280 



GRAVITATIONAL METHODS 



[Chap. 7 



Gravity minima alone are observed on deep-se&ted domes (depth to cap 

 or salt, 5000 feet or more). An example is the Shepards-Mott dome illus- 

 trated in Fig. 7-113. No salt was encountered at depths of 6000 feet. 

 The regional effect is quite large and keeps the gradients from showing 

 the typical reversal. The reflection contour map indicates a closure of 

 1400 feet. A well-defined gravity minimum has also been found on the 

 Tomball field, showing good closure with and against the regional dip but 



weak closure in the direction 

 ^"^ ^ of strike, as is the case in other 



domes along the Conroe trend. 

 No evidence of salt has been 

 found by driihng. 



Fig. 7-114 illustrates the 

 gravity picture of the Esperson 

 dome (discovered by the tor- 

 sion balance late in 1928 ) 

 and the geologic section as- 

 sumed in the interpretation 

 calculations. The gradients 

 are of the order of 2 to 7 E.U. ; 

 the gravity minimum on the 

 left is due to the Esperson 

 dome. Because of the inter- 

 ference from the salt mass of 

 the adjoining South Liberty- 

 Dayton dome, a maximum is 

 produced between them. In 

 the calculations, the average 

 density of salt was assumed 

 to be 2.19 and that of the cap 

 rock 2.6. For the sediments 

 the assumptions were: from 

 to 300 feet, 1.9 to 2.05; from 

 2000 to 4000 feet, 2.20; from 

 4000 to 8000 feet, 2.25; from 8000 to 12,000 feet, 2.3. This results in the ap- 

 parent salt-density differences indicated in the figure. It is seen that a grav- 

 ity maximum above a dome may be due not only to the effect of the cap but 

 also to that part of the salt which exceeds the density of the surrounding 

 sediments. Such interdomai maxima, as revealed by this survey, may be 

 a source of interpretation difficulties. As a matter of fact, maxima of this 



Fig. 7-110. Torsion balance anomalies on 

 Fiore§ti salt dome, Rumania (after Ghituiesco). 



1" D. C. Barton, A.A.P.G. Bull., 14(9), 1129-1143 (1930). 



