by Donald E. Hattin
Originally published in 1975 as Kansas Geological Survey Bulletin 209 and reprinted in 1979.
In Kansas the Greenhorn Limestone consists predominantly of dark colored, impure shaly chalk and has an average thickness of 94.8 feet in surface exposures studied. The basal, Lincoln Member is characterized by abundance of skeletal grainstone and numerous thin seams of bentonite. Based on 11 measured sections the member ranges from 16.2 to 33 feet in thickness. The overlying Hartland Member has little skeletal grainstone but in central Kansas contains three widely traceable, time-parallel beds of burrow-mottled chalky limestone and four equally widespread bentonite seams. Based on 14 measured sections the Hartland ranges from 8.6 to 68 feet in thickness. In central Kansas the Hartland is overlain by the Jetmore Member, which is characterized by thirteen ledge-forming beds of chalky limestone that can be traced confidently for hundreds of miles. In 18 measured sections the Jetmore ranges from 12.9 to 22.6 feet in thickness. Comprising the upper part of the Greenhorn in central Kansas, the Pfeifer Member is characterized best by its content of concretions and concretionary beds of chalky limestone. In 14 measured sections the member ranges from 15.2 to 25.8 feet. Presence of widespread marker beds makes possible the recognition of exact equivalents of the Pfeifer, Jetmore, and most of the central Kansas Hartland in the Bridge Creek Member of western Kansas. Chalky strata of the Greenhorn Limestone are related genetically to the superjacent Fairport Member of the Carlile Shale.
The Greenhorn Limestone is disconformable on the Graneros Shale across most of central Kansas. The Graneros-Greenhorn contact is diachronous, ascending temporally in a northeasterly direction. Most of the type Hartland grades northeastward into the Lincoln Member of central Kansas, and the lower part of the Bridge Creek Member passes northeastward into the central Kansas Hartland. The Hartland-Jetmore, Jetmore-Pfeifer, and Pfeifer-Fairport contacts are time parallel.
Inoceramid bivalves are the only ubiquitous Greenhorn macroinvertebrates and almost everywhere are represented only by the prismatic shell layer. Ammonites are common in some beds and are preserved mostly as molds. Diversity of macro- invertebrates is very low in most beds. A sequence of assemblage zones has been established, based on common, readily identifiable forms. The major assemblage zones (ascending) are characterized by the following species: Acanthoceras wyomingense; Calycoceras? canitaurinum-Exogyra aff. E. boveyensis; Sciponoceras gracile; Mytiloides labiatus-Watinoceras reesidei; M. labiatus-Mammites nodosoides wingi; and M. labiatus-Collignoniceras woollgari-Inoceramus cuvieri. In respective order these zones occur in (1) the lower Lincoln of Hodgeman, Ford, and Kearny counties and upper Graneros of Mitchell County; (2) lower Lincoln of central Kansas and middle or upper Lincoln in Hodgeman, Ford, and Kearny counties; (3) near middle of Hartland Member in central Kansas and in lower part of Bridge Creek in western Kansas; (4) upper part of Hartland and lower part of Jetmore in central Kansas and stratally equivalent part of Bridge Creek in western Kansas; (5) upper two thirds of Jetmore and equivalent part of Bridge Creek; and (6) middle part of Pfeifer and equivalent part of Bridge Creek to basal part of Fairport Member, Carlile Shale. Greenhorn beds below the top of the S. gracile zone are of late Cenomanian age; the remainder of the formation is of early Turonian age.
Thin section study is the basis for recognition of three major carbonate rock types, viz. skeletal grainstone, micritic shaly chalk having both packstone and wackestone textures, and micritic to microsparitic chalky limestones that have mudstone, wackestone, and packstone textures with wackestone predominating. Principal allochems are foraminifer tests, Inoceramus debris, and fecal pellets, with locally abundant calcispheres. Unaltered micritic matrices are composed largely of coccoliths. During diagenesis compaction was greatest in shaly chalks, cementation was greatest in skeletal grainstones, and neomorphism was widespread among chalky limestones.
Greenhorn carbonates are predominantly of pelagic origin and were deposited on the broad, flat eastern shelf area of the Western Interior region, far from major sources of terrigenous detritus. The formation was deposited near and at the peak of an eastwardly directed marine transgression that began in Cenomanian time and reached its maximum during early Turonian time. Concentrations of skeletal grainstones in the basal part of the Lincoln Member reflect a far-offshore high-energy depositional environment that was bordered on the east by a shallower water area of terrigenous mud deposition (Graneros) and on the west by a deeper water, lower-energy environment in which most sediment was carbonate mud.
Although wide diversity of opinion is evident in literature on depth of Cretaceous chalk deposition, and even for chalks of the American Western Interior region, the Greenhorn of Kansas was probably laid down at depths considerably less than 600 feet. During deposition, bottom conditions were generally unfavorable to development of a diverse epibenthos, and reducing conditions precluded an endobenthos in most shaly-chalk-producing sediments. Widespread, time-parallel beds of chalky limestone resulted from periods of reduced rates of sedimentation which generally fostered development of a burrowing endobenthos. Generally poor circulation of bottom water is manifest in the more or less laminated, dark-colored shaly chalk strata that dominate the formation (energy level 1). Zones of small lenses of fairly well-washed skeletal limestone and beds with great abundance of Inoceramus reflect times of increased circulation by moderate-strength bottom currents (energy level 2). Discontinuous beds and zones of lenses of skeletal grainstone, mostly in the Lincoln Member, represent a still higher level of energy expenditure on the sea floor (energy level 3). At the base of the Lincoln Member, beds of crossbedded skeletal grainstone, commonly containing a variety of rudaceous components, represent the highest energy levels of the Greenhorn (energy level 4).
Broad uniformity of paleobiotic composition suggests stable salinity. Diverse assemblages of coccoliths and planktonic foraminifera, as well as ammonites, pycnodont oysters and probably the inoceramids as well, suggest water of normal salinity in the far-offshore area of Greenhorn deposition.
Zoogeography of Greenhorn invertebrates suggests the area lay near the southern limit of the North Temperate realm with corresponding warm-temperate waters. The gross imbalance between numbers of benthonic and planktonic foraminifera is a result of poor oxygenation on the sea floor. Most Greenhorn ammonites seem to have been nektobenthonic forms. Occurrence of abundant epizoal oysters parallels that of skeletal limestones and is related to periods of better circulation of bottom waters. The inoceramids were predominantly free-living benthonic forms that owe their ubiquitous distribution to environmental adaptability.
Kansas Geological Survey, Geology
Placed on web June 5, 2010; originally published May 1975.
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