52 



Subsurface Geologic Methods 



fault along which the hanging wall has moved down relative to the foot- 

 wall.46 



Billings ^^ lists the following criteria to aid in the recognition of 

 faults: (1) the discontinuity of structures, (2) the repetition or omission 

 of strata, (3) features characteristic of fault planes, (4) silicification and 

 mineralization, (5) sudden changes in sedimentary types and (6) physio- 

 graphic data. Only certain of these criteria may be applied in evaluating 







m^mmMiif- 



Wafer iai 



..a. 



/7 ^- <. 









Figure 22. Complexly faulted Miocene shales exposed in a road cut in Grimes 

 Canyon, Ventura County, California. This sketch clearly demonstrates complex 

 structural conditions which may develop in the subsurface as a result of thrust 

 faulting. Note structural relations above and below thrust zone. Decipherment 

 of this type of geologic problem requires close coordination between the struc- 

 tural and stratigraphic geologists. Drawing was sketched from a projected 

 35-mm. Kodachrome slide. Graphic symbols used do not represent true lithology 

 but illustrate only minor compositional variations in the shale section. 



the presence of a fault in the subsurface. To establish the type and char- 

 acter of faulting in well sections the following indications have proved 

 applicable: (1) anomalous profiles of lithologic, electric, and radioactive 

 logs; (2) a change in dip and disturbed phases as exhibited by cores; (3) 

 mineralogic and paleontologic irregularities; (4) lost circulation; (5) a 

 caving hole; (6) an increase in penetration rate; (7) an increase in por- 

 osity and permeability; (8) slickensided fragments in ditch samples; (9) 

 a sudden increase of drilling-mud temperature; (10) poor core recovery; 

 and (11) an abrupt deviation of the hole from the vertical. 



" Billings, M. P., op cit., p. 152. 

 " Billings, M. P., op. cit., p. 155. 



