432 MAGNETIC METHOD [Cuap. 8 



depth of the basement in the case of the Amarillo fault on the downthrow 

 side was assumed to be about 4000 feet and the depth of the basement in 

 the case of the Beckham County fault about 2500 feet. The Hazeldean 

 fault (Miller^") in the pre-Cambrian, with a throw of 700 feet, caused a 

 positive peak of about I2O7 on the upthrow side and a flatter negative 

 anomaly of — 160 7 amplitude on the downthrow side. Theoretical calcula- 

 tions checked the maximum as to amplitude and position, but gave for 

 the negative anomaly about half the observed amplitude and a negative 

 peak much closer to the fault. 



Igneous intrusions have been frequently mapped in connection with 

 magnetic oil exploration. Extensive batholithic intrusions may give rise 

 to doming of the oil-bearing strata. Fracture and fault zones are often 

 accompanied by intrusions; laccolithic wedges produce warping in the 

 sedimentary beds. Examples of magnetic effects of intrusives have been 

 frequently cited in the literature and but a few examples will suffice. 

 Lynton^^^ reports a number of surveys on basic igneous intrusions from 

 California whose susceptibility is given as around 7000 • 10~^ units. In the 

 vicinity of Paso Creek in the San Joaquin Valley the magnetometer out- 

 lined a strong high (anomaly not stated). A well subsequently drilled 

 encountered plutonic rocks at 2700 feet. In the Ventura basin near 

 Oxnard an anomalous area with peak value of 7OO7 was outlined, and a 

 well put down in the high area encountered basalt at 1915 feet. The 

 susceptibility of the basalt as determined on well samples was 700 • 10~ . 



On the Jackson uplift in Mississippi an extensive magnetometer survey 

 was made by Spraragen. A difference of QOOr in vertical intensity from 

 the lowest to the highest point corresponds to a difference in depth to the 

 chalk of 560 feet. A similar survey was made by Barret^**" on the Caddo- 

 Shreveport uplift. The maximum in vertical intensity is 25O7 and a 

 minimum of —2507 occurs to the north. The structural high is much 

 broader than the magnetic high, and there appears to be a considerable 

 .shift (to the south) of the place of maximum vertical gradient compared 

 with the place of maximum structural gradient. The area of the 600-foot 

 contour (on top of the producing sand in the Nacatoch formation) is dis- 

 placed as much as 10 miles to the northwest from the area of the 225-7 

 contour. A rather unusual condition exists in the serpentine fields of 

 southwest Texas (Yoast field, Bastrop County; and Dale field, Caldwell 

 County) because the oil occurs in the magnetically active formation. 

 In the Yoast field^"^ the serpentine is at a depth of about 1500 feet 



»" Canad. Geol. Survey Mem., 170, 99-118 (1932). See also Fig. 7-117. 



198 Loc. cil. 



"9 Oil and Gas J., 30(26) (Jan. 21, 1932). 



2«»A.A.P.G. Bull., 14(2), 175-183 (1930). 



^oiD. M. CoUingwood, A.A.P.G. Bull., 14(9), 1191-1198 (Sept., 1930). 



