GEOLOGIC STRUCTURE 755 



and 20, the strata dip westward on the west side of the range at angles 

 varying from 40 degrees to 80 degrees, while on the east side of the range 

 they dip eastward at angles of 20 degrees to 70 degrees. The crest of the 

 anticline is approximately in the middle of the mountain range and ex- 

 tends in a broadly crescentic curve from a point 4 or 5 miles northeast 

 of Limon to a point 10 or 12 miles northeast of Boyuibi, as indicated on 

 the accompanying map, figure 19. The fold is more than 85 miles in 

 length and is about 10 miles wide. It is apparently not connected with 

 any other structural feature, but is roughly parallel to similar folds situ- 

 ated several miles to the westward. 



Along the crest of the Charagua anticline the rocks appear to be much 

 broken, but no faults of any great significance were observed. The ex- 

 posure of the Espejos formation in the gorge of Rio Tacuru is due to a 

 fault, the exact nature of which could not be ascertained from the one 

 exposure. It is probably a thrust-fault, which complicates the western 

 limb of the anticline at that locality. 



OQUITA-TAIMBERMI FAULT 



The geologic structure of the Cuestas de Oquita and their northward 

 extension in the Cuestas de Taimbermi is typical "Great Basin" structure. 

 The steep escarpment along the eastern margin defines a major fault 

 trending in a general north-south direction roughly parallel to the 

 Charagua anticline. At the point where Rio Parapiti crosses this fault 

 its plane is vertical, and it is probable that throughout its extent the 

 fault-plane does not depart far from vertically. The western side is the 

 upthrow side and the movement along the fault-plane amounts to about 

 10,000 feet. 



At the locality just mentioned on the north bank of Rio Parapiti, a 

 photograph of which forms figure 21, the fault has resulted in the forma- 

 tion of a sheer zone a hundred feet or so in width, within which the beds 

 are greatly shattered and the prevailing dip is vertical. This is true of 

 the Tatarenda shales on the margin of the downthrow block east of the 

 fault-plane, as well as of the Oquita sandstones on the upthrow side. A 

 few rods farther west the strata are inclined westward at an angle of 85 

 degrees, and for nearly a mile west of the fault-plane the dip is 80 de- 

 grees or higher. Beyond that point the dip gradually lessens, until at 

 the western margin of the group of hills, 2% miles west of the fault- 

 plane, the top beds of the Tacuru formation are inclined westward at an 

 angle of only 14 degrees. This dip continues westward, as indicated by 

 the attitude of the Tatarenda strata, for several miles. Throughout the 

 entire group of the Oquita Hills the strike of the beds is within a few 



