762 K. F. MATHER FRONT RANGES OF THE ANDES 



PERMO-CARBONIFEROUS AND PERMO-TRIASSIC TIME 



This region seems not to have again become a site for sedimentation 

 until shortly before the episode of glacial action, which is ordinarily re- 

 ferred to Permo-Carboniferons time. Then the sands, gravel, and muds 

 of the Oquita formation were brought from contiguous highlands and 

 spread broadcast throughout the entire area between Santa Cruz and 

 Embarcacion. In most localities the deposits were probably nuviatile, 

 but in places there were shallow lakes or lagoons of considerable extent, 

 and it is possible that for brief intervals parts of the area may have been 

 submerged beneath an epicontinental sea. 



With the advance of great ice-sheets over the adjacent land areas, there 

 seems to have been marked augmentation of the loads brought by rivers 

 to this basin. Fluvio-glacial accumulations were piled in great beds until 

 the tremendous thicknesses of the Mandiyuti conglomerate were effected. 

 Glacio-lacustrine beds alternated in some localities with the fluvial de- 

 posits; large boulders and occasional masses of till were borne by ice- 

 bergs to many places remote from the glacier-capped lands. 



For some time after the final withdrawal of the Permo-Carboniferous 

 ice this region received deposits of sand and mud, brought by the same 

 streams which had previously been clogged with glacial debris. Great 

 deltas were built near the river mouths, and the finer sediments Avere 

 sjDread widely by lake and sea currents. Occasional invasions of marine 

 embayments, apparently from the north, account for the fossil lingulas 

 found in the Machareti beds in the Sierra de Santa Cruz. With apparent 

 abruptness the long episode of clastic sedimentation seems to have given 

 way to widespread seas in which the limestones and cherts of the Vitiacua 

 formation were deposited. This marine condition seems to have been of 

 short duration, however, for it lasted only long enough to permit the ac- 

 cumulation of a comparatively slight thickness of these fossiliferous beds. 

 Its date seems now rather definitely fixed as very late Triassic or early 

 Jurassic. 



LATE ME 80 ZOIC TIME 



Again the regions of sedimentation were changed to areas of erosion. 

 In some localities there must have been rather definite crustal warping, 

 following the deposition of the Vitiacua limestone and chert, but more 

 commonly the change was effected without notable disturbance. After 

 an interval of unknown duration the accumulation of the Tacuru forma- 

 tion was begun. These beds seem to have been in the main composed of 



