Stratigraphic, Structural, and Correlation Considerations 49 



basin showing the function of a regional unconformity in localizing pro- 

 duction in the Woodbine sand. 



Major oil structures are often completely obscured by nonconform- 

 able deposition of later sediments. The geologic history of the older 

 rocks may have been such that great oil and gas accumulations were formed 

 below regional unconformities. For example, the Central Kansas uplift is 

 obscured by a uniform cover of north- and west-dipping Permian and 



Figure 21. Idealized section west-east of Woodbine sand in Tyler basin of northeast 

 Texas showing relation of unconformities to production. In Mexia field to west, 

 production is from oil trapped between a fault and an unconformity. In East 

 Texas field to east, two unconformities intersect on flank of Sabine uplift to 

 form oil trap. This section illustrates importance of regional unconformity to 

 production of oil. 



Cretaceous rocks. Again, the Bend arch of Texas is obscured by overlap- 

 ping and west-dipping Pennsylvanian strata. 



Two areas offer great possibilities for new geologic conditions con- 

 cealed below unconformities. The Comanche rocks of northern Louisiana, 

 northeast Texas, and southern Arkansas, where 5,000 to 10,000 feet of 

 sediments are folded, tilted, and completely overlapped unconformably 

 by Upper Cretaceous rocks. Pre-Carboniferous rocks of west Texas have 

 been folded and eroded with great lateral changes in porosity and are 

 widespread beneath a cover of Carboniferous rocks. 



Ore Deposits 



Types of deposits that may be found associated with unconformities 

 are residual and placer ores. Surfaces of unconformity may also in- 

 fluence the localization of hydrothermal deposits. 



Residual Deposits: Erosion surfaces of regional extent may be the 

 loci for residual deposits of bauxite, manganese ores, lateritic iron ores, 

 and phosphates. If this surface is then buried completely or partly, an 

 unconformity results, the tracing and mapping of which may lead to the 

 discovery of commercial concentrations of ore. 



