GEOLOGIC STRUCTURE 757 



degrees of north, but farther to the north, in the hills west of Taimbermi, 

 the strike swings westward, so that it varies between north 20 degrees 

 west and north 30 degrees west. 



MANDIYUTI FAULT AND ANTICLINE 



The Mandiyuti fanlt presents a sigmoid plan and is known to extend 

 from the vicinity of Pipi southward more than 30 miles, passing 3 or 3U> 

 miles west of Cuebo. At the north it forms the eastern margin of the 

 tilted fault-block responsible for the Cuestas de Pipi. The structure of 

 these hills is quite similar to that of the Cuestas de Oquita. The fault- 

 plane along the east front is nearly vertical; displacement was more than 

 10,000 feet. As elsewhere, the west side is the upthrow side. Imme- 

 diately west of the fault zone in the Parapiti Canyon the lower beds of 

 the Bermejo series dip westward at the comparatively low angle of 23 

 degrees. The inclination steepens farther to the west, so that a half mile 

 from the fault the beds are tilted at an angle of 54 degrees, the steepest 

 dip observed within this group of hills. Still farther westward the dip 

 becomes more gentle, and midway of the width of the cuestas the inclina- 

 tion is 27 degrees toward the west. This dip remains fairly constant 

 throughout the western half of the series of ridges and presumably con- 

 tinues beneath the Guarui Valley. 



Ten miles south of Wirandi the Mandiyuti fault has much less dis- 

 placement, so that only the uppermost beds of the Tacuru formation are 

 brought above drainage by it. Still farther southward the fault-plane 

 curves strongly toward the west and ihe displacement increases, so that 

 all of the Bermejo-Tacuru series is exposed in the Sierra de Mandiyuti. 

 Where crossed by Eio Cuebo, the fault-plane is inclined at an angle of 

 about 60 degrees toward the west and the displacement is at least 10,000 

 feet. The hanging wall on the west is the upthrow side and the fault is, 

 therefore, a thrust. East of the fault-plane the topmost beds of the 

 Tacuru formation are overturned, so that they dip at an angle of 74 de- 

 grees toward the west in close proximity to the fault zone. A few hun- 

 dred yards to the east the strata are vertical, and beyond that they dip at 

 successively lower angles toward the east. West of the fault-plane the 

 strata dip westward at angles varying between 40 degrees and 45 degrees, 

 the inclination toward the west becoming more gentle farther from the 

 fault-plane, until at the western margin of the range the dip is only 10 

 degrees. The Mandiyuti range is, therefore, a tilted fault-block which 

 has been pushed bodily eastward and overrides the shattered and bent 

 strata along the western margin of the Cuebo lowland. 



