STRATIGRAPHY 729 



because of the great differences in resistance offered by the weaker and 

 stronger formations. At some localities within the various mountain 

 groups the former graceful, meander curves have been changed into 

 sharply angular chevrons, so that canyons like the Yitiacua Gorge zigzag 

 here along the strike and there at right angles to it in response to the 

 attempt made by the rivers to traverse the mountains along the easiest 

 channels. 



Stratigraphy 

 general statement 



The entire area of the Front Eanges of the Andes is underlain by sedi- 

 mentary rocks of late Paleozoic, Mesozoic, and Cenozoic age. There are 

 no igneous nor metamorphic rocks exposed within these mountains. Be- 

 neath the lowlands, both within the Cordillera and on the plains to the 

 east, the solid rocks are nearly everywhere concealed beneath a veneer of 

 stream gravel, wind-blown sand, and soil, but exposures along the moun- 

 tain slopes and cuesta fronts are excellent, so that an accurate idea of 

 the succession of strata beneath the entire area may be obtained with 

 comparative ease. To facilitate the study and description of the geology 

 of this region, concerning which very little reliable information has been 

 published, the sedimentary strata have been divided into the following 

 formations and series, to which local geographic names have been applied. 

 The formations are named in ascending order, the oldest at the bottom. 



Quaternary gravels, sands, clays, and silts. 



Unconformity. 

 Tatarenda formation ; soft sandstones, shales, unconsolidated sands and clays, 

 occupying in general the lowland areas ; probably Tertiary ; 3,000 to 4,000 

 feet thick. 



Unconformity. 

 Tacuru formation ; resistant sandstones and interbedded shales, the "hogback 

 makers" along the margins of the sierras ; late Mesozoic or possibly in 

 part early Tertiary ; 2,000 to 4,000 feet thick. 

 Unconformity. 

 Vitiacua limestone and chert ; Upper Trias or Lias ; 10 to 100 feet thick. 

 Bermejo series ; Permo-Triassic ; comprising the following formations : 



Machareti formation ; massive, cross-bedded sandstones and inter- 

 bedded non-fissile shales and clays ; 800 to 1.600 feet thick. 

 Mandiyuti conglomerate ; massive conglomerates, grits and sandstones, 

 with interbedded sandy shales ; predominantly red ; in large part 

 fluvio-glacial ; 1,500 to 3,200 feet thick. 

 Oquita formation ; extremely variable sandstones and shales, some of 

 which are probably glacio-lacustrine ; at many localities, including 

 lavender or maroon sandy shales; 1,000 to 2,000 feet thick. 

 Unconformity. 



XLVIII — Bull. Geol. Soc. Am., Vol. 33, 1921 



