ABSTRACT OP PAPERS 139 



The entire region is underlain by sedimentary strata, which may be grouped 

 into the following formations : 



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. 



Vitiacua limestone and chert; middle or late Mesozoic; 10 to 100 feet 

 thick. 



Unconformity. 



Bermejo series ; Permo-Carboniferous, and possibly in part Early Meso- 

 zoic comprising the following formations : 



Machareti formation ; massive, cross-bedded sandstone and interbedded 

 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 2,500 feet thick. 



Los Monos shale ; blue gray to black, fissile shale, with interbedded 

 micaceous sandstones ; base not exposed ; more than 1,000 feet thick. 

 Unconformity. Fault contact. 



Totora series ; Devonian ; including, probably near or at the top, the 

 Espejos formation ; dark bluish or greenish black, carbonaceous fissile 

 shale with intercalations of dark, fine grained, brittle limestone in 

 beds two or three inches thick ; total thickness unknown. 



Structurally, the Front Ranges of the Andes consist of closely compressed 

 anticlines extending for long distances north and south, separated by com- 

 paratively broad synclines. Each of the sierras is a long, narrow, sharply 

 folded, doubly plunging anticline, the limbs of which are inclined at angles of 

 25 to 75 degrees and the axial plane of which is somewhat inclined toward 

 the east. All but one of these anticlines is strongly sheared along a major 

 fault plane which trends parallel to the longer axis of the fold and is situated 

 a short distance east of its crest. The faults are thrusts, with the overhang- 

 ing wall on the west and with displacements of 3,000 to 10,000 feet. Each of 

 the cuestas, or groups of hogback hills, is a tilted fault block with monoclinal 

 dip toward the west at angles of 20 degrees to more than 70 degrees. Each 

 is terminated on the east by a major fault plane, in approximately vertical 

 position, with the upthrow on the west and with displacement of several 

 thousand feet. 



The structure is evidently the result of strong lateral compression resulting 

 from a great thrust which came from the west. The diastrophic movements 



