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Graneros Shale in Central Kansas

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Stratigraphy of the Graneros Shale (Upper Cretaceous) in Central Kansas

By Donald E. Hattin

Cover of the book; cream paper with black text.

Originally published in 1965 as Kansas Geological Survey Bulletin 178. This is, in general, the original text as published. The information has not been updated. An Acrobat PDF version (38 MB) is also available (large plate available separately).


The Graneros Shale of central Kansas was examined at more than 60 places, fossils were collected wherever possible, and sections were measured in detail at 42 localities. Six complete sections were sampled for laboratory analysis. Thickness of the formation ranges from 23.6 feet to 40.4 feet and averages 30.5 feet for 20 sections including three that are composite. Stratigraphy of the formation and distribution of key macroinvertebrate fossils is depicted in two series of graphic columns, one of which is oriented northeast-southwest, parallel to the outcrop, and the other perpendicular to the first. Marker beds include a thick bentonite lying near the top of the formation throughout the central and northern part of the outcrop and a zone of thin beds and lenses of silty sandstone overlain bv bentonite that lies near the middle of the formation in the central part of the outcrop.

The Dakota-Graneros contact generally lies at the top of an evenly bedded ferruginous sandstone unit but elsewhere is transitional through alternating beds of sandy shale, shale, and thin sandstone beds. The Graneros-Greenhorn contact lies at an abrupt change in lithology above which skeletal limestone and chalk predominate.

The Graneros consists mainly of medium dark-gray shale that contains numerous very thin layers, laminae, and lenses of very fine quartz sand or silt. Sand and silt content diminishes irregularly upward in the section. Shale in the upper part of the formation is slightly calcareous at two localities where the rock has been freshly exposed. Chief clay minerals in the shale are kaolinite, illite, and montmorillonite with relative abundance of the later generally greater in the upper part of the formation. Beds of noncalcareous quartzose sandstone containing small quantities of chert, feldspar, rock fragments, mica, glauconite, and heavy minerals characterize the lower part of the formation. These beds are supplanted in the upper part of the formation by calcareous quartzose sandstone which contains widely ranging quantities of shell debris, and skeletal limestone consisting largely of Inoceramus prisms and some terrigenous detritus. Coquinoidal limestone and beds containing conspicuous quantities of rounded bone and tooth pebbles are scattered through much of the formation. Layers of bentonite occur in most exposures, but only two can be traced widely with assurance.

Fossils in the Graneros are grouped into two assemblages characterizing the lower and upper parts of the formation respectively. The lower assemblage is characterized by Callistina lamarensis (Shumard) and consists of 15 species including 3 brachiopods, 10 pelecypods, 2 gastropods, and 1 scaphopod. Endobenthonic forms are most abundant. The upper assemblage, consisting largely of epibenthonic and nektonic forms, is characterized by Ostrea beloiti Logan and comprises at least 8 species, including 3 pelecypods, 2 gastropods, 2 ammonites, and 1 cirriped. The lower assemblage cannot be correlated precisely with the standard sequence of the Western Interior because distinctive species of Inoceramus and ammonites are lacking, but it probably represents all or part of the Zone of Borissiakoceras compressum or is a local biofacies of the Plesiacanthoceras amphibolum Zone. The upper assemblage zone embraces part or all of the P. amphibolum Zone and possibly a few feet of younger strata. Absence of Plesiacanthoceras wyomingense between Graneros strata containing P. amphibolum and Greenhorn strata containing molds presumed to be Dunveganoceras pondi supports lithologic and stratigraphic evidence for an unconformity between the two formations. The hiatus may be greater at the southern end of the outcrop where the C. lamarensis assemblage occupies nearly all of the formation and the O. beloiti assemblage is not represented in the Graneros.

Environmental conditions changed gradually through the time of Graneros deposition in consequence of eastward or northeastward marine transgression, which left the present outcrop area in a progressively farther offshore position with respect to the advancing shoreline. Widening distance to shoreline is reflected in the lesser quantities of quartz sand and silt upward in the Graneros section. Change of salinity from less than normal in the lower part of the formation to normal in the upper part is indicated by distribution of clay minerals, limestone beds, foraminifers, inarticulate brachiopods, and ammonites. Change in water depth during deposition from possibly less than 30 feet initially to a probable maximum of 100 feet ultimately was determined by study of sedimentary structures and foraminifers. Change in bottom conditions from those of rapid deposition and frequent reworking by waves and currents during deposition of the lower part of the formation to relatively slower deposition and less frequent sediment disruption during deposition of the upper part is indicated by abundance and distribution of benthonic macroinvertebrates, burrows, and thin layers, laminae, or lenses of very fine sand and silt.

Graneros sediments are polygenetic. Vertebrate and invertebrate skeletal debris is of local derivation. Montmorillonite probably came mostly from volcanos in the Nevadan orogenic belt. First-cycle detritus including angular quartz, feldspar, rock fragments, and kaolinite is believed to have come from the southern part of the Canadian Shield. Illite, chert, and rounded grains of quartz and heavy minerals were reworked from an early Paleozoic terrain lying along the eastern edge of the Western Interior Sea. The detrital sediments were transported into central Kansas by one or several streams systems that flowed generally southwestward. The stream systems built a deltaic complex in which submarine topset beds arc represented by the upper part of the Dakota and, locally, the lowest part of the Graneros. Foreset beds are best represented in the central part of the outcrop where laminated silty shale in the lower and middle parts of the formation contains conspicuous sandstone beds attributed largely to changes in river discharge, and softsediment flow structures which suggest greater sea-floor slope than elsewhere. Strata analogous to bottom set beds are represented by the more sparingly silty and less well laminated shale beds that lie near the top of the formation.

Graneros Shale comprises transgressive phases 2 and 3 of a seven-phase marine cyclothem beginning with the upper part of the Dakota and terminating at the top of the Codell Sandstone Member of the Carlile Shale. For an unknown time interval following deposition of phase 3. the sea floor in central Kansas was an area of nondeposition and sublevation which resulted in the stratigraphic hiatus at the Graneros-Greenhorn contact.

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Kansas Geological Survey, Geology
Placed on web Dec, 15, 2014; originally published December 1965.
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