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Mineralogy
Major Constituents
Quartz
The quartz grains of sand and coarse silt size show a wide range in roundness from extremely angular (particularly in the finer sizes) to very well rounded. Most of the grains are subangular, except for the scattered large grains in the upper part of the section, which are almost invariably very well rounded. The degree of rounding, as will be shown in the section on petrography, seemingly is not entirely a function of differences in particle size.
The surfaces of some grains are polished and have an orange color. Some grains are slightly etched and are partly replaced by carbonates or clay minerals. Some of the large well-rounded grains have frosted surfaces. Much of the frosted appearance seems to be produced by small quartz overgrowths.
Nearly all the quartz grains throughout the section are of silt size or very fine-grained sand size (less than 125 microns in diameter). The larger rounded grains are approximately 0.2 to 1.0 mm in diameter and are most abundant in the upper part of the section (Whitehorse, Taloga formations), although they are found as low as the upper Salt Plain formation. Degree of sorting is variable. In general the quartz (and feldspar) grains themselves are rather well sorted but they are commonly associated with clay minerals. The sorting of quartz in those samples which contain the large rounded grains is seemingly bimodal. Relatively straight extinction in most of the grains indicates that they are nearly strain-free, but from 5 to 35 percent show undulose extinction. Nearly all the large rounded quartz grains show strain shadows. Flamboyant extinction typical of some vein quartz is extremely rare.
Many types of inclusions are observed, indicating several modes of origin. Bubble planes form the most common type, but several varieties of idiomorphic and allotriomorphic crystalline inclusions also are observed. Apatite is particularly common, and the following minerals were also identified: rutile, biotite, hornblende, tourmaline, and chlorite. A very few grains contain carbonaceous inclusions. Large fluid inclusions are also somewhat rare. In summary, most grains have the internal characteristics of igneous (plutonic) quartz (Krynine, 1946).
Small quartz overgrowths are fairly common in some specimens, although in no case are they pronounced or well developed. A few grains exhibit worn overgrowths.
Feldspars
The feldspar grains are angular to rounded, and in general are only slightly better rounded than the quartz. There are almost no large very well-rounded feldspars corresponding to the large very well-rounded quartz grains in the upper part of the section; only two such grains were observed (Pl. 15A).
Most of the feldspar is orthoclase. Very few grains of sodic plagioclase and even fewer of microcline were observed; this may be in part a function of the very fine particle size of the grains, so that small grains might not include the twinning boundaries. Chemical and petrographic evidence, however, suggests that the paucity of plagioclase feldspar may be attributed to penecontemporaneous alteration and reorganization of the grains shortly after deposition.
Much of the feldspar is fresh, but many degrees of weathering are observed in any one sample; some are partly replaced by chlorite. Replacement by carbonates is not common. Large overgrowths are rare but in some thin sections incipient overgrowths may be observed on most of the orthoclase grains. "Worn" feldspar overgrowths (Pl. 15B) are rare in most parts of the section. It is possible that even those which seem to be worn are actually xenomorphic sedimentary overgrowths (Krynine, personal communication, April 4, 1954).
Plate 15--Photomicrographs, Whitehorse sandstone. A, Overgrowth on large orthoclase grain, lower Marlow member; sec. 4, T. 32 S., R. 14 W., Barber County. x76. B, Feldspar grains with worn overgrowths, mounted in glycerine and water; Relay Creek? member, NW sec. 18, T. 33 S., R. 16 W., Comanche County. x100. C, Friable sandy clastic limestone from Marlow member; SW sec. 4, T. 32 S., R. 14 W., Barber County. x76. D, Red micaceous feldspathic siltstone, upper shale member; 10 miles north of Freedom, Oklahoma. x76. [Note: web versions are enlarged to show more detail.]
Dolomite and Calcite
Carbonates are present in most of the specimens which were examined in thin section, and are abundant in many of them. Formations or beds consisting predominantly of carbonate rocks are thin but relatively persistent; dolomite is the most common mineral in these beds. Dolomite rhombs are common in many of the clays and shales (particularly the greenish-gray shales). Calcite is a common cementing material in many siltstones and sandstones, and also occurs as veins in some of the silty shales and sandstones. Calcite of chemical clastic origin is present in some sandstones.
Gypsum and Anhydrite
Gypsum occurs in beds ranging in thickness from a featheredge to 30 feet (in the Blaine gypsum), as the cementing material in certain sandstones, and as individual selenite crystals and satin spar veins in clay shale (particularly in the Flowerpot shale). Gypsum also occurs as vug fillings and as replacement of oolites or parts of oolites in certain dolomite beds. Anhydrite is common in the subsurface, particularly in the Blaine and Stone Corral formations. Thin discontinuous lenses of anhydrite also crop out in the middle part of the Medicine Lodge gypsum. The relationship between gypsum and anhydrite in the Medicine Lodge gypsum has been studied by McGregor (1948) and is reviewed in the present report in the section on petrography.
Halite
Halite occurs interbedded with gray shale in the Wellington formation in central Kansas (in the subsurface), and it is also found in large quantities stratigraphically higher (Stone Corral, Blaine formations) farther west. Evidence of salt at various horizons throughout the Permian redbeds is found in the form of small casts of cubic crystals. Most are well under 1 cm in diameter (Pl. 14B).
Layer Lattice Silicates
The clay minerals, coarser micas, and chlorite are all grouped together here although they seemingly have divergent origins. In general the coarser grained flakes are of detrital origin, but this is not invariably true.
Coarse detrital micas (i.e., of very fine sand size) are abundant in some siltstones and sandstones, particularly in the lower part of the section. These are predominantly muscovite, but biotite also occurs. Some of the biotite is partly altered to chlorite. Most of the coarse micas are well rounded. Chlorite flakes of coarse silt size are present in many samples; they may be only in part detrital. Chlorite is also an important constituent of nearly all the clays. Illite is the predominant clay mineral in nearly all samples from which diffraction patterns were obtained. Some beds in the upper part of the section contain montmorillonite, and some beds contain small quantities of kaolinite (Pls. 16, 17). Detailed descriptions of these clay minerals and a discussion of their relationship to the other constituents of the rocks is deferred to the section on petrography.
Small green grains of glauconite are observed rarely in certain white sandstones (e.g., Relay Creek member, Whitehorse sandstone), dolomites, and nonred siltstones. The origin of this mineral is not clear, but its associations may be significant genetically.
Plate 16--Electron micrographs of particles finer than 1 micron, shadowed with Cr from angle of 17°. A, Wellington shale; SW sec. 4, T. 33 S., R. 1 E., Sumner County. x13,000. B, Kingman red siltstone; sec. 14, T. 35 S., R. 7 W., Harper County. x15,000. C, Salt Plain red silty shale; sec. 9, T. 32 S., R. 10 W., Barber County. x15,000. D, Flowerpot gray shale; sec. 1, T. 32 S., R. 14 W., Barber County. x13,000. Note aggregates suggestive of kaolin mineral. [Note: web versions are enlarged to show more detail.]
Plate 17--Electron micrographs of particles finer than 1 micron, shadowed with Cr from angle of 17°. A, Red Ninnescah shale; sec. 33, T. 26 S., R. 4 W., Reno County. x13,000. B, Cedar Hills red sandy siltstone; sec. 9, T. 32 S., R. 10 W., Barber County. x15,000. C, Marlow white sandstone; sec. 4, T. 32 S., R. 14 W., Barber County. x14,500. D, Taloga silty shale; sec. 3, T. 34 S., R. 26 W., Meade County. x15,000. [Note: web versions are enlarged to show more detail.]
Hematite
Hematite occurs as a stain on sand and silt grains, and as the principal coloring matter in the red clay shales. Although large quantities of pure hematite do not occur, the mineral is responsible for the red coloration in all the redbeds under consideration. The only evidences of hematite other than stain are occasional small areas of bright-red opaque material around a few sand and silt grains observed in thin section, and problematical particles from the Kingman siltstone observed in the electron microscope. The less than 1-micron fraction of some samples from the Kingman contains extremely thin (as determined by chromium shadows) flaky particles with density so great that they are nearly opaque to the electron beam. The bright-red-stained sandstones and siltstones contain generally less than 3 percent Fe2O3, and the red shales contain less than 6 percent.
The mineral is identified as hematite rather than one of the hydrated iron oxides because diffraction patterns of less than 2-micron fractions of some of the brightest red shales and siltstones show weak reflections at 3.67, 2.69, 2.51, 2.20, and 1.45 A, rather than reflections for goethite or lepidocrocite.
Aggregates and Rock Fragments
The scarcity of rock fragments is attributable in part to the fine grain of most of the sediments. A few small fragments of granite were observed in the coarser sandstones in the upper part of the section. Rare phyllite and fine-grained schist occur in sandstones in several formations. Chert is generally rare, but forms a rather large proportion of the large rounded grains in the upper formations. The most common aggregates are colorless, almost clear grains, some of the crystalline units of which show low birefringence similar to that of chert, and which are tentatively attributed to the alteration of sodic plagioclase feldspars under alkaline conditions.
Accessory Heavy Minerals
Nonopaque Minerals
Apatite--Small rounded grains of apatite are observed in some sandstone specimens. The acid treatment necessary to remove iron oxide destroyed most of the apatite.
Chlorite--Relatively large flakes of chlorite (up to very fine sand size) occur in many of the argillaceous siltstones. Most of these flakes are judged to be detrital, and many are probably altered biotite. A few flakes show aggregate polarization. Much chlorite, particularly in the finer sizes, may not be detrital.
Epidote--This mineral is observed rarely. It occurs as small pale-yellow subrounded grains with characteristic "compass-needle" interference figure.
Garnet--Garnet is present in all the heavy mineral concentrates examined. Most of it is colorless, but some grains are pink and a few are pale yellow. The grains are characteristically pitted, although some are smooth, and others have roughly fractured surfaces. A few grains have deep embayments and are an irregular shape. Inclusions are rare.
Rutile--Rutile is a rare mineral in the heavy mineral concentrates studied. Where observed, it consists of small subrounded to subangular elongate or equant grains having a brownish-red or amber-yellow color.
Sillimanite--A few elongate colorless grains of sillimanite are observed in some concentrates. They are extremely rare.
Staurolite--Staurolite is one of the more common minerals and is present in nearly all the concentrates, particularly in the fractions coarser than 62 microns. It is invariably deeply etched and has delicate saw-toothed edges which seemingly could not have survived transportation. No broken sawteeth are observed. A few of the grains contain dark carbonaceous inclusions.
Titanite--Small rounded dusky grains having high relief and high birefringence occur sparingly in a few samples. They are provisionally identified as titanite. Some of them are partly coated with white opaque material ("leucoxene"?).
Tourmaline--Many varieties of tourmaline are present in the heavy mineral concentrates from the sandstones. Some grains are worn and a few are very well rounded, others are prismatic, and still others have irregular angular shapes. Most of the grains are brown, with few inclusions, and are rounded to subrounded. Also fairly common are well-worn brown prismatic grains, with few inclusions. Another common type is subrounded to rounded green tourmaline with few inclusions.
The following types are also present: black, rounded to subrounded; golden to reddish-brown, rounded to subrounded and worn prismatic: brown with acicular (rutile?) inclusions, rounded to subrounded, brown with many inclusions, subrounded to rounded and prismatic (some nearly idiomorphic); green with many inclusions, rounded to subrounded; blue, subrounded to rounded and subangular (indicolite); and varicolored, brown-green, and brown-pink.
Some of the grains are olive green, and the boundary line between brown and green tourmaline is indistinct. Most of the tourmaline is of granitic origin (Krynine, 1946a). Some obviously is reworked from older sediments. No overgrowths are observed. The mineral is most common in the coarser sandstones.
Zircon--Zircon is common in the finer fractions. It occurs principally as subangular to well-rounded, colorless worn prismatic grains, but some grains are worn only slightly. A few grains are pale yellow and a few are zoned. Large black irregular inclusions and slender rodlike crystalline inclusions are present in some grains. Zoning is more common, or more obvious, in angular grains than in round ones.
Opaque Minerals
Hematite--Hematite, the mineral responsible for the red coloration of the sediments, occurs as stain in clay coatings around sand and silt grains, and as staining material in the red clays. The mineral is identified as hematite because some of the brighter red clays give the prominent hematite reflections in diffraction patterns. The mineral is so common that it is described in greater detail in the section on major constituents.
Ilmenite, "leucoxene," and magnetite--Well-rounded grains of ilmenite are common in the finer fractions of the heavy concentrates. Magnetite is also present, but rare. White opaque, well-rounded "leucoxene" grains are nearly as common as, or more common than, ilmenite in many samples. Some ilmenite grains are partly white. Some white opaque grains are quite angular and "fluffy" or porous in appearance and probably have not been formed by the alteration of well-rounded ilmenite grains. In studying the leucoxene content in several Permian sandstones of Oklahoma, Coil (1933) noted that this mineral is the most common of all the accessories, sometimes constituting more than 2.5 percent of the sample. Coil found leucoxene to be five times as common as ilmenite, and attributed its formation to the alteration of ilmenite by carbonate waters.
Authigenic Minerals (Other Than Layer-lattice Silicates)
Anhydrite
Other than in the anhydrite-gypsum formation (Blaine), this mineral occurs as small crystals in the Hutchinson salt and in some shales, particularly those associated with salt. Much of the gyspum may have been anhydrite at one time.
Barite
Barite is not common, but occasional clear, irregularly shaped grains are found in some sandstones and shales. It is an important constituent of the Milan member of the Wellington formation, where it fills vugs in the dolomite. Norton (1939) reports barite nodules in the Whitehorse sandstone. It is also present in salt.
Calcite and Dolomite
Calcite is the cementing material in some sandstones and siltstones, and commonly forms veins in many of the red shales and sandstones. The calcite veins in the red shales occur at a low angle to the bedding and the calcite has a columnar structure. The peculiar structure has led some workers to describe the veins as aragonite. Their diffraction pattern, however, is that of calcite. Most of these veins are only a few millimeters thick. Calcite also occurs as disseminated particles in clay shales.
Dolomite, other than that in the bedded dolomite units, occurs as the cementing material in some sandstones and siltstones, and as rhombohedral crystals in some of the clays and clay shales, particularly in some of those which are greenish gray (Pl. 18A). Curved rhombohedral calcite-cemented red siltstone casts in red siltstone at one horizon attest to the former presence of large dolomite crystals at that place (Pl. 7A).
Plate 18--Photomicrographs of thin sections of sale and sandstone. A, Dolomite rhombs in shrinkage cracks of greenish-gray dolomite shale, horizon of Stone Corral dolomite; SE SE sec. 10, T. 31 S., R. 6 W., Harper County. x8.5. B, Gypsum vein in gypsum-cemented feldspathic sandstone of upper Flowerpot shale; sec. 11, T. 31 S., R. 15 W., Barber County. x8.5. C, Calcite-cemented Verden sandstone; 1.6 miles east of Tegarden, Oklahoma. x8.5. D, Light-red, noncalcareous, very fine-grained feldspathic sandstone, lower Marlow member, Whitehorse sandstone; sec. 4, T. 32 S., R. 14 W., Barber County. x8.5. [Note: web versions are enlarged to show more detail.]
Chalcopyrite and Malachite
Scattered small spots (less than 3 mm diameter) of bright-green copper carbonate are observed at various horizons in the lower part of the redbeds, particularly in thin silty limestones. They are identified by x-ray diffraction as malachite. According to Norton (1939, p. 1757), chalcopyrite is the original copper mineral in the unweathered rock. I was unable to find any copper mineral other than malachite.
Gypsum
Other than in the massive beds, gypsum occurs as cement and veins in a few sandstones (Pl. 18B), and as selenite crystals and satin spar veins in the Flowerpot shale and clay of the Blaine formation. It forms large subround sand-free crystals in a few sandstones in the upper part of the redbeds (Pl. 7C).
Halite, Polyhalite, and Glauberite
The only evidences of soluble salts in the surface exposures are occasional zones of halite casts in some of the silty shales. These occur at various horizons throughout the section. Minor quantities of finely crystalline polyhalite and glauberite have been identified from the Hutchinson salt in the Carey mine at Hutchinson, Kansas (Swineford and Runnels, 1953).
Pyrite
Pyrite is extremely rare, but is observed at one or two localities. It occurs as irregularly-shaped particles filling interstices between sand grains, and is associated only with deposits which are not red.
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Kansas Geological Survey, Geology
Placed on web Aug. 25, 2006; originally published May. 1955.
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