FIELD STUDIES OF CONCRETIONS IN SHALE 379 



CONCRETIONS IN THE CRETACEOUS (PIERRE) SHALES IN SOUTH DAKOTA 



AND WYOMING 



Concretions occur in the Pierre shale in tlie same region mentioned 

 above for the Graneros and Carlisle. Although not so common, when 

 they do occur it is always along one bedding plane. This distribution 

 along one plane is well shown in an illustration in an article on the con- 

 cretions of the Pierre shale by Carrie A. Barbour.^- They are extremely 

 ferruginous, calcareous concretions and are always found in dark shales. 



CONCRETIONS IN THE CRETACEOUS (CLAGGETT) SHALES IN HILL COUNTY, 

 MONTANA, AND SOUTHERN SASKATCHEWAN, CANADA 



Scattered calcareous concretions occur in the black Claggett shales in 

 northern Montana and, southern Saskatchewan. Although they are not 

 numerous, they are found along one plane and are usually elliptical and 

 small, less than a foot in diameter. Nothing could be determined as to 

 their relationship to the bedding planes, beyond the fact that different 

 lines of concretions followed one plane. 



Origin of the Concretions 



The significant points noted above are (1) concretions in shales are 

 dominantly along planes which are variously spaced vertically; (2) the 

 bedding planes curve around the concretions in every case where this 

 relationship was determinable; (3) in at least one instance the concretion 

 contained bedding planes not continuous with the surrounding planes 

 which curved around the concretion; and (4) they may show slickensides. 



These points are believed to indicate a syngenetic origin for the con- 

 cretions for the following reasons : The recurrence of planes of concre- 

 tions, without isolated concretions between, points to a varying source of 

 material in the water in which the sediments were being deposited. 

 When the concretions are mainly calcium carbonate, as they are in the 

 Carlisle, Pierre, and Claggett shales, deposition occurred when the lime 

 salts in the water readied the saturation point. Since the concretions 

 are distributed along each plane, it is readily seen that had this amount 

 of calcium carbonate been greater, a thin bed of limestone would have 

 resulted. Geologists have long accepted without question the fact that 

 alternating beds of shale and limestone are due to differences in the rate 

 of supply of calcium carbonate and of its reaching the saturation point. 

 Why the calcium carbonate should take the concretionary form instead 



^- Carrie A. Barbour : Observations on the concretions of the Pierre Shale. Proc. Neb. 

 Acad. Sci., vol. 7, 1897, pp. 36-38. 



