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Petrographic, X-Ray, Differential Thermal, and Firing Analysis of Core
By P. C. Franks and Norman Plummer
General
Following are descriptions of selected samples taken from the core. The samples were studied by petrographic and x-ray examination, differential thermal analysis, and firing tests to provide lithologic checks of the megascopic core logging and to provide data on clay-mineral composition of the various stratigraphic units.
Description of lamination is based on the system of McKee and Weir (1953 p. 383). Thus laminated refers to strata 2 millimeters to 1 centimeter thick and thin-laminated to strata less than 2 millimeters thick.
Petrographic Descriptions
All the following petrographic descriptions are based on thin-section studies. Measurements cited in the descriptions are, therefore, those made in the plane of the thin section. All percentage compositions cited are estimates based on visual inspection of both hand samples and thin sections. Likewise, roundness of detrital grains was estimated by use of charts published by Krumbein (1941).
In many samples it was impossible to estimate original roundness of the detrital grains because the grains had been extensively modified by overgrowths and sutures. Commonly the overgrowths are not delineated by changes in kind or amount of inclusions.
X-Ray Data
X-ray diffraction data were obtained by use of a General Electric XRD-3 proportional counter diffractometer. Patterns were run at a scanning rate of 2 degrees 2θ per minute and at 0.2 degree 2θ per minute where more detail was needed for interpretation. A 1° beam slit, 0.2° detector slit, and nickel-filtered copper radiation were used. The unit was operated at 50 kv and 15 ma.
Oriented clay particles having equivalent spherical diameters of 2 microns or less were studied. Samples were prepared by settling the clay from a water suspension onto glass slides. The slides were then air-dried and diffraction patterns were obtained. Depending on clay-mineral composition, the slides were treated with glycerol and heated to 450° and 575°C. Some samples containing 14-angstrom minerals other than montmorillonite were treated by boiling in hydrochloric acid and in ammonium chloride for determination of chlorite and vermiculite.
Estimates of the percentage of each clay mineral in the samples were made from the diffractometer patterns and are based in part on the method outlined by Murray (1954). The 001 reflection of glycerated montmorillonite was arbitrarily assigned the same reflecting power as the 001 illite reflection. The estimates are not intended to be accurate but only to serve as a means of comparison between samples (Table 7).
The term "degraded illite", as used in this report, refers to mixed-layer clay whose weak reflections between 10 and 13Å are apparent as skew shoulders on 001 illite reflections at 10Å as shown, for example, in Figure 4, sample 2034, pattern A, and in Figure 6, sample 2176, pattern A. On treatment with glycerol, it was found that the reflections between 10 and 13Å shifted to 14Å and that in many samples a weak reflection also appeared at about 28Å as shown in Figure 4, sample 2034, pattern B, and in Figure 6, sample 2176, pattern B. Some samples of air-dried degraded illite had a suggestion of a reflection between 24 and 26Å. On glyceration, the 24Å reflection shifted to 28 or 29Å as shown in Figure 6, sample 2191, patterns A and B.
Many of the clay samples studied gave weak but distinct reflections between 11 and 13Å. Some of the samples also had weak reflections at about 24Å. On treatment with glycerol, as in the case of the degraded illite, the reflections shifted to 14Å and 28 or 29Å, respectively (Fig. 4, sample 2074, patterns A and B; Fig. 8, sample 2341, patterns A and B). In accordance with work by Weaver (1956, p. 217) these reflections, as well as those from the degraded illite, are attributed to irregularly interstratified mixedlayer clay that is composed of approximately equal amounts of illite and montmorillonite.
Differential Thermal Analysis
The differential thermal analysis apparatus used to obtain the curves described in this report consists of a program controller, recorders, and preamplifiers. The unit is similar to that described by Kerr and Kulp (1948)) and was obtained from Leeds and Northrup Company. Furnace temperatures are recorded on a Micromax, and differential temperatures on a Speedomax. The furnace is a platinum-wound tube enclosed in suitable insulation. The sample holder is made of sintered alumina. Both furnace and sample holder were fabricated in the ceramic laboratory of the State Geological Survey.
A mixture of alumina and calcined kaolin was used as the inert reference, and the furnace thermocouple was placed between the sample and the inert reference. Accuracy of the furnace temperature and its correlation with the differential thermal record were checked by the alpha to beta quartz transformation at 573°C.
The samples were heated at a uniform rate of 10°C per minute, and all the curves were recorded at the same amplification. The curves shown in this report were reproduced from tracings of the originals. No corrections were made for base-line drift, nor were minor irregularities smoothed out.
Interpretation of curves produced by the differential thermal analysis of clay materials is severely limited by several factors. The fact that the various clay minerals show endothermic and exothermic reactions at approximately identical temperatures is the chief source of confusion. For example, the kaolin group, the illite group, mixed-layer clays, and some of the montmorillonoids show a major endothermic reaction producing a peak between 500° and 620°C. A further limitation to accurate interpretation of the curves is the greatly varying size of the areas enclosed by the curves produced by the various clay minerals. Rocks containing only one clay mineral, or in which one clay mineral greatly predominates, can be identified with considerable accuracy, and under some conditions fairly close estimates of quantity can be made.
Carbonaceous material, pyrite and marcasite, and calcite and dolomite add to uncertainty in interpretation of differential thermal analysis curves. All of these materials produce large peak areas when present in comparatively small amounts. Carbonaceous materials and the iron sulfide minerals give strong exothermic reactions beginning at temperatures as low as 200° and extending as high as 700°C. Carbonaceous material seriously interferes with the endothermic peaks of the clay minerals in the 500° to 620°C region, and in some samples may permit only a small endothermic swing on a larger exothermic peak. The differential thermal method also is sensitive to calcite and dolomite. Traces of calcite as small as 1 percent and of dolomite as small as 0.3 percent can be detected in the absence of conflicting peaks (Mackenzie, 1957, p. 329). The peak temperatures for calcite vary from 840° to 940°C in a diluted sample. This endothermic peak may be confused with the higher endothermic peaks for illite, montmorillonite, or even chlorite.
Firing Tests
Pieces broken from several core samples were fired slowly in an oxidizing atmosphere to about 1000°C. The fired color and the relative hardness of the samples so treated are recorded in Table 8. In a series of tests on samples of Kiowa Shale, Dakota Formation clays, and Graneros Shale it was found (Plummer and Romary, 1947) that the fired color of clay samples is a good empirical criterion for approximate determination of the clay-mineral content and of stratigraphic units.
As the result of firing a large number of clay and shale samples of known clay-mineral composition, it has been found that any clay material that fires white or buff contains a fairly high percentage of kaolinite unless the material is calcareous, or in rare samples, contains a high percentage of iron-free montmorillonite. No confusion need arise from the exceptions, however, because the presence of calcite is easily detected in the raw sample, and montmorillonite shrinks excessively on firing and becomes almost glasslike. Illite, chlorite, and most montmorillonite fire to colors ranging from salmon or light orange pink to dark brown, or brownish red, owing to the presence of iron. Of course the various iron compounds that may be present with kaolinite can produce pink or reddish colors in the fired material, but such compounds are fairly conspicuous in the raw sample, and the color produced on firing tends to be a fairly clear red or pink resulting from various dilutions of the color produced by hematite. The iron in illite, montmorillonite, and chlorite usually produces orange-pink to brown or' brownish red shades.
The colors of fired samples given in Table 8 are the conventional names used by brick manufacturers. These colors were compared to the Soil Color Name Charts, a selected group of colors from the Munsell color standards. The buffs are close to the Munsell very pale brown (10 YR 8/4), although the notation ranges from 10 YR 7/4 to 8/4. The "very pale brown" is between white and light gray on the left of the names chart and yellow on the right of the names chart, and is much closer to white than the color commonly thought of as brown. The salmon used as a color name in Table 8 ranges from pink (5 YR 7/4) to reddish yellow (7.5 YR 7/6). The red of the tables ranges from light reddish brown (2.5 YR 6/4) through light red (2.5 YR 6/6) to red (2.5 YR 5/8).
Descriptions of Samples
Graneros Shale
Sample from depth of 2001 feet
Megascopic description—The rock is a very light gray claystone that has conchoidal to blocky fracture. Where the sample has been worn by coring it displays markedly parallel thin laminae. The sample swells in water and is bentonite.
Petrographic description—In thin section (Pl. 2A, B), it can be seen that the bentonite is composed of montmorillonite showing orientation along wavy laminae that are subparallel to bedding. Locally, the montmorillonite is segregated into coarse wormlike books that amount to about 5 percent of the rock and measure 0.1 to 0.3 millimeter in length. The rock is vertically jointed and shows distinct bedding. There is gradation in grain size and color from the centers to the margins of the joint blocks; centers of the joint blocks are in part coarser grained than the margins and are brown in transmitted light, whereas the margins are uniformly very fine grained and light brown. Some areas in the block centers are white under reflected light, whereas the rest of the rock is gray to light brown.
Plate 2—Photomicrographs of thin sections, x 65. A. Sample 2001, bentonite, plane polarized light, Montmorillonite book, M, and quartz grains, Q, embedded in fine-grained montmorillonite. Black grains are pyrite. B. Sample 2001, bentonite, crossed nicols. Note composite structure of montmorillonite book. C. Sample 2017, calcareous mudstone, plane polarized light. Circular bodies, F, are fossilized foraminifera. Black specks and stringers include carbonaceous matter and pyrite. D. Sample 2017, calcareous mudstone, crossed nicols. Light-gray areas, C, are calcite. E. Sample 2095, sandstone, plane polarized light. Detrital grains, mostly quartz, embedded in clay matrix, C, that also contains organic matter, O. F. Sample 2095, sandstone, crossed nicols. Q, quartz; F, feldspar; and M, metaquartzite. Note irregular shape of most grains and sutured margins shown by some.
In addition to montmorillonite, the thin section contains about 5 percent small angular fragments of quartz and plagioclase that range in length from 0.07 to 0.18 millimeter. A small percentage of pyrite is present in the rock as irregular patches and minute blebs ranging in diameter from 0.05 to 1 millimeter. Pyrite also occurs as veinlets measuring about 0.05 millimeter in length.
X-ray data—In the fraction finer than 2 microns, the sample is composed completely of sodium montmorillonite as shown by a 12.6Å basal spacing of the air-dried sample (Grim, 1953, p. 57). On glyceration the basal spacing expanded to 18.0Å, and collapsed to 9.8Å on heating to 450°C (Fig. 4).
Figure 4—Diffractometer patterns of fraction finer than 2 microns from selected claystone and mudstone samples. Sample 2001: sodium bentonite, Graneros Shale; A, air-dried; B, glycerated. Sample 2017: interlaminated sodium bentonite and calcareous mudstone, Graneros Shale; A, air-dried; B, glycerated; C, heated to 575°C. Sample 2034.5: mudstone, Omadi Formation (Gurley member); A, air-dried; B, glycerated; C, heated to 575°C. Sample 2074: mudstone, Omadi Formation (Gurley member); A, air-dried; B, glycerated; C, heated to 450°C; D, boiled in hydrochloric acid and air-dried.
Differential thermal analysis—Differential thermal curves indicate (Fig. 5) that the only clay mineral present in the bentonite is sodium montmorillonite. The single absorbed-water peak at 145°C is exceptionally strong. The first dehydration peak at 697°C is fairly strong, and the second at 885°C is fairly weak. There is only a suggestion of an exothermic peak at 925°C. The small percentage of pyrite in the sample produced small but sharp exothermic peaks at 477° and 537°C.
Figure 5—Differential thermal curves of core samples taken at 2001 feet (Graneros Shale), 2017 feet (Graneros Shale), 2034.5 feet (Gurley member of Omadi Formation), and 2074 feet (Gurley member of Omadi Formation).
Sample from depth of 2017 feet
Megascopic description—The rock consists of interlaminated medium-gray claystone and light-gray calcareous mudstone.
Petrographic description—Two distinct rock types are apparent in thin section: one is claystone; the other, calcareous mudstone, is composed mainly of clay and calcite.
The claystone is composed mainly of montmorillonite and shows good preferred orientation parallel to bedding. The clay is light brown in transmitted light. Embedded in the claystone are rounded books of montmorillonite (?) that transect the lamination of the rock, but that have not disturbed the orientation of the surrounding clay flakes. The books amount to about 15 percent of the claystone and range in diameter from 0.05 to 0.3 millimeter. They are dominantly clear in transmitted light but are transected by pleochroic brown material (X and Y = dark brown and Z = light brown) that forms distinct bands parallel to the layering of the books.
Pyrite, which amounts to about 7 percent of the claystone, occurs as minute euhedral or subhedral grains measuring less than 0.01 millimeter in diameter and as irregular aggregates measuring as much as 0.15 millimeter in diameter. The claystone also contains trace amounts of quartz as angular fragments ranging in length from 0.05 to 0.15 millimeter.
The calcareous part of the rock consists of approximately equal amounts of clay and calcite (Pl. 2C, D). The clay is mainly montmorillonite and has good preferred orientation parallel to bedding. Trace amounts of clay also are present as books identical to those described above.
The calcite is seen as finely divided grains ranging in diameter from 1 to 10 microns. In addition to finely divided calcite, irregular patches of optically continuous calcite that measure as much as 0.2 millimeter in length approximate 10 to 15 percent of the calcareous mudstone. Optically continuous calcite also fills numerous foraminiferal tests. The calcite filling approximates 10 or 15 percent of the calcareous mudstone.
Pyrite similar to that in the claystone amounts to about 10 percent of the calcareous mudstone. Trace amounts of quartz also are present as small, angular, sutured grains.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 70 percent montmorillonite and 15 percent each kaolinite and illite. The basal spacing of air-dried montmorillonite was 12.6Å. The montmorillonite basal spacing expanded to 18Å on treatment with glycerol and collapsed to 9.8Å on heating to 450°C (Fig. 4). It is a sodium montmorillonite (Grim, 1953, p. 57). Minor amounts of calcite also are present in the -2 micron fraction.
Differential thermal analysis—Montmorillonite is the only clay mineral definitely indicated by the differential thermal analysis curves (Fig. 5). The single strong peak at 132°C indicates a sodium montmorillonite. The second endothermic montmorillonite peak at 675°C is fairly short and broad; its small size may be due in part to the extremely strong exothermic reaction (caused by both pyrite and organic matter), which reaches a peak at 475°C. Kaolinite and illite, known from x-ray to be present, would tend to broaden and lower the second endothermic peak for montmorillonite. The third endothermic montmorillonite peak at 874°C is unusually strong and doubtless was produced by both montmorillonite and calcite. The usual exothermic reaction at or above 900°C is very weak.
Omadi Formation—Gurley Member
Sample from depth of 2034.5 feet
Megascopic description—The rock is a medium- to dark-gray thin-laminated mudstone that has slight fissility. Sparse minute mica flakes are apparent on the laminae. Pyrite is present as crystalline aggregates measuring less than 3 millimeters in diameter.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 60 percent partly degraded illite, 30 percent kaolinite, and 10 percent chlorite (Fig. 4). Minor amounts of quartz also are present in the -2 micron fraction.
Differential thermal analysis—Pyrite and carbonaceous material dominate the differential thermal record of this sample (Fig. 5) from about 200°C to 700°C. An endothermic peak at 543°C and a very small one at 605°C are superimposed on the broad exothermic peaks produced by pyrite and carbonaceous material. The moderately sharp endothermic peak at 543°C reflects the presence of degraded illite and kaolinite. The 605°C inflection probably is caused by a variation in the intensity of the large exothermic reaction. The very small absorbed-water peak at 110°C indicates that very little montmorillonite can be present. Very small deflections to the endothermic side of the record at 750° and 790°C may be caused by the presence of chlorite. The high-temperature endothermic peak at 850°C is extremely weak, but the exothermic reaction at 950°C is fairly strong, probably owing to the presence of kaolinite.
Sample from depth of 2039 feet
Megascopic description—The rock is a medium-gray, fine- to medium-fine-grained sandstone in which irregularly spaced mudstone films impart a wavy lamination to the rock. The sample contains trace amounts of mica as small flakes, and abundant interstitial clay. Sparse concentrations of minute grains of pyrite locally give the rock a mottled appearance.
Petrographic description—In thin section, the rock is seen to be composed mainly of subangular to subrounded grains of quartz that are embedded in a clay matrix. The detrital grains measure 0.02 to 0.40 millimeter on their long axes; most range from 0.15 to 0.25 millimeter in length. The detrital grains show only slight tendency for orientation of their long axes parallel to bedding. Bedding is indicated in thin section by alignment of the interstitial clay and its concentration into thin layers.
Detrital quartz amounts to about 84 percent of the rock and occurs as subangular to subrounded grains that show abundant silica overgrowths as well as sutured margins. The overgrowths locally are delineated from the original detrital quartz grains by lines of inclusions. Apatite, rutile-like needles, "dust planes", and fluid bubbles are present as inclusions in the quartz. A few of the quartz grains show a mosaic pattern under crossed nicols and probably are detrital meta quartzite.
Other detrital grains include chert, feldspar, and mica. Grains of detrital chert that is both fine and coarse grained amount to about 2 percent of the rock and hold abundant small inclusions of clay. The detrital feldspar includes orthoclase, microcline, and plagioclase and amounts to about 1 percent of the rock. Muscovite is present in trace amounts as slender books measuring as much as 0.4 millimeter in length. The books have shredded ends and most are bent.
Trace amounts of tourmaline, zircon, staurolite, and phyllite fragments are present as detrital grains. The tourmaline, which is both brown and blue green and is strongly pleochroic, occurs as subangular grains. The zircon is clear and well rounded. The staurolite occurs as light-yellow-brown subrounded grains. The phyllite fragments are well rounded, measure about 0.2 millimeter in length, and are composed of various amounts of sericite and anhedral quartz.
The matrix is composed mainly of clay but locally contains minor amounts of calcite and pyrite. The clay, which fills pore spaces and occurs as thin films surrounding the detrital grains, amounts to about 7 percent of the rock. The clay is brown in transmitted light, strongly birefringent, and oriented subparallel to the bedding. Calcite is present as anhedral networks between the detrital grains and has replaced the margins of the detrital grains. It amounts to about 1 percent of the rock. Pyrite, which amounts to as much as 3 percent of the rock, has replaced both the matrix and parts of the detrital grains and occurs as minute subhedral grains and anhedral patches.
Glauconite is present in trace amounts as rounded masses measuring as much as 0.2 millimeter along their long axes. Rounded grains of leucoxene as much as 0.1 millimeter in length also are present in trace amounts.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 84 |
| Chert | 2 |
| Feldspar | 1 |
| Muscovite | Tr |
| Tourmaline | Tr |
| Zircon | Tr |
| Staurolite | Tr |
| Phyllite fragments | Tr |
| Clay | 7 |
| Calcite | 1 |
| Pyrite | 3 |
| Glauconite | Tr |
| Leucoxene | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 45 percent partly degraded illite, 35 percent kaolinite, and 20 percent chlorite. Minor amounts of quartz, feldspar, and calcite also are present.
Sample from depth of 2074 feet
Megascopic description—The rock is a medium-gray, slightly silty, thin-laminated mudstone showing slickensides and slight fissility.
X-ray data—In the fraction finer than 2 microns, the clay minerals include approximately 55 percent illite that is partly degraded, 15 percent kaolinite, 20 percent chlorite, and 5 percent each montmorillonite and mixed-layer clay (Fig. 4). The mixed-layer clay has basal reflections between 11 and 13Å and at about 25Å. On treatment with glycerol, the basal reflections shift to 14 and 29Å respectively. Also present are minor amounts of quartz.
Differential thermal analysis—The only peak of any size produced on differential thermal analysis of this sample (Fig. 5) was a moderately broad peak at 535°C indicating mixed-layer clay or degraded illite. The endothermic peak at 890°C is very weak. The exothermic peak at 955°C is small but definite. The strong exothermic reaction between about 200° and 500°C, which produces peaks at 327° and 460°C, is caused by the presence of relatively large amounts of pyrite and organic material in the sample. The weak exothermic deflection at 745° could be attributed to chlorite, but this interpretation is not positive, inasmuch as similar peaks appear in the thermo grams of samples containing no indication of chlorite on analysis by x-ray. A small quartz peak at 567°C is superimposed on the large endothermic clay peak. This alpha to beta quartz inversion temperature recorded is about 6° below the actual inversion temperature and may indicate a temporary lag at the junction of the control thermocouple.
Sample from depth of 2082 feet
Megascopic description—The rock is a light-brownish-gray thin-bedded to thin-laminated fine-grained sandstone. Wavy films of dark-gray mudstone occupy the bedding planes. Small flakes of muscovite are disseminated through the rock and scattered on bedding surfaces.
Petrographic description—In thin section, the rock is seen to be composed mainly of subangular to subrounded grains of quartz that are embedded in a clay matrix. The detrital grains measure 0.02 to 0.35 millimeter on their long axes; most measure 0.1 to 0.2 millimeter in length. The detrital grains show slight tendency for preferred orientation of their long axes subparallel to bedding. Bedding is indicated by local concentration of interstitial clay and carbonaceous matter into thin bands.
Detrital quartz amounts to about 83 percent of the rock and occurs as subangular and subrounded grains. The quartz grains show abundant silica overgrowths, some of which are delineated from the rest of the grain by lines of inclusions. In addition, some of the grains have sutured margins. "Dust planes", apatite, tourmaline (?), chlorite (?), and fluid bubbles are present as inclusions in the quartz grains. A few of the quartz grains show a mosaic pattern under crossed nicols and probably are detrital metaquartzite.
Other detrital grains include feldspar, chert, mica, and tourmaline. Feldspar amounts to about 3 percent of the rock and includes plagioclase, microcline, and orthoclase, and shows differing degrees of argillic alteration. About 2 percent of the rock consists of grains of detrital fine-grained chert that is clouded by small inclusions of clay or organic matter. Muscovite, which constitutes about 2 percent of the rock, is present as slender books having shredded ends. Many books are bent and measure as much as 0.7 millimeter in length. Much of the muscovite is concentrated in layers that contain abundant interstitial clay. Trace amounts of detrital tourmaline also are present. The tourmaline grains are subrounded, pleochroic, and clear or brownish green.
Phyllite fragments approximate 2 percent of the rock. They are composed of mica, chlorite, and anhedral quartz, and measure as much as 0.2 millimeter in length.
The matrix is composed mainly of clay but locally includes minor amounts of chert, calcite, and pyrite. The clay, which constitutes about 5 percent of the rock, occurs as thin films separating the detrital grains. It is brownish green in transmitted light and its birefringence is typical of intermixed illite and kaolinite. Locally, small books of kaolinite are present in pore spaces. Secondary chert, which amounts to about 1 percent of the rock, occupies pore space and is clouded by small particles of clay or organic matter. Calcite is present in trace amounts as optically continuous patches that replace both detrital grains and matrix. Pyrite, also present in trace amounts, replaces both detrital grains and matrix and encrusts particles of carbonaceous matter.
Carbonaceous matter is present as elongate irregular particles that are interstitial to the detrital grains and also are incorporated in the layers containing abundant clay. Individual particles are partly pyritized and measure as much as 0.7 millimeter in length. Carbonaceous matter amounts to less than 1 percent of the rock.
Trace amounts of glauconite are present as rounded masses that are interstitial to the detrital grains and measure as much as 0.25 millimeter along their long axes.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 83 |
| Chert | 2 |
| Feldspar | 3 |
| Muscovite | 2 |
| Tourmaline | Tr |
| Phyllite fragments | 2 |
| Clay | 5 |
| Secondary chert | 1 |
| Calcite | Tr |
| Pyrite | Tr |
| Carbonaceous matter | <1 |
| Glauconite | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 40 percent kaolinite, 40 percent partly degraded illite, 10 percent chlorite, and 10 percent montmorillonite. Also present are traces of quartz, feldspar, and calcite.
Sample from depth of 2095 feet
Megascopic description—The rock is a light-gray fine-grained sandstone that has abundant irregularly spaced, contorted medium- to dark-gray thin mudstone laminae. Sparse flakes of mica are apparent in the hand sample.
Petrographic description—In thin section (Pl. 2 E, F), the rock is seen to be composed mainly of fine-grained detrital quartz and about 15 percent interstitial clay as matrix. The detrital grains range in length from 0.02 to 0.3 millimeter; most are 0.1 to 0.2 millimeter along their long axes. The detrital grains are subrounded to subangular. The clay matrix is brown in transmitted light and may contain organic matter.
Quartz amounts to about 72 percent of the rock. Most of the quartz is detrital, but trace amounts occur as overgrowths on detrital grains and as irregular patches in the matrix. Overgrowths on quartz grains generally are limited to areas of least abundant interstitial clay. Locally some quartz grains have sutured borders and are partly replaced by calcite. Most of the quartz shows uniform extinction, but some grains have a mosaic pattern under crossed nicols and probably are detrital metaquartzite. Inclusions in the detrital quartz include rutile-like needles, small subhedral crystals of apatite, sphene (?), zircon, "dust planes", and spherical fluid inclusions.
Detrital feldspar amounts to about 5 percent of the rock and includes plagioclase, microcline, and orthoclase. The feldspar grains are subrounded to subangular and show differing degrees of argillic alteration. Chert, in subrounded to subangular grains, constitutes about 3 percent of the rock.
Rounded phyllite fragments, present in traces, are composed of well-oriented sericite flakes and various amounts of anhedral quartz, and measure as much as 0.2 millimeter in length.
Interstitial to the detrital grains is an argillic matrix that amounts to about 15 percent of the rock. The matrix is brown in transmitted light and is irregularly distributed through the rock as contorted stringers and blebs. It is composed mainly of illite that is oriented subparallel to the laminae of the rock. Included in the matrix are trace amounts of calcite and quartz that form irregular patches. Irregularly distributed through the matrix are trace amounts of chlorite (?) and about 1 percent well-crystallized illite that is segregated as clear books and patches. Subhedral books of muscovite, which amount to about 1 percent of the rocks, also are present in the matrix. The books range in length from 0.05 to 0.3 millimeter and locally are bent around quartz grains.
Magnetite or ilmenite (?), or both, as irregular patches and blebs, appear as replacements of the detrital grains and the matrix. They amount to about 2 percent of the rock. Limonite or hematite, or both, in similar patches amount to about 1 percent of the rock. Leucoxene is present in traces as small blebs scattered through the rock.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 72 |
| Feldspar | 5 |
| Chert | 3 |
| Phyllite fragments | Tr |
| Clay | 15 |
| Calcite | Tr |
| Chlorite (?) | Tr |
| Illite books and patches | 1 |
| Muscovite | 1 |
| Magnetite or ilmenite (?), or both | 2 |
| Limonite or hematite, or both | 1 |
| Leucoxene | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 50 percent illite that is partly degraded, 30 percent kaolinite, and 20 percent chlorite (Fig. 10). Also present are minor amounts of quartz and feldspar.
Differential thermal analysis—The differential thermal curve (Fig. 11) for the fraction finer than 2 microns from this sandstone sample shows a fairly strong endothermic peak at 555°C and a definite exothermic peak at 957°C, both of which are characteristic of kaolinite. A fairly large percentage of degraded illite in the sample is definitely indicated by the double absorbed-water peak at 95° and 105°C, and by a slight broadening at the 555°C peak. The small 740°C endothermic swing could be ascribed to chlorite if similar peaks were not present on the curves for samples not containing chlorite. The large exothermic swing producing peaks at 325° and 415°C was produced by carbonaceous matter and perhaps pyrite as well.
Omadi Formation-Huntsman Member
Sample from depth of 2118 feet
Megascopic description—The core sample is dark-gray to very dark gray thin-laminated mudstone that has good fissility parallel to the laminae. Stringers and blebs of very light gray micaceous siltstone parallel the lamination. Sparse specks of carbonaceous matter are apparent in the sample.
X-ray data—In the fraction finer than 2 microns, the clay minerals include approximately 60 percent illite that is partly degraded, 25 percent kaolinite, and 15 percent chlorite. The degraded illite has a superlattice reflection at about 27Å in the air-dried samples. On glyceration, the reflection shifted to about 30Å. Also present are minor amounts of quartz.
Omadi Formation-Cruise Member
Sample from depth of 2146 feet
Megascopic description—The rock is light- to medium-gray conglomeratic sandstone that grades downward to conglomerate. The conglomeratic sandstone is mainly fine grained but contains smooth well-rounded pebbles that measure as much as 2.5 centimeters in length. The conglomerate is composed of pebbles that are well rounded, measure as much as 3 centimeters in length, and are embedded in a matrix of very fine grained to coarse-grained sandstone. The pebbles in both the conglomerate and the conglomeratic sandstone have black exterior shells and darkgray to light-brown interiors. The conglomeratic sandstone has irregularly spaced wavy laminae. Bedding in the conglomerate is indistinct. Both rock types contain abundant interstitial clay, sparse muscovite flakes, and angular black fragments. The black fragments, which are thin, somewhat curved, and marked with slender arcuate striae, range in length from about 1 to 10 millimeters and are less than 1/2 millimeter thick. They may be shell fragments from phosphatic bivalved organisms. Sparse fragments of light- to dark-gray fossilized bone (?) also are apparent in the core sample.
Petrographic description—The sandstone and conglomerate are composed mainly of detrital quartz grains embedded in a brown clay matrix. The detrital grains range in long diameter from 0.02 to 0.56 millimeter and are poorly sorted. Incorporated in the rock are large rounded masses of collophane, the thin borders of which are black and dark brown in transmitted light. Also present are rectangular and gently curved fragments of interlayered collophane and birefringent apatite. Other fragments, which may be fossilized bone, are composed mainly of collophane and birefringent apatite (Pl. 3 A, B).
Plate 3—Photomicrographs of thin sections, x 65. A. Sample 2146, conglomeratic sandstone, plane polarized light. Quartz grains embedded in matrix of clay, C, and collophane, P. Large striped grain, S, composed of collophane and apatite may be shell fragment. B. Sample 2146, conglomeratic sandstone, crossed nicols. C, clay, P, collophane, S, shell(?) fragment, and Q, quartz. C. Sample 2146, collophane pebble, plane polarized light. Clear grains are quartz; black specks include hematite and pyrite. D. Sample 2146, collophane pebble, crossed nicols. E. Sample 2146, collophane pebble, plane polarized light, Round patches, G, glauconite; light-gray grains quartz; black specks, hematite and pyrite. F. Sample 2189, coal, plane polarized light. Rock composed of clay, C, and translucent and opaque attritus. Note black spicules in translucent attritus at bottom of photograph. Bright round spots and round spots showing light margins and dark centers are resin globules.
The detrital quartz grains are subangular to subrounded and have sutured margins and silica overgrowths. The overgrowths are not delineated from the original detrital grains by lines of inclusions. Some of the quartz grains show mosaic patterns under crossed nicols and probably are detrital metaquartzite. Present as inclusions in the quartz grains are fluid bubbles, "dust planes", and sparse rutile-like needles.
Sparse chert and even lesser amounts of feldspar are present as detrital grains in the rock. The chert is both fine and coarse grained. Margins of the grains commonly are uneven and indistinct; they seem to have been modified by secondary solution or by deposition of silica, or by both. The feldspar includes microcline, plagioclase, and orthoclase, whose grains show differing degrees of argillic alteration.
Other detrital minerals include trace amounts of tourmaline and muscovite. The muscovite is present as slender shredded books that are bent about other detrital grains and that measure as much as 0.2 millimeter in length. The tourmaline is subangular and strongly pleochroic. Some grains are blue, some brown, and some are both blue and brown.
Small rounded fragments of quartz-sericite phyllite also are present in trace amounts. The fragments are as much as 0.15 millimeter long.
The pebbles are translucent and light brown in transmitted light. X-ray and optical data show that the light-brown material is collophane. The borders of the pebbles are partly opaque owing to pyrolusite (?) and have a submetallic steel-gray color under reflected light. Included in the pebbles are abundant angular grains of quartz, chert, and feldspar, and rounded masses of glauconite (Pl. 3 C, D, E). The included detrital grains range in length from 0.05 to 0.1 millimeter, and the rounded masses of glauconite are as much as 0.2 millimeter long. Also included in the pebbles are irregular blebs of pyrolusite (?).
Numerous shell (?) fragments are irregularly distributed; most are subrectangular and curved. They range in thickness from 0.05 to 0.2 millimeter and are composed of alternate layers of apatite and collophane. Some of the collophane is opaque and seems to contain pyrolusite (?). Fragments of bone 0) are sparse. They too are composed mainly of collophane but also contain minor amounts of anisotropic apatite. The haversian canals (?) are partly filled with pyrolusite (?) and pyrite.
The matrix is composed mainly of strongly birefringent brown clay (illite) that seems to be poorly oriented. Also present in the matrix is abundant glauconite, as irregular blebs that grade imperceptibly into the rest of the matrix and as discrete round masses as much as 0.25 millimeter in length. Pyrite and limonite are present in traces as anhedral blebs in the matrix. Locally, adjacent to the phosphate pebbles, the matrix is composed of collophane and pyrolusite (?). The distribution and interstitial occurrence of collophane in the matrix indicate that there may have been secondary deposition of collophane.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 40 percent illite that is partly degraded, 30 percent kaolinite, 20 percent chlorite, and 10 percent mixedlayer clay. The mixed-layer clay has reflections between 11 and 13Å and at 26Å. On treatment with glycerol, the reflections shifted to 14 and 31Å respectively. Trace amounts of quartz also are apparent.
The black pebbles were studied by x-ray diffraction. They were ground to pass a 74-micron sieve, and diffractograms were made. The diffractograms show reflections for apatite, lesser amounts of quartz, and minor amounts of feldspar. The apatite has reflections at 1.723 and 1.783Å that indicate that it is carbonate-fluorapatite (Silverman, Fuyat, and Weiser, 1951, p. 6). Optical data indicate that the mineral is collophane, an isotropic form of apatite.
Sample from depth of 2166 feet
Megascopic description—The rock consists of alternating thin laminae of medium-gray mudstone and light-gray siltstone; abundant minute mica flakes and sparse carbonaceous specks are disseminated on the laminae. Locally the siltstone laminae have been deformed into wavy layers and small elongate pods that transect bedding at angles as great as 40°.
Petrographic description—In thin section, the rock is seen to be composed mainly of silt and clay particles that are segregated into distinct laminae. The siltstone laminae locally contain as much as 30 percent interstitial organic matter and clay, whereas the mudstone laminae locally contain as much as 30 percent quartz and are brownish green in transmitted light. The laminae range in thickness from 0.02 to 3 millimeters and locally are deformed so that scroll-like projections of silt penetrate the mudstone. Clay flakes in the mudstone are aligned subparallel to the siltstone laminae.
Quartz constitutes about 57 percent of the rock. It occurs as subangular silt-sized detrital grains in the siltstone laminae and in lesser amount in the mudstone laminae. Long axes of the quartz grains measure 0.15 to 1.5 millimeters, but most grains are 0.15 to 0.55 millimeter long. The quartz grains contain inclusions of apatite (?), zircon, "dust planes", and fluid bubbles. The matrix of the siltstone laminae is composed mainly of small flakes of illite as much as 0.1 millimeter long.
The mudstone laminae are composed mostly of strongly birefringent flakes of clay (illite?). The flakes are about 0.03 millimeter in length and are aligned subparallel to the laminae. Total clay content of the rock is estimated to be 27 percent.
Disseminated flakes of muscovite, ranging in length from 0.05 to 0.5 millimeter, amount to about 4 percent of the rock. Trace amounts of green biotite or chlorite, or both, are present mainly in mudstone laminae. Traces of leucoxene occur in the siltstone laminae.
Organic matter, probably macerated coal, although scattered through the thin section, is concentrated mainly in the mudstone laminae. It appears as small irregular particles and as flakes or films as much as 2 millimeters long. Most of the organic matter is opaque, but some appears translucent red brown. Organic matter amounts to about 11 percent of the thin section.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 57 |
| Clay | 27 |
| Muscovite | 4 |
| Biotite, chlorite, or both | Tr |
| Leucoxene | Tr |
| Organic matter | 11 |
X-ray data—In the fraction finer than 2 microns, the clay minerals include 60 percent illite that is partly degraded, 25 percent kaolinite, and 15 percent chlorite. Also present are minor amounts of quartz.
Sample from depth of 2176 feet
Megascopic description—The rock is a medium-gray, somewhat silty, thin-laminated mudstone having very little fissility. Diffuse limonitic stains parallel the laminae. Sparse minute mica flakes and specks of carbonaceous matter are apparent on bedding planes.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 60 percent partly degraded illite and 20 percent each of kaolinite and chlorite (Fig. 6). The rock also contains minor amounts of quartz in the fraction finer than 2 microns.
Figure 6—Diffractometer patterns of fraction finer than 2 microns from selected mudstone samples. Sample 2176: mudstone; A, air-dried; B, glycerated; C, heated to 575°C; D, boiled in hydrochloric acid and air-dried. Sample 2191: argillaceous siltstone; A, air-dried; B, glycerated; C, heated to 450°C. Sample 2233.5: mudstone; A, air-dried; B, glycerated; C, heated to 575°C. Sample 2308: mudstone; A, air-dried; B, glycerated; C, heated to 575°C. All samples from Cruise member, Omadi Formation.
Differential thermal analysis—A small endothermic peak below 100°C, a fairly large, broad endothermic peak at 540°C, and a small endothermic peak at 967°C indicate that the dominant clay mineral in this sample is mixed-layer clay or degraded illite (Fig. 7). In addition, organic material produced a broad exothermic curve from 200° to about 500°C showing peaks at 332° and 465°C. A very small quartz peak at 573°C is superimposed on the large 540°C endothermic peak.
Figure 7--Differential thermal curves of core samples taken at 2,176 feet, 2,191 feet, 2,233.5 feet, and 2,308 feet. All samples from Cruise member of Omadi Formation.
Sample from depth of 2189 feet
Megascopic description—The rock is a dark-gray to black subbituminous coal showing dull luster and thin wavy laminae. Small lenses and wisps of light- to medium-gray clay parallel the bedding. Scattered areas having woody texture show on bedding planes. The coal has conchoidal fracture and abundant randomly oriented slickensides as well as incipient cubical fractures.
Petrographic description—In thin section (Pl. 3 F) the coal is seen to be composed mainly of macerated carbonaceous matter and clay. The clay has faint birefringence and forms discrete blebs, lenses, and layers that are almost free of carbonaceous matter and in which the clay mineral is oriented parallel to the bedding. Clay also forms a poorly oriented groundmass that includes abundant finely divided carbonaceous matter. Color of the clay in transmitted light depends upon the content of carbonaceous matter and ranges from very light brown to dark brown. Trace amounts of quartz as anhedral grains measuring as much as 0.1 millimeter in diameter also are apparent in thin sections of the coal.
The coaly material is macerated and includes resistant translucent and opaque attritus, abundant resin globules, and sparse fusain. The resin globules, which measure as much as 0.15 millimeter in diameter, contain minute tubular structures about 2 microns in diameter. The tubular structures may be fungus hypha (Robert W. Baxter, oral communication, March 1957).
Other structures observed in the coal include elongate translucent layers or bands that are bounded by opaque attritus and contain numerous opaque branching curved spicular carbonaceous particles. The bands measure as much as 0.4 millimeter in width, and the included spicular particles measure as much as 1 millimeter in length and 5 microns in diameter. Under crossed nicols, the translucent material assumes a rippled sheen similar to that exhibited by mica books. Birefringence may be partly masked by brown coloration, but seems to be first order gray. Extinction is parallel to the length of the layers and bands. The translucent material is slightly pleochroic from light brown to brown, and it probably is an aggregate of well-oriented clay.
X-ray data—In the fraction finer than 2 microns, the mineral constituents of the coal include a kaolin mineral, mixed-layer clay, and trace amounts of quartz. The kaolin mineral has a broad 001 peak at 7Å and seemingly is poorly crystallized. The mixed-layer clay has a broad diffraction band that extends from 10 to 15Å. On treatment with glycerol, the broad band is resolved into two small peaks at 10.1 and 14.7Å. On heating to 450°C, the reflection at 14.7Å disappears and the reflection at 10.1Å is intensified somewhat. Percentages of clay components estimated from the diffractogram of the glycerated sample are 40 percent kaolin and 60 percent mixed-layer clay.
Sample from depth of 2191 feet
Megascopic description—The rock is a light- to medium-gray argillaceous siltstone or very fine grained sandstone that contains abundant carbonaceous matter as fragments and films and as small veinlets steeply inclined to locally distinct bedding.
Petrographic description—In thin section (Pl. 4 A, B), the rock is seen to be composed mainly of silt-sized quartz grains embedded in a matrix of clay. Bedding is indistinct in thin section except where delineated by alignment of carbonaceous flakes and mineral matter. Bedding commonly is deformed: arcuate silty lenses and silty blebs merge with horizontal and gently inclined planar bedding.
Plate 4—Photomicrographs of thin sections, x 65. A. Sample 2191, siltstone, plane polarized light. Quartz grains embedded in clay matrix. Black material is organic matter. B. Sample 2191, siltstone, crossed nicols. C. Sample 2259, sandstone, plane polarized light. Quartz grains cemented mainly by calcite, C. D. Sample 2259, sandstone, crossed nicols. C, calcite. E. Sample 2421, sandstone, plane polarized light. Dark grain is tourmaline. F. Sample 2421, sandstone, crossed nicols.
Detrital quartz approximates 60 percent of the rock; the grains are subangular to subrounded and measure 0.01 to 0.25 millimeter along their long axes. The grain size distribution is bimodal; maxima lie between 0.01 and 0.05 millimeter, and between 0.1 and 0.2 millimeter. Other detrital minerals include traces of orthoclase (?) and plagioclase.
Clay matrix amounts to about 30 percent of the rock and is composed mainly of coarsely crystalline illite (?) whose flakes measure as much as 0.05 millimeter in length. Trace amounts of muscovite also are present in the matrix as books that range in length from 0.05 to 0.15 millimeter. The matrix shows two distinct directions of orientation of the clay and muscovite flakes, which are inclined between 30° and 45° to the axis of coring. Because of the preferred orientation of the constituent flakes, the matrix appears as a rhomboidal plexus under crossed nicols.
Allochthonous coalified matter amounts to about 10 percent of the rock. The coal is composed mainly of opaque and translucent attritus, which forms stringers parallel to the bedding as well as nearly vertical anastomosing veinlets. The apparent opaqueness of much of the coalified material seemingly is due to the thickness of the thin section. Locally the translucent attritus includes resin globules that measure as much as 0.12 millimeter in diameter. In addition to attritus, abundant carbonaceous matter has impregnated the siltstone matrix and imparted a brown color to it.
Examination of the hand sample indicates that the coalified material is lignite or subbituminous coal and that in addition to attritus the rock contains small amounts of fusain as stringers and films along bedding surfaces.
X-ray data—In the fraction finer than 2 microns, the clay minerals include approximately 65 percent illite that is partly degraded and 35 percent kaolinite. The x-ray patterns (Fig. 6) also show a weak reflection at 25Å. On glyceration, the 25Å reflection shifts to 29Å, a second order reflection appears at 14Å, and the shoulder on the illite 001 reflection disappears. Also present are minor amounts of quartz.
Differential thermal analysis—The differential thermal curve for this sample is similar to that for the sample at 2,176 feet (Fig. 7). The endothermic peak at 542°C for this sample is somewhat larger and sharper than the corresponding peak for the sample from 2,176 feet, and reflects the slightly smaller percentage of degraded illite and greater percentage of kaolinite indicated by the x-ray data for this sample. The thermal curve for this sample also shows a weak endothermic reaction at 900°C and a small exothermic peak at 975°C. An absorbed-water peak, such as is present on previously discussed curves, is not shown by this one. The exothermic reactions for carbonaceous matter, which produce peaks at 323° and 425°C, are strong, but the area under the peaks is somewhat less than for samples taken above a depth of 2,191 feet. It is probable that pyrite and marcasite, present in previously discussed samples, have increased the exothermic reaction in the range between 323° and 425°C; under the circumstances it is impossible to separate the reactions due to carbonaceous matter from those due to iron sulfides.
Sample from depth of 2193 feet
Megascopic description—Ths rock is a light- to medium-gray mudstone containing coalified material as films and irregular fragments. The mudstone has indistinct deformed bedding and numerous randomly oriented slickensides. It is locally silty, and the silty parts form indistinct irregular patches within the mudstone.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 65 percent partly degraded illite and 35 percent kaolinite. Minor amounts of quartz also are present.
Sample from depth of 2206 feet
Megascopic description—The rock is a light-gray to light-greenish-gray mudstone that has no obvious lamination and has conchoidal fracture. Sparse carbonaceous stringers and one fern-leaf imprint were observed in the sample. Numerous brown subspherical grains of siderite are apparent. The grains are about 0.5 millimeter in diameter and their surfaces are irregular, owing to crystal growth.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 70 percent illite that is partly degraded and 30 percent kaolinite. The degraded illite has a superlattice reflection at about 24Å, which shifts to about 30Å on glyceration. Also present in the -2 micron fraction are minor amounts of quartz and calcite.
Sample from depth of 2226 feet
Megascopic description—The rock is a light-gray to light-greenish-gray mudstone that has no obvious lamination. It is somewhat silty and breaks with a conchoidal fracture. Sparse subspherical brown grains of siderite are visible in the sample. Individual grains are about 0.5 millimeter in diameter, and their surfaces are irregular, owing to crystal growth.
X-ray data—In the -2 micron fraction, the clay minerals include approximately 65 percent partly degraded illite, 25 percent kaolinite, and 10 percent chlorite. The degraded illite has a superlattice reflection at about 25Å, which shifts to about 30Å on treatment with glycerol. Also present in the fraction finer than 2 microns are minor amounts of quartz.
Sample from depth of 2233.5 feet
Megascopic description—The rock is a light-gray mudstone having sparse irregular siltstone laminae, conchoidal fracture, and numerous slickensides, but no apparent fissility.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 65 percent illite that is partly degraded and about 35 percent kaolinite (Fig. 6). Minor amounts of quartz also are present.
Differential thermal analysis—A fairly large endothermic peak at 550°C dominates the differential thermal record of this sample (Fig. 7). Although the peak temperature is low, the shape of the peak is characteristic of kaolinite. The partly degraded illite indicated by x-ray data probably accounts for the slight broadening of the 550°C peak and for the small endothermic peak at 900°C. The small exothermic peak at 970°C is more or less normal for a mixture of these two clay minerals. A very broad exothermic shift below 400°C probably was produced by a small amount of organic matter. The small exothermic peak at 392°C indicates a minute amount of pyrite.
Sample from depth of 2239 feet
Megascopic description—The rock is a light-gray fine-grained sandstone showing indistinct wavy bedding. It contains abundant interstitial clay and abundant carbonaceous matter as thin seams, films, and fragments. Sparse mica flakes are disseminated through the rock.
Petrographic description—In thin section, the rock is seen to be composed mainly of subangular to subrounded detrital quartz grains embedded in a clay matrix. Wavy bedding is indicated by variations in amount of interstitial clay, by stringers of interstitial carbonaceous matter, and by diverse subparallel orientation of the long axes of detrital grains. The detrital grains measure 0.02 to 0.20 millimeter long.
Detrital quartz, which amounts to about 80 percent of the rock, is present as subangular and subrounded grains that have abundant silica overgrowths. The overgrowths locally are delineated by lines of inclusions, but most commonly they are not. Some of the quartz grains have sutured margins. Present as inclusions in the quartz grains are brownish-green tourmaline, apatite, chlorite (?), "dust planes", fluid bubbles, and rutile-like needles. Some grains have a mosaic texture under crossed nicols and probably are detrital metaquartzite.
Other detrital minerals include about 2 percent feldspar, 1 percent chert, and trace amounts of muscovite and tourmaline. The feldspar includes microcline, orthoclase, and plagioclase. Some feldspar grains show signs of secondary growth. Muscovite is present as slender books having shredded ends. The books measure as much as 0.3 millimeter in length and are bent around the detrital grains. The tourmaline is present as subangular, somewhat prismatic, gray-blue and brown grains.
The clay matrix approximates 11 percent of the rock but locally amounts to as much as 15 percent. The matrix is brown in transmitted light where abundant carbonaceous matter is present; elsewhere it is light green. The birefringence and green tinge indicate that the clay is mainly illite. The clay fills voids and forms thin films surrounding detrital grains; it lacks preferred orientation.
Carbonaceous matter approximates 5 percent of the rock and occurs as irregular stringers that are interstitial to the detrital grains and extend parallel to bedding, and as irregular blebs as as much as 0.3 millimeter long. Pyrite, which amounts to less than 1 percent of the rock, is present as small irregular patches and stringers in the carbonaceous matter.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 80 |
| Feldspar | 2 |
| Chert | 1 |
| Muscovite | Tr |
| Tourmaline | Tr |
| Clay | 11 |
| Carbonaceous matter | 5 |
| Pyrite | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include 55 percent illite that is partly degraded, 40 percent kaolinite, and 5 percent montmorillonite. Also present in the -2 micron fraction are minor amounts of quartz.
Sample from depth of 2254 feet
Megascopic description—The rock is a medium-gray mudstone containing abundant thin sandy siltstone laminae. The laminae are wavy and locally contorted and have little fissility. Traces of carbon are apparent as specks on bedding planes.
Petrographic description—In thin section, lenses and layers of siltstone and mudstone are inclined at differing angles, partly truncating and partly paralleling one another. The rock is composed of almost equal amounts of mudstone and siltstone.
The mudstone is composed mostly of brown strongly birefringent clay that has fair orientation parallel to the lenses and stringers. Included in the mudstone are books of illite, which measure about 0.01 millimeter in length and approximate 3 percent of the rock. Small grains of silt-sized quartz (0.02 to 0.05 millimeter in diameter) amount to 5 or 10 percent of the mudstone. The quartz grains are subangular to angular, in part corroded, and some show overgrowths of silica. Traces of shredded muscovite are present as flakes measuring as much as 0.05 millimeter in length. Traces of carbonaceous (?) matter are apparent as black specks measuring as much as 0.1 millimeter in length.
The layers and stringers of sandy siltstone are composed mainly of quartz grains ranging from 0.01 to 0.10 millimeter long. The grains are angular and sub angular and have been modified in part both by silica overgrowths and by suturing. Interstitial to the quartz grains are small flakes of light-green faintly pleochroic illite, which amount to about 20 percent of the rock and measure as much as 0.01 millimeter in length. In addition to quartz and clay, the siltstone layers contain trace amounts of feldspar (orthoclase?) and mica. The mica occurs as small shreds measuring as much as 0.25 millimeter in length.
X-ray data—In the fraction finer than 2 microns, the clay minerals include 60 percent partly degraded illite, 30 percent kaolinite, and 10 percent chlorite. Minor amounts of quartz also are present.
Sample from depth of 2259 feet
Megascopic description—The rock is a light-brownish-gray medium-fine-grained sandstone that has indistinct cross-lamination. Locally it contains abundant muscovite flakes and specks of interstitial argillic material. Blebs of pyrite that measure as much as 5 millimeters in diameter locally are apparent.
Petrographic description—In thin section (Pl. 4 C, D), the sandstone is seen to be composed mainly of subangular to subrounded detrital quartz grains cemented by calcite, silica, and clay. The detrital grains are 0.02 to 0.30 millimeter long, 'but most grains measure 0.1 to 0.2 millimeter. Bedding is indicated by some segregation of detrital grains according to size, by distribution of interstitial clay in layers, and by diverse subparallel orientation of the long axes of detrital grains.
Quartz amounts to about 85 percent of the rock. The detrital quartz grains have abundant silica overgrowths that commonly are delineated from the original detrital grains by lines of inclusions. The silica overgrowths commonly interlock to cement the rock. Initial roundness of the quartz grains is estimated to be 0.4 to 0.6. Trace amounts of detrital quartz show mosaic patterns under crossed nicols and probably are grains of metaquartzite. "Dust planes", fluid bubbles, and sparse rutile-like needles are present as inclusions in the quartz grains.
Other detrital minerals include chert, feldspar, muscovite, tourmaline, and staurolite. The chert, which amounts to about 1 percent of the rock, is fine grained and clouded by particles of clay. The feldspar includes microcline, orthoclase, and plagioclase, and amounts to about 2 percent of the rock. Muscovite approximates 1 percent of the rock. Locally it is concentrated along bedding planes but is otherwise present only in trace amounts. It occurs as slender books as much as 1 millimeter long, locally bent around the detrital grains. Pleochroic green tourmaline and yellow-brown staurolite are present in trace amounts as subrounded grains.
Phyllite or schist fragments, or both, approximate 2 percent of the rock. They are composed of anhedral quartz and sericite flakes that show diverse orientation.
In addition to siliceous cement, the rock contains about 7 percent yellow-brown calcite, as distinct rhombohedra and as aggregates of rhombohedral grains. The aggregates are 0.02 to 0.2 millimeter in diameter. Locally the calcite seems to have formed at the expense of clay matrix and quartz grains.
The clay matrix amounts to about 2 percent of the rock. It is composed of coarse clay flakes measuring as much as 0.01 millimeter in length, which occupy pore space and are diversely oriented. Birefringence indicates that the clay may include both illite and kaolinite.
Trace amounts of glauconite occur as rounded patches that are, interstitial to detrital grains and measure as much as 0.2 millimeter in diameter. Pyrite was observed in the hand sample, but none was found in thin section.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 85 |
| Chert | 1 |
| Feldspar | 2 |
| Muscovite | 1 |
| Tourmaline | Tr |
| Staurolite | Tr |
| Phyllite or schist fragments, or both | 2 |
| Calcite | 7 |
| Clay | 2 |
| Glauconite | Tr |
| Pyrite | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 45 percent partly degraded illite, 40 percent kaolinite, 7 percent each chlorite and mixed-layer clay, and 1 percent montmorillonite (Fig. 10). The mixed-layer clay has a basal reflection between 11 and 13Å; on treatment with glycerol, the basal reflection shifts to 14Å and a weak reflection appears at 28Å. Also present in this size fraction are minor amounts of quartz and calcite.
Differential thermal analysis—The endothermic peaks at 95°, 527°, and 940°C indicate degraded illite or mixed-layer clay, although the curves (Fig. 11) are not clearly diagnostic. Both the 95° and 527°C peaks are abnormally low, whereas the 940°C peak is high. It is probable that the fairly large percentage of kaolinite, confirmed as present by x-ray data, produced the fairly sharp point on the 527°C peak. The high-temperature exothermic reaction does not form a peak but drifts in the exothermic direction. Carbonaceous material and pyrite produced a definite and strong exothermic reaction that produced small peaks at 270°, 308°, and 388°C. The minor amount of calcite detected by the x-ray in the fraction finer than 2 microns should have produced a small endothermic deflection but did not. This sample sintered and shrank in the sample well—a fact that may account for the decided base line drift in an exothermic direction.
Sample from depth of 2280 feet
Megascopic description—The rock is a light-brownish-gray, medium-fine-grained friable sandstone. Sparse mica flakes and light-yellow argillic specks are discernible in the sample. No bedding is apparent.
Petrographic description—The thin section is composed mainly of subangular to subrounded grains of quartz, feldspar, and chert embedded in a matrix of clay minerals and chert. Slight tendency toward subparallel orientation of long axes of detrital grains is suggestive of bedding.
Detrital quartz amounts to about 84 percent of the rock and occurs as subangular and subrounded grains measuring 0.02 to 0.4 millimeter long; most grains are 0.2 to 0.3 millimeter long. Quartz grains show sparse local overgrowths that do not surround the grains. Some overgrowths are delineated from the original quartz grains by inclusions of the matrix. Many grains have sutured outlines that indicate secondary solution of quartz. "Dust planes", rutile-like needles, apatite, tourmaline or staurolite, or both, chlorite (?), and fluid bubbles are present as inclusions in the quartz grains. Some quartz grains show a mosaic pattern under crossed nicols and probably are detrital metaquartzite.
Other detrital grains include feldspar, chert, and mica. The feldspar, including plagioclase, microcline, and orthoclase, amounts to about 3 percent of the rock and shows differing degrees of argillic alteration. Grains of detrital chert amount to about 1 percent of the rock. Macerated muscovite and biotite in flakes as long as 0.7 millimeter occur in trace amounts. Also present in trace amounts are rounded grains of pleochroic blue-green tourmaline, yellow-brown staurolite, and clear zircon.
The matrix is composed mostly of clay, which approximates 7 percent of the rock. The clay is mainly brownish-green illite and occurs as interstitial flakes as much as 0.02 millimeter long. Aggregates of cryptocrystalline silica and clay also are present in the matrix. Locally, irregular patches of sutured quartz seem to have replaced clay. The quartz and cryptocrystalline silica in the matrix approximate 3 percent of the rock. Traces of calcite have replaced clay matrix locally. Barite (?), clouded with small specks of carbonaceous matter, forms about 1 percent of the rock as minute grains that line and fill voids interstitial to the quartz grains.
Present as secondary minerals are trace amounts of leucoxene and pyrite. The pyrite appears as small irregular replacements of the matrix and of detrital grains. The leucoxene appears as small irregular and rounded blebs 0.07 to 0.15 millimeter in length.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 84 |
| Feldspar | 3 |
| Chert | 1 |
| Muscovite and biotite | Tr |
| Zircon | Tr |
| Tourmaline | Tr |
| Staurolite | Tr |
| Clay | 7 |
| Secondary silica | 3 |
| Barite | 1 |
| Calcite | Tr |
| Pyrite | Tr |
| Leucoxene | Tr |
Sample from depth of 2304 feet
Megascopic description—The rock is a light-brown, medium-fine-grained sandstone containing abundant pyrite blebs surrounded by. small nodular aggregates of calcite cement, which give the rock a warty appearance where worn by the core barrel. Sparse small flakes of mica are apparent in the hand sample. No bedding was observed.
Petrographic description—In thin section, the rock is seen to be composed mainly of subangular and subrounded quartz grains, locally cemented by calcite and lesser amounts of pyrite, but more commonly by clay and silica. Size of the detrital grains ranges from 0.05 to 0.40 millimeter in long diameter; most grains are 0.15 to 0.30 millimeter in long diameter. Bedding is not apparent in thin section.
Quartz constitutes about 86 percent of the rock. The quartz grains have abundant silica overgrowths, some of which are delineated by lines of inclusions. Locally the overgrowths interlock to cement the rock. Many of the quartz grains have sutured margins, particularly where the rock contains abundant calcite and pyrite. Some of the quartz grains reveal a mosaic pattern under crossed nicols and are thought to be particles of metaquartzite. Present as inclusions in the quartz grains are chlorite (?), rutile-like needles, apatite, fluid bubbles, and "dust planes".
Other detrital minerals include chert, feldspar, muscovite, and tourmaline. Detrital chert amounts to about 2 percent of the rock; feldspar constitutes about 1 percent and includes microcline, orthoclase, and plagioclase; muscovite is present in trace amounts as slender books measuring as much as 0.3 millimeter long. Trace amounts of subangular green tourmaline also are present.
Detrital grains of phyllite or schist, or both, amount to about 1 .percent of the rock. They are composed of parallel sericite and muscovite flakes, but contain various amounts of quartz.
Irregularly distributed patches of clay, calcite, and pyrite constitute the matrix. The clay is interstitial, diversely oriented, and amounts to about 4 percent of the rock. Its birefringence indicates that it may be mainly illite and kaolinite. Calcite, constituting 4 percent of the rock, in optically continuous rounded patches as much as 10 millimeters in diameter is irregularly distributed in horizontal zones. Most of the calcite is interstitial to the detrital grains, but locally it has formed at their expense. Pyrite, which amounts to about 1 percent of the rock, forms anhedral blebs and stringers within the rounded patches of calcite. It is mainly interstitial to the detrital grains that are incorporated in the patches of calcite but locally has replaced them.
Trace amounts of glauconite and leucoxene also are present. The glauconite occurs as rounded interstitial patches as much as 0.3 millimeter long. Leucoxene is present as subrounded grains as much as 0.2 millimeter in diameter.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 86 |
| Chert | 2 |
| Feldspar | 1 |
| Muscovite | Tr |
| Tourmaline | Tr |
| Phyllite or schist, or both | 1 |
| Clay matrix | 4 |
| Calcite | 4 |
| Pyrite | 1 |
| Glauconite | Tr |
| Leucoxene | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 30 percent kaolinite, 30 percent partly degraded illite, 25 percent chlorite, 10 percent mixed-layer clay, and 5 percent montmorillonite. The mixed-layer clay has basal reflections between 11 and 13Å. On treatment with glycerol, the basal reflections shift to 14Å, and a weak reflection appears as a shoulder on the trace at 28Å. Minor amounts of quartz and calcite also are present.
Kiowa Shale
Sample from depth of 2308 feet
Megascopic description—This sample is a medium-gray horizontally thin-laminated mudstone.
Petrographic description—In thin section, the rock shows both even and somewhat wavy horizontal lamination and is composed mainly of strongly birefringent clay (illite?) well oriented parallel to the bedding. In plane-polarized light, the thin section is light to dark brown.
Small books of faintly pleochroic light-green illite are scattered through the section, making up 1 to 2 percent of the rock. The books measure 0.01 to 0.05 millimeter in length. Angular to subangular grains of quartz, which measure 0.01 to 0.08 millimeter in length, constitute 5 to 10 percent of the rock. Most of the grains are oriented subparallel to the lamination of the rock, and many show delicate suturing.
Small fragments of black carbonaceous matter, which are disseminated through the thin section and measure as much as 0.05 millimeter in length, amount to about 3 percent of the rock. Traces of very finely divided pyrite encrust some of the black fragments.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 70 percent partly degraded illite and 30 percent kaolinite (Fig. 6). Also present are trace amounts of quartz.
Differential thermal analysis—Endothermic peaks were produced by the clay minerals in this sample at 115° and 895°C (Fig. 7). All peaks are relatively small. The 560°C peak was probably subdued to some extent by the strong exothermic reaction from about 200° to some point above 500°C. Although the dominant mineral is degraded illite, it is likely that kaolinite produced most of the 560°C endothermic peak and the 960°C exothermic reaction. Small peaks on the broad exothermic swing below 500°C could have been produced by either pyrite or carbonaceous matter.
Sample from depth of 2341 feet
Megascopic description—The rock is a medium-gray silty mudstone that has blocky fracture and sparse slickensides. Bedding is not apparent in the sample.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 65 percent illite that is partly degraded, 20 percent kaolinite, and 5 percent each of chlorite, montmorillonite, and mixed-layer clay. The mixed-layer clay has distinct but weak reflections between 11 and 12Å and may have a weak reflection at about 25Å (Fig. 8). On treatment with glycerol, the reflections between 11 and 12Å shift to 14A, and a shoulder is apparent at 28 or 29Å. Also present are minor amounts of quartz.
Figure 8—Diffractometer patterns of fraction finer than 2 microns from selected mudstone samples. Sample 2341: mudstone, Kiowa Shale; A, airdried; B, glycerated; C, heated to 575°C. Sample 2425: mudstone, Cheyenne Sandstone; A, air-dried; B, glycerated; C, heated to 575°C; D, boiled in hydrochloric acid and air-dried.
Differential thermal analysis—The differential thermal record (Fig. 9) for this sample is characterized by a strong exothermic reaction to carbonaceous matter and pyrite, which produced peaks at 320°, 422°, and 445°C, and by a decided base line drift to the exothermic side above 550°C. The base line drift probably was caused by the sintering and shrinking of the sample from the wall of the sample holder. The 120° absorbed-water peak, the broad 550° peak, and the small 882°C endothermic peaks are characteristic of degraded illite or mixed-layer clay but certainly could not be diagnosed without the aid of x-ray data. The 940°C exothermic peak indicates a small percentage of kaolinite. The apparent endothermic peak at 740°C does not fit the clay minerals present in the sample and may represent only a swing back to base line at the end of an extended exothermic reaction produced by carbonaceous matter.
Figure 9—Differential thermal curves of core samples taken at 2,341 feet (Kiowa Shale) and 2,425 feet (Cheyenne Sandstone).
Sample from depth of 2397 feet
Megascopic description—The rock is a light-gray aphanitic limestone containing numerous clay-lined cavities ranging in length from about 1 millimeter to 1 centimeter. Locally the rock shows fibrous structure. It contains no fossils. Etched surfaces show irregularly lobate rounded patches of calcite 1 to 5 millimeters long, which stand in relief. Cupric nitrate and cobalt nitrate staining techniques show that the rock contains no dolomite or aragonite.
Petrographic description—In thin section, the rock shows an indistinct irregular mosaic of calcite. Patches of calcite are locally separated by films of unidentified clay and contain very fine grained calcite as irregular blebs about 0.01 millimeter in diameter as well as fibrous calcite showing both radial and parallel alignment of fibers. Trace amounts of quartz are present as sutured grains about 0.05 millimeter long.
Cheyenne Sandstone
Sample from depth of 2421 feet
Megascopic description—The rock is a light-gray-brown fine- to medium-grained friable sandstone having only faint traces of bedding.
Petrographic description—In thin section (Pl. 4 E, F), the rock is seen to be composed mainly of subrounded to subangular quartz grains; amounts of detrital chert and feldspar are minor. The matrix is mainly clay. Bedding is not apparent in the thin section.
Quartz constitutes approximately 92 percent of the rock. Grains range from 0.05 to 0.5 millimeter in length, but most are 0.20 to 0.35 millimeter long. About 5 percent of the quartz grains show local overgrowths not completely surrounding the original subrounded grains. Most of the overgrowths are not delineated from the rest of the quartz grain by inclusions of matrix. Roundness is estimated to have been 0.4 to 0.6 prior to development of the overgrowths. A few of the quartz grains have sutured outlines. "Dust planes", apatite, fluid bubbles, and tourmaline or staurolite, or both, are present as inclusions in the quartz grains. Rutilated inclusions are sparse. Some of the quartz grains show mosaic patterns on extinction and probably are detrital metaquartzite.
The section contains 1 percent detrital chert and about 1 percent feldspar, including orthoclase, microcline, and plagioclase (?). Also among the detrital minerals are trace amounts of subrounded pleochroic blue-green tourmaline and rounded zircon.
The matrix, about 4 percent of the rock, is composed mainly of clay as minute brown flakes that barely separate the detrital grains. X-ray data show that the clay is approximately 45 percent montmorillonite. Also present in the matrix is about 1 percent barite (?) as minute crystals in the voids between quartz grains.
Leucoxene occurs in trace amounts as rounded and irregular grains 0.02 to 0.14 millimeter long.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 92 |
| Chert | 1 |
| Feldspar | 1 |
| Tourmaline | Tr |
| Zircon | Tr |
| Clay | 4 |
| Barite (?) | 1 |
| Leucoxene | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 15 percent illite, 25 percent kaolinite, 15 percent chlorite, and 45 percent montmorillonite (Fig. 10). Also present are minor amounts of quartz and calcite.
Figure 10—Diffractometer patterns of fraction finer than 2 microns from selected sandstone samples. Sample 2095: sandstone, Omadi Formation (Gurley member); A, air-dried; B, glycerated; C, heated to 575°C. Sample 2259: sandstone, Omadi Formation (Cruise member); A, air-dried; B, glycerated; C, heated to 575°C. Sample 2421: sandstone, Cheyenne Sandstone; A, air-dried; B, glycerated; C, heated to 575°C. Sample 2440: sandstone, Cheyenne Sandstone; A, air-dried; B, glycerated; C, heated to 575°C.
Differential thermal analysis—The differential thermal record (Fig. 11) of the fraction finer than 2 microns is dominated by the strong exothermic reactions in the temperature range of pyrite, marcasite, and carbonaceous matter, although none of these constituents were found by petrographic examination. The exothermic swing starts below 200°C and continues almost to 500°C. It probably extends as high as 700°C but is partly neutralized by the endothermic reaction from the clay minerals. The absorbed-water peak at 100°C is comparably strong, but the second endothermic reaction, reaching a peak at 576°C, is small and there is a faint indication of an endothermic peak at 695°C. Seemingly the general endothermic swing from about 500° to about 800°C was produced by a gradual loss of water from the clay minerals. There is no clear indication of a high-temperature endothermic peak or the usual exothermic reaction at 900°C or higher. The 100°C peak indicates montmorillonite, as does the minute 695°C endothermic deflection. The 576°C endothermic peak normally indicates degraded illite or mixed-layer clay.
Figure 11—Differential thermal curves of fraction finer than 2 microns, sandstone core samples taken at 2,095 feet (Gurley member of Omadi Formation), 2,259 feet (Cruise member of Omadi Formation), 2,421 feet (Cheyenne Sandstone), and 2,440 (Cheyenne Sandstone).
Sample from depth of 2425 feet
Megascopic description—The rock is a dark-gray wavy-laminated mudstone that contains sparse siltstone laminae. Sparse minute mica flakes are apparent on bedding surfaces. The rock exhibits blocky fracture and slight fissility.
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 65 percent partly degraded illite, 20 percent kaolinite, and 15 percent chlorite and montmorillonite (Fig. 8). Also present are minor amounts of quartz.
Differential thermal analysis—With the exception of slight differences in size and shape of the peaks and some variation in peak temperatures, the differential thermal record for this sample is similar to that of the sample taken at 2,341 feet (Fig. 9, 2425 and 2341). Both the endothermic peak at 544°C and the exothermic peak at 940°C indicate a slightly larger ratio of kaolinite to degraded illite than the same peaks produced by the sample from 2,341 feet. The exothermic reactions to carbonaceous matter and pyrite are fairly strong. The endothermic peak at 740°C is missing from the record of this sample, indicating that all the carbonaceous matter had been oxidized below that temperature.
Sample from depth of 2440 feet
Megascopic description—The rock is a light-brownish-gray fine-grained sandstone that shows faint traces of bedding. Apparent in the hand sample are trace amounts of muscovite flakes, opaque grains, and green grains that include glauconite and fragments of reworked schistose or phyllitic rock.
Petrographic description—In thin section, the rock is seen to be composed mainly of subangular to subrounded grains of quartz in a clay matrix. The detrital grains are 0.05 to 0.35 millimeter long, but most are 0.13 to 0.21 millimeter long. Estimated original roundness of the detrital grains is 0.4 to 0.6, but the grains have been extensively modified by overgrowths and corrosion. Bedding is suggested in the thin section by partial segregation of the finer detrital grains into irregularly spaced subparallel stringers that contain more interstitial clay that the rest of the rock.
Quartz approximates 84 percent of the rock. About 70 percent of the quartz grains show overgrowths, which commonly are delineated by inclusions of the clay matrix along the juncture between the detrital grain and the overgrowth. Many grains show both overgrowths and suturing. About 2 percent of the quartz is metaquartzite, as is indicated by the mosaic pattern under crossed nicols. Present as inclusions in the quartz grains are "dust planes", apatite, and rutilated needles, but most grains are relatively free of inclusions.
Detrital chert amounts to about 1 percent of the rock. Both fine- and coarse-grained chert are present. Detrital feldspar includes microcline, plagioclase, and orthoclase in amounts ranging from traces to 1 percent. Traces of detrital muscovite consist of macerated and bent books as much as 0.3 millimeter long.
Also present in the rock are rounded detrital aggregates, 0.08 to 0.20 millimeter long, composed offine-grained mica and quartz. The mica, probably biotite, is green parallel to X and Y axes and light green parallel to Z. The quartz forms blebs and stringers commonly penetrated by the mica flakes. The fragments constitute about 2 percent of the rock and are probably reworked schist or phyllite, or both.
Heavy minerals include trace amounts of rounded and subrounded zircon, light-yellow-brown pleochroic staurolite, pleochroic blue to blue-green and clear tourmaline, and sphene that is partly altered to leucoxene.
The matrix is composed mainly of strongly birefringent clay (illite?), which constitutes about 5 percent of the rock and in most places forms films that barely separate the detrital grains. Elsewhere the clay occurs in subparallel stringers together with fine-grained detrital quartz. A small amount of calcite as irregular blebs partly replaces quartz grains and matrix. Traces of microcrystalline barite (?) speckled with minute flecks of carbonaceous matter are interstitial to the detrital grains. Irregular stringers and blebs of chert also occur in trace amounts in the matrix.
Leucoxene, which amounts to about 2 percent of the rock, is present as rounded irregular grains 0.02 to 0.15 millimeter long. Glauconite, which constitutes about 3 percent of the rock, is present as rounded and irregularly rounded grains ranging in length from 0.19 to 0.23 millimeter.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 84 |
| Chert | 1 |
| Feldspar | 1 |
| Muscovite | Tr |
| Schist or phyllite fragments | 2 |
| Staurolite | Tr |
| Zircon | Tr |
| Tourmaline | Tr |
| Sphene | Tr |
| Clay | 5 |
| Calcite | Tr |
| Barite (?) | Tr |
| Leucoxene | 2 |
| Glauconite | 3 |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 35 percent illite that is partly degraded, 30 percent kaolinite, 25 percent chlorite, and 10 percent montmorillonite (Fig. 10). Also present are minor amounts of quartz, feldspar, and calcite.
Differential thermal analysis—The differential thermal curve for this sample (Fig. 11) is fairly typical for a clay containing a large proportion of illite and a small amount of montmorillonite or chlorite. The fairly large proportion of kaolinite shown by the x-ray data is not definitely indicated by the curves. The large absorbed-water peak at 105°C is indicative of montmorillonite but perhaps also equally indicative of a degraded illite or mixed-layer clay. The small endothermic peak at 700°C probably was produced by chlorite, and the small endothermic peak at 875°C could be attributed to mortmorillonite, illite, chlorite, or calcite. The small exothermic peak at 917°C could have been produced by any of the common clay minerals. The large exothermic deflection marked by peaks at 316° and 405°C was produced by the oxidation of carbonaceous material.
Sample from depth of 2473 feet
Megascopic description—The rock is a light-gray to brownish-gray very fine grained to fine-grained sandstone showing indistinct subhorizontal laminae 2 to 10 millimeters thick. Small amounts of carbonaceous matter occur as sparse films and specks locally on the laminae, and muscovite flakes as large as 1 millimeter in diameter are concentrated along the laminae. Abundant specks of interstitial white material are apparent in the sample. The rock is indurated and effervesces slightly with dilute hydrochloric acid.
Petrographic description—The thin section is composed mainly of subangular to subrounded quartz grains and includes lesser amounts of detrital chert and feldspar. The matrix is mainly clay but contains some calcite and chert. Stringers of clay and subparallel alignment of the detrital grains readily define the bedding. The detrital grains range from 0.024 to 0.35 millimeter in length, but most are 0.1 to 0.2 millimeter long.
Quartz amounts to about 83 percent of the rock. About 60 percent of the quartz grains show obvious overgrowths, some of which are delineated by inclusions of clay at the contact between the grains and the overgrowths. Suturing has modified some of the grains, and many grains show overgrowths that have been modified by later suturing. Present as inclusions in the quartz grains are "dust planes", zircon, rutile-like needles, fluid bubbles, and minute irregular dark-green, almost opaque shreds. A few quartz grains show a mosaic texture under crossed nicols and are thought to be detrital metaquartzite.
Chert amounts to about 5 percent of the rock and occurs in roughly equal amounts as detrital grains and as matrix. It is commonly clouded by included clay particles. Detrital feldspar, including orthoclase, microcline, and plagioclase, is present in amounts ranging from traces to about 1 percent. The grains commonly are clouded by argillic alteration. Rounded fragments of phyllite ranging from 0.1 to 0.2 millimeter in length approximate 3 percent of the rock. They are composed of various proportions of fine-grained mica, quartz, and chlorite, and show subparallel orientation of the micaceous flakes.
Muscovite as shredded and bent books ranging in length from 0.2 to 0.6 millimeter constitutes a trace to 2 percent of the rock.
Other detrital minerals include trace amounts of rounded grains of magnetite or ilmenite, or both, and subangular to subrounded grains of garnet, brown and blue-green tourmaline, and yellow-brown staurolite. Leucoxene, as rounded and irregular grains 0.05 to 0.15 millimeter in diameter, makes up about 2 percent of the rock.
The matrix approximates 7 percent of the rock and consists mainly of small flakes of illite that occupy voids and form films between the detrital grains and between the small grains of secondary chert also present in the matrix. Calcite in trace amounts occurs as both optically continuous and cloudy patches in the matrix and as replacement of the margins of detrital grains. Trace amounts of pyrite as small irregular blebs replace other parts of the matrix.
Traces of carbonaceous matter locally appear as irregular masses elongate parallel to the bedding, which they thus help to define. Glauconite is present in trace amounts as rounded green grains about 0.1 millimeter in diameter.
| Estimated percentage composition: | |
|---|---|
| Constituent | Percent |
| Quartz | 83 |
| Feldspar | 1 |
| Chert | 5 |
| Phyllite fragments | 3 |
| Muscovite | 1 |
| Leucoxene | 2 |
| Magnetite or ilmenite, or both | Tr |
| Garnet | Tr |
| Tourmaline | Tr |
| Staurolite | Tr |
| Clay | 4 |
| Pyrite | Tr |
| Calcite | Tr |
| Carbonaceous matter | Tr |
| Glauconite | Tr |
X-ray data—In the fraction finer than 2 microns, the clay minerals include about 35 percent each illite and kaolinite, 25 percent chlorite, and 5 percent montmorillonite. Also present are minor amounts of quartz, feldspar, and calcite.
Summary
The petrographic, x-ray, differential-thermal-analysis, and firing-test studies of the Cretaceous rocks in the Guy F. Atkinson No. 1 Beaumeister well show only minor differences between similar rock types, from the base of the Graneros Shale to the base of the Cretaceous. The only truly distinctive rock types found in the Dakota Group are the phosphatic conglomeratic sandstone of the Cruise Sandstone member of the Omadi Formation, the coal near the top of the Cruise, and the limestone from the Kiowa Shale.
Petrographic studies show that the sandstone samples examined are essentially similar, both in composition and grain size. The Cheyenne Sandstone, however, commonly has less clay matrix and less silt-sized quartz and seems to be better sorted than sandstone from the Omadi Formation. No diagnostic mineral assemblages were noted. Almost all sandstone examined contained some glauconite, leucoxene, fragments of schist or phyllite, and detrital tourmaline in addition to grains of quartz, feldspar, and chert. Some samples contained trace amounts of zircon and staurolite, but the absence of these minerals in other examples probably reflects the small size of the samples examined.
Variations in clay mineralogy determined by x-ray diffraction are shown in Table 7. The table indicates that the clay minerals of the Graneros Shale include considerably more montmorillonite than the clay minerals of the Dakota Group. Relative proportions of illite and kaolinite in sandstone samples from the Dakota Group seem to be virtually uniform except at the top of the Cheyenne Sandstone, where montmorillonite seems to have formed at the expense of illite. It is suggested that erosion and weathering of the topmost parts of the Cheyenne Sandstone prior to deposition of the Kiowa Shale converted much of the illite to a montmorillonite-like clay.
Clay-mineral composition of mudstone and claystone samples from the Dakota Group also is almost uniform, but differences are to be noted in the thick mudstone sequence in the Cruise Sandstone member of the Omadi Formation. Samples of mudstone between depths of 2,190 and 2,234 feet contain little or no chlorite and montmorillonite (Table 7). The paucity of montmorillonite may reflect the low permeability of the unit and accordingly the absence of circulating ground water that could have decomposed illite to produce montmorillonite-like clay. In this respect, it should be noted that low-temperature endothermic water peaks in the differential thermal analysis curves of samples from the same sequence are weak in comparison with those from mudstone samples from other parts of the Dakota Group. The coal in the Cruise member contains mainly kaolinite and mixed-layer clay, a composition totally unlike other rocks in the Dakota Group.
Table 7—Clay-mineral composition of selected core samples.
| Depth (feet) |
Lithology | Percentage of clay minerals in samples | ||||||
|---|---|---|---|---|---|---|---|---|
| Illite | Kaolinite | Chlorite | Montmorillonite | Mixed layer |
||||
| Graneros Shale |
2,001 | Bentonite | 100 | |||||
| 2,017 | Claystone | 15 | 15 | 70 | ||||
| Omadi Formation |
Gurley member |
2,034.5 | Mudstone | 60 | 30 | 10 | ||
| 2,039 | Sandstone | 45 | 35 | 20 | ||||
| 2,074 | Mudstone | 55 | 15 | 20 | 5 | 5 | ||
| 2,082 | Sandstone | 40 | 40 | 10 | 10 | |||
| 2,095 | Sandstone | 50 | 30 | 20 | ||||
| Huntsman member |
2,118 | Mudstone | 60 | 25 | 15 | |||
| Cruise member |
2,146 | Sandstone | 40 | 30 | 20 | 10 | ||
| 2,166 | Mudstone | 60 | 25 | 15 | ||||
| 2,176 | Mudstone | 60 | 20 | 20 | ||||
| 2,189 | Coal | 40 | 60 | |||||
| 2,191 | Siltstone | 65 | 35 | |||||
| 2,193 | Mudstone | 65 | 35 | |||||
| 2,206 | Mudstone | 70 | 30 | |||||
| 2,226 | Mudstone | 65 | 25 | 10 | ||||
| 2,233.5 | Mudstone | 70 | 30 | |||||
| 2,239 | Sandstone | 55 | 40 | 5 | ||||
| 2,254 | Mudstone | 60 | 30 | 10 | ||||
| 2,259 | Sandstone | 45 | 40 | 7 | 1 | 7 | ||
| 2,304 | Sandstone | 30 | 30 | 25 | 5 | 10 | ||
| Kiowa Shale |
2,308 | Mudstone | 70 | 30 | ||||
| 2,341 | Mudstone | 65 | 20 | 5 | 5 | 5 | ||
| Cheyenne Sandstone |
2,421 | Sandstone | 15 | 25 | 15 | 45 | ||
| 2,425 | Mudstone | 65 | 20 | -15- | ||||
| 2,440 | Sandstone | 35 | 30 | 25 | 10 | |||
| 2,473 | Sandstone | 35 | 35 | 25 | 5 | |||
Differential-thermal-analysis curves for samples from the Dakota Group show minor variations, but there are no consistent features common to anyone of the stratigraphic units that distinguish it from the others. As noted above, however, absorbed-water peaks of mudstone samples between depths of 2,190 and 2,234 feet are weak in comparison with those of samples from the other stratigraphic units. The curves obtained for the two Graneros Shale samples are distinctly different from the others in that the peaks are chiefly characteristic of montmorillonite only.
Fired colors of claystone and mudstone samples (Table 8) allow easy recognition of the mudstone sequences in the Cruise Sandstone member of the Omadi Formation. The samples fire to some shade of buff, whereas claystone and mudstone samples from the Cheyenne Sandstone, Kiowa Shale, other parts of the Omadi Formation, and Graneros Shale fire to darker, reddish colors, with the exception of the sample of calcareous Graneros Shale taken at 2,009 feet, which fired buff.
Table 8—Color and hardness of selected core samples after heating to 1000° C.
| Depth (feet) |
Unfired samples | Fired samples | ||||
|---|---|---|---|---|---|---|
| Lithology | Color | Color | Hardness as to steel |
|||
| Graneros Shale |
1,997 | Claystone | Very dark gray | Light pink buff | Harder | |
| 2,001 | Bentonite | Very light gray | Light rose | Harder | ||
| 2,009 | Claystone (calcareous) |
Dark gray | Buff | Harder | ||
| Omadi Formation |
Gurley member |
2,034.5 | Mudstone | Gray | Salmon | Harder |
| 2,074 | Mudstone | Dark gray | Salmon | Harder | ||
| Huntsman member |
2,118 | Claystone | Dark gray | Salmon | Harder | |
| Cruise member |
2,145 | Claystone | Dark gray | Salmon and gray | Harder | |
| 2,166 | Mudstone | Gray-red layer | Salmon and dark red | Softer | ||
| 2,176 | Mudstone | Gray | Light red and salmon | Harder | ||
| 2,193 | Claystone | Gray to dark gray | Cream | Softer | ||
| 2,205 | Mudstone | Light gray | Buff | Softer | ||
| 2,206 | Mudstone | Light gray | Buff | Softer | ||
| 2,209 | Mudstone | Light gray | Buff | Harder | ||
| 2,213 | Mudstone | Gray | Buff | Harder | ||
| 2,214 | Claystone | Gray | Buff | Harder | ||
| 2,220 | Mudstone | Gray | Salmon | Harder | ||
| 2,227 | Mudstone | Gray | Pink buff | Harder | ||
| 2,233.5 | Mudstone | Gray | Buff | Harder | ||
| 2,247 | Mudstone | Gray | Salmon and brown | Harder | ||
| 2,254 | Mudstone | Gray | Salmon | Harder | ||
| Kiowa Shale |
2,308 | Mudstone | Dark gray | Salmon buff | Harder | |
| 2,341 | Mudstone | Dark gray | Salmon | Harder | ||
| 2,419 | Mudstone | Dark gray | Salmon | Harder | ||
| Cheyenne Sandstone |
2,425 | Mudstone | Dark gray | Red | Harder | |
| 2,476 | Mudstone | Gray | Salmon red | Harder | ||
| 2,484 | Claystone | Dark gray | Red | Harder | ||
The presence of subspherical grains of siderite in mudstone samples from the Cruise Sandstone member of the Omadi Formation at depths of 2,206 and 2,226 feet should be mentioned, inasmuch as siderite "pellets" are common in mudstone from the Dakota Formation at the outcrop in central Kansas (Plummer and Romary, 1942, p. 326; Swineford and Williams, 1945, p. 116).
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Kansas Geological Survey, Geology
Placed on web July 27, 2017; originally published April 15, 1959.
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