STRATIGRAPHY 735 



sierra are found in the valley of Rio Cuebo at the point where it emerges 

 from its mountain canyon, 3 miles west of Cuebo. The lowest beds on 

 the upthrow side of the Mandiyuti fault at this point are Devonian sand- 

 stones and sandy shales, and the contact between them and the base of 

 the Oquita formation was not definitely located. The lower third of 

 the Oquita formation has not been observed in the Sierra de Charagua, 

 although it may be exposed in the gorge of Rio Parapiti. 



The middle third of the Oquita formation, at the type locality in the 

 Cuestas de Oquita, is composed chiefly of cream and red sandstone, in 

 somewhat thicker and more irregular beds than the gray strata which 

 underlie them. There is less mica in this portion of the formation and 

 the sand grains are much more evenly sized. In the Sierra de Aguarague, 

 at this same stratigraphic horizon, there are massive white sandstones 

 which in places have a resistance to erosion almost equivalent to that 

 of quartzites. These beds form the narrows at El Chorro, in the 

 Vitiacua canyon (see figure 8), and they likewise may be recognized in 

 the Quebrada de Los Monos between 200 and 400 feet above the base of 

 the formation. At that locality they are variegated and streaked with 

 many shades of red on weathered surfaces. 



The resistant sandstones midway in the sequence of beds referred to 

 the Oquita formation are overlain in the northern part of the Sierra de 

 Aguarague by a peculiar structureless clay which outcrops immediately 

 upstream from El Chorro, in the Vitiacua gorge. This clay is dark gray, 

 rather firmly indurated, and contains occasional tiny pebbles of quartz 

 or igneous rock. In places it is finely laminated in a manner strongly 

 suggesting the varve structure of glacial lake beds. The entire thickness 

 of this member of the Oquita formation is about 300 feet. It was not 

 observed at other localities than this. 



The upper six or eight hundred feet of the Oquita formation at the 

 type locality in the Cuestas de Oquita consist chiefly of thin beds of fine- 

 grained sandstone and sandy shales which display a variety of tints of 

 vermilion, maroon, and greenish gray. The sandstones contain much 

 clay and are finely laminated as well as thinly bedded. The shales con- 

 tain only a little less sand than the interbedded sandstones and are com- 

 monly in beds an eighth to a quarter of an inch in thickness. These 

 strata are richly micaceous and in places contain carbonaceous material 

 which is believed to represent finely comminuted vegetable matter. This 

 member is the most persistent of those forming the Oquita formation 

 and varies only slightly in thickness throughout the entire region from 

 the Sierra de Charagua southward to the Eio Pilcomayo. In the gorge 

 of Rio Charagua these strata are in somewhat thicker beds and are much 



