LITHOLOGIC CORRELATION IN BEND SERIES, TEXAS. 



21 



Where the percentage composition of the 

 beds above and below the contact of two units 

 is the same, however, it may be necessary to 

 depend on the glauconite bed to mark the 

 boundary. 



Directly observable lithologic peculiarities 

 have generally been found useful for separa- 

 ting only the larger, chronologically most wide- 

 ly separated portions of the section, not for 

 differentiating the lesser units. 



I wish to emphasize that I do not claim that 

 the methods used here will be applicable every- 

 where. In regions of deposits formed very 

 near shore, including much sandy material, 

 larger fluctuations may be so confused by 

 local variations of conditions that it may not 

 be possible to disentangle them, though I be- 

 lieve that the method of study by means of 

 the percentage log is always worth tr\ ing. 



Under these near-shore conditions also 

 glauconite probably does not form, as is indi- 

 cated by its absence at the base of the Strawn 

 in both wells and at the base of unit B in the 

 nearer-shore Seiiman well. On the other hand, 

 as noted aboA^e, a typical glauconite layer was 

 found at the base of a sandy formation, prob- 

 ably the Strawn, near Ilichland Springs, in 

 San Saba County. At least this basal type of 

 formation would be worth looking for in every 

 section until the conditions under which it 

 occurs are better known. 



Glauconitic basal beds mark the contact of 

 the Mississippian "Lower Bend" shale with the 

 Ordovician Ellenbui'ger limestone and the con- 

 tact of the Pennsylvafiian Marble Falls lime- 

 stone with the Mississippian "Lower Bend" 

 limestone. Here they separate units whose 

 distinctness is beyond question. What is then 

 the significance of the units tlefined by other 

 basal glauconite beds 'i- I think these must be 

 accepted as definite and persistent strati- 

 graphic elements, many of which paleontologic 

 evidence has not yet discriminated. Maybe 

 these elements can not be recognized paleonto- 

 logically, but that does not invalidate them, if 

 they are persistent and can be recognized litho- 

 logically. Probably these units are not all of 

 the same order. The determination of their 

 more exact chronologic and genetic significance 

 in an analysis of sedimentary processes such as 

 that outlined by Barrell -^ must await the ac- 

 cumulation of many more facts. Meanwhile, 



« Barrel!, Joseph, r.eol. Soc. America Bull., vol. 28, pp. 776-834, 1917. 



it must be realizeil that faunas and sediments 

 are both complicated responses to complicated 

 contlitions and are to the investigator merely 

 tools the accm'acy of whose product depends 

 on the skill of the hands that use them. Time 

 and environment are two independent factors 

 in the change of faimas. Wlien the time is 

 relatively short and the changes in environment 

 slight the changes in faunas may be slight, 

 though the lithologic change is widespread and 

 distinct. On the other hand similar lithologic 

 facies may be characterized by faunas that can 

 not be chfferentiated, though the times at which 

 the similar facies were deposited may be rather 

 widely separated. Therefore, the conclusions 

 that appear to be indicated by fossils can not, 

 I believe, offhand and without critical analysis, 

 be taken to supersede those derived from the 

 rocks themselves. 



Lithology has an especial advantage in the 

 correlation of well sections, because hthologic 

 material is obtained from the entire, well, but 

 fossils, even micro-fossils, are generally found 

 only at intervals. It is therefore always pos- 

 sible to ascertain a good ileal about the lith- 

 ology and consequently not only to recognize 

 imits but, where distinct basal beds are present, 

 as here, to place the boimdaries of these units 

 with precision within the limits of a single 

 sample. 



Some of the problems awaiting solution by 

 the study of the lithology of well driUings have 

 been referred to in the preceding pages. The 

 most fundamental of these problems is the ex- 

 tension and development of correlation by the 

 work of jietrologists and micropaleontologists. 

 The method and conclusions presented here 

 need to l)e checked, refined, and given greater 

 precision. Other microscopic criteria will doubt- 

 less be developed, and chemical and physical 

 tests may be expected to furnish additional 

 criteria. One of the criteria I hope to take up 

 next is the mineralogy of the units. The possi- 

 bility of finding hoi'izon-marking index minerals, 

 especially among the rarer heavy minerals of 

 sedimentary rocks, has been tested by several 

 investigations. The results have not on the 

 whole been very satisfactory. The great va- 

 riety of minerals present in any sediment and 

 local variation due to currents or independence 

 of drainage areas feeding into a common basin 

 tend to confuse the differences corresponding 

 to differences in age. However, as index min- 



