Stratigraphic, Structural, and Correlation Considerations 47 



in the "Wilcox" sand. The Nemaha buried ridge of Kansas and Okla- 

 homa is perhaps the largest "bald head" of all. 



Shoestring Sands: Very long sand lenses a few hundred feet wide, 

 a few score feet thick, and seveial thousand feet to several miles long, 

 are called "shoestring sands," when they are found buried in mud de- 

 posits or in shale formations. They may be fillings of old stream chan- 

 nels or buried offshore bars. If the former, the cross section of the de- 

 posit will have its greatest width at the top and will have a base convex 



Figure 18. Idealized section west-east across Oklahoma City field showing relation 

 of unconformities to oil and gas production. A typical "bald-headed" struc- 

 ture. 



downward. In the latter case, the cross section will have a flat base and 

 a top convex upward. These sands will have disconformable relation- 

 ships with the surrounding rocks. 



An example of a shoestring-sand oil pool is the Bush City field in 

 Anderson County, Kansas. Oil is trapped in a sand body 13 miles long, 

 about one-fourth mile wide, and buried 30 to 40 feet below the top of the 

 Cherokee shale of Pennsylvanian age. The sand has been folded into 

 minor anticlines and synclines. Production is from both because of the 

 water-free nature of the sand. A typical cross section and plan of the 

 sand is shown in figure 19. 



Disconf ormities : Oil may be found at the surfaces of disconform- 

 ities where impervious beds overlie an erosion surface of some relief 

 and will be found in structural adjustment in the highs of the older 

 strata. Figure 20 shows diagrammatically how petroleum has been trapped 

 in hills of the Wilcox formation buried under the impervious clays of the 

 Cane River formation; both are Tertiary. 



Regional Unconformities: Unconformities whose surfaces can be 

 traced over wide areas are termed "regional unconformities." The inter- 

 section of two unconformable surfaces against the flanks of the Sabine 



