LA PUERTA, Group

See LA PUERTA, Formation

LA PUERTA, Formation

TERTIARY (uppermost Oligocene to lower middle Miocene)

State of Falcón, Venezuela

Author of name: C. R. Nichols, 1922 (private report).

Original reference: A. H. Garner, 1926, p. 680.

Original descripcion: F. Hodson, 1926, p. 174.

Garner (1926, p. 680) cited the neighborhood of the village of La Puerta, State of Falcón, as the type locality of the La Puerta formation. He described the formation as consisting of "metamorphosed and contorted, black shale and slate with quartz crystals. Occasional thin, quartzitic sandstone beds and fossiliferous, thin limestones". Garner considered the formation to be Eocene in age. It seems that some kind of a mistake must have happened in the printing of Garner's paper because the lithologic description cited there for the La Puerta formation does not agree with the Ethology of the beds cropping out in the vicinity-of La Puerta but fits the Ethology of the "San Juan de Los Morros formation" which Garner mentioned in his paper just above the La Puerta formation. The lithologic description of the ''Sah Juan de Los Morros formation" is omitted in Garner's paper and it is therefore very well possible that the lithologic description given for the La Puerta formation actually belongs to the "San Juan de Los Morros formation" and that the lithologic description of the La Puerta formation was actually omitted.

Hodson (1926, p. 174) stated that the type locality of the "La Puerta series" is at La Puerta, southeast of Dabajuro, District of Buchivacoa, State of Falcón. He described the unit as consisting of "variegated, gypsiferous clays and sands".

Liddle (1928, p. 320-321) considered the "La Puerta beds" to represent the "western phase of the La Vela beds". He also referred to the unit as La Puerta formation and stated that the type locality is at Cerro La Puerta, 15 kilometers southeast of Dabajuro. Liddle described these beds as consistof a small amount of variegated mottled clay and predominance of gray, micaceous, sandy shale interbedded with gray, white or brown-stained, soft micaceous, cross-bedded sandstone containing ferruginous concretions. Liddle stated that the formation rarely exceeds from 1,500 to 2,000 feet in thickness. He considered the age of the formation to be upper Miocene. Liddle gave western Falcón as the outcrop area of the formation.

Halse (1937, p. 177-178) divided the "La Puerta Series" into an "Upper Section" and a "Lower Section". He described the "Upper Section" as consisting almost entirely of mottled clay with few thin sand streaks. Halse pointed out that these beds appear to be considerably younger than the lower part of the La Puerta and that there is probably an unconformity between them. He stated that this upper section is 2,000 feet thick. Halse described the "Lower Section" as consisting of a series of pebble beds, with pebbles showing wind polish, at the top. Below the pebble beds are about 1,000 feet of massive micaceous and highly "falsebedded" sandstones with iron cemented and ferruginous bands containing clay and quartz pebbles, and gray and mottled shales. These beds pass downward into a lignitic sand and gray shale group with a 500-foot mottled clay, and, finally just below this clay there are more sandstones. Gypsum bands up to one inch thick and leaf impressions are common in this "Lower Section" which comprises a thickness of 2,300 feet according to Halse. He pointed out tnat the entire "La Puerta series" consists of at least 4,300 feet and mentioned that it is believed that rocks corresponding in age to the La Puerta attain a thickness of over 17,000 feet further east in the State of Falcón. Halse emphasized the finding of remains of Gyrinodon quassus, a large extinct herbivorous mammal of the sub-order Toxodontia, about 3 miles south of El Mene by Dr. Parkinson who considered these remains to be middle or lower Pliocene in age. Halse stated that these remains were f ound in the Barranca Basin at a locality where rocks about half way down the La Puerta column were exposed at the surface, but were probably associated with a gravel deposit. Halse considered the "La Puerta series" to be Mio-Pliocene in age.

González de Juana (1938, p. 130-133) stated that "the subdivision of the middle and upper Miocene Socorro, Caujarao, and La Vela formations is practically impossible in western Falcón, where this great thickness has been called La Puerta group". González de Juana pointed out that the subdivision of the middle and upper Miocene in central Buchivacoa is only possible into the "Upper and Lower La Puerta". González de Juana described the lower-most La Puerta as consisting of brackish to paludal deposits changing in ascending order to fossiliferous neritic sediments, which in general terms are the time equivalent of the Socorro and Caujarao formations. Between the lower and the upper La Puerta is an unconformity recognized in many places in the District of Buchivacoa and called the "La Danta Unconformity" which corresponds to Halse's inter-La Puerta unconformity of western Buchivacoa. González de Juana pointed out that the angle of unconformity is small when both the upper and lower La Puerta are present, but may be large when the upper La Puerta lies directly on Eocene sediments. He cited approximately 3,500 feet as the thickness for the "La Puerta group" stating, however, that it is impossible to give a reasonably accurate figure for the thickness because of the character of the sediments and because of the hiatus within these beds.

Sutton (1946, p. 1704-1705) in his discussion of the La Puerta formation stated that "in the Bolívar Coastal field the name is restricted to the equivalent of the Upper La Puerta of the type locality and this restriction should be adopted everywhere". Sutton described the formation as composed of bright blue, yellow, green and red, mottled claystones; yellow-brown and red, mottled siltstones; brown siltstones, and light greenish gray, fine to locally coarse grained, friable, massive sandstones. Marine intercalations are locally present and the absence of lignite differentiates the La Puerta from the underlying Lagunillas formation according to Sutton. He cited a thickness varying from 170 to 700 meters (558 to 2,297 feet) for the La Puerta formation. Increases in thickness are noted from north to south in the Bolívar Coastal field and eastward into Falcón. Sutton pointed out that the La Puerta is transitional with the underlying Lagunillas formation in the Bolívar Coastal field. Where the latter is not present, an unconformable relationship exists with older sediments. In Buchivacoa the restricted La Puerta rests unconformably on the Lagunillas equivalent according to Sutton. The relationship with the overlying El Milagro formation appears to be unconformable. Sutton pointed out that sediments referred to as the La Puerta formation are restricted to the Districts of Bolívar and Miranda in northeastern Zulia and the District of Buchivacoa in western Falcón. He stated that only reworked foraminifera and a few brackish-water fossils have been found in the La Puerta in the Bolívar Coastal field. He claimed, however, that an upper middle Miocene age had been established for the La Puerta formation based on marine fossiliferous beds in Falcón which represent the lateral equivalent of the La Puerta. Sutton mentioned that the La Puerta of his usage has been called "Upper La Puerta" by González de Juana (1938, p. 130-133) and "El Mene sandstone series" by Williston and Nichols (1928, p. 449) in western Buchivacoa. Then Sutton claimed, the La Puerta formation is equivalent to the La Villa formation of western Zulia and to the Isnotú formation of the southeastern part of the basin.

Information taken from private sources indicates that later studies have revealed that the La Puerta of the type locality should be considered a synonym of the Socorro formation of an uppermost Oligocene to lower middle Miocene age. The "La Puerta beds" and "La Puerta formation" of Liddle (1928, p. 320-321) are synonymous with the upper Miocene Codore formation. So are the "Upper Section" of the "La Puerta series" of Halse (1937, p. 177178) and the "Upper La Puerta" of González de Juana (1938, p. 130-133). The "Lower Section" of the "La Puerta series" of Halse (1937, j. 177-178) and the "Lower La Puerta" of González de Juana (1938, p. 130-133) are a synonym of the combined Socorro and Urumaco formations. The La Puerta formation of Sutton (1946, p. 1704-1705) is at present believed to be synonymous with the Isnotú formation.

Halse (1937, p. 178) mentioned that the remains of Gyrinodon quassus, found in rocks about half way down the La Puerta column, were considered to be indicative of a lower or middle Pliocene age by Dr Parkinson. Romer (1945, p. 618), however, listed the genus Gyrinodon as restricted to the Miocene of South America. This latter determination is more in agreement with the present practice of considering the upper Miocene Codore formation and the lower section of Halse's La Puerta series to be synonymous with the combined Socorro and Urumaco formations.

Leo Weingeist