LITHOLOGIC CORRELATION IN ' BEND SERIES, TEXAS. 



9 



Although these two distinct types of shale are 

 characteristic of units B and C, neither of them is 

 absolutely restricted to its respective unit. Each 

 type is found in smaller amounts in the other unit. 



In the Rudd well the microscopic differences 

 between the shales of units B and C' are very 

 slight. Sand is not at all abundant in unit B; 

 in the thin sections of many chips there is none. 

 The groundmass of the shales of both units 

 appears equally flocculent. The lime in the 

 shales of unit B is coarsely granular, distinctly 

 not the fine dust that is characteristic of the 

 shale of unit B in the Seaman well. Neverthe- 

 less, it is not on the average as coarse as the 

 lime in the directly underlying shales of unit V 

 in the Rudd well, in which a few recognizable 

 coarse fossil fragments are inchuled. Under 

 the hand lens the difference between samples 

 from adjacent parts of the two units is corre- 

 spondingly slight. These observations on ma- 

 terial from the Rudd well are in agreement with 

 the evidence of continuity between the two 

 imits in the Seaman well. 



DISTINCTION BETWEEN BLACK SHALE AND 

 BLACK LIMESTONE. 



From what has been said above of the grada- 

 tions between black shale and black limestome 

 it follows, as pointed out by many previous 

 workers,''' that the distinction between black 

 shale and black limestone is more or less arbi- 

 trary. In my work I have differentiated them 

 mainly by the appearance of the chips. Angular 

 chips mth sharp edges, more or less concave 

 surface, and generally harder appearance I 

 have called limestone. The flatter, more 

 rounded, and more regular chips I have called 

 shale. WTiere the appearance of the thin sec- 

 tion seemed definitely opposed to the classifi- 

 cation based on the appearance of the chip I 

 have followed the indications of the thin section. 

 The opinion of the driller, who in addition to 

 seeing the chips knows how the bed drilleil, may 

 often be more reliable than an opinion deduced 

 merely from an examination by eye. On the 

 other hand, most drillers appear inclined to gen- 

 eralize the character of a considerable succes- 

 sion of materials. In the synthetic log I have 

 incorporated only my o^xn conclusions, but the 

 facts above set forth should be kept in mind 

 when the synthetic logs are examined and com- 

 pared with the driller's log. 



1* See Am. Assoc, Petr. Geologists Bull., vol. 3, passim, 1919. 



UNITS C AND C 



In the Seaman well unit (' is far from coher- 

 ent. In the percentage log it appears very 

 iri-egular. The only generalization that sug- 

 gests itself is that this unit is distinctly calca- 

 reous in the lower part, containing very little 

 sand, but grows sandy toward the top, appar- 

 ently approaching, more or less by oscillations, 

 the rise of base-level that separates it from 

 unit B. There is more room for considerable 

 diflerence of opinion about the homogeneity 

 of this unit in the Seaman well than about that 

 of any other unit in the section. If it is to be 

 subdivided, the dividing line would be drawn 

 at 3,320 feet, separating a distinctly^ sandy and 

 shaly unit above from a succession of black 

 limes' ones and shales below. To me these two 

 units have not appeared sufficiently distinct, 

 and no glauconite bed was found to indicate 

 a boundary. The correlation with the Rudd 

 well supports this view. 



In the Rudd well unit C is very regular and 

 coherent, although it has five sandy zones which 

 may correspond to the five maxima of sand in 

 the Seaman well. If, in accordance with this 

 correlation, the sand between 2,440 and 2,450 

 feet in the Rudd well corresponds to the sand 

 between 3,440 and 3,450 feet in the Seaman 

 well, the ratio between the part of the section 

 below this sand in the Rudd well compared 

 with the same part of the section in the Seaman 

 well is slightly less than the ratio between 

 the combined units C and C in the two 

 wells. Thus : 



In the Rudd well 2,445 to 2,525 = 80 feet. 



In the Seaman well 3,445 to 3,640= 195 feet. 



The ratio is therefore 80 : 195= 1 : 2.44. 

 The combined thickness of units C and C is: 



In the Rudd well 2,245 to 2,525 = 280 feet. 



In the Seaman well 3,045 to 3,640 = 595 feet. 



The ratio is therefore 280 : 595=1 : 2.12. 



This slight difference may be accounted for 

 by the greater sandiness, implying more rapid 

 deposition, in the upper part of unit C in the 

 Seaman well, as noted above. 



In the upper part of this unit there is in both 

 wells a sandstone in the midst of shales — in 

 the Seaman well between 3,180 and 3,190 feet 

 and in the Rudd well between 2,290 and 2,300 

 feet. In both wells it is largely dark gray, 

 medium grained, and more or less calcareous. 

 In the Seaman well there is, in addition, a very 

 little coarse white sandstone. Similar calcu- 



