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Upper Cambrian and Lower Ordovician Rocks in Kansas

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Method of Study

The difficulty encountered in attempts to subdivide the older Paleozoic beds in the subsurface of Kansas is the result of a number of factors. These include lack of fossils in well cuttings, absence of easily recognizable lithologic zones in a vertical section, rapid lithologic changes laterally within the same bed, and absence in the area of certain formations known in the general sequence.

Although fossils are fairly abundant in a few zones in these beds, their relatively large size prevents recovery of unbroken specimens in drill cuttings. A few identifiable specimens have been found in cores and have been used in determining the age of the enclosing formation, but the number so obtained is too small to permit zoning of the beds by paleontological methods. Differences in lower Paleozoic beds at different outcrop areas in other states suggest the impracticability of extrapolating the surface information for any considerable distance into the subsurface.

The methods of subsurface study commonly used by oil company geologists are adequate for the study of a sequence of formations in which easily recognizable "breaks" occur. More specialized methods, however, are desirable for the study of a great thickness of beds in which well-defined markers are absent.

Interbedded shales and sandstones make up only a small percentage of the lower Paleozoic section of Kansas. The shales, for the most part, are thin and some are dolomitic. With few exceptions, the sandstones are highly dolomitic, resembling a sandy dolomite that may grade either laterally or vertically into beds or lenses of relatively pure sandstone. In addition, the lateral change in lithology, seemingly within the same bed, may be greater within a short distance than the vertical change in lithology from bed to bed. All these factors contribute to the difficulty of zoning the Upper Cambrian and Lower Ordovician beds in Kansas.

The absence of faunal material required that study be based entirely upon lithology in order to differentiate zones within the older beds.


Samples from 110 wells were examined during the course of this investigation. Cuttings from nearly all wells drilled in the eastern part of the state prior to 1942 that penetrate significant amounts of the pre-St. Peter formations were studied. Relatively few of the wells that have been drilled to the top of the Lower Ordovician in western Kansas penetrate the dolomite more than a few feet. Some of the wells studied penetrate the entire section but some reach only the upper part of the Lower Ordovician section. Generalized descriptions of the dolomites are given in the part of this report dealing with the correlation of the formations. Detailed descriptions of the insoluble residues from 15 representative wells, the locations of which are shown in Figures 10 and 12, are given at the end of the report.

Most of the samples used in this study were obtained from the Kansas Geological Survey. Samples and insoluble residues from wells in western Nebraska and Colorado were borrowed from oil companies. Outcrop samples and specimens, for the most part, were collected by the writers although a few were borrowed from oil companies.

Certain inaccuracies connected with the method of drilling may cause seeming anomalies greater than actual lateral and vertical lithologic changes in the rocks. Cuttings from cable tool wells drilled while the hole is full of water are of very fine size. This affects the interpretation of the samples in several ways. (1) Much of the relatively soft dolomite that has been reduced to a fine rock flour is lost during the sample washing operation. The amount of chert remaining in the sample therefore indicates a much higher percentage than was contained in the original rock. (2) It is impossible to determine from such fine material the color, texture, or other characteristics of the dolomite. (3) Insoluble residues from these fine samples are of very little value, especially in those parts of the section in which the diagnostic characteristic is an insoluble residue of soft dolocastic chert or shale. The difficulty of drilling the hard Pre-Cambrian rocks with cable tools causes less resistant material, such as feldspar, to be ground to a fine powder which may be lost when the samples are washed. The fine angular quartz grains which then compose the sample resemble a sand or quartzite. Traces of feldspar and mica may be interpreted as fragments of weathered granite that were included in a basal sand during deposition.

Samples from wells drilled with rotary tools are contaminated by material derived from beds higher in the well. Therefore, formation boundaries are not sharp but may extend over a zone 30 feet or more in thickness. Because of the lag in bringing samples to the surface where drilling is deep, the boundaries of formations are logged 10 to 30 feet below the actual depth at which they were encountered. The weight of the drilling mud may not bring cuttings of a limestone or dolomite to the surface as rapidly as it will bring cuttings of shale; the samples then consist almost entirely of caved shale. The sorting of extraneous material from rotary cuttings is not entirely satisfactory. It is impossible to exclude material from upper formations within the Upper Cambrian and Lower Ordovician beds or to recognize in every case new material indigenous to the formation. For example, it is probable that much green shale interbedded with the formations has been overlooked because of similarity to green shales encountered higher in the well.

Comparison of Dolomites

Microscopic examination of the dolomites in the well cuttings reveals many variations in color, texture, fracture, and other features. Attempts to devise a method of describing these variations in a manner that would permit consistent recognition of each variant were so notably unsuccessful that a method was devised that would permit direct comparison of each variant with some unvarying standard.

Each distinctive kind of dolomite was designated as a type and was assigned an arbitrary color symbol which, when plotted on the log form, would represent graphically that particular variant in any subsequent sample in which it might be found. A few representative fragments of each variety of dolomite were glued on a small card with the adopted color symbol, the name and location of the well, and the depth from which the type was taken. In use, these type cards, together with the samples to be examined, were placed in a small tray on the stage of the microscope and direct comparisons were made under the microscope between the dolomite cuttings in the tray and the fragments of the type dolomite on the card. In addition to making possible close comparison between the dolomite in the cuttings and the fragments selected as dolomite types, these cards formed a permanent, easily accessible reference file of the large number of different kinds of dolomite in the samples examined.

The first dolomite types were selected from cable-tool wells located nearest the outcrop section in Missouri. As the studies progressed westward new dolomite types were encountered and these were differentiated in the same manner as those from wells that had been studied earlier.

Comparison of Insoluble Residues

The distinguishing characteristics of chert, sand, and shale in the Cambrian and Lower Ordovician dolomites are obscured to a great extent in samples as they come from the well but may be brought out satisfactorily by the use of insoluble residues. McQueen (1931) and others have shown that, in many cases, dissolving the dolomite from rock samples by means of hydrochloric acid leaves an insoluble residue that may be identified more readily than the original sample. Briefly summarized, McQueen's method of preparing insoluble residues consists of (1) measuring volumetrically in a straight-sided glass bottle a sample of drill cuttings, (2) dissolving the measured sample in an excess of hydrochloric acid in a 250 cc. beaker, (3) washing the remaining insoluble part by decantation, (4) drying the insoluble residue, and (5) measuring volumetrically by means of a suitable scale the residue in the same bottle in which the original sample was measured. The bottle, when fitted with a cork stopper, serves as a storage receptacle for the residue.

This method of preparation is rapid and is satisfactory when the samples are uncontaminated, as are outcrop samples or cuttings from wells drilled with cable tools. If this method is used it is desirable that the amount of sample be sufficient to permit the use of a large fraction (25 to 35 grams) for the preparation of the residue.

The practice in Kansas of dividing the well samples in order that each of several interested oil companies and State agencies may have a set reduces considerably the amount of sample available to each. In rotary cuttings, extraneous material included in some samples may compose all but a few fragments of rocks from the lower beds. As a result, the residue fraction available for use in this study usually was too small to permit accurate determination of the volume in a vial large enough in diameter to hold the largest rock fragments. Therefore, instead of following the volumetric method developed by the Missouri Geological Survey, the original fraction from which the residues were to be prepared and the residues that remained after solution of the dolomite in acid were weighed on an analytical balance to the closest 0.1 gram and the percentage of residue was then calculated.

Residues made from samples as they came from rotary wells were useless. To prevent the large amount of shale, sandstone, gypsum, and anhydrite that had caved from higher beds from obscuring the relatively small amount of residue from the dolomites in the lower part of the well, the samples were hand-picked and only the dolomite was retained for making the residue. This method has the obvious disadvantage of removing some material, especially shale, that might be indigenous to the lower beds, and leaves in the residue only the insoluble material that was included in the dolomite fragments. There probably is some contamination within the dolomite beds. This results in less precise determination of formational boundaries.

Notwithstanding all the difficulties inherent in the use of rotary samples, stratigraphic zones that were recognizable from residues in eastern Kansas were traceable across the state.

The kinds of chert present in the insoluble residues were divided into a relatively small number of types, mainly on the basis of such characteristics as color, degree of transparency, luster, texture, and presence of dolocasts. Each type was assigned a color symbol, which was plotted on log forms. In general, shale and sand grains in the residues were classified according to standard methods. Each type was represented by symbols on the logs, both as to type and as to relative amount. It should be noted that the color of some shales is considerably modified by the action of the acid, and that calcareous or dolomitic shales may be disintegrated by the action of the acid, with the result that fine shale fragments are lost when the residues are washed.

The presence of accessory minerals, such as pyrite and glauconite, was also recorded.


Adoption of arbitrary symbols representing various types of dolomite or residues obviated the necessity for detailed description during study, but in order to publish the results of observations, it is necessary to use a number of descriptive terms. Most of these have been adopted from descriptions by Lee (1940, pp. 16-17, and 1943, pp. 14-16) and from publications of the Missouri Geological Survey (McQueen, 1931). The use of such terms in this report is indicated by the following definitions.

Cotton rock is a soft porous siliceous rock or insoluble residue composed of white opaque uncemented microscopic particles of silica.

Dolocast is a term introduced by McQueen (1931) for the impression left by a dolomite crystal removed from chert or other materials in the insoluble residues. It is widely used in spite of being a misnomer. Dolocasts may occur singly or be so numerous that they give rise to a porous or lacy texture.

Drusy texture applies to deposits of crystalline quartz, usually on chert fragments, or to aggregates of quartz crystals, probably originating in microscopic cavities.

Granular texture applies to microscopic crystals of dolomite or chert sparsely distributed in a dull opaque cryptocrystalline or earthy matrix.

Oöcast is used in this report to indicate the cavity from which an oölitic grain has been removed by weathering or by acid.

Porcelaneous chert is smooth, even-textured, homogeneous, somewhat vitreous, and opaque.

Porous texture is used to describe aggregates of quartz or chert or other materials in which the individual cavities are microscopic.

Quartzose chert has the glassy appearance and sharp fracture of crystalline quartz but lacks crystal faces.

Spongy texture applies to aggregates of quartz, chert, silt, or clay in insoluble residues from which the soluble matrix has been removed and in which the individual openings are submicroscopic.

Sucrose texture applies to microscopically coarse or fine crystals, usually dolomite, packed closely (without matrix) and resembling the grains of lump sugar.

Tripolitic texture refers to chert or siliceous rock that is soft, crumbly, rotten, and has a mealy appearance.

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Kansas Geological Survey, Geology
Placed on web Jan. 22, 2010; originally published June 1948.
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