Figure 23-12. Isopach and sand-shale for the Dakotan and Coloradoan rocks (lower sub- 

 unit of the section analyzed) (Krumbein and Nagel, courtesy American Association of 

 Petroleum Geologists) . 



To initiate construction of a series of stratigraphic maps, the following pro- 

 cedure is recommended. Measure, describe, and sample, in detail, surface and 

 subsurface sections; define the boundaries of stratigraphic units and establish 

 their lateral equivalents; prepare the following maps: isopach, which is the 

 basis for all subsequent stratigraphic -type maps; litho fades, including clastic 

 and non-clastic (percentage or ratio) ; bio fades (percentage or ratio of vari- 

 ables) ; and tecto fades and paleogeographic. When these maps are completed — 

 then comes the stage of integration and interpretation of the data. 



Figures 23-11, 23-12 are examples of typical maps prepared by Krumbein 

 and Nagel (1953) during their stratigraphic studies of the Cretaceous of the 

 Rocky Mountain region. Basic data for this study were obtained from many sur- 

 face and subsurface sections. The purposes of the maps were "to present the 

 broad pattern of thickness and lithologic variation in the Upper Cretaceous and 

 to introduce methods for expressing the vertical variability, as well as the lateral 



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