Lithogenetic or rock units involve specific rock masses such as the group, 

 formation, tongue, and bed-units with which the geologist is concerned primarily 

 in his mapping programs. 



Time-stratigraphic units are represented by deposits that have accumulated 

 during a specific interval of geologic time. The boundaries of these units are 

 time surfaces, which are established most commonly on paleontological evidence ; 

 however, these surfaces may be delineated sometimes by unconformities. A 

 time-stratigraphic unit frequently contains, within its boundaries, various types 

 of deposits, (sandstone, reef limestone, and shale) as shown in Figure 23-1 

 between Ti and T 2 . Time stratigraphic units may transect lithogenetic units, as 

 illustrated in Figure 23-2. Hedberg (1948) states that "it is desirable to be 

 able to express as a time-stratigraphic unit the sediments equivalent in age to 

 the same time scope of any recognizable feature of sedimentary rocks which may 

 be useful as a stratigraphic measuring stick." 



Time units (Era, Period, and Epoch) involve segments of geologic time 

 and are related to the lithologic and paleontologic aspects of a stratal sequence 

 only indirectly. To define these units it is necessary, first, to evaluate the total 

 aspect of lithogenetic units. Next, the boundaries of the time-stratigraphic units 

 are delineated, and they, in turn, permit erection of the time units. 



Amphistegina Zone 

 Bolivina Zone 



Uvigerina Zone 



Cibicides Zone z= 



Elphidium Zone 



Figure 23-3. Biostratigraphic zonation. The boundaries of biozones frequently do not 

 coincide with lithologic boundaries. The paleontologist is concerned, primarily, with 

 the former and the field geologist with the latter. Paleontologic and lithologic unit 

 relationships should be properly evaluated and integrated in all correlation investiga- 

 tions. 



441 



