Chap. 7] 



GRAVITATIONAL METHODS 



285 



with torsion balance, magnetic, and electrical methods in an attempt to 

 delineate step-faulted blocks in the Limagne-Graben zone have been pub- 

 lished by Geoff roy/^^ 



The torsion balance has been rather successful in the Mexia-Luhng 

 fault zone. In a profile illustrated by Barton,"^* the Edwards limestone, 

 at a depth of between 1000 and 2400 feet, is in contact with lighter Eagle- 

 ford, Del Rio, and Austin chalk, while the Eagleford, Del Rio, and Austin 

 are against lighter Taylor and Navarro shales. Hence, the maximum 

 gradient occurs above the subsurface point where this density contrast 

 occurs, that is, down dip in the fault from its surface trace. Two faults 

 in the Paleozoic area near Ottawa were investigated by Miller and 



reet 



no 



•+ + + 4- + + 3 +4 + + + + 



+ + + + + + +■»■ + + + + + 

 + + + + + + + + ++■)■■(- 



+ + -f- + + + + <-+ + 4 + 

 + + <■ + + + + + + + + •»- 



•^+ + + + +J' ++ + + + + 

 § +-(-+++ + ■«•+ + + + 



+ -»■ + + + + + + + + + + 

 + -f + +•♦• ■)- + + + + + + 



+ + +'+ +-(■ * + + + + .» 



Fig. 7-117. Torsion balance results on Hazeldean fault, Canada (after Miller). 

 1. Black River, Chazy limestone (5 = 2.7); 2. Chazy shale and sandstone (5 = 2.5); 

 3. Beekmantown dolomite (5 = 2.8); 4. Potsdam sandstone (5 = 2.5); 5. Precambrian 

 (5 = 3.0). 



showed a distinctly different response. The Hull-Gloucester fault near 

 Leitrim (Fig. 7-116) in the Paleozoic and Pre-Cambrian has a 900 foot 

 displacement and produces large gradients (about 150 E.U.) because of a 

 near-surface contrast in density. Theoretical values calculated for an 

 assumed subsurface section agreed well with the experimental results. 



In the same Paleozoic section the Hazeldean fault (575 foot displace- 

 ment) gave a distinctly different picture. A "minimum" gradient was 

 observed, since the effects of the lower beds (density contrast in upthrow 

 direction) are overcompensated by a near-surface effect of density contrast 



"3 M. P. Geolfroy, Ann. Office Nat. Comb. Liqu., 4, 617-647 (1929). 

 154 D. C. Barton, A.I.M.E. Geophysical Prospecting, 416-466 (1929). 

 "B A. H. Miller, Canad. Geol. Survey Mem., 165, 197-208 (1931). 



