THE CLAY MINERALOGY OF THE SHALY PORTIONS OF THE BRASSFIELD LIMESTONE

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1 THE CLAY MINERALOGY OF THE SHALY PORTIONS OF THE BRASSFIELD LIMESTONE ERNEST G. EHLERS AND KARL V. HOOVER Department of Mineralogy, The Ohio State University and Indstrial Minerals Section, Ohio Division of Geological Srvey, Colmbs, Ohio INTRODUCTION The Brassfield limestone otcrops over a wide area in sothwestern Ohio. It shows considerable variation in physical characteristics, laterally as well as vertically. The clay minerals associated with this formation have never been examined. It was hoped that a stdy of these minerals might provide additional information FIGURE 1. Locations of cores and stratigraphic sections. ^Brassfield Otcrop Section * Core THE OHIO JOURNAL OF SCIENCE 61(4): 227, Jly, 1961

2 228 EHLERS AND HOOVER Vol. 61 on the variable lithology of this formation, and perhaps indicate distinctive differences among the varios nits. REGIONAL GEOLOGY The Brassfield limestone is of Silrian age and is the only formation of the Albion grop which otcrops in Ohio. The area of its srface exposre is limited to sothwestern Ohio (fig. 1); the otcrop area trends northeastward from the vicinity of Fairhaven, in Preble Conty, to central Miami Conty, and thence sotheastward to the Ohio River near Rome, in Adams Conty. The otcrop area of the Brassfield limestone is divided lithologically into northern and sothern zones. The bondary between these zones is a covered area in sothern Clinton Conty. The Brassfield formation is nderlain by the Elkhorn formation, which sally consists of red and green shales, althogh locally it contains qartz or calcite-rich lenses. Stot (1941) reported that the Elkhorn generally contains the minerals sericite, biotite, celadonite, and hematite. Above the Brassfield lies the Dayton formation. This is commonly a blish-gray silicios calcite-rich dolomite; locally it is a limestone. U. Brassfield Brassfield Elkhorn FIGURE 2. Typical Brassfield limestone section in the sothern otcrop zone (locality 234). The Brassfield formation is divided into three stratigraphic nits, the (which is a transitional zone with the Elkhorn), the, and Upper Brassfield. The nit is a greenish-gray, fine grained argillaceos dolomite or dolomitic limestone. These carbonate beds contain intercalated greenish-gray clay shales, which make p from 3 to 30 percent of the total thickness of 1 to 8 ft. Glaconite, either disseminated or in the form of pellets, is generally present. Sphalerite, pyrite, and chert are locally abndant as accessory minerals. The nit is considered to be limited to the eastern flank of the Cincinnati arch. In the western otcrop area the correlative bed is a dense limestone or argillaceos

3 No. 4 BRASSFIELD LIMESTONE CLAY MINEROLOGY 229 dolomite (0.2 to 1.2 ft thick) which is occasionally present between typical Elkhorn and beds. The Upper and nits are broken into northern and sothern zones by a hiats in otcrop pattern in sothern Clinton Conty, as well as differences in lithology. The northern zone of the Brassfield consists of two distinct nits the Lower Brassfield and. The is typically a light gray to white, fine to coarse-grained crystalline limestone. The crystalline textre is a reslt of recrystallization of fossil fragments. Green clay is locally present, TABLE 1 Analyses of the shaly portions of the Brassfield limestone Section Sample Statigraphic nit no. no. CO3 loss with acid digestion Analyses (in percent) Newberry MgCO 3 Insolble material Upper Brass field Dayton Upper Brass field Lower Brass field Dayton Elkhorn either in the disseminated state or as thin partings. Bedding varies from thin to massive, with crossbedding common. Field observations indicate that the Lower Brassfield limestone is extremely dolomitic in the Clark Conty area; the dolomite content decreases gradally in the direction of the Indiana border. In general, the dolomite content also decreases from the bottom to the top of the nit. Pyrite is a characteristic accessory, and sphalerite is locally present in minor qantities.

4 230 EHLERS AND HOOVER Vol. 1 The thickness of the nit varies from a minimm of 6 ft in Preble Conty to greater than 20 ft in Miami Conty. The of the northern zone is characteristically a pink, red, or gray, medim to coarse grained bioclastic limestone. Green clay, as partings, lenses, or disseminated material is mch more abndant than in the. Hematite-rich oolites and fossil fragments are present in Clinton and Greene Conties; this zone grades into a red clay in the extreme northwestern portion of Greene Conty. Pyrite is present as a characteristic accessory. Glaconite is occasionally present in the pper portions of the nit. The thickness varies from 1 to 22 ft and shows rapid lateral variation. The dolomite content of the Upper Brassfield parallels that of the previosly described. The Brassfield limestone of the sothern zone has several featres which are distinctly different from those of the northern zone. These are the following: lack of sharp lithologic differences between the pper and lower parts, a large LE IN FEET I I I I I ^ ^^^ s FIGURE 3. Stratigraphic sections and sample locations of the Brassfield limestone. increase in the shale content, an increase in the nmber of chert nodles, and differences in many textral and strctral featres. Althogh the two rock types of the northern zone do not remain distinct in the sothern zone, many of the distingishing stratigraphic featres persist, sch as the ferrginos oolite zone, the basal brown limestone, and the corse bioclastic textre of the Upper Brassfield. The most common rock type is a light to medim gray, fine to medim grained limestone. The interbedded green shale in some sothern exposres constittes the major portion of the nit (fig. 2), and as in the northern zone, the shale is more abndant in the pper portions. Chert, which occrs near the base, has no fixed stratigraphic position. Glaconite is fairly common throghot the nit, both as discrete beds as well as irreglarly-distribted grains. Calcite is prevalent

5 No. 4 BRASSFIELD LIMESTONE CLAY MINEROLOGY 231 throghot the nit, with the exception of the West Union area in Adams Conty, where a large proportion of dolomite is present. PROCEDURE Clay samples were collected from thin clay seams in limestone, from both drill cores and otcrops. The type of sample, its locality, and stratigraphic horizon are shown in figres 1 and 3. The carbonate minerals which occr in abndance within the clay seams were removed by treatment with dilte (1:3) glacial acetic acid; acetic acid treatment, in contrast to hydrochloric acid treatment, does not appreciably alter the claymineral assemblage. The samples were sieved with a 60-mesh screen, weighed, and placed in contact with acid for a minimm of 30 hr. The reside was filtered, given a triple washing with distilled water, evaporated, and reweighed. Newberry analyses for CaCO 3, MgCO 3, and insolble material percentages were rn on most of the samples. The reslts are given in table 1. Portions of each sample were monted on glass slides for x-ray stdies. Part of the sample was x-rayed by means of a Norelco low angle diffractometer. Copper radiation with a nickel filter was sed. The speed was 2 20 per min; critical reflections were rn at 1 20 per min. The area examined ranged from 2.31 to A. All of the significant clay mineral peaks, as well as those of many common imprities fall within this range. A typical x-ray chart is shown in figre 4. The relative amonts of kaolinite and illite were determined on the basis of intensities of (001) reflections, as sggested by Mrray (193). This techniqe yields approximate differences in the form of a ratio, bt the reslts are not qantitative.

6 232 EHLERS AND HOOVER Vol. 61 x'ul of the samples were treated with ethylene glycol in order to determine the presence of montmorillonite, and to observe the natre of the mixed-layer strctres. In addition, all samples were rern after being fired for one hr at 600 C in order to distingish between chlorite and other clay minerals, as well as to observe effects on mixed-layer strctres. The reslts of the x-ray analysis are given in table 2. DISCUSSION OF RESULTS Comparison of the Newberry analysis in table 1 with the sample locations clearly shows that the samples collected in the sothern zone are richer in insolble material (mainly clays) than those of the northern zone. It is also interesting to note, when comparing these analyses with the clay-mineral determinations, that Relative Intensity FEGURE Degrees Typical x-ray diffractometer pattern (C radiation) of a shaly portion of the Brassfield limestone. the composition of the clays is virtally the same, whether the sample was collected in a clay-rich band, or in a section with a high carbonate content. As can be noted from table 2, most of the samples collected in the sothern part of the otcrop region contain illite and mixed-layer illite-chlorite as the predominant clay minerals, with lesser amonts of kaolinite, and occasional traces of chlorite. The more northern samples contain a similar site, with the difference that the kaolinite is either absent or present in only trace amonts. These reslts are in agreement with similar clay-mineral stdies by other investigators. Stdies on the clay minerals associated with limestones and dolomites, sch as those of Grim, Lamar, and Bradley (1937), Millot (1949), Grim (191), and Robbins and Keller (192), indicate that illite is the predominant clay mineral associated with carbonate rocks. Weaver (198), in smmarizing

7 No. 4 BRASSFIELD LIMESTONE CLAY MINEROLOGY 233 mch of the work done on clay minerals, conclded that althogh this relationship is correct, illite is also the predominant clay mineral in all sediments and is not noticeably more abndant in limestones than in shales. In most cases where shales and limestones are interbedded, they contain the same type of clay minerals. Treatment of all samples with ethylene glycol failed to reveal the presence of montmorillonite. It can be seen from figre 4 that the 10 A illite peak is noticeably skewed in the direction of the wider spacings. This is often indicative of mixed-layer clay TABLE 2 Mineral composition of the shaly portions of the Brassfield limestone Sample nmber 10-14A C86-2A C86-6A C86-7A C86-10A C86-22A C86-4A lOA A A 229-A A C863-3 C863-9 C C C C C C C C863-24A C863-27A C863-28A C863-30A C863-32A C863-33A C863-3A-41A C863-44A C863-47A C863-48A C863-1 Illite: kaolinite ratio ry i Tr. Kaol. Tr. Kaol. Tr. Kaol. Other minerals Pyr. Felds. Felds., Felds., Pyr., Felds., Felds., Tr. Pyr.. Ch., Felds., Pyr., Pyr., Felds. Ch., Pyr. Ch. Stratigraphic nit Dayton Dayton (I U it U i a a Elkhorn Qartz is present in all samples. Ch. = Chlorite, Felds. = Feldspar, = Illite-chlorite mixed-layer strctre, Kaol.= Kaolinite, Pyr. = Pyrite, Tr.= Trace, and = Illite. strctres. A mixed-layer clay is derived from stripping or degradation of preexisting clay minerals dring weathering; cations removed from the clay strctre dring weathering are commonly replaced by others, either in the sorce area or in the basin of deposition, to form a mixed-layer strctre. Treatment by ethylene glycol did not prodce any broadening or shifting in the peak, indicating that the mixed-layer strctre did not contain a clay with an expandable lattice. The shape of the peak either increased in skewness after firing or there was a general increase U i

8 234 EHLERS AND HOOVER Vol. 61 in backgrond between 13.8 and 10. A, which is indicative of chlorite in mixedlayer arrangement with illite. This increase in intensity wold be expected, as the firing of chlorite minerals which are not in a mixed-layer arrangement increases the intensity of the (001) reflection, as well as depressing the higher order reflections. The increase in intensity over sch wide spacings indicates a very random arrangement of Mg or Fe within the strctre. Althogh illite is often in mixed-layer arrangement with montmorillonite, an illite-chlorite mixed-layer strctre is relatively ncommon. The general distribtion of illite to kaolinite shows the presence of kaolinite in the sothern samples, and either none or merely trace amonts in the northern samples. This relationship appears to hold tre for the Dayton formation, and the,, and nits. Althogh only a relatively small nmber of samples have been taken, the consistency of this relationship wold indicate that the clay mineralogy does differ in these two areas. Weaver (198) states that kaolinite tends to be most common in continental and near-shore sediments. It is ths possible that the presence of kaolinite in the sothern samples may indicate that this area is closer to the sediment sorce than the northern samples. This wold be in agreement with the mch greater abndance of clay in the sothern samples. No significant differences are present in otcrop samples as compared to core samples. For the most part the same clay mineralogy exists among the samples collected from the Dayton, Brassfield, and nits. CONCLUSIONS Clay-mineral analyses were condcted on the shaly portions of the Brassfield and adjacent limestones. The predominant clay mineral is illite. Kaolinite, mixed-layer illite-chlorite, as well as trace amonts fo chlorite, were also determined. The reslts indicate that the more northern samples contain either traces of kaolinite or none at all, whereas those in the soth sally contain significant amonts. REFERENCES CITED Grim, R. E The depositional environment of red and green shales. J. Sedim. Petrol. 21: , J. E. Lamar, and W. F. Bradley The clay minerals in Illinois limestones and dolomites. J. Geol. 4: Millot, G Relations entre la constittion et la genese des roches sedimentaires argileses. Geologie Appliqee et Prospection Miniere, Univ. of Nancy, 2, 32 pp. Mrray, H. H Genesis of clay minerals in some Pennsylvanian shales of Indiana and Illinois. Proc. Second Natl. Conference on clays and clay minerals, Univ. of Missori, Natl. Acad. Sci. Pbl. 327, Robbins, C. and W. D. Keller Clay and other non-carbonate minerals in some limestones. J. Sedim. Petrol. 22: Stot, W Dolomites and limestones of western Ohio. Ohio Geol. Srv. Bll. 42: 468 pp. Weaver, C. E Geologic interpretations of argillaceos sediments. Am. Assoc. Petrol. Geol. Bll. 42:

Notes Sedimentary Rocks.notebook. May 10, magma / lava INTERLOCKING. crystal. fine. derived. land. banding. chemically. Foliated Nonfoliated

Notes Sedimentary Rocks.notebook. May 10, magma / lava INTERLOCKING. crystal. fine. derived. land. banding. chemically. Foliated Nonfoliated Sedimentary Rocks Quick Review: Igneous ( fire rock ) Solidification of INTERLOCKING crystals Vesicular Glassy magma / lava TEXTURE based on size fine crystal Non crystalline,, coarse, very coarse Quick