38 



Subsurface Geologic Methods 



sediments related to depth. Thus a coast line will be characterized by 

 roughly parallel belts of beach sands, muds, and limy oozes. As the sea 

 moves in on the land, these three types of deposits will retain their rela- 

 tive positions. Muds will be deposited on sands and oozes on muds. A 

 vertical cross section through a marine transgressive overlap will show 

 coarse elastics overlain by finer elastics. The relief on a surface of trans- 

 gressive overlap is essentially that of the submerged surface. Figure 15 

 shows a marine transgressive overlap. 



The slow rise of the sea may result in a different kind of progressive 

 overlap. Erosion may take place before deposition and a wave-cut sur- 

 face extending a long distance from shore may be developed. As the 

 sea level continues to rise, the wave-cut surface is brought below the base 

 level of erosion, and sediments are deposited. There will be fine-grained 



Figure 15. A marine transgressive overlap; lines parallel to ocean floor are called 

 "time lines" (ab) , for they run through material deposited contemporaneously; 

 lines essentially parallel to gravel, sand, and mud deposits are called "formation 

 lines" icd) . A-B — Erosion surface overlain unconformably by overlapping sedi- 

 ments. 



sediments, and the basal conglomerate produced by rapid submergence 

 will not be present. 



Unconformities resulting from overlap may be found in lacustrine 

 and alluvial fan deposits as well as marine. 



Ofilap : Offlap or regressive overlap is produced when the sea recedes 

 from the land. The beach zone will migrate over earlier offshore muds 

 and the muds over earlier deposited oozes. If sufl&cient time has elapsed 

 between deposition of offlap beds and the earlier deposited sediments, 

 a marine regressive unconformity may be formed. This is likely to be a 

 disconformity which might be diflScult to distinguish from conformable 

 deposition. 



Other Marine Unconformities: The rise of sea level may be inter- 

 rupted by periods of stability during which the sea bottom may be built 

 to the base level of deposition for a considerable distance from shore. 

 Equilibrium established over a long period of time will result in a dis- 

 conformity of nondeposition. 



A falling sea may lower the base level of erosion so that previously 



