ISNOTU, Formation

TERTIARY (middle nad upper Miocene)

State of Trujillo, Venezuela

Author of name: Q. A. Abadilla, 1928 (private report).

Original reference: F. A. Sutton, 1946, p. 1706-1707.

Original descripcion: ibid.

Sutton (1946, p. 1706-1707) referred the name to a predominantly clay and sandstone formation which crops out at the west of the village of Isnotú on the road between Motatán and Betijoque in the west-central part of the State of Trujillo. He described the formation as principally composed of interbedded clays and sandstones, with subordinate amounts of clay shale, coal, and conglomerate. The clays, which comprise about 65 per cent of the formation, are massive, locally sandy, soft and light gray in color. Mottling is common. Some of the clays are carbonaceous and plant-fossiliferous. The sandstones, occurring in beds up to 2 to 3 meters in thickness, are white to light gray, fine-grained to finely conglomeratic, locally micaceous and ripple-marked. Embedded pellets of white clay are common. The bulk of the formation was deposited in fresh to brackish water with short local marine incursions. Sutton cited a thickness of about 1,000 meters which remains fairly constant throughout the outcrop belt. The formation thins basinward to the northwest. The Isnotú appears to overlie the Palmar formation conformably from the Río Buena Vista in southern Trujillo southwestward to the Colombian border. On the north, in Trujillo and the District of Baralt, State of Zulia, it overlaps the Palmar formation and rests unconformably on older sediments. Sutton stated that the Isnotú formation is overlain conformably, and locally overlapped, by the "Betijoque formation". The outcrops of the formation occupy a narrow belt along the northwest front of the Mérida Andes through the States of Trujillo, Mérida and Táchira. In the subsurface, Sutton claimed, the Isnotú passes northward through the District of Baralt and grades into the "La Puerta" in the District of Bolívar, State of Zulia. Fossils are meagerly present (see Sutton, 1946, p. 1707). Sutton cited an upper middle Miocene age for the formation based on stratigraphic position and paleontology. Sutton stated that the Isnotú formation "is the same as the Lower Maracalbo series of Tash (1937, p. 163) in the Mene Grande region".

Schaub (1948, p. 220) expressed the opinion that Sutton's Isnotú and Palmar formations are one and the same. He proposed to eliminate the name Isnotú formation claiming that the outcrops of the Palmar formation in the Río Buena Vista were better suited for a type section than those of the Isnotú formation.

Mencher et al. (1951, correlation chart) showed the Isnotú formation as the middle formation of the Betijoque group in Trujillo and Táchira resting conformably on the Palmar formation and being overlain conformably by the Sanalejos formation. At Mene Grande they showed the Isnotú formation to be unconformably overlain by the "Sanalejos formation". They indicate a middle Miocene age for the Isnotú.

Information taken from private reports indicates that the age of the Isnotú formation is middle and upper Miocene. The "La Puerta formation" as used by Sutton (1946, pp. 1704-1705) is synonymous with the Isnotú formation. See also BETIJOQUE, Group.

Leo Weingeist

BETIJOQUE, Group

TERTIARY (Oligocene to Pliocene)

State of Trujillo, Venezuela

Author of name: W. G. Argabrite, 1919.

Original reference: A. H. Garner, 1926, p. 682.

Original description: ibid.

Garner (1926, p. 682) listed the "Betijoque formation" in the Miocene giving as its type locality the low hills west of the town of Betijoque, State of Trujillo. He described the formation as consisting of "alternating beds of white to red clays and shales, coarse, massive, poorly sorted sandstones and reddish brown, sandy shales. Lower part darker shales and thinner bedded sandstones. Whole series micaceous and ferruginous."

Liddle (1928, p. 279-280) stated that Argabrite, in 1919, was probably the first to apply the name "Betijoque beds" to a unit consisting of chiefly brown, cross-bedded sandstones and shales forming bad-land topography south of Betijoque, State of Trujillo. Liddle commented that the original application, as well as his own, of the term, probably included beds of more than one formation. He cited a lower to upper Miocene age for the unit.

Liddle (1946, p. 391-394) raised the Betijoque to group rank pointing out again that probably more than one formation were included in the original application, as well as his own, of the term "Betijoque beds". He stated that the Betijoque group probably ranges in age from upper Eocene through middle Miocene.

Sutton (1946, p. 1709-1710) stated that Liddle (1928, p. 279-280) had described the "Betijoque beds" which included the "Betijoque formation", as used by Sutton, plus the Isnotú and Palmar formations. Sutton explained that he followed Q. A. Abadilla (1928, private report) in the usage of the term "Betijoque formation". Abadilla had restricted the Betijoque to the upper or conglomeratic part of the section and had applied the name Isnotú to the underlying mottled clays and white sandstones. Sutton pointed out that the Palmar formation, which includes the fossiliferous marine shales at the base of Liddle's, Betijoque beds, was not named until 1943, when P. P. Wolcott introduced it in a company report. Sutton described his "Betijoque formation" as consisting of massive beds of conglomerate, sandstone, siltstone and clay. The conglomerates are the most conspicuous and comprise about 25 percent of the formation. They are coarser in the upper half of the formation, loosely cemented, poorly sorted and occur in beds up to 12 meters thick. The sandstones are massive, locally cross-bedded, gray to brown, loosely cemented and friable, poorly sorted, fine to coarse-grained, micaceous and locally clayey, lignitic, and plant-fossiliferous. The siltstones are massive to thin-bedded, locally well laminated, light gray to greenish gray and micaceous. The clays constitute the mayor part of the unit. They are massive, dark greenish gray to locally brown and black, generally sandy and locally carbonaceous and plant-fossiliferous. Lateral gradation from clays into siltstones, sandstones and conglomerates is common. Sutton cited a maximum thickness of 4,365 meters for the "Betijoque formation" of his usage. He stated that the formation appears to be conformable with the underlying Isnotú formation although in some areas it overlaps the Isnotú and lies unconformably on older sediments. The "Betijoque formation" is unconformably overlain commonly with high angularity, by the Carvajal formation. The "Betijoque formation" occupies a narrow belt along the northwest flank of the Mérida Andes. Sutton cited an upper Miocene age for the formation based on its stratigraphic position and on paleontological evidence (see Sutton, 1946, p. 1710). He pointed out that the "Betijoque formation" is the exact equivalent of the combined upper and middle Maracaibo series of Tash (1937, p. 162) and the Boscán formation of Garner (1926, p. 682). The type Onia beds of Hedberg and Sass (1937, p. 106-107) form the lower part of the Betijoque section on the Río Onia in southwestern Mérida. According to Sutton, the "Betijoque formation" is also the same as the upper part of the Guayabo group in the Onia region of southwestern Mérida and in Táchira.

Mencher et al. (1951, correlation chart) showed the Betijoque group to range from Oligocene through lower Pliocene in Trujillo and Táchira. They include, from top to bottom, the Sanalejos, Isnotú and Palmar formations in the group. The Betijoque group is shown to overlie unconformably the Paují formation in Trujillo and the "Lobaterita formation" in Táchira. In both Trujillo and Táchira the Betijoque group is shown to be overlain unconformably by the Pleistocene Carvajal formation.

Mencher et al. (1953, p. 774) showed the Betijoque group in the State of Táchira unchanged from Mencher et al. (1951, correlation chart). In Trujillo, however, the group is shown to include, from top to botton, the Sanalejos, Vichú and Hormiga formations. The lower contact of the Betijoque group in Trujillo is indicated as either unconformable or conformable with the underlying Paují formation, probably depending on the area. They showed the Vichú formation of Trujillo as corresponding exactly to the Isnotú formation in Táchira. The Hormiga formation is shown to include more section than the Palmar formation which is shown to correspond to the upper part of the Hormiga formation approximately.

Present day usage favors considering the Betijoque as a group of Oligocene to Pliocene age including either the Sanalejos, Vichú and Hormiga formations or the Sanalejos, Vichú, Isnotú and Palmar formations depending of Mencher et al. (1951), the Sanalejos of Mencher et al. (1953) in Táchira, the Sanalejos and Vichú of Mencher et al. (1953) in Trujillo and the "Betijoque formation" of Sutton (1946) are all equivalent to each other. The Hormiga formation shown by Mencher et al. (1953) corresponds to the combined Isnotú and Palmar formations of Sutton (1946, p. 1706-1707 and 1701-1703) plus an additional section at its base depending on the area of outcrop. A number of geologists prefer to follow Sutton applying the name Betijoque to the reduced section and carrying it as a formation. They include two members in their "Betijoque formation", the Sanalejos and Vichú, which correspond in section exactly to the two formations of the same names. This number of geologists consider the age of the "Betijoque formation" to be upper Miocene and Pliocene.

Leo Weingeist