LITHOLOGIC CORRELATION IN " BEND SERIES, TEXAS. 



11 



unit C, and thus it affords one of the chief con- 

 firmations of the principle assumed above (p. 3) 

 to govern in a general way the relation of clay 

 and lime. In the .Seaman well it was deposited 

 nearer shore and is therefore less calcareous 

 than the lower part of unit C. In the Rudd 

 well also it was laid down nearer shore and is 

 therefore more calcareous than the apparently 

 deep-water shales of the lower part of unit C. 

 In the Rudd well unit D is distinguished through- 

 out by the abundance of calcareous spicules, a 

 characteristic of the Marble Falls limestone. '' 

 In the Seaman well this characteristic was not 

 noted in unit D, which is probably too argilla- 

 ceous and sandy. 



Although this is the highest unit to show 

 abundant spicular limestones and shales it does 

 not represent the highest horizon at which 

 they appear. In the material from the Rudd 

 well they were first noted, though in small 

 amount, in the sample from 2,490 to 2,500 feet, 

 and in that from the Seaman well in the sam- 

 ple from 3,210 to 3,220 feet, but in these 

 higher occurrences they were of a narrower 

 type than those in unit D in the Rudd well. 



The base of unit D in the Seaman well is 

 marked by a typically developed glauconite bed 

 with coarse quartz sand, coarse fossil fragments, 

 phosphate grains, and abundant sulphides. 

 In the Rudd well, on the contrary, though the 

 lithologic difference on the two sides of the 

 boundary is sharp, the basal phase of imit D 

 does not appear pronounced. All that was 

 noted was a small amount of sulphidic black 

 limestone or shale, a few coarse quartz grains, 

 and a very few fragments of black shale or 

 limestone with glauconite grains and coarse 

 fossil fragments. 



UNITS E, F. AND G. 



The rocks between the base of unit D and the 

 top of unit J fall naturally into two groups, 

 which will be treated as such in the following 

 discussion. The higher of these groups con- 

 sists of units E, F, and G, which together are 

 characterized by abundant lime deposits with 

 minor detrital accumulations. 



The most striking fact about this group of 

 units is the great irregidarity of the ratios of 

 thickness between the units in the two wells 

 (see graphic logs, PI. I, in pocket, and table, 



IS CI. Udden, J. A., and Waite, V. V., op.cit., pp. 16-19. 



p. 17) and yet their close similarity in details 

 of lithologic succession, as brought out both 

 in the percentage logs and in the synthetic 

 logs. Another characteristic is the thinness 

 of some of the members and the rather poor 

 definition of some of their boundaries, espe- 

 cially as regards the development of glau- 

 conite. 



The in-egular ratios of thickness of the units 

 may be explained by differences in the amount of 

 erosion in the two localities before the formation 

 of each next higher unit. The greatest depar- 

 ture from the prevailing ratios between corre- 

 sponding units in the two wells is shown by 

 unit E and may indicate that the unconformity 

 at the top of that unit represents a particu- 

 larly long time interval. This inference is in 

 harmony with the very characteristic devel- 

 opment of the basal glauconite bed of unit D 

 and with the pronoimced distinctness of imit 

 D from the underlying units, as shown in the 

 percentage log. Another possible interpre- 

 tation is difference in the rate of accumulation 

 of the beds in the regions of the two wells. 

 Unit E is mainly limestone and therefore, not 

 being primarily detrital, might have accumu- 

 lated faster in the deeper water of the region 

 of the Rudd well than the combined detrital 

 and calcareous material in the region of the 

 Seaman well. 



The composition of unit E calls for little 

 special discussion. There is close parallelism 

 in the succession of its components in the two 

 wells — at the top a thin layer of purer lime- 

 stone, then a tliin more argillaceous layer, 

 antl finally purer limestone nearly to the liase. 

 In the Seaman well the first typical spicule 

 limestone or shale was noted in the sample 

 from 3,790 to 3,800 feet. 



The first gray limestone in the Seaman well 

 appeal's at the top of unit F. The strong de- 

 velopment of sandy and argillaceous deposi- 

 tion at the base of this unit is brought out in 

 striking parallelism in the two percentage logs. 

 The sample from the base of this unit in 

 the Rudd well contained a small fragment 

 consisting of sand loosely bound by a brown 

 substance, believed to be oil. 



Unit G is most striking on account of its 

 grreat difference in thickness in the two wells, 

 yet its two main lithologic members, the calca- 

 reous one in the lower part and the argillaceous 

 one in the upper part, are represented in both 



