STRATIGRAPHY 749 



ing considerable coarse, gritty materials heterogeneously intermingled 

 with fine sand and masses of clay. As in the lower beds, the cement 

 which binds the sand particles together is quite inadequate to make a 

 firm rock and the strata weather readily. 



The outermost series of low hogbacks along the flanks of the Sierra do 

 Aguarague and the adjacent lowland areas on either side the range are 

 overlain by soft crumbly beds of the Tatarenda formation. A total thick- 

 ness of upward of 4,000 feet was measured at one locality. Ordinarily, 

 about a thousand feet of Tatarenda strata are present in the low hog- 

 backs. Interspersed through the basal portion of the formation, there 

 are several conglomeratic horizons. These contain subangular fragments 

 of sandstone and other rocks derived from the Bermejo-Tacuru series. 

 In the Quebrada de Los Monos, for example, these conglomerates are well 

 displayed in the last series of portals through which the stream passes 

 after it turns eastward from the center of the range. The conglomerates 

 at that locality are very evidently the result of torrential accumulation. 

 A short distance upstream from these outer portals in the Quebrada de 

 Los Monos, there is a cherty sandstone composed of beds which range up 

 to 2 feet in thickness, containing an abundance of nodules and sheets of 

 brown chert, as well as a large amount of calcite in large crystals filling 

 cavities in the sandstone. This calcite-chert horizon is two or three thou- 

 sand feet above the Vitiacua formation as observed in the gorge of Vitia- 

 cua creek and, as pointed out by Bonarelli (1921, page 78), it is quite 

 distinct from the beds which Steinmann has called the "Horizon te Cal- 

 careo-dolomitico." 



Although, so far as known, the Tatarenda formation is structurally 

 conformable with the underlying Tacuru formation, the presence of a 

 stratigraphic break between the two formations is indicated by the com- 

 position of the conglomerates near the base of the Tatarenda. Presum- 

 ably the episode of sedimentation, responsible for the Tacuru formation 

 was closed by crustal warping which elevated a part of the region -occu- 

 pied by the Bermejo-Tacuru series and formed uplands whence came the 

 debris of which the Tatarenda beds are composed. Much of the accumu- 

 lation was of a torrential nature, but in many localities only the finer 

 sediments were washed broadcast over lowlying plains or into shallow 

 seas. The red and brown colors so commonly observed in this formation 

 are chiefly due to the fact that much of the Tatarenda sediments was 

 derived from erosion of the Bermejo red beds. 



This formation is approximately the equivalent of Bonarelli's "Ter- 

 ciario Subandino" series, which that geologist appears to have referred 

 to the Tertiary period on the basis of its lithology. Unfortunately, the 



