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Graneros Shale in Central Kansas

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Origin of Sediments

Inorganic Sediment

From paleontologic and stratigraphic evidence that the Western Interior Sea was transgressing in a generally eastward direction during Graneros deposition, and from consideration of general aspects of Late Cretaceous paleogeography in the Western Interior Region, the conclusion may be drawn that the source area(s) for Graneros terrigenous detritus lay somewhere to the east of the present central Kansas outcrop area. The regional cross-bedding trend in the Dakota Formation of central Kansas (Ottawa County) is generally southwestward (Franks, et al., 1959, p. 237), suggesting a northeasterly source for the Graneros sediments which would have been transported by the same stream system( s) as the partially contemporaneous Dakota sediments. A similar cross-bedding trend was determined in the type Dakota area by Tester (1931, p. 280). The major source area for Dakota and Graneros sediments of Kansas would thus be the southern part of the Canadian Shield, especially the part that extends into Wisconsin, and surrounding areas of Paleozoic sedimentary rocks. An easterly source of sediment prevailed also in North Dakota where, according to Hansen (1955, p. 28), the sand or silt content of the Belle Fourche Shale (the lithologic equivalent of the Graneros) increases eastward.

Shale in the Graneros is dominated mineralogically by quartz, much of which is in the finer silt sizes. Such fine quartz detritus was probably produced partly by thorough disintegration of parent rocks in the source areas and also by size reduction of quartz particles during transportation to the sites of the late Cretaceous deposition. Kaolinite, illite, and montmorillonite are the only other major constituents of the shale, and the relative proportions of the three differ from bottom to top of the formation. Kaolinite is more abundant in the upper part of the Dakota and lower part of the Graneros and montmorillonite is more abundant toward the top of the Graneros, whereas the amount of illite remains nearly uniform throughout. If clay minerals are mainly of detrital origin, as suggested by Weaver (1958, p. 258; 1959, p. 182), their distribution in Graneros sediments requires explanation in terms of source and depositional environment as well as possible diagenesis. According to Weaver (1958, p. 259), kaolinite is most common in continental and nearshore sediments. Both kaolinite and illite flocculate and settle rapidly when introduced into seawater of even low salinity (Whitehouse, et al., 1960). The general decrease in amount of kaolinite upward from the uppermost part of the Dakota Formation probably reflects gradually increasing salinity during deposition. The nearly uniform distribution of illite in the Graneros may be a result of montmorillonite diagenesis whereby some of the latter mineral was altered to illite by potassium-ion absorption. Milne and Earley (1958, p. 331) believe that in the modern Gulf of Mexico this kind of diagenesis takes place slowly and under conditions of slow sedimentation, but Keller (1963, p. 144, 145) has presented evidence that this reaction occurs with increasing depth of burial. Study of a much larger number of samples may shed more light on this question.

Kaolinite can form by thorough leaching of parent rock, probably under humid climatic conditions. The diversity of plant remains in the famous Dakota flora, the common occurrence of carbonized plant remains in the Graneros Shale, and of calcified, carbonized, and silicified logs in the Greenhorn Limestone and Fairport Chalk Member of the Carlile Shale bespeak a humid, rather than arid, climate in the land areas adjacent to the east edge of the Western Interior Sea during Cenomanian and Turonian time. The most probable source of large quantities of koalinite would seemingly be soils in areas of Precambrian rock in the southern part of the Canadian Shield and its extension into Wisconsin. Adjacent early Paleozoic sedimentary rocks were probably not a major source of kaolinite because, as Weaver (1958, p. 259) has noted, illite is the dominant clay mineral in Paleozoic rocks; this is especially true of pre-Upper Mississippian rocks (Weaver, 1959, p. 1954). Indeed, Paleozoic rocks along the eastern border of the Western Interior Sea may have been the source for much of the illite in the Graneros Shale. Soils developed on these Paleozoic rocks may have contributed some of the kaolinite, however.

Montmorillonite concentrated in Graneros bentonite samples, and possibly some elsewhere in the formation, was derived from volcanic ash by devitrification, but much of montmorillonite in the formation may be detritus from soil. Montmorillonite flocculates and settles most rapidly in water of normal salinity (Whitehouse, et al., 1960) and thus, in a transgressional sequence, detrital montmorillonite would be expected to increase in quantity upward in the section. This is the case in the uppermost Dakota-Graneros sequence and seemingly supports conclusions drawn elsewhere in this report. On the other hand, if the quantity of montmorillonite increased because of increased volcanism, we could expect progressive dilution, initially, of both kaolinite and illite upward in the Graneros. Despite such dilution, the relative quantity of illite could remain nearly uniform owing to montmorillonite diagenesis. The thickest bentonite in the Cretaceous of Kansas, the marker bed in the upper part of the Graneros, is ample testimony to the magnitude of volcanic activity during deposition of that part of the formation. Marked thickening of the presumed western equivalent of this bentonite northwestward into Wyoming and Montana indicates the general direction of the eruptive source.

In a study of late Cretaceous and Tertiary clays of the upper Mississippi Embayment, Pryor and Glass (1961, p. 48) noted the dominance of kaolinite in the fluviatile environment, dominance of montmorillonite in the outer neritic environment, and nearly equal mixtures of kaolinite, montmorillonite, and illite in the inner neritic environment. This distribution pattern is similar to that of the Graneros Shale. Pryor and Glass (1961, p. 50) reasoned that all three clay minerals were detrital and that if the montmorillonite represented ash falls, then large quantities should be found in the fluviatile sediments as well as in the marine, but altered volcanic ash is not necessarily represented by montmorillonite. Under conditions of acid leaching, volcanic ash can be altered to kaolinite (Weaver, 1963, P: 347). An X-ray study of five bentonite samples from the lower part of the Graneros Shale showed kaolinite to be the dominant clay mineral in each. However, montmorillonite is also present in these samples, so not all of the ash was altered to kaolinite. Montmorillonite in shale samples from the lower part of the Graneros could be of either detrital or volcanic origin but the local dominance of this clay mineral in the upper part of the Dakota and lower 'part of the Graneros, involving marginal-marine and nearshore marine environments respectively, suggests a dominantly volcanic origin. It should be noted here that bentonite beds do occur in the upper part of the Dakota Formation (see Pl. 1). It is generally agreed that the abundant montmorillonite in Upper Cretaceous rocks is of volcanic origin (Weaver, 1959, p. 168, 169).

From the mineralogical standpoint, the terrigenous sand and silt in both the calcareous and noncalcareous quartzose sandstones is remarkably mature and contains a limited group of stable heavy minerals. The mineral suite in Graneros sandstones is nearly identical with that reported by Potter and Pryor (1961) for most Paleozoic sandstones of the Eastern Interior Region with the exception of garnet which I did not detect. Earlier Paleozoic sedimentary rocks of the Eastern Interior are attributed to a Precambrian crystalline source in the Wisconsin-MichiganOntario area by Potter and Pryor (1961, p. 1224), who concluded that sediments from this source were recycled during each of the succeeding geologic periods with resultant supermaturity of some Paleozoic sandstones. On the basis of paleocurrent trends in the Dakota Formation mentioned above, and of the mineralogical composition of Graneros sands, the dispersal center for the latter is deemed to be generally the same as that for the earlier Paleozoic, and some of the later Paleozoic, terrigenous detritus of the Eastern Interior Region. The mixture of angular to well-rounded, but mostly subrounded, quartz grains in Graneros sandstones reflects a relatively small contribution of fresh detritus from a crystalline source and a comparatively large amount of sediment recycled from older formations that must have cropped out adjacent to the crystalline rocks. Further evidence of a primary source of sediment during Graneros deposition includes the grains of fresh, unaltered feldspar and sparse rock fragments that are scattered through the sandstone units. However, according to the conclusions drawn by Blatt and Christie (1963, p. 571), the high percentage of nonundulatory quartz grains in Graneros sandstones may indicate that the bulk of this sediment has undergone considerable recycling. Early Paleozoic rocks surrounding the southern part of the Canadian Shield would have been a convenient and likely source for chert grains that are common in the Graneros and the presence of this chert seemingly lends support to the contention that Graneros detritus is largely recycled sedimentary material. Polycrystalline quartz in the Graneros occurs as both rounded and irregular-shaped grains and thus could represent both fresh and recycled sediment, but the low polycrystalline quartz/quartz ratio is an additional index of mineralogical maturity, owing to lesser stability of the former (Blatt and Christie, 1963, p. 570) and supports the concept that Graneros sand is largely reworked detritus.

Glauconite grains in Graneros sandstone units are believed to be largely of detrital origin. The evidence for this conclusion consists of the well-rounded shape of most grains, compatibility with grain size of surrounding rock, absence of grains having the shape of skeletal cavities or the structure of cleavable minerals, and the paucity of radial cracks.

Because the grains in the Graneros sandstones are not predominantly rounded to well rounded, the rock is considered to be texturally submature.

Organic Sediment

Skeletal limestones in the Graneros Shale are interpreted as lag concentrates that were produced by current activity which, during periodic sea-floor sweeping, winnowed away finer detritus and concentrated the coarser grains, consisting mostly of shell debris, in lenses or in thin beds that are generally gently cross-laminated. At sporadic intervals storm activity stirred the sea floor more deeply than usual, resulting in concentration of coarser organic detritus including bone pebbles and invertebrate shells that are now bone beds and coquinas. The degree of rounding of the bone-pebbles suggests that such constituents were subjected to considerable abrasion, possibly along or near the ancient shoreline, before being transported to the sites of ultimate deposition.

Carbonaceous matter occurring in Graneros sediments as minute specks, larger flakes, woody chunks, and a few leaves is regarded as largely the remains of vascular plants which were swept out to sea during flood stages in the stream system (s) that bordered the Kansas portion of the Western Interior Sea.


Pyrite and marcasite in clayey sediments of the Graneros Shale, occurring largely in the lower part of the formation, are probably the result of anaerobic conditions that prevailed shortly below the sediment-water interface. Here, liberation of hydrogen sulphide during decomposition of small quantities of organic matter, which was buried rapidly before thorough oxidation, reacted with dissolved iron compounds in interstitial water and caused precipitation of iron sulphide (Rankama and Sahama, 1950, p. 688). Some of the iron sulphide was concentrated (probably by aggregation of a colloidal precipitate) into blebs and nodules. Some remained scattered through the sediment in a finely divided state and is probably in part responsible for the dark color of the shale (see Table 1). Carbon left in the clayey sediment after decomposition of organic matter also contributes to the dark color of the shale. Upon weathering, the iron sulphide minerals, particularly marcasite, oxidize readily to iron sulphate which imparts the sharply bitter taste of melanterite to partially weathered shale at many localities, especially in the lower part of the formation. Further weathering of the iron compounds produced the limonite and jarosite that characteristically encrusts rock surfaces along joint and bedding planes throughout the Graneros and that are the cementing agents in many of the sandstone bodies. Some of the oxidized iron sulphide, while in the sulphate stage of oxidation, combined with water to produce sulphuric acid. Reaction of this acid with calcareous matter throughout the formation resulted in extensive crystallization of gypsum.

In the Callistina lamarensis Assemblage Zone, calcareous invertebrate fossils are preserved in only one bed and some shells have been replaced by gypsum at a few localities. Crystals and crystalline aggregates of selenite are especially abundant in the lower half of the Graneros; selenite became the cementing agent locally in sandstone and bone beds. The possibility of acid leaching causing alteration of montmorillonite to kaolinite is discussed on page 62.

In the Ostrea beloiti Assemblage Zone selenite is common, but usually as small crystals oriented parallel to bedding planes. In some skeletal limestone beds selenite has partially replaced sparry calcite cement and a few Inoceramus prisms. Oysters in shale have been replaced locally by gypsum. Inoceramus and ammonites in shale of the O. beloiti Zone are preserved mostly as molds except at Locality KG-43 where a deep excavation exposes unweathered shale in which the gypsum-producing process has not yet occurred extensively. Alunite nodules in shale at three localities were probably produced by localized reaction of sulphuric acid with clay minerals.

Sparry calcite cement, especially in quartz sandstone and skeletal limestone beds of the Ostrea beloiti Zone, resulted from early diagenetic crystallization of calcium carbonate from interstitial water, commonly in observable optical continuity with Inoceramus prisms. The calcite probably formed shortly after burial when a maximum amount of seawater could penetrate the then highly permeable sediments. That the calcite precipitation occurred under alkaline conditions is manifested by the common occurrence of quartz grains which have been partly replaced and locally deeply invaded by sparry calcite.

In a few places, alkaline conditions developed within sediments shortly after burial and created a chemical environment favoring the localized precipitation of calcite that ultimately formed the septarian concretions that lie in the upper half of the formation. Anaerobic organic decomposition results in liberation of ammonia which would create the necessary conditions for the inorganic precipitation of calcium carbonate, During enlargement of the concretions, an increasing volume of surrounding sediment was incorporated into these structures and the dark color is due to these included clayey sediments. Lithification of Graneros concretions apparently occurred during early diagenesis because fossils observed in them were not flattened bv compaction, as in adjacent sediments, and enclosing shale layers bend around the concretions.

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