LITHOLOGIC CORRELATION IN BEND SERIES, TEXAS. 



would not conform in detail to this assumption. 

 In a general wa}-, of course, more lime accumu- 

 lated at certain periods than at others, and in 

 the "Bend series" this fact is most obviously 

 indicated by the subdivision of the series into 

 the Smithwick shale and Marble Falls lime- 

 stone, though it expresses itself also in detail 

 throughout the series. But the acciunulation 

 of lime is much more subject to local conditions 

 than that of sand or clay. Furthermore, lime 

 is not related to depth as simply and regularly 

 as sand and shale. Though there used to be a 

 tendency to assume that limestone represented 

 a deposit in deep, clear water, geologists are 

 now beginning to realize that this assumption 

 applies only to certain kinds of limestone and 

 to certain regions. Most of the mollusks, mol- 

 luscoids, and echinoids whose remains form the 

 largest part of our limestones lived in water 

 shallow enough to give them their needed light 

 and warmth, though not so near the shore and 

 especially not so near the mouths of rivers that 

 they would be injured by the accumulation of 

 sediment. If a generalization must be made 

 concerning the relation of lime of organic 

 origin to depth it would probably be truer to 

 say that on gently sloping offshore sea bot- 

 toms limestones are formed between near-shore 

 detrital deposits, which in many areas include 

 clay, and the deep-sea terrigenous clay depos- 

 its, which are fairly well represented l^y the 

 "blue muds" of the Challenger expedition. 

 But this generalization also is subject to many 

 modifications to bring it into accord with local 

 conditions. What is essential for the present 

 purpose is to realize that lime is not likely to 

 vary as regularly from well to well as the true 

 sediments, clay and sand, and that on tliis 

 account logs representing the proportion of 

 lime will require critical interpretation. The 

 ideal correlation would be one based only on 

 the exact proportions of clay and sand. 



The results of my determinations, in addi- 

 tion to being summed up in the percentage log, 

 are represented also in columns 2 and .5 of 

 Plate I (in pocket ) , in terms of definite litho- 

 logic types, in what I have called, for conven- 

 ience of reference, a "synthetic log." In this 

 log the materials found in each sample are, as 

 far as possible, represented by the usual sym- 

 bols. It was of course necessary to generalize 

 very much m prepaiing this log. Obviously, 

 moreover, there is no sure way of determining 



the thickness or relative positions, witliin the 

 interval represented by the samples, of the 

 different types recognized. Where a boundary 

 between two distinct types lies in the interval 

 represented by a sample and such a boundary 

 has been recorded in the driller's log the posi- 

 tion given by the driller's log has therefore 

 usually been accepted. In order to bring out 

 some of the more significant thui beds it has 

 been necessary to exaggerate their probable 

 thickness. Finally in column 1 on Plate I (in 

 pocket) , is given the usual gi-aphic form of the 

 driller's log of the Seaman well. No driller's 

 log of the Rudd well was available. 



The correlation of the percentage logs was 

 aided greatly by the discovery, made in the 

 com-se of the work, that the more significant 

 breaks in a stratigraphic series are likely to be 

 marked by the occurrence of autochthonous 

 glauconite, in many instances associated with 

 phosphate, directly above the break or rarely 

 more than a foot or two above it. By autoch- 

 thonous glauconite I mean glauconite formed 

 in place contemporaneously with the bed in 

 which it occurs. The suggestion that glauco- 

 nite occurs in this association ^ was the result 

 of observations in San Saba County, Tex. A 

 thin sand that contains nodules of phosphate 

 and is full of glauconite was found not more 

 than a foot or two aliove the Ellenburger lime- 

 stone at several places southwest of San Saba. 

 .\ little west of Richland Springs, at the contact 

 between a sandy formation believed to be the 

 Strawn and a limestone believed to be the 

 equivalent of one of the hmestones occurring 

 at Dennis, south of Millsap, in the "Millsap 

 division" as defined by Cummins, a similar 

 very glauconitic sand containing nodules 

 of phosphate was found. To establish the 

 validity of the generalization that glau- 

 conite occurs just above stratigraphic breaks 

 it may be stated in advance that each of 

 the samples in the collections of the United 

 States Geological Survey taken from the base 

 of the "Bend series" in three wells that showed 

 this horizon and that were rather evenly spaced 

 over a distance of 120 miles north of the outcrop 

 contained coarse autochthonous glauconite. 

 As will be seen when the sTOthetic logs are dis- 

 cussed (see PI. I), glauconite occurs at horizons 

 other than breaks in the stratigraphic succes- 

 sion, but at nearly all these horizons it differs in 



3 Ooldmau, M. I., Washingtou Acad. Sci. Jour., vol. 9, p. 502, 1919. 



