Petrology of Smoky Hill Member, Niobrara Chalk (Upper Cretaceous), in Type Area, Western Kansas

Abstract

The Smoky Hill Member of the Niobrara Chalk is composed primarily of olive-gray, well-laminated to nonlaminated impure chalk, which is mostly foraminiferal pelmicrite with packstone or wackestone texture. The rock is characterized by well-stratified grain fabric. Ubiquitous constituents include coccolith-rich fecal pellets, mostly well-preserved tests of planktonic forams, wisps and silt-sized grains of black organic matter, fish bones and scales, and minute framboids of pyrite or its oxidized equivalent. Inorganic detritus is common in all samples, but only sparse grains of angular quartz silt are detected commonly in thin sections. Scattered through the member are thick to very thin beds of lighter colored, bioturbated (probably microbioturbated), and granular chalk. These are mostly foraminiferal and pelletal micritic wackestones, which lack well-stratified grain fabric; in thin sections, fecal pellets are less obvious than in well-stratified chalks, organic matter is much less common, and pyrite framboids are less common in the matrix. Diagenetic features of Smoky Hill stratified chalks include compactional deformation of fecal pellets, forams, burrow structures, and macroinvertebrate remains; dissolution of aragonitic skeletal material; incipient microstylolites; sparry calcite cement in foram chambers; interstitial calcite cement, and secondary calcite overgrowths on coccoliths, especially in the matrix. Bioturbated and granular beds are less well compacted than the stratified chalks and have greater amounts of secondary calcite as interstitial cement and overgrowths on coccoliths. In these chalks lithification was initiated earlier than in the stratified chalks. Bioturbation of these and related Kansas chalk deposits produced textures similar to those ascribed by others to deep-burial diagenesis and extensive sol tion transfer. Thin-section and scanning-electron-microscope (SEM) data suggest that most reduction of original porosity to the present average of 38% was accomplished by gravitational compaction. The remainder of porosity reduction resulted from the limited addition of secondary calcite, which was derived mainly from dissolution of abundant aragonitic skeletal remains and in small part from pressure solution. The Smoky Hill chalk is a pelagic, deeper water outer shelf deposit, which was laid down mostly as a soft, perhaps mostly soupy, coccolith-rich ooze on a nearly flat sea floor under conditions of normal or nearly normal salinity and generally poor bottom-water circulation. At irregularly spaced intervals, deposition of purer ooze furnished substrates suitable for a burrowing endobenthos, which bioturbated sediments and helped to initiate earlier lithification than in nonbioturbated deposits.