Stratigraphy and Depositional Environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the Type Area, Western Kansas
Authors: Hattin Donald E. 
Published: January 2025

Strata of the Smoky Hill Chalk Member (Coniacian-Campanian) are exposed extensively in the type area, where 12 key sections are the basis for a composite section that is 181.8 m (596.3 ft) thick. The member consists mainly of olive gray, well-laminated to nonlaminated, flaky-weathering, fecal-pellet-speckled, impure chalk consisting mostly of foraminiferal pelmicrite with wackestone or, less commonly, packstone texture, and characterized by well-stratified grain fabric. Ubiquitous constituents include micritic matrix, coccolith-rich fecal pellets, planktonic foraminiferal tests, wisps and angular silt-sized grains of black organic matter, skeletal remains of fish, and minute framboids of pyrite or its oxidized equivalent. Sparse grains of angular silt-sized quartz are the only detrital grains detected commonly in thin sections. Scattered through the member are very thin to thick beds of lighter-colored, bioturbated chalk and granular (probably microbioturbated) chalk, which form conspicuous bands on little-weathered exposures. Bioturbated and granular chalks contain the same basic components as stratified chalks, but fecal pellets are less obvious, black organic matter is less abundant, and pyrite framboids are less common in the matrix.
The Smoky Hill composite section contains more than 100 seams of bentonite, which range in thickness to as much as 11.3 cm (0.37 ft). Gypsum, jarosite, and limonite, usually in some combination, are common along weathered seams. Principal clay minerals in the bentonites are kaolinite (dominant) and smectite, and the most common accessory minerals are quartz, gypsum, and calcite.
Diagenetic phenomena of Smoky Hill laminated to nonlaminated chalks include compactional deformation of fecal pellets, foraminifers, sediment-filled burrows, macroinvertebrate remains, and grain fabric around large allochems; incipient microstylolites; dissolution of aragonitic skeletal remains; sparry calcite cement in foraminiferal chambers; interstitial micritic calcite cement; and secondary calcite overgrowths on skeletal remains, especially in matrix portions of the rock. Bioturbated and granular chalks are less well compacted than the well-stratified chalks, and have greater amounts of secondary calcite as interstitial cement and overgrowths on coccoliths. In such chalks, lithification was initiated earlier than in the well-stratified chalks. Bioturbation of these and other Kansas chalk deposits produced textures similar to those ascribed by others to deep-burial diagenesis and solution transfer. Most extensively altered by diagenesis are lenses of Uintacrinus limestone, in which skeletal elements have been altered to microsparite and the rock has been microstylolitized.
Several stratigraphic intervals, none more than about 1.5 m (4.9 ft) in thickness, are exceptionally rich in macroinvertebrate skeletal remains, which litter eroded slopes with shelly debris. Lenses of hard, brittle limestone, consisting mainly of Uintacrinus skeletal remains, occur sparingly in the zone of Clioscaphites choteauensis. Sparse biomicrudite lenses, consisting mainly of inoceramid bivalve debris, occur locally in the lower half of the member.
Bioturbated chalk beds, granular chalk beds, bentonite seams, shelly zones, and organic-rich chalk beds are useful stratigraphic markers. Twenty-three marker units are described in detail, and are indispensable tools for precise determination of stratigraphic position. The Smoky Hill Member contains vast numbers of macroinvertebrate body fossils, but at most horizons diversity is very low. Ammonites, which are the basis for the standard Western Interior zonation of Smoky Hill and equivalent strata elsewhere, are sparse in all but a few intervals, and many of the standard zonal indices have not been recorded in Kansas. The lower half of the composite section contains four easily recognizable zones, namely those of Inoceramus (Volviceramus) grandis, I. (Cladoceramus) undulatoplicatus, Clioscaphites vermiformis, and C. choteauensis. The upper half of the member contains the single, broadly defined zone of Inoceramus (Endocostea) balticus. On the basis of macroinvertebrate fossils and physical correlation (i.e., marker beds) the composite section has been determined to extend from the Upper Coniacian into the Lower Campanian.
Coccolith-rich Smoky Hill muds were deposited on the eastern shelf of the Western Interior Sea. Chalky strata of Kansas grade westward into progressively less calcareous beds, which in the western portion of the ancient seaway are dominated by terrigenous detritus. Stratigraphically upward variations in detrital components reflect varying rates of terrigenous detrital influx from the west. However, the principal component of most Smoky Hill strata is low-magnesium calcite, which consists largely of coccoliths, coccolith debris, coccolith-rich fecal pellets, tests of planktonic foraminifers, interstitial cement, calcite overgrowths on coccoliths, forams and other skeletal remains, and remains of inoceramid and ostreid bivalves.
On the basis of biotic and physical considerations, depth of Smoky Hill deposition is estimated to have ranged from a minimum of 150 or 200 m (492-656 ft) to a maximum that probably exceeded 300 m (984 ft). What is now Kansas lay in a warm- or mild-temperate climatic zone. During deposition, Smoky Hill muds were mainly soft, perhaps even soupy oozes. Evidence of bottom currents is minimal, and bottom waters were at most times poorly circulated. Although interstitial conditions were largely anoxic, near absence of infaunal suspension feeders was apparently owing mainly to substrate fluidity. Wide lateral persistence of marker beds, coupled with absence of organically constructed banks, scour channels, or wedge-shaped stratigraphic units, is evidence for almost perfectly flat depositional topography. Based on data deriving from remains of pelagic organisms (e.g., low diversity of coccoliths and planktonic foraminifers and paucity of crinoids and ammonoids) the salinity is judged to have been somewhat lower than normal.
Inoceramids, rudists, and conchs of dead cephalopods were the principal substrates for other benthic organisms, such as ostreid bivalves, acrothoracican cirripeds, lepadomorph cirripeds, and clionid sponges. Inoceramids were nearly ubiquitous inhabitants of the sea floor, and manifest a variety of growth forms that reflect adaptation to soft substrates. The numerically predominant ostreid bivalves, represented by as many as four generations on a single host, commonly encrusted all available substrate area, and apparently grew even on the undersides of host inoceramids and rudists. Deposit-feeding organisms, lacking or very sparse in well-stratified chalks, produced bioturbated intervals whenever suitable substrates (i.e., purer chalk beds) were developed. Short intervals of increased circulation fostered proliferation of the macro invertebrate epibenthos, and produced the present shelly zones in which biotic diversity is greater than normal. Nonfragmented epibenthic macroinvertebrates are preserved in life position, bivalve articulation ratios are high, and assemblages have remarkably uniform composition. These features suggest that Smoky Hill assemblages are true fossil communities. Except for infaunal deposit-feeding worms(?), Smoky Hill benthic macro invertebrates were exclusively suspension feeders. Exceedingly soft substrates, possibly combined with marginally oxygenated bottom waters, seem the best explanation for absence of infaunal deposit feeders other than worms(?). Slow depositional rates (0.036 mm per year) help to account for the gigantic size of many Smoky Hill inoceramids and heavy encrustation of substrates by epizoans.
The Smoky Hill Member contains significant reserves of biogenic gas, which are being exploited currently in northwesternmost Kansas and adjacent parts of the bordering states. Knowledge of Smoky Hill stratigraphy, petrology, diagenesis, and correlation should enhance exploration for this valuable energy resource.