82 Subsurface Geologic Methods 



Color of Sedimentary Rocks 



The color of sedimentary rocks is controlled by the grain size, by the 

 composition of the grain, and by the chemical pigmentation. Colors may 

 be primary or secondary, or both. 



Several proposals have been made in order to standardize colors of 

 sedimentary rocks more adequately. DeFord's statement,^^ "The descrip- 

 tion by geologists of the colors of rock outcrop and rock cuttings from 

 test wells is completely anarchistic: every man for himself," should be 

 carefully considered and recognized as being the truth. What one individ- 

 ual designates as "brick-red" another might consider "orange-red." The 

 terms "tan" and "buff" are frequently used. Tan, to one person, may be 

 a medium-brown to another. 



In order to improve standardization of rock colors among geologists, 

 it is recommended that the rock-color chart prepared by the Rock-Color 

 Chart Committee and published by the National Research Council in 1948 

 be carefully followed. 



Evaporites, carbonates, and some argillaceous sediments, which range 

 from white to light gray, indicate the total or nearly total absence of 

 bituminous and carbonaceous impurities. The dark coloring (dark gray 

 to black) in rocks is invariably due to the presence of organic matter, 

 black iron sulphides, manganiferous constituents, or dark detrital min- 

 erals. Upon weathering, these dark hues may become lighter as a result of 

 leaching. Sediments derived from basic igneous rocks assume dark colora- 

 tion. 



Iron compounds (limonite, hematite) produce yellow, tan, and red 

 hues. The presence of red feldspar in arkoses are largely responsible for 

 reddish and pinkish colorations. 



Greenalite, glauconite, epidote, olivine, chlorite, and ferrous iron 

 compounds are responsible for greenish colors. 



Special notation should be made during the recording of colors as 

 to whether the sediment at the time of recording is wet or dry. Rocks 

 when wet invariably assume darker colors. Mention should also be made 

 as to whether a color represents a weathered or unweathered surface. The 

 latter case is clearly exemplified by the Apishapa shale (Upper Cretace- 

 ous) of Colorado. In outcrop this member assumes a pale orange to a 

 light-buff color. In the subsurface or on fresh exposure it is gray-black. 

 Many similar examples may be cited. 



DiAGENESIS of SEDIMENTARY RoCKS 



Following the deposition of a sediment, certain physical and chemical 

 processes are initiated which tend to adjust the sediment to its environ- 

 ment. These processes are varied and complex, and their relationships 

 are unknown in many instances. Certain modifications of the sediments 



" DeFord, R. K., Rock Colors: Am. Assoc. Petroleum Geologists Bull., vol. 28, no. 1, pp. 128-137, 

 Jan. 1944. 



