The upper portion of the so-called 'Frontal Sandstone' along the Perijá mountain front is frequently the basal part of the El Fausto formation." This is an expression of a growing confusion between the La Sierra unit and the overlying El Fausto group, which eventually led to separation of the "Basal Sandstone formation". This separation is currently considered to be erroneous; arguments and further data of historic interest are presented in paragraphs below that discuss stratigraphic relationships.
By present definition, the La Sierra formation forms the principal ridgeforming sandstone unit of the "Frontal" ridge in part of the District of Perijá, approximately form the Río Yasa northward to the Río Palmar. Its upper boundary is the base of the El Fausto claystone sequence; its base is the boundary with the Guasare or Guasare-Marcelina sediments, or with Cretaceous or pre-Cretaceous rocks where the Guasare-Marcelina sequence is missing. (The Marcelina-Guasare sequence, where it occurs, generally crops out in the western slope of the "frontal" ridge.
The section on the road to the "finca" La Sierra (north side of the Río Negro see quotation from Hedberg and Sass, above) is retained as the type section; the section is easily accessible and fairly well exposed. It is rather difficult in the section, however, to define the exact base of the unit; the Hedberg and Sass definition of the lower boundary "arbitrarily at the top of the first limestone below the main ridge-forming sandstone" admits possibility for error; an interval of nearly 20 meters immediately above the limestone bed, extending to the oil seepage from a prominent sandstone bed, is concealed. It is suggested that the sandstone bed at the seepage may be the base of the La Sierra. Thickness of the section is estimated at 150 meters. (This estimate pertains to a section which excludes about 20 meters from the base of the 110 meter sequence reported by Hedberg and Sass, but adds the "Basal Sandstone" unit, about 70 or 80 meters. It consequently compares with the Hedberg and Sass value with error of about 10 to 20 meters.) A section better exposed than the one on the Río Negro, crops out six and onehalf kilometers northeast of the Río Negro section, on Quebrada Caña Brava al Sur.
The La Sierra formation, in most of the mountain front area in the District of Perijá, can be divided into two principal members. The upper member is the so-called "Basal Sandstone"; the lower member has been called the "pebbled garnet member". Descriptions of the members follow:
The zircon in the La Sierra is commonly colorless or pink; a brown variety of zircon, and the mineral clinozoisite, are observed less frequently in the La Sierra than in the underlying Guasare-Marcelina sequence.
In considering stratigraphic relationships of the La Sierra, it is best to further discuss the concepts that have brought about varying definitions for the formation. By this means it is possible to demonstrate why the "Basal Sandstone" unit was separated from the La Sierra. Foremost in early concepts seems to be the tendency to call part of the section "Eocene", and part of it "Oligocene", and construe an unconformity associated with the time boundary. This viewpoint is prompted by evident unconformable relationships on the east side of the Maracaibo basin. It was believed that westward thinning of the Eocene section represented this same period of erosion. An unpublished report by R. P. Miller dated 1945 (Richmond Exploration Company) recalls a meeting of the "Maracaibo section of the American Association of Petroleum Geologists" on June 13, 1931, a discussion of stratigraphy and tectonics, and of conditions encountered in the Zanca and Zancada wells. He quotes from a record of the meeting: "these wells penetrated both the upper and lower members of the Frontal Sandstone but show more or less a gradation between the two. There is apparently an overlapping of the younger sandstones on the older". He construes more importance for the overlap than for the gradation, and concludes: "It seems that Hedberg, Sass and others are correct in their conception that there is no major break between the Cretaceous and lower Eocene (sensu lato) in the Maracaibo Basin region. There are two sandstone formations instead of one along the mountain front from a point west of Machiques to Río Palmar. The older formation is the Eocene 'La Sierra' and is overlapped by the younger Basal sandstone below the Oligocene El Fausto." Much of the above argument was evidently widely accepted; Sutton (1946, p. 1683, 1691) does not contest unconformity between his defined El Fausto and his "La Sierra", a stratigraphic position that corresponds to the base of the upper member of the La Sierra. Mencher (1953, p. 709) also speaks of this unconformity.
Evidence now available invalidates the argument for an unconformity and hiatus within the La Sierra, below of the so-called "Basal Sandstone". Hence the term "Basal Sandstone" implies a condition that does not exist, and its use should be discontinued; furthermore, it is not generally fitting to use this term in the sense of a proper name. On the basis of observed surface and subsurface relationships, the La Sierra appears to be the lateral equivalent of the Las Flores formation and of the overlying "basal" sandstone unit of the Icotea formation, observed at a position near the edge, or maximum transgression, of the Las Flores seaway. A pattern of uniformity and diminishing thickness of the "marine" sedimentary wedge toward the shoreline is striking; lateral changes simply represent differing local aspects of sedimentation. Transgression of the seas progressed northwestward across truncated edges of Paleocene and Cretaceous sediments in a manner that is clearly delineated.
The lower member of the La Sierra represents the transgression, and brackish, marginal-marine sedimentation during maximum invasion. This member thickens eastward into shallow marine sediments of the Las Flores sequence. The upper member belongs to the regressive phase of the Las Flores sedimentary cycle, and corresponds to the "basal" sandstone of the Icotea farther east, e.g. in the Boscán oilfield. It evidently represents sedimentation in a "beach" zone that may involve a barrier bar and sand spits. The opposite side of the "beach" presumably faced a lagoonal area, where claystones of the Peroc formation (basal unit of the El Fausto group) were deposited. Regression of the sea caused the "beach" and the "lagoon" to move progressively eastward, and corresponding eastward encroachment of the Peroc sediments over sandstones in the La Sierra is construed.
Thinning of the La Sierra toward the "turn-around" point of the transgression is apparent in the pattern. On the evidence of well data, the La Sierra is completely missing over parts of the Totumo arch, a condition that may be caused either by non-deposition or by local erosion preceeding deposition of the E1 Fausto claystones. The upper member, which includes sandstones deposited on the back side of the beach, generally should have greater areal extent than the lower member. Measurements in the mountain front area demonstrate a tendency to diminishing thicknesses northward toward the supposed shoreline; first figures are for the lower member, second figures are for the upper member (thicknesses in meters): Río Negro (90-80); Quebrada Caña Brava al Sur (10?-100); Río Macoita (100-100); Zulia 26D-1 (143-15); Zulia 20D-1 (94-50); Caño El Mene (130-20); vicinity Cañada la Ge (50-50); Zulia 17D-1 (0?-6?). (In Zulia 15D-1 and several other wells associated with the Totumo arch, no La Sierra sands whatsoever are evident.)
The term La Sierra is applicable throughout the mountain front area from the Río Yasa to the Río Palmar, and has been used in subsurface work eastward as far as the well Zulia 36E-1, 20 kilometers east of Machiques. Possibly the name can be applied also to a ridge-forming sandstone unit at the top of the Mostrencos formation (or base of the Orumo formation) in the Manuelote syncline. This ridge is fairly prominent south of Riecito de Mache and strikes toward junction of this stream with the Río Socuy. Southward, the La Sierra, on basis of photo-interpretation seems to be related closely to the Carbonera formation. The identity of the Mirador sandstone is largely lost in a confusion of sandstone ridges north of the Río de Oro, but in general sectional appearance, the Mirador may be slightly subjacent to the La Sierra. The unconformity at the base of the La Sierra seems to disappear south of the Río Yasa and east of Machiques.
Age assignment for the La Sierra is made by comparison of sectional relationship to the Las Flores. On this basis the formation is evidently upper Eocene.
John B. Miller