JARILLAL, Shales

TERTIARY (upper Eocene-lower Oligocene)

State of Lara, Venezuela

Author of name: A. Senn, 1932 (private report).

Original reference: A. Senn, 1935, p. 62.

Original description: ibid.

The term Jarillal shales was published by A. Senn (1935, p. 62) for a stratigraphic unit which he called the middle part of his Agua Negra "formation" (now considered a group). It overlies the Santa Rita conglomerate and underlies the Churuguara beds. The name was evidently derived from the town of Jarillal, about 10 km northeast of Baragua, District of Urdaneta, in northern Lara. The type-section (according to the designation by Senn, of the type-section of the Agua Negra) occurs in the Buena Vista anticline, in the Agua Negra Range. Another good section, according to Senn, is exposed some 20 km to the east, on the north side of Cerro de los Indios, between the part of the highway that rises steeply and Cerro Cometa.

The Jarillal beds are described as shales ("Tonschiefer"), more or less sandy, the color variegated to dark gray, weathering red. The lower 50 m, rich in molluscan fossils and crab remains, are distinguished by Senn as the "Raetomya-Shales". The upper part of the Jarillal beds, in the Buena Vista anticline, are barren shales. In the Cerro de los Indios section, beds of nummulitic limestone appear within the Raetomya-shales, and also appear sporadically in the mottled clays that form the upper part of the Jarillal in that section. The total thickness of the Jarillal shales was estimated as 375 m; Senn found it difficult to determine precisely the boundaries of the formation due to the transitional nature of the upper and lower contacts.

Senn, after an extensive discussion of the paleontologic data (see below), refers the Raetomya-shales to the uppermost Eocene, and the overlying upper part of the Jarillal to the lower Oligocene.

Except for its inclusion in correlation tables or in compilations (Seen 1940, p. 1580; Renz, 1942, p. 539-540; Liddle, 1946, Correlation Table), no further reference to the Jarillal shales has been found in later publications.

The Raetomya-shales are said to contain an abundant fauna of mollusks (more than 50 species), which has never been described. Rutsch (1930), and Seitz and Rutsch (1930), have described two species characteristic of these shales, and also of the Santa Rita conglomerate: Mya (Raetomya) schweinfurtbi Rutsch and Rimella (Ectinochilus) gaudichaudi alauda Olssen. (See also Rutsch, 1937, p. 40-43 and fig. 1-3. The name "El Vadillal" on p. 43 is a misspelling for Jarillal). Van Straelen (1933) described four species of crabs (see list in Senn, p. 63). Senn mentions various genera of mollusks. He also cites two genera of foraminifera, identified by Gorter and Van der Vlerk (1932, p. 100) from a limestone in Cerro de los Indios: Nummulites sp. (abundant) and Heterostegina sp. (In Gorter and Van der Vlerk the locality was referred to the Paloma Alta series). Finally, Senn mentions remains of belemnites (these are also mentioned by Rutsch, 1930, p. 607). Rutsch, 1937 (note 1, p. 43), gives the following information; believing that the specimens might represent the genus Bayanoteuthis (a characteristic lower Eocene genus), he sent them to Prof. Stolley at Braunschweig for identification. Stolley stated that tne fossils, although indeed belemnoids, were almost certainly not Bayanoteutbis, but that the material was insufficient for a positive generic identification. Senn believes that these fossils were definitely autocthonous, not redeposited. It would be very interesting to have more specimens from Senn's locality in Cerro de los Indios. The macrofossils above mentioned were found in the Raetomya-shales, but Senn states that the limestones with Nummulites and Heterostegina were found in the upper part of the Jarillal.

In a restored geologic section, Senn (1935, p. 61) indicates that south of the town of Churuguara, the Jarillal shales disappear due to facies changes, so that the Churuguara formation comes to lie directly on the Santa Rita. He indicates in his correlation table that in eastern Falcón the Jarillal shales are replaced by the Guayaval marls and the Tacamire shales (see articles). The Jarillal shales appear to correspond in stratigraphic position and age (upper Eocene-lower Oligocene) to the Guacharaca formation (see article); however, it seems convenient to retain the name Jarillal for the beds described by Senn until such time as detail studies shall have been published on the relations between the eastern Falcón basin and this region of south-central Falcón and northern Lara.

Frances de Rivero