14 



SHORTER CONTRIBUTION'S TO GENERAL. GEOLOGY, 1921. 



stones. In thin section the limestone is much 

 purer than most beds of the Marble Falls 

 limestone and rather evenly crystalline, witii 

 hardly any traces of fossils, but in nearly all 

 fragments of limestones in overlying units at 

 least traces, of fossils are recognizalile. The 

 shales of units K and L are also very distinc- 

 tive, and those of unit L are of the same general 

 character in both wells, but the Seaman well 

 shows no siialcs in unit K. In the solid they 

 have generally a dull brownish-black appear- 

 ance; in thin section they have a peculiar red- 

 dish-brown rusty color and contain an evenly dis- 

 seminated meal of more or less fine angular sand 

 grains in a vaguely granular argillaceous matrix. 

 A third characteristic type of material looks 

 in the solid hard and compact like limestone 

 but in thin section is seen to consist almost en- 

 tirely of coarse rhombs of a colorless carbonate 

 m a dense ai-gillaceous matrix. This material 

 was found in vmit K in both wells and in the 

 glauconitic bed shown between 3,042 and 3,048 

 feet in unit L in the Rudd well. Very probably 

 it is (lolomitic. I hope later to study it further. 

 Black shales or limestones with more or less of 

 rhombic carbonate in them are not restricted 

 to unit K. They are rather common in unit J, 

 and isolated occurrences were noted in material 

 from 2,400 to 2,410 feet in the Rudd well and in 

 the upper part of unit D in the same well. But 

 in these higher positions the rhombs are usually 

 smaller and not so closely packed. 



As may bo seen in the sjaithetic logs, unit K 

 is sepai-ated from unit L in both wells by a bed 

 of very phospliatic limestone in which phos- 

 phate spherules and nodules and phosphatized 

 calcareous skeletons, including many echinoid 

 fragments, occur rather closely crowded in a 

 crystalline calcareous matrix. This is a typical 

 phospliatic contact bed in which no glauconite 

 was foun<l. Evidently there are different con- 

 ditions at stratigraphic breaks which make 

 glauconite predominate at some and phosphate 

 at otiiers. In general, the conditions that pre- 

 vailed during the deposition of units K anil L 

 seem to liave been more favorable to the forma- 

 tion of phosphate. 



Unit L is composed almost entirely of a very 

 uniform succession of shale of the type de- 

 scribed al)Ove, which in the Rudd well seems 

 to be ratlicr phosphatic, the phosphate occur- 

 ing as small brown sphenih^s around 0.2 milli- 

 meter in diameter, very slightly different in 



color in thin section from the shale containmg 

 them. 



At 3,042 to 3,04S feet in unit L in the Rudd 

 weU occurs an exceptionally characteristically 

 developed glauconite and phosphate bed of the 

 contact type. This bed does not, however, 

 appear to separate any distinguishable iniits. 

 It is probably merely an indication that unit L, 

 like miits H and I adjacent to their contact, 

 was formed under conditions approaching those 

 favorable for the formation of accumulations of 

 phosphate and glauconite. so that oidy a slight 

 shift of base-level was necessary to bring on the 

 favorable conditions. By its associations and 

 in its appearance this bed therefore represents 

 merely a renewal of the conditions that formed 

 the basal bed of this unit. In the Seaman well 

 air equivalent bed may occur in the interval 

 between 4,420 and 4,470 feet not represented 

 by samples. 



The well-developed and horizontally exten- 

 sive glauconite bed that marks the base of the 

 "Bend series," which is here the base of unit L, 

 resting on the EUenburger (Ordovician) lime- 

 stone, was mentioned in the introduction. The 

 contact facies is evidently very thin. In the 

 Rudd well only a small amoimt of coarse sand- 

 stone and coarsely glauconitic limestone with 

 some phosphate spheniles was found. A pecu- 

 liarity of the sandstone is that most of the grains 

 were shattered into two to four fragments only 

 slightly displaced and subsequently recemented 

 with what is probably opal. Probably a related 

 phenomenon is the pronounced elongation 

 and straining parallel to the elongation, slight 

 shattering, and penetration of glauconite by 

 sand from the surroTuiding matrix seen in a 

 fragment of the basal glauconitic shale from 

 4,490 to 4, .510 feet in the Seaman well. Does 

 this indicate movement along this contact 

 plane, periiaps as a result of folding? Shat- 

 tering, api)arently less violent, was noted in the 

 basal sandstone of unit H in the Seaman well. 

 (See above, p. 13.) In the Seaman well the 

 amount of the basal glauconitic material is even 

 less than in the Rudd well, so that is hard to 

 find in the sample. A peculiar feature of the 

 contact zone in the Seaman well is that the lied 

 of shale which carries the coarse glauconite was 

 foimd only in the sample from 4,490 to 4..olO 

 feet, though the driDer places the top of the 

 EUenlmrger !) feet below the base of that inter- 

 val, and more than hah of the sample from 



