- The Greenhorn Limestone of Kansas is part of a widespread chalky carbonate unit that is similar lithologically, and related genetically, to the overlying Fairport Member, Carlile Shale. The Greenhorn ranges in thickness from 68.5 to 135.9 feet, averaging 94.8 feet for 11 measured sections. Thinnest in Mitchell County, the formation nearly doubles in thickness to the southwest.
- Shaly chalk, a more or less laminated, impure, olive gray to olive black rock comprises most of the Greenhorn Limestone. The basal Lincoln Member is characterized by abundant beds and lenses of skeletal grainstone and by numerous thin to very thin seams of bentonite. The overlying Hartland Member contains little skeletal limestone, three regionally traceable beds of burrow-mottled chalky limestone, four equally widespread seams of bentonite, and a few beds of soft, usually burrow-mottled chalk. Above the Hartland Member the section is characterized by an abundance of chalky limestone. The Jetmore Member contains 13 regionally traceable beds of mostly burrow-mottled chalky limestone that are separated by thicker intervals of shaly chalk. A widespread bentonite seam lies on the twelfth limestone bed. The uppermost bed of limestone, called the Shellrock limestone bed, is usually concretionary in its upper part and is crowded with Inoceramus valves in the lower part. Shaly chalk directly beneath the Shellrock bed contains beds of lenses of concretionary chalky limestone. The Pfeifer Member is characterized by numerous irregular, discontinuous, shell-rich beds of concretionary chalky limestone and, especially in the upper half, by oblate spheroidal concretions of this rock. Widespread Pfeifer marker beds include a thin bed of chalky limestone lying an average of 6.1 feet above the base, a thin bentonite seam that is overlain in most of central Kansas by a bed of granular calcite known as the sugar sand, and the Fencepost limestone bed which lies at the top of the formation.
- Across most of central Kansas the Greenhorn Limestone lies disconformably on the Graneros Shale; elsewhere the two formations are apparently conformable and lithologically gradational. The Graneros-Greenhorn contact is diachronous, ascending stratigraphically in a northeastward direction. The Lincoln-Hartland contact also is diachronous such that most of the type Hartland, and lower Hartland of Ford County, pass northeastward into the Lincoln Member. In western Kansas the Bridge Creek Member includes strata equivalent to the Pfeifer, Jetmore, and most of the Hartland of central Kansas. By northeastward disappearance of most chalky limestone beds the lower Bridge Creek passes into the main part of the central Kansas Hartland. The Jetmore and Pfeifer Members of central Kansas are readily recognizable as being equivalent to the middle and upper parts of the type Bridge Creek.
- Fossils are abundant throughout the Greenhorn, but with few exceptions diversity of macroinvertebrates is low in most beds. A sequence of assemblage zones is recognized including (ascending): Acanthoceras wyomingense Assemblage Zone, known in the Greenhorn only in Ford and Kearny counties; Calycoceras? canitaurinum-Exogyra aff. E. boveyensis Assemblage Zone, occurring in the upper part of the Lincoln in Ford and Kearny counties and in the lower Lincoln of most central Kansas localities; an undesignated assemblage zone, occurring in the type Hartland, in the lower half of the Hartland of Ford County, and in the middle and upper Lincoln and lower few feet of the Hartland across most of central Kansas; the Sciponoceras gracile Assemblage Zone, occurring in the lower few feet of the type Bridge Creek, near the middle of the Hartland of Ford and southern Hodgeman counties, and in the lower half of the Hartland in most central Kansas localities; a second undesignated assemblage zone, occurring near the base of the type Bridge Creek and in the upper part of the central Kansas Hartland; the Mytiloides labiatus-Watinoceras reesidei Assemblage Zone, extending from near the top of the Hartland and an equivalent part of the Bridge Creek to the top of Jetmore marker bed JT-6; the M. labiatus-Mammites nodosoides wingi Assemblage Zone, extending from Jetmore marker bed JT-6 to the top of the Jetmore Member; a third undesignated assemblage zone, occupying the lower few feet of the Pfeifer Member and an equivalent part of the type Bridge Creek; and the M. labiatus-Collignoniceras woollgari-Inoceramus cuvieri Assemblage Zone, extending from the base of marker bed PF-1 through the lower part of the Fairport Member, Carlile Shale. Greenhorn assemblage zones up to and including that of S. gracile are of Late Cenomanian age; above this zone the Greenhorn is of Early Turonian age.
- The Greenhorn limestone is composed of three major kinds of carbonate rock including skeletal limestones, laminated impure shaly chalk, and chalky limestone. Skeletal limestones, including biosparites and biosparrudites, are most abundant in the Lincoln Member and are common also in the upper half of the Jetmore member and lower one-half to two-thirds of the Pfeifer Member. The principal skeletal grain types are inoceramid debris and tests of planktonic foraminifera but calcispheres predominate locally. The skeletal limestones are mostly cemented by sparry calcite, and qualify as grainstones, but where incompletely washed such rocks include some micritic matrix. Shaly chalk is a poorly consolidated carbonate ooze composed primarily of coccolithophore skeletal debris. The main allochemical grains are inoceramid debris, tests of planktonic foraminifera, and fecal pellets. The last everywhere manifest the effects of sediment compaction. Organic matter, pyrite, and very fine grained terrigenous detritus are common in most shaly chalks. Chalky limestone includes rocks having micritic or microsparitic matrix and allochemical grains consisting of inoceramid bivalve debris, tests of planktonic foraminifera, fecal pellets and calcispheres. Soft beds of nonlaminated chalk have a predominantly micritic matrix. Depositional texture in the chalky limestones ranges from mudstone to packstone, with wackestones or wackestone-packstone transitional rocks predominating. Depending on allochem content, chalky limestone and chalk beds are classifiable mostly as biomicrite or biomicrosparite, biopelmicrite or biopelmicrosparite, and fossiliferous micrite or fossiliferous microsparite.
- Greenhorn shaly chalk beds have been little altered by diagenesis. An inhospitable interstitial environment largely prevented development of a burrowing infauna so original stratification is well preserved. Compaction flattened most fecal pellets so they are now largely fusiform in vertical section; bivalves and sparse ammonites were also flattened. Diagenetic loss of skeletal aragonite, especially from bivalves and ammonites, may have furnished the CaCO3 that now fills chambers of foraminifer tests. Pressure solution of skeletal grains was a possible source of late diagenetic cement. Diagenesis of skeletal grainstones consisted largely of cementation by void-filling sparry calcite cement. Sources of cement may have included interstitial pore water (early diagenesis), dissolution of aragonitic skeletal material (early to late? diagenesis), and pressure solution (late diagenesis). Large size of most crystals of sparry calcite, and general absence of centripetal increase in crystal size of void-filling calcite suggests slow development of sparry cement from relatively few growth centers, but present morphology of cement crystals may reflect diagenetic inversion to low-magnesium calcite. Chalky limestone and chalk beds have a generally homogeneous texture and most are extensively burrow mottled or have an isotropic fabric that suggests bioturbation. Exceptions are beds of soft, secondary granular chalk associated with bentonite seams, and laminae that are preserved locally in concretions or irregular, concretionary beds of chalky limestone. Burrow-mottled chalk beds usually have a micritic matrix and represent incompletely developed beds of chalky limestone. In these the original matrix of coccolith-rich ooze has been partially cemented but only partly neomorphosed. The harder, usually thicker, chalky limestone beds have been largely converted to microsparite, a process that has obliterated many of the original nannoplankton skeletons. Textural differences associated with burrow structures suggest that burrowing organisms were a factor in facilitating neomorphism; bioturbation also may have helped bring into the sediment fresh supplies of seawater rich in CaCO3 for early diagenetic cement. Early diagenetic dissolution of skeletal aragonite is implied by almost complete absence of void space where these skeletons lay. Early lithification of chalky limestone beds is demonstrated in beds having in-the-round or little-compacted specimens of macroinvertebrates and fecal pellets. The best-cemented rocks (Hartland marker bed HL-2, Jetmore marker beds JT-1, JT-6, and JT-10 to 12) also contain the greatest concentrations of fossils that had at least partially aragonitic skeletons.
- The Greenhorn Limestone was deposited on a broad, flat, gently subsiding cratonic shelf that lay along the eastern side of the Western Interior Sea. The western side of the seaway was a deeply subsiding trough in which Late Cretaceous deposits locally reach thicknesses of nearly 20,000 feet. During Late Cretaceous time the interaction of subsidence and sediment supply from the Sevier orogenic belt imposed a broad pattern of cyclicity on deposits in and along the western margin of the seaway.
- The Greenhorn Limestone represents deposition during the first Late Cretaceous transgression. The sea spread eastward or northeastward from Colorado, reaching Kansas in Late Cenomanian time. From the upper part of the Dakota Formation through the Greenhorn Limestone the vertical succession of lithofacies manifests upward change from fluviatile and marginal marine environments (Dakota), through open sea, brackish to normal or nearly normal marine environments in which very-fine-grained terrigenous mud was deposited (Graneros Shale), and culmination ultimately in far offshore carbonate-mud producing environments representing maximum transgression (Greenhorn Limestone).
- Nearly everywhere in Kansas, Greenhorn deposition began with deposition of skeletal sands, silts, and conglomerates in a high-energy, far offshore zone of wave and/or current impingement on the sea floor. As the transgression approached its peak, and the sea floor reached its greatest depth, energy levels at the sediment-water interface decreased to minimum levels that were interrupted occasionally by impingement on the sea floor of weak to moderate currents. These produced zones of poorly to well-washed skeletal silt and sand. Shaly chalk beds reflect deposition under low-energy conditions on a nearly stagnant sea bottom that supported few benthonic macroinvertebrates other than inoceramid bivalves. Chalky limestones and nonlaminated chalk beds represent periods of slow sedimentation during which organic matter could not accumulate in sufficient quantities to produce interstitial reducing conditions. During most such times sediments were extensively bioturbated, particularly in the Hartland and Jetmore Members and equivalent parts of the Bridge Creek Member. Coincidence of reduced rates of sedimentation and improved circulation of bottom waters produced chalky limestones that are exceptionally rich in benthonic macroinvertebrates, including especially marker beds HL-2, JT-10 to JT-12, part of the Shellrock limestone bed, and many irregular, more or less concretionary beds of chalky limestone in the lower half to two thirds of the Pfeifer Member. In the upper Jetmore and through much of the Pfeifer, diagenetic segregation of calcium carbonate produced many early diagenetic concretions and concretionary beds of chalky limestone, possibly related to extremely slow rates. of sedimentation.
- The consensus of current European opinion is that the Cretaceous chalks of that continent are not deep sea deposits; most European authors interpret chalk as a deposit of seas less than 50 m to about 300 m in depth. Although depths greater than 500 m have been suggested for Greenhorn deposition, initial depths on the order of 30 m and maximum depths of perhaps 90 m at the height of transgression are more in line with paleoecologic, stratigraphic, and tectonic considerations. Diverse assemblages of coccoliths, planktonic foraminifera, ammonites, and certain bivalves indicate open-sea conditions and normal or nearly normal salinity during accumulation of Greenhorn sediments. During Greenhorn deposition the Kansas area lay near the southern edge of the north temperate realm and enjoyed a warm temperate climate. That the seaway was open to the world ocean is manifest in inclusion of several cosmopolitan species in Greenhorn fossil assemblages.
- The major invertebrate groups represented in Greenhorn rocks are foraminifera (mostly specimens of planktonic species), ammonites, oysters, and inoceramid bivalves. The enormous imbalance between numbers of planktonic and benthonic specimens is believed owing to generally poor oxygenation at the sea floor. Despite a great diversity in shape and size, most of the Greenhorn ammonites seem best interpreted as nektobenthonic forms. Distribution of oysters in Greenhorn rocks essentially parallels that of lenses of skeletal limestone and seems related to episodes of current action when the sea floor was better oxygenated than usual. Inoceramid valves are the almost exclusive hosts for these bivalves. Nearly all the inoceramids lay free on the sea floor, with plane of commissure parallel to the substrate. Inflated umbos prevented juveniles from sinking into soft sediment. Broad, nearly flat or only moderately biconvex valves enabled adult inoceramids to exist on the dominantly soft carbonate substrates. Inoceramids were remarkably well adapted to life on the muddy bottom and tolerated the low oxygenation levels on the floor of the Western Interior Sea.
Kansas Geological Survey, Geology
Placed on web June 5, 2010; originally published May 1975.
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