LAS LAJAS, Formation

TERTIARY (Miocene)

State of Falcón, Venezuela

Author of name: unknown.

Original reference: A. H. Garner, 1926, p. 5.

Original description: ibid.

The name, Las Lajas formation, appears in Garner (1926, p. 5) with the following description: "Type locality: surrounding small town of Las Lajas, State of Falcón. Fairly hard, well-bedded, carbonaceous, ferruginous, brown sandstones, dark brown shales, thin beds of lignite and bituminous coal". The age is given as Miocene.

We have not been able to locate the town of Las Lajas, so we are not sure which formation, in modern nomenclature, is indicated, but from the description, it seems that it might be the coal-bearing beds of El Isiro (see article). This seems the more likely, since Liddle (1928, p. 263) mentions that east of the road from Las Lajas to Carrizal, the San Luis formation dips below the Agua Clara formation. Liddle, in his Falcôn correlations, includes the El Isiro horizon, now in the Agua Clara formation, now in the Socorro, (see EL ISIRO CARBONACEOUS "SERIES"). On the other hand, we presume that the references by Liddle (1928, p. 340-341) to the "Rondón formation" and to "Pleistocene near Las Lajas" must refer to another town of the same name in the plains. In 1946 (p. 427, 526), Liddle repeats the same references, in neither case mentioning Garner's fõrmation.

Garner's name appears in the list of Hedberg (1938) as undesirable, and moreover, it is obsolete.

Frances de Rivero

EL ISIRO, Coal "Series"

TERTIARY (upper Oligocene)

State of Falcón, Venezuela

Author of name: possibly H. G. Kugler, 1926 (private report).

Original reference: S. H. Williston and C. R. Nichols, 1928, p. 446, 450.

Original description: C. Wiedenmayer, 1937, p 69.

The name "Upper Isiro beds" appears in a private report by H. G. Kugler in 1926, but it seems possible that it might have been used even earlier, since the coal workings in the Coro river valley in the region of El Isiro and El Saladillo (Mesa Saladillo, Mesa de Coro) are well known, and Williston and Nichols (1928) mention names such as Cerro Pelado as dating back to at least 1923. These coal-bearing beds are mentioned by Liddle (1928) in various contexts, but not given formational names. His correlation of these beds is rather confusing. They are said to occur in the "Socorro formation of Lower Middle Miocene age" (Liddle, 1928, p. 384); but on p. 267-268, coal-bearing beds at Curamichate in eastern Falcón which are said to represent Agua Clara or possibly Cerro Pelado "are thought to correlate with the coal beds at El Isiro". Liddle compounds the confusion with mention of the Agua Salada clays at Antón Coro" which appear to come in the top of the Agua Clara" but which he then correlates "with beds in Mesa Saladillo" which (on p. 267) are considered the upper part of the Socorro formation, representing 1500 feet of section above the horizon of the El Isiro coals.

Williston and Nichols (1928) criticize Liddle's correlations of the Oligo-Miocene formations of Falcón, especially of the Agua Clara. They introduce the term "El Isiro coal series" in their discussion and in a table of columnar sections (their fig. 1, p. 446). In this figure, in the column of the Distrito Colina, there appears the following sequence (descending order): La Vela, type Damsite, Agua Salado (sic), Socorro and El Isiro undivided, Agua Clara, San Luis. In the text, the name appears in the following context:

"These beds" (i. e., the Mesa Saladillo beds outcropping south of Coro) "have been detailed and traced eastward from this vicinity into the typical Agua Salada clays at Anton Coro. At Mesa Saladillo these beds have largely lost their lithologic character, the upper part having graded into the upper part of the El Isiro coal series, the equivalent of the upper part of the El Isiro coal series, the equivalent of the Cerro Pelado-Socorro." (Williston and Nichols, 1928, p. 450).

Senn (1935, p. 79), without mentioning El Isiro by name, describes the Cerro Pelado formation (correlated with the Aquitanian) as being coal-bearing.

C. Wiedenmayer (1937, p. 67) describes briefly the "tramo carbonífero El Isiro". (Since in this report the La Vela, Caujarao, Agua Clara and San Luis are all called "tramos", the word may be taken as equivalent here to "formation"). The "tramo" El Isiro is referred to the lower Miocene, and said to overlie the Agua Clara and underlie the "tramo Valle o Querales" (Socorro). It is described as consisting of sands and sandstones, sandy and carbonaceous clays, lignites and laminated gypsiferous clays, of lagoonal or brackish-water origin. Its thickness is given as 213 meters in Mina de Coro, 251, in El Isiro. A geological map and structural sections are included in the report.

González de Juana (1937, p. 208) includes the lignite beds of El Saladillo (Mesa de Coro) and El Isiro in the Cerro Pelado formation.

Liddle (1946, p. 581) accepts the correlation of Senn and González de Juana, referring the Coro valley coals to the "Cerro Pelado formation of Aquitanian age".

The paper by Payne (1951) on the Cumarebo area does not include the E1 Isiro region, and the beds are not mentioned.

As far as we know, the term El Isiro is not at present in use as a formational name.

Frances de Rivero