Stratigraphic, Structural, and Correlation Considerations 51 



property at the upper levels, a hill of lava projects stratigraphically into 

 the sediments and is marked by a vertical contact. In the lower levels the 

 contact flattens where an embayment of sediments cuts into the old lava 

 surface. The No. 1 vein system occurs opposite the steep contact. Below 

 this the ore is absent along the flatter-dipping contact but recurs where the 

 unconformity starts to steepen. 



Conclusion 



Because of the stratigraphic and economic value of unconformities, 

 the geologist should become adept in using the criteria by which uncon- 

 formities may be detected. Undiscovered reservoirs of petroleum of the 

 magnitude of the East Texas field may lie beneath the anonymity of 

 regional unconformities on the Gulf Coast, in west Texas, and in some of 

 the Rocky Mountain States. Unconformities may prove to be important 

 factors in controlling and localizing ore deposition in many mining areas 

 where their potentialities have been ignored. 



Faults 



Geologists are aware of the need of fault control before stratigraphic 

 problems can be solved. Fault patterns and their relationships may be 

 extremely complicated (fig. 22) and may play such a role that both sur- 

 face and subsurface structural and stratigraphic trends can be evaluated 

 only after voluminous data become available for interpretation. 



A fault represents a surface along which one rock segment has moved 

 with respect to the other. The magnitude of faults ranges from millimeters 

 of displacement to several thousands of feet or even miles. Faults are 

 developed in all rock types; their displacement may increase or decrease 

 with depth or vice versa, and their relative movement may be vertical, hori- 

 zontal, or rotational. 



The apparent movement along a fault is a function of many variables, 

 and depends not only on the net slip, but also on the strike and dip of the 

 fault, the dip and strike of the disrupted stratum, and the attitude of the surface 

 on which the observations are made.'*^ 



Common fault types include strike faults, in which the strikes are 

 more or less parallel to the strike of the rocks involved; oblique faults, 

 in which the strikes are diagonal to the strike of the rocks involved; 

 longitudinal faults, in which the strikes are roughly parallel to the strike 

 of the regional structural fabric; and transverse faults, in which the strikes 

 are perpendicular or oblique to the strike of the regional structural fabric. 

 Other fault varieties are en echelon, peripheral, and radial. High-angled 

 faults are those with surfaces that dip greater than 45 degrees. Low-angled 

 faults dip less than 45 degrees. 



A thrust fault (reverse) or thrust is a fault along which the hanging 

 wall has moved up relative to the footwall. A gravity fault (normal) is a 



^ Billings, M. P., Structural Geology, p. 137, New York, Prentice-Hall, Inc., 1942. 



