734 K. F. MATHER FRONT RANGES OF THE ANDES 



mistakably demonstrate that these beds are late Triassic or early Jurassic. 



The only fossils found by us within the Bermejo series were inde- 

 terminable fragments of lingulas from beds overlying the fluvio-glacial 

 Mandiyuti conglomerates in the northern part of the Sierra de Santa 

 Cruz. The presence of glacial and fluvio-glacial beds among the Bermejo 

 strata is in perfect harmony with the conclusion reached by Bonarelli 

 and based on its stratigraphic relation to the underlying Devonian and 

 overlying Liassic beds, that this thick series of clastic sediments is of 

 Permo-Triassic age. 



As here defined, the Bermejo series is the equivalent of Bonarellr's 

 "Areniscas Inferiores" and Steinmann's "Formacion Petrolifera," and 

 may be correlated with the lower part of the "Sistema de Salta" as 

 defined by Brackebusch. Because of rather uniform differences in the 

 lithology of successive beds within this series wherever the greater part 

 of it is exposed, it has been divided into three formations, as follows : 



Oquita formation. — This formation is exposed in each of the sierras 

 and cuestas where detailed examinations were made. It is named from 

 the type locality, in the eastern part of the Cuestas de Oquita, along the 

 walls of the Parapiti gorge. It includes the sandstone and shales between 

 the Devonian strata and the Mandiyuti conglomerate. Its thickness 

 varies from about 1,000 feet, as exposed in the Quebrada de Los Monos, 

 to about 2,000 feet, as in the Cuestas de Oquita. 



The Oquita formation is dominantly sandstone, but includes a con- 

 siderable amount of shale. It is extremely variable in composition, both 

 with regard to the succession of beds at any one locality and with refer- 

 ence to the nature of any single bed when traced from place to place. 

 Where its base is exposed, the lower beds of this formation are gray 

 sandstone one-half to 2 feet thick, which are generally irregularly 

 laminated or cross-bedded. These sandstones may be stained various 

 shades of maroon or vermilion on weathered surfaces. 



Above these basal sandy beds there is ordinarily a thick series of shales. 

 In the northern part of the Sierra de Aguarague these shales are dark 

 and micaceous, fissile or nearly so, and contain lenses and concretions of 

 sideritic or limonitic material. Along the eastern front of the Cuestas 

 de Oquita, near Rio Parapiti, these same shales are quite sandy and in 

 places might better be called thin-bedded argillaceous sandstones. They 

 are interstratified with beds of gray, or greenish gray, grit and fine 

 pebbly conglomerate. Here, also, they contain an abundance of mica, 

 but the sand grains are not very well sorted according to size. Similarly, 

 in the Sierra de Mandiyuti this portion of the Oquita formation is thin- 

 bedded, sandy and micaceous. The lowermost strata exposed in this 



