1072 



Subsurface Geologic IVTethods 



(1) accurate field data, (2) the proper removal of the regional gradient 

 and other disturbing effects, and (3) sufficient additional control to limit 

 the depth from which the anomaly may arise. 



Minimum and Maximum Control 



Structural uplifts may be expressed by either gravity minima or 

 maxima, depending upon the geographic location. In the Gulf Coast for 

 example, uplifts are indicated by gravity minima (excluding maxima ob- 

 served over cap rocks of piercement salt domes and igneous plugs) ; in 

 Oklahoma or the Rocky Mountains, however, structural uplift is indicated 

 by gravity maxima. The approximate limits of salt or minimum control 



Scale of miles 



100 200 300 



Figure 564. An approximate outline of an area of minimum gravity 

 control in the Southern States. 



are shown in figure 564. In the rest of the United States, with a few local 

 exceptions such as portions of the San Joaquin Valley in California, 

 maximum control exists. Gravity minima associated with uplifts are 

 caused by the presence of a thick bed of light material at depth, such as 

 the Louann salt of Permian age, which consists of more than a thousand 

 feet of rock salt and extends from southwest Texas to western Alabama. 

 Local thickening or actual intrusion of this salt bed into overlying sedi- 

 ments is associated with most of the oil fields in the area underlain by the 

 salt and thus produces minimum gravitational effects. 



In other areas the warping of a dense limestone at depth or the 

 actual uplift of the basement itself will produce gravity maxima over 

 structures. 



