ORTIZ, Sandstones

See ORTIZ, Formation.

ORTIZ, Formation

UPPER CRETACEOUS (Maestrichtian to Danian ?)

State of Guárico, Venezuela

Author of name: R. A. Liddle, 1928.

Original reference: R. A. Liddle, 1928, p. 208.

Original description: ibid.

The Ortiz formation was originally named and described by Liddle (1928, p. 208): "In north central Venezuela Lower and Middle Eocene beds, which to the west are included in the Misoa-Trujillo formation, are called the Ortiz sandstone from the village of Ortiz in the Serrania del Interior in the northern part of the State of Guárico. The formation outcrops in the rugged sandstone hills to the south of the type-locality and from this place extends east and west."

M. Kamen-Kaye (1942a, p. 126-132) noticed the occurrence within the type-area mentioned by Liddle of at least two main sedimentary sequences set in contact in the region of the "galeras" by the Guárico frontal thrust. The author helped to make Liddle's original description clear by restricting the use of the name "Ortiz sandstones" to the sediments that outcrop immediately to the north of the main thrust, which is located to the south of Ortiz; therefore he excluded the sequence of predominant Oligocene age (Guarumen group) that outcrops to the south of said thrust.

Kamen-Kaye (p. 126) admits that the "Ortiz sandstones" (restricted) are intimately associated with "dark shales and quartzitic flags" in outcrops near Ortiz. In another report (1942-b, p. 34) he states that the "dark clay facies" form a band that rarely exceeds 40 kms in northern Guárico, meaning that it can include a complex of outcrops composed of Querecual and San Antonio beds, the upper member of the Ortiz formation (present definition) and some facies of the Guárico formation, all of them associated during intense tectonic movements in the region. In fig. 1, KamenKaye (1942-a, p. 127) indicates that the outcrop area of the "dark shale facies" is the region northeast of Ortiz which is limited southward by the frontal thurst; in the fig. 2 (p. 128) the "dark shales" are also in contact with the "Ortiz sandstones", immediately to the south of Ortiz. The author says (p. 126) that "part of the dark shales outcrop is of proved Upper Cretaceous age on the basis of ammonites, mollusks and foraminifers". He most certainly refers to blocks (tectonically uplifted or perhaps slumpblocks) of the Querecual formation. According to Kamen-Kaye: "The remainder of the shales may represent Paleocene deposition. Certainly there is a lithological similarity between some of the flaggy members near Ortiz and beds which contain Paleocene type mollusk farther north in the foothill section". Dark shales of unquestionable Paleocene and perhaps of lower Eocene age outcrop some two kilometers north-northeastward of Ortiz in association with biostroms whose faunal content was originally described by Mme. de Cizancourt (1951, p. 22, fig. 9) under the name "Ortiz flysch series" (p. 9) (see). It seems convenient to separate those beds from the underlying upper Maestrichtian or Danian sediments and to correlate them with the San Juan de los Morros member of the Guárico formation of which they constitute a southerly extension possibly transgressive. On the other hand, it is obvious that flaggy shales similar to those of Paleocene age, that is, the so called "dark shales", pertain actually to the Guayuta group.

The name Ortiz formation should actually refer to a section of massive shales, intercalated at its base with a sandstone section which outcrop typically in the Ortiz syncline. The southern flank of this syncline crops out immediately to the south of the town of Ortiz and extends for 5 kilometers into the southwest. According to recent unpublished reports by students of the Universidad Central (U. M. Petzall and M. M. and B. Natera), the section mentioned above contains a microfauna, mainly arenaceous, that shows distinctly the Rzehakina - "Spiroplectammina" "zone" defined by Hedberg in the Santa Anita group. The writer of the present article proposes to restrict the name Ortiz formation to the shaly-sandy unit that outcrops typically in the Ortiz syncline, the top of which is indicated by the first reef limestones containing corded nummulites and the conglomeratic facies oi the overlying "Ortiz flysch series" (Guárico formation).

The rocks which outcrop in the vicinity of Ortiz have been studied in detail by students of the Department of Geology and Mines of the Universidad Central de Venezuela (unpublished reports).

The lithologic studies and measurements carried-out by these students permits the description of the following characteristics of the Ortiz formation (sensu strictu) in the type area of the Ortiz syncline.

1) Lower Member: 780 m. thick. It is easily distinguishable for its relatively larger sandstone content, forming beds 0,5 to 1,5 m. thick, grouped in sections a 100 m. thick, which are resistant to erosion, topographically prominent, hard, well stratified, saccharoidal, coarse to fine "rained, locally glauconiferous, light to greenish gray, weathering yellow or reddish brown. The sandy sections are intercalated between massive shales little resistant to erosion, impure, sandy, gray to dark gray, weathering yellon to light brown, containing scarce foraminifera of the type Vidoño and some allochthonous removed from Querecual. Pebbles from the Querecual limestone are observed at the base of the Lower Member. There are other indications of the basal transgression of the Ortiz formation over the truncated surface of the Guayuta group. Blocks 2 to 3 m. thick of sandy biohermal, grayish-white limestone, containing calcareous algae (Archaeolitbothamnian cf. gosaviense Rothpl, etc.), small foraminifera and bryozoa are sporadically observed at the lower sequence of the Ortiz formation. The lower Member outcrops typically on the southern flank of the Ortiz syncline which in its turn forms the northern flank of an overturned anticline located southward. A good section outcrops some 3.5 to 6 km along the road from Ortiz to San Francisco de Tiznados, in the area located to the south of said road which extends to the axis of the overturned anticline.

2) Upper Member: Predominantly shaly; rests on top of the sandy section. Its little resistance to erosion and topographic expression makes it difficult to distinguish its outcrops. In the above mentioned section the top of the Upper Member is concealed by alluvia and only a section 150 m thick is observed. Better outcrops are located in the valley of the Quebrada Peñas Negras, some 2.5 km southwestward of Ortiz and on the northern flank of the syncline. The upper contact of the Ortiz formation with the Guárico formation is about 4 kilometers to the west of Ortiz, near the road to San Francisco. (see "Ortiz Flysch Series".) The shales are massive, sandy-ferruginous, gray to dark gray, containing scarce arenaceous microfauna. In the younger section of the sequence, they are finely intercalated with thin (5 centimeters) sandstone layers. The total thickness of the upper member can exceed 300 m.

In the area westward of Ortiz, the Ortiz formation (present definition) is 1.100 m thick; its average composition is 40 % (maximum) sandstones and 60 % (minimum) shales.

Some geologists have also referred to the Ortiz formation as "Vidolio", based on an approximate correlation with said formation. Actually, there is a considerable distance between the two units; the change in the facies is also evident. The sandy Lower Member of the Ortiz formation might well be an age equivalent, at least in part, of the San Juan sandstone; the younger and thinner sands of the Ortiz formation can be equivalent to some section of the Caracas formation. The lack of evidences for a precise correlation with formations whose ages vary laterally, such as those of the Santa Anita group located far off, indicate the convenience of having a distinctive name in central Guárico.

The Ortiz formation, implicated in the emersion of the Oligo-Miocene geoanticline of Guárico, seems to have been an important peripheric source of sediments of the northeastern margin of the Guarumen basin, it is so suggested by the abundance of allochthonous foraminifera (Rzehakina "Spiroplectammina" assemblage) removed from the Chaguaramas formation and deposited on the present foothill area. Later, probably towards the end of the Pliocene time, the Ortiz formation was dragged southward overthrusting the same Oligo-Miocene sediments.

A good section of the Ortiz formation is observed in the Guárico River from a point located 4 kilometers (aereal) to the east-southeast of San Francisco de Cara to another point (downstream) 2.5 kilometers to the northwest of Camatagua.

A normal sequence extends northeastward (perpendicular to the strike of the Ortiz and from the frontal thrust at the base of the section) from the latter mentioned point to a probably transgressive conglomerate, 4.5 kilometers apart, which constitutes the basal part of the Guarico formation. This is an identical situation to that in the region of Ortiz. The Ethology and microfauna of the Ortiz formation near Camatagua are very much similar to those in the type-area.

The lithology of the well distributed Ortiz formation is characterized by a shale predominance (over 60 %) although the sandstones (less than 30 %) being more resistant to erosion form elongated hills and give the false impresion of being predominant.

The observable thickness varies between a minimum of 1,100 m. in the Ortiz type-area and 3.500 to 4.000 m in the vicinity of Camatagua.

The lower contact, frequently concealed by surface faults, is actually in marked unconformity with rocks of the Guayuta group. The upper contact seems to be in slight unconformity with the Guarico formation; there are slight indications of overlapping.

The probable Maestrichtian to Danian unit, to which is here proposed to restrict the use of the name Ortiz formation, is clearly distinguished from the overlying Guárico formation, called "Ortiz Flysch Series" by Mme. de Cizancourt through the following lithologic and faunal characteristics:

1 - The "Ortiz Flysch Series", also called "San Francisco de Cara beds", the base of which is marked by a small conglomerate, contains more shales, generally laminar (average 80 %) and less sandstones (average 15 %); the shales are typically greenish-gray; some beds are calcareous and present cone-in-cone structure.

2 - The "Ortiz Flysch Series" presents typically some micro-conglomerates and granule conglomerates rich in clastic elements of metamorphic, igneous, and sedimentary origins; graywackes and tabular or lenticular intercalations of conglomeratic limestone and small biostroms containing corded and orbitoidal nummulites and algae, not older than Paleocene. These facies are not observed in the underlying Ortiz formation.

A common feature to both units is the presence of an assemblage of small foraminifera, predominantly arenaceous, of the Rzehakina-"Spiroplectammina" zone.

The Ortiz formation outcrops extensively to the northwest of the frontal thrust, overlapping always the Guarumen group; it is observed to the west and southwest of San Francisco de Cara, to the northwest of Camatagua and extends into the northwest forming a wide belt that includes part of the Taguay and Memo Rivers and it is located to the northwest of a line that goes approximately from Taguay to Altagracia. The formation is identified with at least part of the so-called Macaira group of Evanoff, 1951, and extends from Guanape and San Francisco de Macaira to a wide band located to the south of El Guapo and Cúpira which ends in the Unare lagoon. At this latter region there are indications of lateral differentiation of the facies with a transition to at least part of the equivalent formations of the Santa Anita group.

J. M. Sellier de Civrieux