STRATIGRAPHY 741 



though Steinmann described this formation as Cretaceous on the evidence 

 of fossils which he found within it, the more extensive collections recently 

 described by Bonarelli, to which reference has been made in a preceding 

 paragraph, indicate that it is to be correlated with the European Lias and 

 should therefore be considered as Late Triassic or Early Jurassic. The 

 hiatus is, therefore, presumably due to an erosion interval subsequent to 

 the deposition of the Vitiacua beds and preceding the accumulation of the 

 Tacuru sediments. 



It is believed that the Vitiacua limestone and chert is the equivalent of 

 the Cajones limestone of the Sierra de Santa Cruz, 11 but because of the 

 impossibility of definitely verifying this tentative working hypothesis it 

 is deemed wise to use separate names for the two similar zones in the two 

 widely separated localities. 



TACURU FORMATION 



X 



Eesistant sandstones overlie the Vitiacua sandstone and chert, or the 

 Bermejo series where that thin formation is wanting, and form the sharp 

 crested, steeply inclined hogbacks which border the mountain ranges, at 

 least on their western slopes, and are present in the western portion of 

 each group of cuestas (see figure 14). They have been observed in all 

 parts of the region under discussion. The type locality is the eastern 

 part of the Tacuru gorge, in the Sierra de Charagua. The formation 

 may be defined as including all the beds between the "portals," one mile 

 west of the village of Tacuru, and the outcrop of the Vitiacua (?) lime- 

 stone, l 1 /^ miles farther upstream. 



The Tacuru formation is a variable series of sandstone and sandy shales 

 with a small amount of clay shale recurring at irregular intervals through- 

 out its various members. The sandstones are commonly light gray or 

 even white, but include also a considerable amount of red beds. The beds 

 are generally quite irregular; many of them are strongly cross-bedded. 

 Some appear to be eolian deposits; the rest are presumably fluvial and 

 possibly lacustrine sediments. The shales are commonly mottled or varie- 

 gated with numerous tints of maroon, carmen, or purplish gray, but in 

 places they display a brown monotone. All of them are non-fissile and 

 many are in reality thick beds of uncemented but closely compacted clay. 



This formation is generally between 3,000 and 4,000 feet in thickness, 

 but at the type locality it is probably somewhat greater than 4,000 feet, 

 and in certain other places its thickness is only about 2,000 feet. 



In the northern part of the Sierra de Charagua the lower 200 feet of 

 the Tacuru formation is thin-bedded clay shale, yellow brown to deep red, 



11 Heald and Mather, 1922, pp. 565-567. 



