568 EXPLORATION GEOPHYSICS 



Gillespie Dam. Figure 350 shows a portion of the area covered and 

 a typical cross section obtained during the survey. 



The Gila River flows between high lava abutments situated in Gillespie 

 Gap, a natural pass formed by the Gila Mountains to the west and the 

 Buckeye Hills to the east. The present course of the river is controlled 

 by the high granitic peaks of the Buckeye Hills on the east and a flow 

 of late Tertiary lava, known as the "Gillespie Flow" on the west. The 

 Gila River channel, both ancient and present, is filled with a series of 

 volcanic flows and various phases of flood plain detritus deposited by 

 the river. 



The granitic rocks and cropping on the east and west of the dam 

 abutments and underlying the area of the "gap" form the boundaries 

 which limit the flow of subsurface water. Above the granite, in order 

 from oldest to youngest, lies a series of relatively more porous formations : 

 (1) older volcanic rocks forming the dam abutments, (2) older flood- 

 plain material, (3) younger volcanic rocks, (4) younger flood-plain 

 material, and (5) recent alluvium. 



The general purpose of the investigation was to determine the distri- 

 bution and structural relationship of the above-named formations with 

 particular attention to the effect they might have on subsurface flow of 

 water. Specifically, it was desired to determine the location and cause 

 of leakage that was taking place around the dam. 



A study of the areal geology indicated that possible subsurface water 

 flow was limited to two gaps bounded by the outcropping granite rocks : 

 (1) that between the points where later were placed the geophysical 

 stations £-1 and £-8, and (2) that extending from station E-9 eastward 

 across the present channel of the Gila River to the granitic outcrop on 

 the east abutment. The geophysical study was confined, therefore, to a 

 study of subsurface conditions of these two locations. 



The results of the geophysical studies are shown in Figure 350 by 

 the cross section. 



Cross section ABCD : From stations £-1 to £-8 the geophysical work 

 indicated that the lava capping is relatively thin and that no appreciable 

 amount of porous material such as gravel or alluvium lies between the lava 

 and the underlying granite. From stations E-9 to £-23 a much more 

 complicated subsurface condition was indicated. The most important 

 structural features are the presence of the fault paralleling the stream 

 channel and the presence of porous materials below the lava which forms 

 the dam abutments. 



The examination comprised geological, geoelectrical, and geomagnetic 

 studies. As a result of the work it appears that the leakage occurred through 

 the subsurface underneath the dam. The leakage probably occurred through 

 a layer of porous material which is indicated to underlie the lava flow 

 upon which the dam is built. 



