914 Subsurface Geologic Methods 



the map, the spacing of contours and the nature of thickening and thin- 

 ning may be guided largely by other known factors concerning the source 

 of sediments, their relative rates of deposition, truncation, and so forth. 

 An isopach map drawn strictly to the numerical values and without re- 

 gard to the geologic reasons for thickening and thinning of the formations 

 is likely to present a picture difficult to reconcile with other geologic facts. 



The isopach map, A, in figure 483 is drawn according to thicknesses 

 shown at the wells. No consideration is given to the reasons for the body 

 of thicker sediments in the central part of the area or the changes in 

 rate of thickening in the two regions where the formations are absent. 

 The same points are contoured in B of figure 483 with much better effect. 

 The close spacing of the contours from zero to 200 feet on the west side 

 of the map indicates the area of truncation where the formations are tilted 

 along the granite mass. The cross section shows that the limestones are of 

 about the same thickness here as at points much farther into the basin, 

 and there are no conglomerates that would suggest a near-shore-sedimenta- 

 tion environment. The conclusion is that the higher rate of thinning is 

 caused by erosion of the upturned edges of the strata, and the close spacing 

 of the contours is, therefore, maintained parallel to the granite area. 



Around the uplift on the east side of the map area, the control points 

 show a high rate of thickening. The well samples contain large quantities 

 of coarse arkosic sands and conglomerates, and it is assumed tliat the 

 granite mass was the source of the sediments. With this knowledge at hand, 

 the contours are drawn so that the nature of these deposits indicates the 

 size and shape of the "blank" granite area within the zero line. Finer 

 sediments in wells 190, 450, and 590 suggest a higher and less precipitous 

 terrain; hence, the "nose" plunging to the southeast corner of the area. 



It might be pointed out that the map, B, and section, C, together 

 clearly show that the central uplift is older than the sediments and at the 

 time of sedimentation was higher than the granite area on the west side 

 of the area. Conversely, the western arch is probably younger than the 

 sediments, because the flanking rocks are similar to those in the central 

 portion of the structural basin. 



A common source of error in subsurface isopach maps is the too- 

 great stratigraphic interval caused by steeply dipping strata at the point 

 where the well is drilled. Obviously, a correct interval is obtained in a 

 straight hole only where the beds are level. Since most wells are drilled 

 on structures, the chances are favorable for penetrating the formations 

 where appreciable dips do exist, and, if the dips are quite steep, the error 

 in interval may be large enough to affect the regional aspects of the 

 isopach map. There is little doubt that the thinning of the section on 

 the tops of some structures is only apparent and is the result of this con- 

 dition. If core dips are available, the true stratigraphic thicknesses can 

 be determined by Busk's method for obtaining stratigraphic thicknesses in 

 inclined strata. 



