REPETITION AND ELIMINATION OF STRATA 817 



Stratigraphic significance of faults. 



From the stratigraphic viewpoint, strike faults are of the great- 

 est importance, for they often lead to a duplication of strata or to 

 the elimination of certain beds. Many mistakes in stratigraphy have 

 been made because of the nonrecognition of such faults. The Og- 

 den quartzite, for example, regarded as a distinct formation in the 

 western Ordovicic, has been shown to be a repetition of a lower 

 formation due to overthrust. In the Helderberg region of New 

 York, near Kingston, duplication by overthrust has led to the con- 

 fusion of the stratigraphy. The overthrust New Scotland beds 

 were originally described as the Upper Shaly (Port Ewen), and the 

 higher formation was held to have the same fossils as the lower one. 



In discussing the effects of strike faulting on the apparent suc- 

 cession of strata, eight principal cases may be considered : 



A. Gravity faults. (Figs. 199, A-G.) 



1. Dip of fault plane with dip of strata, but at greater angle. 

 (Fig. A.) In this case elimination of beds will take 

 place. 



2. At smaller angle. In this case repetition of beds will 

 take place. (Fig. B.) 



3. Dip of fault planes against dip of strata. In this case 

 repetition of beds will result. (Figs. C-E.) 



4. Dip of fault plane vertical (hade o). The "down dip" 

 portion descends. In this case elimination of beds results. 



(Fig- F-) _ . 



5. The "up dip" portion descends. In this case repetition 



results. (Fig. G.) 



B. Thrust faults. (Figs. 199, H-J.) 



6. Dip of fault plane with dip of strata, but at greater 

 angle, repetition results. (Fig. H.) 



7. Dip at smaller angle — elimination results. (Fig. I.) 



8. Dip of fault plane against dip of strata. In this case 

 elimination of strata results. (Fig. J.) 



A consideration of the diagram. Figs. A to J, will show that 

 the plane of faulting cuts the inclined strata so as to leave a portion 

 whicl; is limited below by the fault-plane, but may extend indefi- 

 nitely upward except as limited by the earth's surface. This por- 

 tion may be called the "up-dip" end of the strata (see the fig- 

 ures). The other part is limited above by the fault plane, and may 

 extend indefinitely downward. This is the '"down-dip" end {d) of 



