Stratigraphic, Structural, and Correlation Considerations 55 



regularities complicate and, in many instances, control the accumulation 

 pattern of oil and gas and other concentrations of minerals. Fault rela- 

 tionships should always be viewed in three dimensions. Fault surfaces 

 may be contoured and fault gaps outlined.^^ 



Marine Onlap and Offlap 



Transgressions and regressions of the sea across continental segments 

 produce sedimentary onlaps and oflflaps. These depositional features 

 are of local or regional magnitude and have been responsible in many 

 areas for conditions permitting the accumulation of large quantities of 

 oil and gas. Problems of onlap and offlap of varying complexity are com- 

 monly encountered in stratigraphic work. 



Patterson ^^ has clearly demonstrated the transgressive and regressive 

 relationships of the Cook Mountain, Yegua, and Fayette formations (upper 

 Eocene) developed across Webb, Zapata, and Starr Counties, Texas. These 

 relationships are idealistically shown in figure 24. 



Swain's definitions ^° of various terms relating to transgressions and 

 regressions follow : 



1. Onlap: The progressive pincliing-out, toward the margins of a deposi- 

 tional basin, of the sedimentary units of a conformable sequence of rocks. 



2. Offlap: The progressively offshore degression of the updip termina- 

 tions of the sedimentary units of a conformable sequence of rocks. 



3. Overstep: The regular truncation of older units of a complete sedi- 

 mentary sequence by one or more later units of the sequence. The resulting 

 unconformity may be either marginal to the basin of deposition, or within the 

 basin as a result of local uplifts. If more than one unit rests on those beneath 

 the unconformity, both overstep and onlap are involved. 



4. Complete overstep: The entire blanketing with unconformable rela- 

 tionship of the older rocks of a basin by younger rocks. 



To obtain the true trend and magnitude of marine onlaps and offlaps, 

 one must give full consideration to the third-dimensional concept of the 

 region. 



Oil and Gas Traps 



Oil and gas accumulations (fig. 25) occur under favorable structural, 

 stratigraphic, and structural-stratigraphic conditions. Examples of various 

 types of entrapments are graphically illustrated in figures 26-32. Pros- 

 pective oil and gas reservoirs must be considered from a three-dimensional 

 viewpoint. The closure factor should under no circumstances be min- 

 imized. To evaluate closure, whether it be controlled by structural or 

 stratigraphic conditions or a combination of both, requires the interpre- 

 tative ability of both the surface and subsurface geologist. 



•"*Reiter, W. A., Contouring Fault Planes: Oil World, July 14, 1947. 



^^ Patterson, J. M., Stratigraphy of Eocene between Laredo and Rio Grande City, Texas: Assoc. Pe- 

 troleum Geologists Bull., vol. 26, no. 2, pp. 256-274, Feb. 1942. 



^^ Swain, F. M., Onlap, Offlap, Overstep and Overlap, Am. Assoc. Petroleum Geologists Bull., vol. 

 33, no. 4, pp. 634-635, 1949. 



