42 Subsurface Geologic Methods 



of features that might give dip variation are undetected faults, hole devia- 

 tion, cross bedding, flowage, and crossing of the axial surface of a fold. 

 Figure 17 illustrates several situations in which a nonconformity would be 

 apparent from cores taken at intervals, if angular discordance were the 

 only criterion. 



Truncation of Faults or Intrusives: Faults and intrusive bodies in 

 older strata may be truncated by erosion and end abruptly at unconform- 

 able contacts. There may be a definite contrast in the number of intrusives 

 and the faulting present above and below an unconformity. Again caution 

 should be used in using this criterion, as faulting may produce the same 

 contrasts. 



Degree of Folding or Metamorphism : There may be definite contrasts 

 in the degree of deformation or metamorphism on both sides of an uncon- 

 formable contact. Faulting again, however, may produce the same result. 



Evidences of Erosion or Weathering. Basal Conglomerate: It was 

 previously mentioned that a basal conglomerate may not be present in 

 either major type of unconformity. When present, the basal conglomerate 

 is composed of the coarser debris of the transgressed erosional surface. 

 It may be definitely arkosic if that surface is underlain by granite rocks 

 and may even be a blended unconformity if the transgressed surface is 

 deeply weathered. 



If it is a marine unconformity, where the older strata were below sea 

 level before deposition of the younger, there may be bones, teeth, and shell 

 fragments, in addition to pebbles, in the basal formations of the younger 

 sediments. 



Autoclastic rocks might be confused with basal conglomerates, for 

 example, fault breccias. However, they are usually more angular than con- 

 glomeritic pebbles. They will contain material from the rocks on both sides 

 of the fault and may be filled with vein material. 



Lag Gravels: Lag gravels are an accumulation of coarse debris left 

 after the removal of the finer material by wind or water. They may repre- 

 sent an eolian unconformity where concentration has been effected by the 

 scouring action of the wind under desert conditions. As such they may con- 

 tain wind-polished and faceted pebbles called "ventifacts" or pebbles coated 

 with "desert varnish" and may be tightly fitted together from an old 

 "desert pavement." 



Wave and current action may also concentrate lag gravels under con- 

 ditions of submarine erosion. The presence of pebble bands in a marine 

 section suggests a diastem, if found within a formation, or a disconform- 

 ity of larger magnitude, if located at the top of a formation. 



Buried Soils: One of the strongest proofs of unconformity is the 

 identification of ancient soils in the geologic column. This identification 

 may be very difficult because of the changes that take place in soils sub- 

 sequent to burial. They may be partly or completely incorporated into the 

 basal part of the new formation and may lose the zoned characteristics 



