Palmer — Notes on the Andes of N. W. Argentina. 321 



Part II. 

 The Calchaqui Graben. 



It was the good fortune of the expedition to remain at La 

 Poma in the Calchaqui Valley for several days, so that it was 

 possible to study a little more closely the structure of the 

 graben there developed. The details in hand therefore war- 

 rant giving somewhat more space to this than to the other 

 parts of the section covered by the expedition. 



There is a block of Jurassic red sandstone about eight kilo- 

 meters wide and at least forty kilometers long, bounded by two 

 closely parallel normal faults, which separate it from the 

 Paleozoic metamorphic rocks to the east and west. These 

 faults were actually observed : the one at Cienaga in the south- 

 east corner of the map (fig. 4) and the other a little west of 

 Pircas in the northwest corner. 



The Rocks. 



The name "Jurassic red sandstone" is used in this article 

 as a convenient and comprehensive term for a series of greatly 

 varying sediments, having a total thickness of at least three 

 thousand feet. They consist of red sandstones, shales and con- 

 glomerates, well cemented by a mixture of iron oxide and 

 calcite. The color is brick-red, of much the same quality as 

 that of the Triassic of the eastern United States, but of greater 

 intensity. In the Calchaqui valley there is a great increase in 

 coarseness toward the north end of the valley, and with this goes 

 an increase in the amount of cross-bedding. Thus in the neigh- 

 borhood of Cajoncillo there are about equal amounts of sandstone 

 and shale, with only a very few lenses of conglomerate. North 

 of Orusca this condition is reversed — the sandstone predominates 

 over the conglomerates, but the shale is almost absent. In 

 the Quebrada do la Paya (the canyon in which Pircas lies), 

 with an extremely rapid change from a sandstone, the rock 

 becomes a solid mass of conglomerate with mere partings of 

 sandstone. The conglomerate seems to -be especially char- 

 acteristic of the upper portions and the marginal portions of the 

 formation, for in ascending this Quebrada one passes through 

 the western flank of an anticline until the great fault near 

 Pircas is reached. The change from sandstone to conglomer- 

 ate is very striking. Cross-bedding is everywhere developed 

 and can be found in almost every outcrop, but is especially 

 characteristic of the conglomerates. The angle of inclination 

 of the cross-bedding is quite steep, ranging from 10° to 30°, 

 indicating that the deposits were laid down in running water. 

 Were these wind-laid deposits, the angle of the cross-bedding 

 would be much lower. 



