Abstracts Earth and Environmental Sciences

Dolomitized Pahasapa Limestone (Mississippian), northeastern sector of the Black Hills, South Dakota, petrography and geological setting

by Abdullatif Najjar

"The carbonates of the Mississippian Pahasapa Limestone in the northeastern sector of the Black Hills outcrops were examined in order to ascertain specific characters of the lithology in this area where the formation is predominantly a dolomite in contrast to adjacent areas where it is mainly a limestone. The Pahasapa seems to be uniform in thickness at the study areas and maintains a thickness of about 550 feet. Because of poor exposures, usually at the base or top, complete sections could not be measured. Much of the Pahasapa consists of coarsely crystalline dolomite containing varying proportions of crinoid plates, gastropods, bryozoans, corals, and ostracodes, all dolomitized to varying degrees. Eight rock types are recognized in the Pahasapa carbonates in the study areas. These rock types are: 1. Dolosparite 2. Biodolosparite 3. Crinoidal biodolosparite 4. Biodolomicrite 5. Dolomicrite 6. Sparite 7. Intrasparite 8. Dolointramicrite. These rock names were constructed from a modified Folk terminology and are based on semi-quantitative analysis of about 200 thin sections of rock samples. Bar diagrams were drawn alongside the stratigraphic sections which proved to be useful in comparing rock types between different outcrops. However, there was sufficient variability in the constituents from section to section so that correlations based on the bar diagrams were not feasible. The original Pahasapa sediment is inferred to have consisted of inorganic calcitic ooze with considerable to dominant amounts of calcitic organic debris resulting from the disintegration and abrasion of hard parts of organisms that lived in shallow and relatively clear water. Abundant bedding planes and cross-bedding attest to relatively high energy conditions. The process by which and the time at which these sediments became dolomitized is not well understood, as is the case with other dolomitized limestones. The uniformity and the widespread distribution of the dolomite in the area of study is used to support the contention that the change to dolomite took place early in the history of the sediment, i.e., it was essentially penecontemporaneous with the sediment" – Abstract, pages ii-iii.