LA MESA, Formation

MESA, Formation

MESA, Formation

TERTIARY? (Pliocene?) - QUATERNARY? (Pleistocene?)

States of Anzoátegui and Monagas, Venezuela

Authors of name: H. D. Hedberg and A. Pyre, 1944.

Original reference: H. D. Hedberg and A. Pyre, 1944, p. 25.

Original description: ibid.

The sediments which form the "mesas" at the eastern "llanos" of Anzoátegui and Monagas have been described, although without a formation name, since the beginning of the last century. Their widespread topographical features constituted by flat-topped hills with abrupt sloping sides, are so typical that they engaged the attention of Humboldt, 1825; Codazzi, 1841; Karsten 1858 and 1886; Sievers, 1896; Jahn, 1921; Weidenmayer, 1937, Hedberg, 1937, Kamen Kaye, 1937 and 1938). Such deposits were included by Garner (1926, p. 7) in his Llanos formation, which he considered of Quaternary age. It is possible that part of the Llanos group (Pliocene, Quiriquire formation), described by Regan (1938, p. 190 and 191), would pertain to the Mesa formation (Renz, 1944, p. 561 and 563) as well as some deposits of Manger's Rondon formation (1932, p. 31), the formation described by Liddle (1946, p. 524), and gorger's Quiriquire formation (1952, p. 2304 and 2306).

The Mesa formation was distinguished and described by Hedberg and Pyre (1944). According to these authors (1944, English edition, p. 25; Spanish translation, p. 44). "The youngest stratigraphic unit of northeastern Anzoátegui is the Mesa formation. This formation is widespread over the Eastern Venezuelan basing, constituting the cap rock of the broad mesa which forms such a striking physiographic feature of this region. The sediments of this formation are apparently of diverse origin. Near the mountain front they consist dominantly of several hundred feet of pebble, cobble, and even boulder gravels and evidently represent a series of coalescing alluvial fans formed during rapid erosion of the Cretaceous and Tertiary rocks of the Serranía and deposited across the eroded edges of all older formations. The formation thins southward to 50-100 feet in the interior plains and decreases in grade size. Here it evidently also includes locally reworked stream gravels, eolian deposits, and residual deposits and its relation to the underlying Sacacual group varies locally from distinct erosional unconformity to apparent transition".

"Fossils in the Mesa formation are largely limited to local occurrences of petrified wood. The formation is commonly considered of Pleistocene age but without any definite basis. Certainly it is older than the last major uplift of the mountains as it shows gentle tilting and wasping and is also locally faulted. Only eroded remnants such as Cerro Corozal remain in the foothills areas and these are definitely much older than the alluvium of the present rivers, which have cut several hundred feet below the base of the Mesa beds."

Later on, this formation was thoroughly studied by González de Juana (1946, p. 9-15) who extended it across the "llanos" of Monagas. He considers it of Quaternary age and says that the type-locality could be designated "in any one of the flat-topped eastern "mesas" such as the Mesa de Guanipa in Anzoátegui and the Mesas de Tonoro and Santa Barbara in Monagas, or even better, in the escarpments at the east and north-east of Santa Rosa, Anzoátegui". As far as the origin is concerned, Gonzalez de Juana (p. 10) coincides with Hedberg and Pyre, but he thinks that it is "largely a product of deltaic and paludal sedimentation resulting from an extensive delta which advanced eastward in the same way the Orinoco delta is seen to advance today, with the only difference that the old delta was flanked by the northern mountains of Monagas and the Guayana lands, which so contributed to the sedimentation of coarser - grained clastics". He describes several geologic sections and goes so far as to assign to the formation a thickness of 275 mts. in the Mesa de Maturin. According to González de Juana, it is mineralogically characterized by its abundance of Fe and Zr minerals (magnetite specially), the frecuency of muscovite, staurolite, tourmaline, banded zircon, andalusite, kyanite, and sillimanite; rutile and red zircon are less frequent and there is scarcely any biotite, brookite, corindon, monzonite, spinel, tremolite, chlorite, epidote, glaucophane, tita- nite, garnet, and hypersthene. Such mineralogic characteristics vary according to the source-sediments. The metasediments are predominant in the south and disappear toward the north. González de Juana indicates that some fresh-water fossils have been found generally associated with lignitic clays and petrified wood remains. As far as the age is concerned he says (p. 15) that it is generally considered of Quaternary age on account of its unconformable position over the Pliocene Las Piedras and Sacacual formations. Within that age it is preferrable to call it Pleistocene, although there is no scientific basis for it, besides the fact that it is certainly older than the recent sediments of the valleys.

Further information concerning the Mesa formation would be found in Davey (1946-a), Hedberg, Sass and Funkhouser (1947), Hedberg (1950), Mencher and others (1951 and 1953), and Moore and Shields (1952).

The name of this formation is not specific of any locality but descriptive of a topographical feature which is neither specific of the formation. Although eastern Venezuela is where the "mesas" are better developed there are also important ones at the Guayana region (Roraima formation) and at the Andes, where the fluvial and fluvio-glacial terraces took such topographical features (Sievers, 1888; Jahn, 1921; Davey (1946-2). There is a small town near Timotes which is called La Mesa. There is also the Mesa de Carora, the Mesa de Mara near Maracaibo, the Mesa del Saladillo (Miocene) at Coro, etc. According to Davey (1946-a), in the southern "llanos" the name "mesas" is given to small elevations which are not limited by abrupt vertical sides.

The total inclusion of the Mesa formation in the Pleistocene is dubious. No fossils have been found and the petrified woods (xylopal and xyloid silex) present such an advanced fossilification that it could indicate an older age. It is stratigraphically above the Sacacual group, the Pliocene age of which is not yet well determined. The contact is transitional and sometimes disconformable. Up to four different levels of Pleistocene fluvial terraces, besides the present level of the valleys, have been noted above the Mesa formation. This indicates that the latter is somewhat older. It is also difficult to explain the fact that, at the foot of such low hills (1000 to 1500 m. or exceptionally 2595 m. at Cerro Turumiquire) as those north of Anzoátegui and Monagas, a 275 m. thick Pleistocene sedimentation was produced while at the foot of the Andes, where the Quaternary glaciation was so important, nothing but piedmont alluvial cappings and alluvial terraces were deposited. It is possible that something similar to what happened to the Santa Barbara de Zamora beds of the Andean foothill area could happen to the Mesa formation. The Santa Barbara de Zamora beds, similar to the Mesa formation and to the El Milagro formation of Maracaibo, were first believed to be of Quaternary age (Christ, 1927, p. 406; Liddle, 1928, p. 354) and then assigned to the upper Miocene to Pliocene time (Liddle, 1946, p. 532). Only the study of mammal fossil remains, which are yet to be found, will bring out the final decision. Therefore, it may be established that the cover of lateritic conglomerate is perhaps of Pleistocene age and is underlain by sediments of more probable Pliocene age.

José Royo y Gómez