1074 



Subsurface Geologic Methods 



years previously. By combining such data with gravity-meter information 

 around the area, a satisfactory picture may be obtained. 



Applications 



Grand Saline Salt Dome, Texas — A residual gravity map of the Grand 

 Saline dome in Van Zandt County, Texas, is shown in figure 565. This 

 salt plug is a piercement-type dome rising to within 215 feet of the 

 surface and is capped by a very thin layer of limestone and anhydrite. 

 The gravity minimum observed is roughly symmetrical, with a magnitude 

 of about 3.8 milligals centered on the salt mass. From the areal extent 

 of the anomaly it is apparent that the structure must lie close to the 



Figure 565. Residual gravity map of Grand Saline salt dome, Van Zandt County, 

 Texas. Position of upper salt mass is shown by cross hatching. (After Peters 

 and Dugan. Reproduced permission Geophysics.) 



surface. Density determinations made on the salt and on the surrounding 

 sediments give a maximum contrast between the salt and the sediments 

 of 0.75 grams per cubic centimeter. ^^ 



Hawkins Field, Texas — A gravity-meter survey over the Hawkins 

 field in Wood County, Texas, is shown in figure 566. The structure has 

 probably resulted from the arching of formations through the uplift of a 

 large, deep-seated salt mass. A pronounced gravity minimum is noted over 

 the field, which by observation of the areal extent of the anomaly indicates 

 a deep density interface. The producing Woodbine formation has been 

 uplifted approximately 1,200 feet above its normal depth for the area, 

 and geologic evidence would indicate that the structure is even more pro- 



^^ Peters, J. P., and Dugan, A. F., op. cit., p. 382. 



