STRUCTURAL GEOLOGY OF NORTH AMERICA 



Salt domes also differ on the basis of their cap rock. Overlying the 

 massive rock salt of the core of the dome is an irregular layer or cap 

 of limestone, gypsum, and anhydrite. Limestone is generally at the top, 

 and anhydrite at the bottom of the capping layer. The cap rock of a few 

 domes contains immensely valuable native sulfur deposits. 



The sedimentary layers over the salt domes have been domed up 

 gently, and the layers adjacent to the intrusive salt plugs have been 

 dragged upward to a greater or lesser extent. Oil is found, therefore, 

 over the salt plug in the domed strata, on the flanks against the plug, 

 under overhangs, and in associated fault traps. 



Deep drilling of certain salt domes of southern Louisiana shows that 

 very large volumes of salt are involved, and that one structure at a 

 depth of 20,000 feet has an area of 200 square miles, and contains 265 

 cubic miles of salt above the 20,000 foot datum (Atwater and Forman, 

 1959). Further, the salt is intrusive like major igneous discordant 

 plutons; the country rock has been "replaced" rather than shoved aside. 

 The manner of emplacement is an enigma. Also, large masses of contorted 

 shale have been carried up far above their normal stratigraphic position 

 and look like intrusive masses themselves. The intrusive action has been 

 localized to one part of the large dome at one time, and then to another 

 part at another time. 



Origin. Salt domes result from the plastic intrusion of sedimentary 

 rock salt into overlying beds. Rock salt under pressure deforms easily 

 and flows from places of greater pressure to places of lesser pressure. 

 Many geologic observations confirm the concept that salt flows easily 

 and that the associated shales are commonly intensely deformed. Ac- 

 cording to Nettleton (1936) it is reasonable to assume that the present 

 shape of the dome is due to ( 1 ) initial configuration that localized the 

 dome, (2) the thickness of the mother salt layer, (3) the strength or vis- 

 cosity of the overlying rocks, and (4) the strength or viscosity of the 

 salt. Figure 41.10 shows the theoretical development of salt domes under 

 three conditions. 



A number of deep-seated salt domes are marked by faults that cut 

 and offset the arched beds over the salt core. Perhaps the faulting is a 

 general characteristic. The faults are normal and form a complex graben 



through the central part of the dome. Wallace (1944) believes that the 

 common fault patterns are simple offsets and simple and complex 

 graben such as illustrated in Fig. 41.11. The first fault that occurs is 

 called the principal fault, which produces the simple offset. The next is 

 the complementary fault ( also called minor fault ) , and the next is another 

 minor fault. The generalized diagrams of Fig. 41.11 give the impression 

 that all graben cut across the domes; but as more is learned of the detail 

 of the deep domes, more faults are recognized, and their ground pat- 

 tern may be somewhat concentric in certain domes, somewhat radial 

 in others, and crosscutting in still others. 



The intruding salt has also caused small reverse faults on the sides 

 of certain domes. These are significant in forming oil traps (Halbouty 

 and Hardin, 1954, 1956). 



Wiggins Anticline and the Deep Wells 



Just 50 miles north of the Gulf of Mexico in southern Mississippi a 

 well was drilled 20,450 feet deep in a subsurface structure called the 

 Wiggins anticline. It is known as the George Vasen's Fee well and was 

 completed in 1951 (Applin and Applin, 1953). At the total depth it 

 reached rock salt of pre-Smackover (Jurassic) age. Nearly 5500 feet of 

 consecutive cores of unmetamorphosed Jurassic strata were obtained. 

 From a depth of 14,670 to the bottom the formations penetrated have 

 been identified as follows: 



Lower Cretaceous 



Lower part of Hosston fm. 



275 



feet 



Dark brownish-red shale 







Upper Jurassic 







Cotton Valley group 



2053 



feet 



Mostly nonmarine or detaic 







deposits in upper part; lower fourth is 







marine and fossiliferous 







Cotton Valley (?) group and Buckner (?) fm. 



1700 



feet 



Nonmarine sandstone and shale 







Smackover formation 



105 



feet 



Dark sandstone, siltstone, and shale 







Dips 25° to 60° 







