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Early Diagenetic Calcareous Coal Balls and Roof Shale Concretions from the Pennsylvanian (Allegheny Series)1

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Early Diagenetic Calcareous Coal Balls and Roof Shale Concretions from the Pennsylvanian (Allegheny Series)1 EARLY DIAGENETIC CALCAREOUS COAL BALLS AND ROOF SHALE CONCRETIONS FROM THE PENNSYLVANIAN (ALLEGHENY SERIES)1 LON A. McCULLOUGH, Department of Geology and Mineralogy, Ohio State University, Columbus, OH 43210 Abstract. The calcareous concretions studied formed in different depositional en- vironments associated with the Pcnnsylvanian eyclothems of cast central Ohio. The late syngenetic to early diagenetie coal balls from Lower Frecport Coal in Jefferson County were formed by cellular permineralization of decaying terrestrial plants in a peat swamp. These coal balls, composed of N9.N% calcite, 7.0% pyrite, and 2.(3% organic material, preserve six identifiable plant genera, Sphenophyllum, Stigmaria, Medulosae, Taxospermum, Psaronius, and Myeloxylon, as well as fusain and spores. The early diagnetic nodules from roof shale overlying Middle Kittanning Coal in Tuscarawas County, Ohio were formed by authigenic cementation of marine muds that contained brachiopods, including Mesolobus, Lingula, and several productid species, bryozoan fronds, ostracodes, gastropods, crinoids, cephalopods, as well as coprolites, terrestrially-derived fusain, root casts, and megaspore coats. These nodules, composed of 84.5% calcite, 11.3% illite, 1.5% quartz, 0.9% pyrite, and 0.5% organic material, have calcite-filled septarian fissures. OHIO J. SCI. 77(3): 125, 1977 Coal balls, or concretions that formed 1976) including large-scale marine inun- in peat, and roof shale nodules preserve dations that carried marine organisms a portion of sediment that was not se- into a peat swamp environment (Mamay verely compressed before lithification. and Yochelson 1962). Most coal balls, As such, they provide a key for decipher- however, including those so far described ing the diagenetie history of the strata from Ohio, were formed in situ under in which they formed and also for recog- relatively quiescent conditions in peat nizing original depositional features. Al- swamps. Roof shale nodules, such as though roof shale nodules, such as those those described in my study, were formed found in the blue shale overlying the Mid- in marine muds that often transgressed dle Kittanning Coal (No. 6) in Tusca- over peat deposits (Stopes and Watson rawas County, are common in near shore 1909). At the coal ball locality in Jef- shales of Pennsylvanian age rocks of ferson County, Ohio there is no known Ohio, coal balls have been described from example of a marine limestone directly coals only in the Monongahela (Good and above the Lower Freeport Coal (Lam- Taylor 1974) and Conemaugh Series born 1930). Instead, the coal commonly (Rothwell 1976) in Ohio. My study occurs below a sandstone that shows no compares the diagenetie concretions noted indication of marine origin. Weller (1930) above and coal balls found near Wells- observed in Illinois that such sandstone ville, Ohio in the Lower Freeport Coal deposits above coals signify a period of (No. 6a) of the Pennsylvanian Allegheny erosion in which sediments originally cov- Series. ering the coal were removed before the Some coal balls show textural varia- sandstone was deposited. The coal prob- tions that indicate formation under a ably was represented at that time by com- wide range of energy conditions (Perkins pressed peat that resisted erosion. Ma- rine deposits that may have occurred 1Manuscript received lanuary 10, 1977 and in above the Lower Freeport Coal in the revised form March 25, 1977 (#77-1). Wellsville area probably were removed 125 126 LON A. McCULLOUGH Vol. 77 The coal contains coal balls for a dis- during the diastemic interval prior to the tance of about 18 m in a north-south deposition of the overlying sandstone. trend (fig. 1). The coal balls appear DESCRIPTION OF THE WELLSVILLE either as aggregated masses or as isolated EXPOSURE units in the coal with ratio of height to The coal ball collecting locality near width on the order of 1:5. Bandings in Wellsville, Ohio is a deep sidehill roadcut the coal, which are prominently warped on the west side of Ohio Route 7 in Saline around individual coal balls, show dis- Township, Jefferson County within T. 9 turbance vertically in either direction up N. and R. 2 W. in the north central part to 0.25 m from isolated concretions. of Section 7, Wellsville Quadrangle, 0.74 km south of the Columbiana and Jeffer- DESCRIPTION OF THE GNADENHUTTEN son County line. The locality was recog- LOCALITY nized in 1961 by Horace Collins, Chief of Roof shale nodules occurred in 2 strip the Ohio Geological Survey. Several pits operated by the Empire Coal Com- specimens were collected at that time and pany of Gnadenhutten, Ohio. They are a few preliminary peels were prepared. located in Clay Township, Tuscarawas A brief mention of the deposit appeared County within T. 6 N. and R. 2 W. in in Denton et al (1961). the northeastern part of Section 3, 2.12 At the collecting locality, the concre- km southwest of Gnadenhutten and in tion-bearing coal rests upon 0.5 m of the western part of Section 1, 0.74 km light gray, non-bedded underclay. The southwest of the village (Gnadenhutten coal varies in thickness from 1.4 m in Quadrangle). Both localities are just areas where coal balls are more prominent south of Tuscarawas County road 16. to 0.6 m where they are more lenticular, Concretionary nodules occur in the flattened, or absent. The coal is over- shales above the Middle Kittanning Coal lain by a massive sandstone, 3 to 3.5 m (No. 6) and to a lesser extent above the thick, which, in the outcrop, extends be- Lower Kittanning Coal (No. 5) in this yond the coal. area. At the two localities nodules for SOUTH NORTH Lower Freeport Coal - Arenaceous Shale Calcareous Coal Balls Underclay SCALE IN METERS Tal us FIGURE 1. Schematic drawing to scale of the Wellsville coal ball outcrop showing the three mam concretionary pockets, A, B, and C, in the Lower Freeport Coal (No 6a) The scale shown applies to both horizontal and vertical profiles. Ohio J. Sci. COAL BALLS AND ROOF SHALE CONCRETIONS 127 my study were found along the highwalls confirm the presence of calcite and pyrite in the coal balls. in place in the shale above the Middle Six roof shale nodules were sawn and the slices Kittanning Coal. The shale is about 20 crushed and thoroughly mixed before separating m thick at the first locality and 2.5 m into 2 portions. Each sample was weighed and thick at the second. The concretions, ex- placed in 10% HC1 to remove the carbonate fraction, then dried and reweighed. One sub- posed irregularly on the highwalls, are sample was mixed into 1,1,2,2-Tetrabromo- seldom aggregated but are roughly ori- ethane (Sp. Gr. 2.89) to separate the heavy ented in horizontal lines separated ver- and light mineral fractions. The lighter por- tically by 15 to 30 cm intervals. Where tion was isolated, dried, weighed and the concretion is superficially exposed (fig. organic material was completely oxidized with dilute H2O2. The resultant residue, after dry- 2), the major axis is oriented horizontally ing and reweighing, was washed until all the and the shale is visibly arched around it. clay was removed and the final residue was again dried and weighed. The other subsample METHODS AND MATERIALS was analyzed chemically for average composi- Individual concretions were cleaned of mud tion and by x-ray diffraction to determine car- and fragments of coalified material and then bonate-free mineral constituents. numbered to assure proper identity in all sub- The evaluation of organic material in the coal sequent procedures. Each concretion was balls was based on peel studies from 50 ran- sawed (with a diamond-bladed slab saw) normal domly selected coal balls from all areas of the to its major axis, into roughly parallel 3 cm crop. Each coal ball regardless of size was slices. The sawed surfaces were smoothed of considered as one unit. From many of the coal imperfections with carborundum powder (#600) balls numerous peels were obtained but only 2 and etched in 10% HC1 for 20 to 45 seconds. peels oriented normally to each other were The wet etched surfaces were studied with a evaluated from each concretion. Each identi- low-power binocular microscope. Dilute saf- fiable plant structure or other component was ranine Y dye (2%) was used to stain the plant tabulated only once regardless of the number of material exposed on the etched surfaces of the occurrences within that peel. Percentage of coal balls (Kosanke 1945). occurrence of plant material in the coal balls The rapid peel technique of Joy el al (1956) was calculated on the basis of all 50 units in the was employed to remove material from the study. etched surface. Before the peels were re- moved, the mineralogy and organic content was RESULTS AND DISCUSSION recorded. Some peel sections were removed and mounted on slides with Permount for high COAL BALLS power microscopic examination (Stewart and The initial mineralization, which Taylor 1965). formed the coal balls, was the result of Vertically and horizontally oriented thin sections were prepared for mineralogical study cellular permineralization of the plant of the concretions to show morphological struc- material present (Schopf 1975). The tures, septaria, and other types of secondary yellow-brown primary calcite, which com- mineralization that were not distinct in the prises the bulk of the matrix, was char- acetate peels. Detailed analysis of the micro- scopic organic constituents of the concretions acteristically bladed or fibrous in crystal was accomplished by using solution residues form. In thin section the calcite crystals (Schopf 1965). Rock fragments were dissolved appear to radiate outward from various in dilute HC1 and the carbonate-free residue loci within the concretion, passing un- was washed and decanted several times, re- sulting in a sludge which was examined with a disturbed through interstitial spaces and microscope.
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