Sandstone 



Established on paleontology 



i i i i i i 



I I I zr 



Sandstone 



_L Reef limestone 

 xzzr 



i i i zzr 



-y^-^rr— 



Shale 



Figure 23-1. Time-stratigraphic unit. The sandstone, limestone and shale between times 

 surfaces T 1 and T 2 constitute a time-stratigraphic unit whose upper boundary is 

 defined paleontologically and whose lower boundary involves an anhydrite bed. To 

 establish contemporaneity of these three lithologies (facies) within the time-strati- 

 graphic unit requires careful surface and subsurface mapping, particularly in areas 

 of interfingering. 



© Conglomerat 

 OSiltstone 

 ©Mudstone 

 ©Sandstone 



BASEMENT 



Figure 23-2. Relationship of time-stratigraphic and lithogenetic units. Depositional onlap 

 as illustrated results from a transgressive sea. In this sketch the base of the sedimentary 

 section is marked by an unconformity which becomes progessively younger towards the 

 apex of the sedimentary wedge and thus does not represent a true time surface; 

 however, under certain conditions, unconformities do correspond to time surfaces. Assume 

 that time surfaces T 1; T 2 , and T 3 have been established paleontologically from the 

 center of the basin to a point (short arrows) where they intersect the basement. The 

 four lithologic facies (conglomerate, siltstone, mudstone, and sandstone) between 

 T 1 and T„ constitute a time-stratigraphic unit. The same applies for deposits be- 

 tween T and T 3 . The deposits between the unconformity and T x is not a time- 

 stratigraphic unit because the unconformity surface is time transgressive. 



440 



