Subsurface Methods as Applied in Geophysics 



1083 



Although it is possible that granite lies at the core of this structure, 

 it is more probable that density contrast within the sedimentary section 

 accounts for the gravity anomaly. Large density contrasts are known to 

 exist between the Permian-Pennsylvanian series and the Mississippian and 

 older sediments, which are predominantly limestones. 



Contour Interval = 5 m.g. 



Figure 574. Thomasville gravity anomaly in northern Clarke County, Alabama. 

 (Courtesy Magnolia Petroleum Company.) 



Altus Field, Jackson County, Oklahoma — The Altus structure in 

 Jackson County of southern Oklahoma was discovered by geophysical 

 exploration and subsequent core-drill information. The field lies to the 

 west of the Wichita Mountains on the trend of the buried Amarillo granite 

 ridge. The structure, which lies on an igneous uplift, is clearly indicated 

 on the residual-gravity map. (See fig. 572.) Although the observed-gravity 

 map does not give the striking picture presented by the residual map, 

 owing to the masking effect of the strong northerly regional gradient, the 

 presence of the structure is betrayed by the pronounced change in gra- 

 dient. The field had five active wells in 1936 with 340 proved acres and 

 was producing from a limestone at about 1,200 feet. 



Thomasville Anomaly, Clarke County, Alabama — Further evidence of 

 an intra-basement variation in mineralogic composition at the Thomasville 

 anomaly in Clarke County, Alabama, is offered by the gravity map shown 

 in figure 574. From the gravity data it would appear that the causative 



