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Notes on the origin of altered macerals in the Ragged Edge of the Pennsylvanian (Asturian) Herrin coalbed, Western Kentucky
International Journal of Coal Geology 115 (2013) 24–40
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Notes on the origin of altered macerals in the Ragged Edge of the Pennsylvanian (Asturian) Herrin coalbed, Western Kentucky Bruno Valentim a, James C. Hower b,⁎, Jennifer M.K. O'Keefe c, Sandra Rodrigues d, Joana Ribeiro d, Alexandra Guedes a a Centro de Geologia da Universidade do Porto and Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências da Universidade do Porto, Rua Campo Alegre, 687, 4169-007 Porto, Portugal b University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, United States c Department of Earth and Space Sciences, Morehead State University, Morehead, KY 40351, United States d Centro de Geologia da Universidade do Porto, Rua Campo Alegre, 687, 4169-007 Porto, Portugal
a r t i c l e
i n f o
Article history: Received 24 January 2013 Received in revised form 2 May 2013 Accepted 4 May 2013 Available online 17 May 2013 Keywords: Geochemistry Marine margin Hydrothermal Carbonate Coal metamorphism
a b s t r a c t The Ragged Edge of the Herrin (Western Kentucky No. 11) coal bed has a variety of maceral textures and assemblages that do not fit in the conventional vitrinite and inertinite definitions as established by the International Committee for Coal & Organic Petrology. Within a narrow zone at the marine margin of the coalbed, the coal is brecciated and cemented by carbonate with some of the brecciated clasts metamorphosed to anthracite-level vitrinite reflectances. This is in contrast to the ambient high volatile C bituminous coal rank in the region. Low-rank peat-like textures are preserved in the clasts, suggesting that the metamorphism took place soon after deposition of the peat. Geochemical and mineralogical evidences suggest that the increase in reflectance was the consequence of the channeling of hydrothermal fluids through the breccia. The narrow zone of metamorphism and, further, the juxtaposition of breccia clasts of varying degrees of metamorphism suggest that the hydrothermal influx occurred at shallow depths of burial, perhaps shortly following deposition of the peat. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The so-called “Ragged Edge” sections of the Herrin coalbed in western Kentucky are known to have maceral forms and assemblages not seen elsewhere in the coalfield (de Wet et al., 1991, 1997; Hower and Williams, 2001; Hower et al., 1987; O'Keefe et al., 2008), including the brecciated and metamorphosed coals from a fault zone in Union County, Kentucky (Hower et al., 2001) (Fig. 1). Within the studied “Ragged Edge” sections (de Wet et al., 1991, 1997; Hower and Williams, 2001; Hower et al., 1987; O'Keefe et al., 2008), there are some fundamental facts, which more or less define the nature of the known marginal Herrin coalbed settings: 1. Coal is brecciated, including the entire Millport section (Hower et al., 1987), core P-181 in the de Wet et al. (1997) study area, the Dixon cores (Hower and Williams, 2001), and the east side of the Cardinal mine exposure (O'Keefe et al., 2008). The left side of the latter entry, about 7-m distance, is faulted, but not brecciated.
⁎ Corresponding author. Tel.: +1 859 257 0261. E-mail address: [email protected] (J.C. Hower). 0166-5162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2013.05.001
The brecciated coal is flanked on one side by normal Herrin coal and on the other side by marine limestone (de Wet et al., 1991, 1997; Neuder, 1984). Overall, the disturbed zone is confined to a narrow marine margin of Herrin, with the lateral extent of the zone well-defined by coring (de Wet et al., 1997; Hower et al., 1987) or mining (O'Keefe et al., 2008). 2. The vertical extent of brecciation is also well known at de Wet et al.'s (1997) site, where a truncated, brecciated Herrin coal (their P-181 core) and sections with breccias in limited portions of the Herrin coal (P-193 and P197) are found. Further, some brecciated sites, such as some of the de Wet et al. (1997) cores and the Ohio County site described in Hower et al. (1987), are not oxidized. There is a mix of normal (ambient Rmax) and heated/oxidized clasts in the Millport core (Hower et al., 1987). 3. Coal clasts range in texture and reflectance from the regionally ambient high volatile C bituminous rank (ca. 0.55% Rmax) to massive vitrinite-like particles with anthracite reflectances. The latter have moderate to abundant sigmoidal-slits dominantly parallel and/or oblique to the bedding plane (Benedict et al., 1968). Textures similar to those in low-rank coals are preserved in higher-Rmax clasts. In addition to the high-Rmax vitrinite, fusinite and semifusinite are present.
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Fig. 1. Map of locations of study sites with locations of faults and interpolated line delineating paleoshoreline during deposition of the Herrin coal (after O'Keefe et al., 2008) (1: Hower et al., 1987; 2: Hower et al., 2001; 3: de Wet et al., 1991, 1997; 4: Hower and Williams, 2001; 5: O'Keefe et al., 2008).
4. The coal-clast breccia generally is cemented by carbonate. At the Millport site, carbonate varies from a MgO-/Mn-rich (>0.24MgO/CaO, >4800 ppm Mn) carbonate in the bottom five sections to MgO-poor/lesser-Mn (b0.07MgO/CaO, b2200 ppm Mn) in the top five sections. Pyrite is less common and total S is lower than in ‘normal’ Herrin coal. 5. The de Wet et al. (1997) study shows the thinning of the Herrin coal towards the margin. Austin (1979) demonstrated a similar thinning of the overlying Paradise coal in Muhlenberg County, Kentucky, mines. Together, both coals, along with the limestone between the coals and the limestone above the Paradise, represent a transgressive sequence. 6. Even though the sections are brecciated, the basin-wide Blue Band parting appears in its usual place within the sequence. Limestone partings occur within the brecciated Herrin coal (Hower et al., 1987) and within the Paradise coal (Austin, 1979), the latter identical to the roof rock. A shelly clay parting, identical to the roof rock, occurs within the Paradise in a Coiltown 7 1/2′ quadrangle mine (observations by Hower, circa 1981). 7. In general, the sub-Herrin-to-post-Paradise stratigraphic section ranges from silty shale/underclay/Herrin coal/limestone/Paradise (not always with underclay)/limestone. Limestone is not present in the sub-Herrin section. Evaporites are not known in the section. Limestone above brecciated coal is also brecciated in de Wet et al. (1997) sections. There are a number of problematical features as well; points that will be addressed in this paper: 1. Is the lack of pyrite an indicator of oxidation or less reducing conditions; or did brecciation and enclosure by carbonate happen prior to formation of secondary pyrite? Or, did heating devolatilize sulfides?
2. If there was heating, why is there no apparent thermal affect nearby? Is sphalerite/pyrite/calcite in semifusinite in the de Wet et al. (1997) core P-197 an indication of elevated temperature for emplacement? Is the minor occurrence of textures resembling low-T stages of coke formation an indicator of heating? In this paper, we are conducting a critical examination of the maceral textures, mineralogy, and chemistry of the Ragged Edge sites with the objective of coming to a better understanding of the mechanisms of their formation.
2. Procedure The original samples were collected and analyzed by personnel of the University of Kentucky Center for Applied Energy Research (CAER). The original petrographic analyses were conducted on polished blocks and polished particulate pellets using oil-immersion, reflectedlight optics with 32 × (with 1.25 × tube factor) and 50 × objectives on Leitz Orthoplan microscopes (de Wet et al., 1997; Hower and Williams, 2001; Hower et al., 1987; O'Keefe et al., 2008). The blocks and pellets are stored in the Applied Petrology Laboratory at the CAER. Additional petrographic analyses of five of the Millport (Hower et al., 1987, study) blocks were made at the Centro de Geologia da Universidade do Porto (CGUP) using oil-immersion, reflected-light optics with a 50 × objective on a Leitz Orthoplan microscope coupled to a DISKUS-FOSSIL system. Additional CGUP studies made at Centro de Materiais da Universidade do Porto (CEMUP) utilized a high-resolution (Schottky) environmental scanning electron microscope equipped with EDS, FEI Quanta 400FEG ESEM/EDAX Genesis X4M.
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Fig. 2. Normal vitrinite. A) High volatile C bituminous collotelinite with resinite (r) and other liptinite macerals. P-197 middle block 01; scale = 50 μm. B) High volatile C bituminous collotelinite and corpogelinite with liptinite. P-197 bottom block 04; scale = 50 μm. C) High volatile C bituminous telinite and transitions in semifusinite (sf) reflectance. P-197 middle block 03; scale = 50 μm. D) Collotelinite in the form of a stem flanked by cutinite. Some of the macerals, particularly at the top, show signs of a disrupted structure without accompanying oxidation. P-197 middle block 04; scale = 50 μm.
A Horiba Jobin-Yvon LabRaman spectrometer interfaced to an Olympus microscope was used. Spectra were obtained using the 632.8 nm emission line of HeNe laser (20 mW). The objective lens (× 100) of the microscope was used to focus the laser beam on the sample and also to collect the scattered radiation. 3. Basic petrology Ambient-rank vitrinite (Fig. 2) of the Herrin coal, as defined by the coal rank in the nearby mines, ranges from high volatile C bituminous (Millport core; Hower et al., 1987) to high volatile B bituminous (de Wet et al., 1997; O'Keefe et al., 2008) to high volatile A bituminous (Hower and Williams, 2001). The forms of the ambient-rank vitrinite at those sites correspond to vitrinite textures defined by ICCP (1998). Aside from fusinite and semifusinite in the latter, unaltered coal, probable fire-derived fusinite and semifusinite can be found in the carbonate cement (Fig. 3a) and in the altered vitrinite (Fig. 3b). Fusinite also occurs in a type developed after the vitrinite maceral corpogelinite (Fig. 3c–d). Macrinite (Fig. 4a) occurs in forms similar to the types described by Hower et al. (2009, 2011), Hower and Ruppert (2011), and O'Keefe and Hower (2011). A rare weathered form (Fig. 4b) may not be macrinite, but rather a high-reflectance form of vitrinite with some resemblance to weathered forms described by Wagner (2007). Secretinite occurs in the classic (Hower et al., 2008a; ICCP, 2001) solid and vesiculated forms (Fig. 5a, b). A secretinite variety with a brecciated inner core and a
more solid rim, although exhibiting some fractures, was observed (Fig. 5c). Secretinite with desiccation cracks is common (Fig. 5d, e), including forms with subtle boundaries with the surrounding degraded fusinite and semifusinite (Fig. 5f). Among the maceral textures associated with degradation are thickened cell walls in fusinite and semifusinite (Fig. 6 a–d). The divisions between the primary formation of fusinite by fire (Bustin and Guo, 1999; Evans, 1929; Guo and Bustin, 1998; McParland et al., 2007; Petersen, 1998; Scott, 1989, 2000, 2002; Scott and Glasspool, 2005, 2006, 2007; Scott and Jones, 1994; Scott et al., 2000; Stach, 1927; Winston, 1993) and inertinite formed by the degradation of wood, as discussed by Hower et al. (2009, 2011), can be diffuse. In the latter case (as seen in Fig. 6 a–d), one possible pathway of maceral development is partial fungal or bacterial degradation followed by conflagration. Coke (Fig. 7), a rare maceral texture in these samples, is formed after the vitrinite has reached bituminous rank. At least superficially, some of the maceral textures resemble ‘pseudovitrinite.’ As defined by Benedict et al. (1968), with further studies by Thompson et al. (1966) and Thompson (2000), pseudovitrinite is marked by (1) higher reflectance than other vitrinite in the same coal, (2) slitted structures, (3) unusual fracture patterns, (4) remnant cell structure, and (5) paucity or absence of pyritic inclusions. The origin of pseudovitrinite seems to be related to (i) pre-coalification oxidation of the woody material, leading to pseudovitrinite-like features (Benedict et al., 1968) (Mackowsky (in discussion by Smith (1980)) suggested that pseudovitrinite may have developed as a result of
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Fig. 3. Fusinite and semifusinite. A) Fusinite and semifusinite in carbonate cement. Vitrinite on right has a higher reflectance than the ambient high volatile C bituminous reflectance of vitrinite in the region. 7873 11; scale = 25 μm. B) Fusinite and semifusinite in carbonate and altered vitrinite. Vitrinite has a higher reflectance than the ambient high volatile C bituminous reflectance of vitrinite in the region. 7867 01; scale = 25 μm. C) Fusinite after collogelinite. 7875 14; scale = 25 μm. D) Fusinite after collogelinite. P-197 top block 11; scale = 50 μm.
primary oxidation during the biochemical phase of coalification), or (ii) post-coalification coal oxidation forming slitted pseudovitrinite as an oxidation product of vitrinite that is intermediate, in terms of degree of oxidation, between vitrinite and oxyvitrinite (Kaegi, 1985), or (iii) thermal influence at a certain coalification stages (Kruszewska, 1998). Benedict and Berry (1964) conducted controlled oxidation experiments, producing material with the same microscopic features as pseudovitrinite. Clark et al. (1984), Kaegi (1985), Diessel and Gammidge (1998), Gurba and Ward (1998), Mastalerz and Drobniak (2005), and Hower et al. (2008b) have made more recent studies of the origin and properties of the maceral.
In the brecciated “Ragged Edge” Herrin coal, pseudovitrinite-like macerals (Fig. 8) are commonly highly slitted (dominantly by sigmoidal-slits parallel or oblique to bedding plane), with higher reflectance than the parental vitrinite. Among the pseudovitrinite characteristics noted by Thompson et al. (1966), certainly features 2–5 (above) are found in the Ragged Edge Herrin coal. Slitted structures, albeit more abundant than in the eastern Kentucky Pennsylvanian coals described by Thompson et al. (1966), are common in the Ragged Edge coals (Fig. 8 a–f) and remnant cell structures are also found (descriptions by de Wet et al., 1997; Hower et al., 1987). The reflectances of vitrinite vary widely. The reflectance of the ‘normal’ vitrinite
Fig. 4. Macrinite. A) High-reflectance coprolite macrinite, possibly after coprolites, in high-reflectance vitrinite/calcite matrix. 7867 08; scale = 25 μm. B) Oxidized/weathered macrinite. 7875 16; scale = 25 μm.
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Fig. 5. Secretinite. A) Round, solid secretinite with fusinite. 7872 12; scale = 25 μm. B) Vesiculated secretinite with round to oblate secretinite. 7872 20; scale = 25 μm. C) Round secretinite with solid rim and fractured center with fusinite in carbonate. 7874 06; scale = 25 μm. D) Oblate secretinite or collogelinite with desiccation cracks with carbonate. 7873 06; scale = 25 μm. E) Oblate secretinite or collogelinite with desiccation cracks with carbonate. 7875 06; scale = 25 μm. F) Oblate secretinite or collogelinite with desiccation cracks; gradation with degraded fusinite is visible. 7875 12; scale = 25 μm.
in the nearby mines and in the unaltered coal clasts is within the high volatile bituminous rank range; for example, 0.55% Rmax in the Hopkins County section studied by Hower et al. (1987). Fig. 8b is a good example of the variation in reflectance, with semi-anthracite/ anthracite-reflectance vitrinite-like macerals adjacent to a liptinite band. As seen in Table 1 (the location key to the reflectance values is provided in Appendix A), a number of vitrinite particles exceed the ambient 0.55% Rmax in the Millport (Hower et al., 1987, study) samples, with values up to 4.63% Rmax reported. The bright, vitrinite-like macerals, while having inertinite-level reflectances, albeit lower than some unequivocal fusinite in the same sample, do not have textures that comfortably fit within the ICCP (2001) inertinite definitions. Several maceral forms test the limits of the ICCP (1998, 2001) vitrinite and inertinite definitions (Fig. 9). Parts of Fig. 9a and b have areas in which the maceral structures suggest devolatilization. Aside from the forms shown in Fig. 7, few macerals suggest coking. Fig. 9f has fusinized and semifusinized cell walls with possible macrinite in the cell interiors. Portions of Fig. 9c, g, and h suggest fungal damage to the vitrinite, but the evidence is not entirely definitive.
Among the mineral evidence for heating are mineral assemblages in the de Wet et al. (1997) cores. Aside from the common Ca–Mg–Fe– Mn carbonates, the sphalerite/pyrite/carbonate assemblage in the Herrin coal in their core P-197 (Fig. 10) resembles assemblages found by Hower et al. (1983, 2001) in mineralized coals in fault zones in the region. 4. Discussion Summarizing arguments in Hower et al. (1987) concerning the origin of the Millport section: • Wood- and herbaceous-derived material was deposited on a swamp located near the marine margin. • In turn, water level variations, due to marine transgressions and flooding from rivers, were responsible for: (i) periodic oxidation of collotelinite precursors, developing pseudovitrinite-like structures; (ii) periodic ripping of the peat and oxidation during transport, preserving the peat/lignite humic textures; followed by re-deposition, and calcite cementation.
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Fig. 6. Degraded fusinite & semifusinite. A) Thickened cell walls in semifusinite suggesting degradation. 7867 09; scale 25 μm. B) Thickened cell walls in fusinite suggesting degradation. 7871 03; scale 25 μm. C) Thickened cell walls in fusinite suggesting degradation. 7874 10; scale 25 μm. D) Thickened cell walls in fusinite suggesting degradation. 7875 18; scale 25 μm.
Each time sea level rose and flooded the swamp, a high-energy environment and varying oxidation potential contributed to coal brecciation. In some periods, the sea level was high enough to form marine carbonates, for example, the marine-carbonate parting in Millport
Block 18 and carbonate-cemented coal breccias, both discussed in Section 4.1, below. The emphasis in the following discussion is on heating by transient thermal fluids. The concept of alteration and preservation in acidic brine, which may also be expected at the intersection of mire and marine waters, could be considered an alternative hypothesis to heating, but will not be discussed further in this paper. 1
4.1. Evidence for heating? When considering thermal-fluid metamorphism of the coal, the way these fluids came in contact with coal must also be addressed. After a “brecciation event”, as described by de Wet et al. (1997), hot fluid flow from deeper levels could have been channelized through high-permeability conduits (such as fracture sets, or individual fractures). If certain macerals or microlithotypes, such as sporinite- and cutinite-rich clarites, were subject to selective devolatilization, then permeability and fluid fluxes would have been greatest and parallel to coal layering, increasing permeability, and thus causing more flow to focus into the reacting area. There is a general trend for increased Rmax of vitrinite and apparent vitrinite macerals towards the bottom of the Millport quadrangle borehole. However, some very high-Rmax particles do occur above the parting in Block 18, possibly an indication of the heterogeneity of fluid flow within the brecciated section. It is also possible that the brecciation and concomitant faulting juxtaposed coal particles of
Fig. 7. Fine carbon resembling a fine borderline isotropic/anisotropic low-temperature coke. 7868 07; scale = 50 μm.
1 Acidification “reduces bacterial activity, and the plant remains are well preserved” (Taylor et al., 1998, pp. 32). In one application of this principle, this is a pickling process applied in the preservation of certain vegetables and of seafood (ceviche). Taylor (1926, 1927, 1928) applied similar principles in his discussion of base exchange as a means of maceral alteration.
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Fig. 8. High reflectance vitrinite with low-rank and desiccation textures. A) Variations in brightness of vitrinite/vitrinite-derived macerals, some with signs of internal fractures/ desiccation cracks. 7866 04; scale = 25 μm. B) Vitrinite layers alternating with darker macerals with inertinite textures and even darker liptinite band. For reference, the vitrinite has reflectances in the semi-anthracite to low anthracite reflectance range. This is in contrast to the high volatile C bituminous reflectance 0.55% Rmax for the coal mined near this site and for unaltered coal clasts in the breccia and the higher reflectance of fusinite in the breccia (ca. 4.5% Rmax). 7867 04; scale = 25 μm. C) Desiccation cracks in high-reflectance vitrinite. 7872 17; scale = 25 μm. D) Desiccation cracks in high-reflectance vitrinite. 7874 13; scale = 25 μm. E) Desiccation cracks in high-reflectance vitrinite. 7874 09; scale = 25 μm.
considerably different degrees of metamorphism. Delicate botanical structures present as fusinite can be largely preserved, despite the brecciation, albeit with some signs of shearing (Fig. 11). Certain structures do bear some resemblance to the heated remains of liptinites. Relict sporinite structures can be seen in Fig. 12. In this case, a void area with the shape of a trilete spore contains 100-nm carbon spheroids. If the void is actually a relict sporinite, the carbons could be the pyrolysis product of the liptinite. Similar carbon spheroids were found in relict cutinite structures.
Liptinite survives, however, in close association with high-reflectance vitrinite, generally in the higher portions of the brecciated coal (Figs. 8b, 13). Rare coke structures (possibly, but not definitively, Fig. 7) and low reflectance anisotropy were found in the brecciated Herrin coal, which could explained by the low rank of the coal at the time of alteration, since one of the most important factors affecting the nature of thermally altered carbonaceous matter is the rank or level of maturity at the time of intrusion. It is also
Table 1 Maximum reflectance of vitrinite and vitrinite-derived macerals in selected samples. Measure location
Collotelinitic
Above B18P parting Middle of B18 beneath B18P Middle of B10, less affect area Bottom of B10 Bottom of B9, less affect area B9 between less affect area and fold
0.55 0.76 0.93 0.99 0.88 0.98
Fused
Fusinite
0.94 0.95 0.87 0.94
2.03 2.74 1.72
Middle of B9, reflectance sequence at chevron fold Bottom of B9, beneath chevron fold Between B6P1 and B6P2 partings Between B6P2 and B6P3 partings Top of B2 bellow B6P3 parting Bottom of B2
1.30 1.23 1.32 1.38 1.81
1.25 1.35 1.27 1.46 1.65
2.05 2.41 1.97 2.38 2.64
High reflectance collotelinitic
High reflectance fused
High reflectance fusinite
2.41 2.81
2.16 2.51
3.59 3.72
4.63
3.23
Collotelinitic material — high reflectance smooth structureless layers like collotelinite. Fused material — high reflectance layers with rounded edges, distorted pores, and relict spore holes. Fusinite — inertinite maceral as defined in ICCP (2001).
3.51 3.08
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Fig. 9. Bizarre structures. A) Desiccation cracks in high-reflectance vitrinite with structures resembling devolatilization vacuoles. 7873 04; scale = 25 μm. B) Remnant of rootlet (?) filled with carbonate surrounded by high-reflectance vitrinite with desiccation cracks. 7874 12; scale = 25 μm. C) Mixed high-reflectance vitrinite and inertinite textures. 7873 07; scale = 25 μm. D) Mixed high-reflectance vitrinite and inertinite textures with corpogelinite with granular interior. 7872 08; scale = 25 μm. E) Mixed high-reflectance vitrinite and inertinite textures with corpogelinites with granular and solid interiors, possibly including macrinite. 7872 08; scale = 25 μm. F) Corpogelinite and secretinite in inertinite breccia. 7868 05; scale = 50 μm. G) Vitrinite and/or inertinite textures with possible fungal contributions to the degradation. 7868 01; scale = 50 μm. H) Vitrinite and/or inertinite textures with possible fungal contributions to the degradation. 7867 12; scale = 25 μm.
possible that the alteration temperatures were not high enough to form structures associated with natural coke, but Hower and Lloyd (1999), among others, have described thermoplastic alteration
of coal at lower temperatures than those associated with natural cokes (Brooks and Taylor, 1966; Mastalerz et al., 2009; Stewart et al., 2005).
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study of the transition from natural coke to unaltered coal, noted less pyrite in the coke than in the normal coal at about 1.2-dike widths from the ca. 10-m-thick intrusion, so, it is possible to see a heating-induced loss of sulfides in coal. More common is the carbonate mineralization pervasive throughout the brecciated sections. The overall carbonate composition for the Millport core noted in the Introduction (MgO-/Mn-rich carbonate in bottom five sections to MgO-poor/lesser-Mn in top five sections) is a bulk composition. On a finer scale, there are differences within the carbonates phases, as shown in Fig. 14 for a portion of Millport Block 2. In this case, the carbonate rhomb (Z1) has Mg > Ca ≫ Fe while the matrix (Z2) has Ca > Mg ≫ Fe (but more Fe than Z1). Millport Block 10 has examples of Mn enrichment in Ca > Mg > Fe carbonates (Fig. 15). Other carbonates in Millport Block 10 have little or no Fe and/or Mg. The parting in Millport Block 18 was analyzed in detail. The severely mylonitized ca. 1 cm-thick parting is intercalated between brecciated coal and is composed of b1 mm layers of calcite intercalated with thin layers of clay. At the top and at the bottom of the parting the contacts with brecciated coal are cemented by Ca-rich carbonate and by Mg–Ca carbonate, respectively (Fig. 16). The characteristic features of this parting are as follows: Fig. 10. Sphalerite (sp), pyrite (py), and carbonate (c) cementing coal-clast breccia (core P-197 from de Wet et al., 1997). Scale = 50 μm.
4.2. Mineralization Mineralization, such as the emplacements of sphalerite, as in de Wet et al.'s (1997) P-197 core (Fig. 10), can lead to an increase in reflectance. Hower et al. (1983) noted increased rank, high volatile A bituminous vs. the ambient high volatile C bituminous, attributed to thermal fluids associated with sphalerite mineralization in coal in a Union County, Kentucky, borehole (see also Hower and Gayer, 2002). Among the specimens observed in all of the Ragged Edge studies, the P-197 sphalerite is an anomaly. Considering the amount of pyrite in the ‘normal’ Herrin coal (for example, Frankie and Hower (1987) discussed pyrite in mine sections of the Herrin within a short distance of the Millport Ragged Edge section), pyrite in the Ragged Edge coals is much less abundant. Stewart et al. (2005), in their
(i) Fragmented calcite layers of ca. 50 μm, with some having pyrite layers or more finely dispersed pyrite; (ii) Rounded Ca-phosphate nodules; (iii) Rounded pyrite nodules with a clay core; (iv) Shells, surrounded and filled by clay, between thin, Mg-free calcite layers (of ca. 50 μm) occurring in narrow band of ca. 0.5 cm in the middle of the parting (Fig. 17); and (v) Ca-phosphate nodules and fragments of calcite layers in the clay associated with the shells. The absence of Mg-rich carbonate in the parting suggests that it was not permeable to the Mg–Ca fluids responsible for the emplacement of the carbonate matrix in the coal breccia. Fragments of rock with characteristics similar to the Millport Block 18 parting, in particular the rounded Ca-phosphate grains, were found several cm above and below the parting. The fragment seen in Fig. 18, however, while in the proximity of the latter parting, does not have the P of the parting and does have quartz grains, not found in the parting. The
Fig. 11. Fusinite in clay layer in Block 2 (Transect2; B2 block; ×60; SEM BSE mode). The “Chevron pattern” zone shows some deformation and shearing, which continues to the right, not evident in the fusinite above the latter zones.
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Fig. 12. Micrographs A (BSE mode) and B (SE mode) show one area were spore holes are abundantly represented. The cross section “trilete spore” shaped hole was selected for analysis (micrograph C; SE mode). After 100,000× magnification of the hole (micrographs D–F; SE mode), ca. 100 nm carbon spheroids are visible. Backscattered electron mode and EDS analysis confirmed that these spheroids are made of carbon.
carbonate vein (Fig. 18, top right) exhibits the compositional heterogeneity observed in other veins. The Mg–Ca fluid path can be traced between two partings in Millport Block 6 (Fig. 19). The Mg–Ca-enriched fluid suspended an illitic clay (with a small Ti signal in the EDS spectra) in the upper parting. The lower parting contains a mineral with a composition suggesting glaucophane. The latter parting acted as a barrier to the migration of the Mg–Ca-enriched fluids (Fig. 20). Pathways of fluid migration can be followed in Millport Block 2 (Fig. 21), with the coal fragments tracing the flow paths through the breccia. As described above, distinct compositions of carbonate minerals are found in the matrix (Fig. 22), with a central zone (Z1) with Ca > Mg ≫ Fe flanking zoned dolomites (Z2) with Mg > Ca (with trace Fe) (see also Fig. 23).
As noted above, a major brecciation evidence described in the strata overlying the Herrin coal (de Wet et al., 1997) formed conduits for the flow of fluids, possibly the fluids responsible for the fracture fill and carbonate cementing of the brecciated Herrin coal. A hypothetical brecciation scenario may be exemplified by considering the Cambro-Ordovician Upper Knox Group (UKG) underlying the entire state of Kentucky. Montanez (1994) described a sequence of episodes (paleokarsting, dolomitization, stylolitization, dissolution, fracturing, and brecciation) occurring in the UKG from the Lower Ordovician up to the Late Pennsylvanian–Permian that could have generated collapse breccias in the overlying formations, including the Herrin coal seam. Conduits could then channel hot fluids from deeper levels of the basin to the upper levels, causing thermal metamorphism in the coal. Considering fluid migration as the source of heat is not
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2002; Hower et al., 1983, 2001). As with the Hower et al. (1983, 2001) sites, this study demonstrates the superimposition of a brecciation event(s) and hot fluid migration on a more local scale than the regional settings discussed by Hower and Gayer (2002). 4.3. Problematic features Returning to the problematical features enumerated in the Introduction, we will use the evidence presented to consider answers to the problems raised.
Fig. 13. Blue-light image of sporinite from Block 18.
extraordinary. This was suggested (Daniels et al., 1990; Harrison et al., 2004; Hower and Gayer, 2002; Oliver, 1986, 1992; Ruppert et al., 2010) for the nearby Appalachian and Ouachita basins, due to conduits opening during the Alleghanian and Ouachita orogenies, as being responsible for high coal rank found in the Appalachian basin. Of particular importance to this study, coal metamorphism and mineralization within the coalfield appear to be related to the mineralization of the Illinois–Kentucky Fluorspar District, immediately to the southwest of the Western Kentucky coalfield (Hower and Gayer,
4.3.1. The general lack of pyrite, at least in comparison with typical western Kentucky coals Both options noted, those being heating-induced oxidation and encasement of the coal breccias prior to the formation of secondary pyrite, seem to be possible causes of the lack of pyrite. The apparent loss of pyrite in heating, as in the case of intrusions into coal (Stewart et al., 2005), is a viable mechanism, but we do not have conclusive evidence for the dominance of either mechanism. 4.3.2. Juxtaposition of heated/oxidized and normal coal and the narrow zone of heating/oxidation Within the coalfield, with the exception of the Gil-30 borehole coals (Hower et al., 1983) and the faulted and mineralized coal studied by Hower et al. (2001) (both in Union County), we know of no other mineralized coals other than the Herrin coals examined here. In the Ragged Edge Herrin brecciated coal, the coincidence of extensive vein carbonate mineralization accompanied by sphalerite, albeit rare, and the metamorphism of the coal corresponds to the trends at the other two sites. In addition, the presence of coke within the Ragged Edge coals is an indicator of heating.
Fig. 14. Scanning-electron microscopy back-scattered electron image of carbonate vein in Millport Block 2. Z1 and Z2 spectra correspond to areas marked on image.
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Fig. 15. Millport Block 10. Z1: Ca–Mg (C–O) (Fe–Mn) spectra, possibly ferroan dolomite. Z2: Ca (C–O) (Fe–Mg–Mn) spectra, calcite. Z3: similar to Z2.
As noted in the Introduction, the zone of brecciation is narrow, as shown in Fig. 24, reproduced from de Wet et al. (1997). Only one borehole, P-181, has a complete, albeit thin, section of oxidized coal breccias. The P-197 section is the perhaps the most complex of any in our studies of the Ragged Edge, passing from normal coal at the base, to unoxidized breccias, to oxidized breccias, and terminating in a fusain lithotype (Fig. 24 and de Wet et al.'s (1997) Figure 8). A thin inertinite or oxidized vitrinite breccia is found within P-193 (Fig. 24 and de Wet et al.'s (1997) Figure 8). In the case of the boreholes in their study, brecciation had to be independent of mineralization; the breccias providing a ready path for the mineralizing fluids, but not necessarily an open path for pervasive heating and mineralization of the entire breccia. The relatively narrow zone of mineralization, as known from coring and mining in at least three of the locations (de Wet et al., 1997; Hower et al., 1987; O'Keefe et al., 2008), may also serve as a clue to the both the limited vertical and lateral extents of the mineralization. If the mineralization was a near-surface phenomenon, then cooling would have been rapid, limiting the extent of the associated heating. The maceral textures, seemingly frozen in the early stages of coal development, if not still in the peat stage, further suggest that the mineralization and consequent coalification was a near-surface process. All four sites in the western part of the Western Kentucky coalfield are close to the Central Faults (Fig. 1), which in turn, originate on the boundary of the extensively faulted and mineralized Fluorspar District. The mineralization in the latter region is imprecisely dated, bracketed as post-Early or Middle Pennsylvanian to pre-Late Cretaceous (Trace and Amos, 1984).
fit in the conventional ICCP (1998, 2001) vitrinite and inertinite definitions. Within a narrow zone at the marine margin of the coalbed, the coal is brecciated and cemented by carbonate. Some of the brecciated clasts have been metamorphosed, with the some clasts having anthracite-rank vitrinite, an increase from the ambient high volatile C bituminous coal rank in the region. In addition to the increase in rank, textures more common in low-rank coals were preserved in the clasts, suggesting that the metamorphism took soon after deposition of the peat. Mineral and major element distributions suggest that the metamorphism was caused by a narrowly restricted influx of hydrothermal minerals. Based on the bulk analyses, the Millport (Hower et al., 1987) section was shown to have a MgO-/Mn-rich carbonate cement in bottom half of the coal and a MgO-poor/lesser-Mn carbonate in top five sections. Among a variety of Ca/Mg/Fe/Mn-carbonate associations, fine-scale analyses, for example, demonstrate that carbonate rhombs with Mg > Ca ≫ Fe are found in the Ca > Mg ≫ Fe matrix. The brecciation at the marine margin provided the pathways for the influx of hydrothermal fluids. Un-metamorphosed breccias, such as in parts of the Nebo cores (de Wet et al., 1997), indicate that mineralization was independent of brecciation. The breccias provided an open path for the mineralization and metamorphism, but mineralization and metamorphism were not universal throughout the breccias. The narrow vertical and lateral extents of mineralization and metamorphism suggest that the process was a near-surface process, with the rapid cooling limiting the extent of metamorphism.
5. Summary
The samples for the first study of the Ragged Edge of the Herrin coalbed were logged in on 25 August 1983. The results of that study were presented at the first meeting of The Society for Organic Petrology in 1984 and published as part of the collected papers of that
The Ragged Edge of the Herrin (Western Kentucky No. 11) coal bed has a variety of maceral textures and assemblages that do not
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conference as Hower et al. (1987). In the 30 years of investigating this sequence throughout the Western Kentucky coalfield, we have been fortunate to be able to collaborate with a number of coal companies including Island Creek Coal and Pyro Coal, both defunct, and Alliance Coal. Our collaborators include Eric Trinkle (now Delaware Department of Natural Resources), Anne Graese (Illinois), Gary Neuder (Louisiana), Carol and Andy de Wet (Franklin & Marshall College), Sean Brennan (US Geological Survey), Steve Moshier (Wheaton College), Anne Raymond (Texas A&M), John Popp (Natural Resource Partners), Mike Shultz (EOG Resources), and Sue Rimmer (Southern Illinois University, Carbondale). Mike McClure (currently with Marshall Miller & Associates) supplied the core samples for the de Wet et al. (1997) study when he was employed by Island Creek Coal Co. The Portuguese authors would like to thank FEDER through the Program COMPETE, FCT project “PEst-OE/CTE/UI0039/201-UI 39” and Ciência 2007 Program.
Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.coal.2013.05.001.
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Fig. 16. Parting in Millport Block 18 (Transect2; B18 block; ×60; SEM BSE mode). (A) Top of parting in contact with brecciated coal (Bc), and fragment of B18P (Fg); (B) middle of parting; and (C) bottom of parting in contact with brecciated coal. Note: each area has a height of 3.8 mm and the gap between lines and areas is 3.8 mm.
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Fig. 17. Parting in Millport Block 18. (A) Micrograph with a general view of aquatic fauna hosting zone (×60; SEM BSE mode). Includes severe mylonitization pattern, calcite layers (Ca) with layered (PyL) or dispersed pyrite, clay (Cl), Ca-phosphate nodules (Pn), and aquatic fauna shell (Afs); (B) magnification of micrograph “A” and location of EDS analyses (×200; SEM BSE mode); (B Z1-4) EDS spectrum of made in areas Z1 to Z4 marked in micrograph B. Z5 is not presented since it is pyrite spectra.
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Fig. 18. On the right is a quartz-bearing clay fragment. Unlike the parting shown in Fig. 17, this fragment has no P. The composition of the zones is as follows. Z4: Ca ≫ Mg-carbonate filling cell voids; Z5 and Z8: Quartz; Z6: Calcite nodule; Z7: Al–Si clay; Z9: Ca ≫ Mg-carbonate vein; and. Z10: Ca > Mg-carbonate vein. The inset, enlarged on the top left, shows a carbonate vein with area Z11 having a Ca > Mg ≫ Fe composition with traces of Al, Si, and S. Area Z12 has a Ca ≫ Mg > Fe composition with traces of Al, Si, and S.
Kruszewska, K.J., 1998. The reactivity of pseudovitrinite in some coals. Fuel 77, 1655–1661. Mastalerz, M., Drobniak, A., 2005. Optical properties of pseudovitrinite; implications for its origin. International Journal of Coal Geology 62, 250–258. Mastalerz, M., Drobniak, A., Schimmelmann, A., 2009. Changes in optical properties, chemistry, and micropore and mesopore characteristics of bituminous coal at the contact with dikes in the Illinois basin. International Journal of Coal Geology 77, 310–319. McParland, L.C., Collinson, M.E., Scott, A.C., Steart, D.C., Grassineau, N.V., Gibbons, S.J., 2007. Ferns and fires: experimental charring of ferns compared to wood and implications for paleobiology, paleoecology, coal petrology, and isotope geochemistry. Palaios 22, 528–538. Montanez, I.P., 1994. Late diagenetic dolomitization of lower Ordovician, upper Knox carbonates: a record of the hydrodynamic evolution of the southern Appalachian basin. American Association of Petroleum Geologists Bulletin 78, 1210–1239. Neuder, G.L., 1984. Controls on Peat Accumulation in the No. 9 to No. 13 Interval in Western Kentucky. (MS thesis) University of Kentucky, Lexington (78 pp.). O'Keefe, J.M.K., Hower, J.C., 2001. Revisiting Coos Bay, Oregon: a re-examination of funginite–huminite relationships in Eocene subbituminous coals. International Journal of Coal Geology 85, 34–42. O'Keefe, J.M.K., Shultz, M.G., Rimmer, S.M., Hower, J.C., Popp, J.T., 2008. Paradise (and Herrin) lost: marginal depositional settings of the Herrin and Paradise Coals, Western Kentucky Coalfield. International Journal of Coal Geology 75, 144–156. Oliver, J., 1986. Fluids expelled tectonically from orogenic belts: their role in hydrocarbon migration and other geologic phenomena. Geology 14, 99–102. Oliver, J., 1992. The spots and stains of plate tectonics. Earth-Science Reviews 32, 77–106. Petersen, H.I., 1998. Morphology, formation and palaeo-environmental implications of naturally formed char particles in coals and carbonaceous mudstones. Fuel 77, 1177–1183. Ruppert, L.F., Hower, J.C., Ryder, R.T., Levine, J.R., Trippi, M.H., Grady, W.C., 2010. Geologic controls on thermal maturity patterns in Pennsylvanian coal-bearing rocks in the Appalachian basin. International Journal of Coal Geology 81, 169–181. Scott, A.C., 1989. Observations on the nature and origin of fusain. International Journal of Coal Geology 12, 443–475. Scott, A.C., 2000. The pre-Quaternary history of fire. Palaeogeography, Palaeoclimatology, Palaeoecology 164, 281–329. Scott, A.C., 2002. Coal petrology and the origin of coal macerals: a way ahead? International Journal of Coal Geology 50, 119–134. Scott, A.C., Glasspool, I.J., 2005. Charcoal reflectance as a proxy for the emplacement temperature of pyroclastic flow deposits. Geology 33, 589–592.
Scott, A.C., Glasspool, I.J., 2006. The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration. Proceedings of the National Academy of Sciences of the United States of America 103, 10861–10865. Scott, A.C., Glasspool, I.J., 2007. Observations and experiments on the origin and formation of inertinite group macerals. International Journal of Coal Geology 70, 53–66. Scott, A.C., Jones, T.P., 1994. The nature and influence of fire in Carboniferous ecosystems. Palaeogeography, Palaeoclimatology, Palaeoecology 106, 91–112. Scott, A.C., Cripps, J.A., Collinson, M.E., Nichols, G.J., 2000. The taphonomy of charcoal following a recent heathland fire and some implications for the interpretation of fossil charcoal deposits. Palaeogeography, Palaeoclimatology, Palaeoecology 164, 1–31. Smith, A.H.V., 1980. An appraisal of the work carried out on pseudovitrinite by the I.C.C.P. and the direction of future work. Proceedings, 28th Annual Meeting, International Committee for Coal Petrology. Stach, E., 1927. The origin of fusain. Gluckauf 63, 759. Stewart, A.K., Massey, M., Padgett, P.L., Rimmer, S.M., Hower, J.C., 2005. Influence of a basic intrusion on the vitrinite reflectance and chemistry of the Springfield (No. 5) coal, Harrisburg, IL. International Journal of Coal Geology 63, 58–67. Taylor, E.M., 1926. Base exchange and its bearing on the origin of coal. Fuel 5, 195–202. Taylor, E.M., 1927. The decomposing of vegetable matter under soils containing calcium and sodium as replaceable bases. Fuel 6, 359–367. Taylor, E.M., 1928. Base exchange and the formation of coal. Fuel 7, 230–238. Taylor, G.H., Teichmüller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic Petrology. Gebrüder Borntraeger, Berlin (704 pp.). Thompson, R.R., 2000. History of applied coal petrology in the United States III. Contributions to applied coal and coke petrology at the Bethlehem Steel Corporation. International Journal of Coal Geology 42, 115–128. Thompson, R.R., Shigo, J.J.I.I.I., Benedict, L.G., Aikman, R.P., 1966. The use of coal petrography at Bethlehem Steel Corporation. Blast Furnace and Steel Plant, September 1966, pp. 1–8. Trace, R.D., Amos, D.H., 1984. Stratigraphy and structure of the Western Kentucky Fluorspar District. U.S. Geological Survey Professional Paper 1151-D (46 pp.). Wagner, N.J., 2007. The abnormal condition analysis used to characterize weathered discard coals. International Journal of Coal Geology 72, 177–186. Winston, R.B., 1993. Reassessment of the evidence for primary fusinite and degradofusinite. Organic Geochemistry 20, 209–221.
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Fig. 19. Path of the Mg–Ca-enriched fluid flow between Millport Block 6 partings. The lower parting is seen on the collage.
Fig. 20. Mg–Ca carbonate vein at the base of parting in Millport Block 6.
Fig. 21. Mg–Ca-enriched fluid path through coal breccias in Millport Block 2.
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Fig. 22. Mg–Ca carbonate vein in Millport Block 2 with two distinct compositions: (Z1) Mg b Ca at light gray areas under SEM BSE mode and (Z2) Mg > Ca at darker gray areas under SEM BSE mode with zoned dolomites.
Fig. 23. Zoned dolomite in same Millport Block 2 vein shown in Fig. 21.
Fig. 24. Idealized lateral and vertical lithologic transitions in the Herrin coal in the de Wet et al. (1997) study area. Figure reproduced from de Wet et al. (1997).
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Notes on the origin of altered macerals in the Ragged Edge of the Pennsylvanian (Asturian) Herrin coalbed, Western Kentucky Bruno Valentim a, James C. Hower b,⁎, Jennifer M.K. O'Keefe c, Sandra Rodrigues d, Joana Ribeiro d, Alexandra Guedes a a Centro de Geologia da Universidade do Porto and Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências da Universidade do Porto, Rua Campo Alegre, 687, 4169-007 Porto, Portugal b University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, United States c Department of Earth and Space Sciences, Morehead State University, Morehead, KY 40351, United States d Centro de Geologia da Universidade do Porto, Rua Campo Alegre, 687, 4169-007 Porto, Portugal
a r t i c l e
i n f o
Article history: Received 24 January 2013 Received in revised form 2 May 2013 Accepted 4 May 2013 Available online 17 May 2013 Keywords: Geochemistry Marine margin Hydrothermal Carbonate Coal metamorphism
a b s t r a c t The Ragged Edge of the Herrin (Western Kentucky No. 11) coal bed has a variety of maceral textures and assemblages that do not fit in the conventional vitrinite and inertinite definitions as established by the International Committee for Coal & Organic Petrology. Within a narrow zone at the marine margin of the coalbed, the coal is brecciated and cemented by carbonate with some of the brecciated clasts metamorphosed to anthracite-level vitrinite reflectances. This is in contrast to the ambient high volatile C bituminous coal rank in the region. Low-rank peat-like textures are preserved in the clasts, suggesting that the metamorphism took place soon after deposition of the peat. Geochemical and mineralogical evidences suggest that the increase in reflectance was the consequence of the channeling of hydrothermal fluids through the breccia. The narrow zone of metamorphism and, further, the juxtaposition of breccia clasts of varying degrees of metamorphism suggest that the hydrothermal influx occurred at shallow depths of burial, perhaps shortly following deposition of the peat. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The so-called “Ragged Edge” sections of the Herrin coalbed in western Kentucky are known to have maceral forms and assemblages not seen elsewhere in the coalfield (de Wet et al., 1991, 1997; Hower and Williams, 2001; Hower et al., 1987; O'Keefe et al., 2008), including the brecciated and metamorphosed coals from a fault zone in Union County, Kentucky (Hower et al., 2001) (Fig. 1). Within the studied “Ragged Edge” sections (de Wet et al., 1991, 1997; Hower and Williams, 2001; Hower et al., 1987; O'Keefe et al., 2008), there are some fundamental facts, which more or less define the nature of the known marginal Herrin coalbed settings: 1. Coal is brecciated, including the entire Millport section (Hower et al., 1987), core P-181 in the de Wet et al. (1997) study area, the Dixon cores (Hower and Williams, 2001), and the east side of the Cardinal mine exposure (O'Keefe et al., 2008). The left side of the latter entry, about 7-m distance, is faulted, but not brecciated.
⁎ Corresponding author. Tel.: +1 859 257 0261. E-mail address: [email protected] (J.C. Hower). 0166-5162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2013.05.001
The brecciated coal is flanked on one side by normal Herrin coal and on the other side by marine limestone (de Wet et al., 1991, 1997; Neuder, 1984). Overall, the disturbed zone is confined to a narrow marine margin of Herrin, with the lateral extent of the zone well-defined by coring (de Wet et al., 1997; Hower et al., 1987) or mining (O'Keefe et al., 2008). 2. The vertical extent of brecciation is also well known at de Wet et al.'s (1997) site, where a truncated, brecciated Herrin coal (their P-181 core) and sections with breccias in limited portions of the Herrin coal (P-193 and P197) are found. Further, some brecciated sites, such as some of the de Wet et al. (1997) cores and the Ohio County site described in Hower et al. (1987), are not oxidized. There is a mix of normal (ambient Rmax) and heated/oxidized clasts in the Millport core (Hower et al., 1987). 3. Coal clasts range in texture and reflectance from the regionally ambient high volatile C bituminous rank (ca. 0.55% Rmax) to massive vitrinite-like particles with anthracite reflectances. The latter have moderate to abundant sigmoidal-slits dominantly parallel and/or oblique to the bedding plane (Benedict et al., 1968). Textures similar to those in low-rank coals are preserved in higher-Rmax clasts. In addition to the high-Rmax vitrinite, fusinite and semifusinite are present.
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Fig. 1. Map of locations of study sites with locations of faults and interpolated line delineating paleoshoreline during deposition of the Herrin coal (after O'Keefe et al., 2008) (1: Hower et al., 1987; 2: Hower et al., 2001; 3: de Wet et al., 1991, 1997; 4: Hower and Williams, 2001; 5: O'Keefe et al., 2008).
4. The coal-clast breccia generally is cemented by carbonate. At the Millport site, carbonate varies from a MgO-/Mn-rich (>0.24MgO/CaO, >4800 ppm Mn) carbonate in the bottom five sections to MgO-poor/lesser-Mn (b0.07MgO/CaO, b2200 ppm Mn) in the top five sections. Pyrite is less common and total S is lower than in ‘normal’ Herrin coal. 5. The de Wet et al. (1997) study shows the thinning of the Herrin coal towards the margin. Austin (1979) demonstrated a similar thinning of the overlying Paradise coal in Muhlenberg County, Kentucky, mines. Together, both coals, along with the limestone between the coals and the limestone above the Paradise, represent a transgressive sequence. 6. Even though the sections are brecciated, the basin-wide Blue Band parting appears in its usual place within the sequence. Limestone partings occur within the brecciated Herrin coal (Hower et al., 1987) and within the Paradise coal (Austin, 1979), the latter identical to the roof rock. A shelly clay parting, identical to the roof rock, occurs within the Paradise in a Coiltown 7 1/2′ quadrangle mine (observations by Hower, circa 1981). 7. In general, the sub-Herrin-to-post-Paradise stratigraphic section ranges from silty shale/underclay/Herrin coal/limestone/Paradise (not always with underclay)/limestone. Limestone is not present in the sub-Herrin section. Evaporites are not known in the section. Limestone above brecciated coal is also brecciated in de Wet et al. (1997) sections. There are a number of problematical features as well; points that will be addressed in this paper: 1. Is the lack of pyrite an indicator of oxidation or less reducing conditions; or did brecciation and enclosure by carbonate happen prior to formation of secondary pyrite? Or, did heating devolatilize sulfides?
2. If there was heating, why is there no apparent thermal affect nearby? Is sphalerite/pyrite/calcite in semifusinite in the de Wet et al. (1997) core P-197 an indication of elevated temperature for emplacement? Is the minor occurrence of textures resembling low-T stages of coke formation an indicator of heating? In this paper, we are conducting a critical examination of the maceral textures, mineralogy, and chemistry of the Ragged Edge sites with the objective of coming to a better understanding of the mechanisms of their formation.
2. Procedure The original samples were collected and analyzed by personnel of the University of Kentucky Center for Applied Energy Research (CAER). The original petrographic analyses were conducted on polished blocks and polished particulate pellets using oil-immersion, reflectedlight optics with 32 × (with 1.25 × tube factor) and 50 × objectives on Leitz Orthoplan microscopes (de Wet et al., 1997; Hower and Williams, 2001; Hower et al., 1987; O'Keefe et al., 2008). The blocks and pellets are stored in the Applied Petrology Laboratory at the CAER. Additional petrographic analyses of five of the Millport (Hower et al., 1987, study) blocks were made at the Centro de Geologia da Universidade do Porto (CGUP) using oil-immersion, reflected-light optics with a 50 × objective on a Leitz Orthoplan microscope coupled to a DISKUS-FOSSIL system. Additional CGUP studies made at Centro de Materiais da Universidade do Porto (CEMUP) utilized a high-resolution (Schottky) environmental scanning electron microscope equipped with EDS, FEI Quanta 400FEG ESEM/EDAX Genesis X4M.
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Fig. 2. Normal vitrinite. A) High volatile C bituminous collotelinite with resinite (r) and other liptinite macerals. P-197 middle block 01; scale = 50 μm. B) High volatile C bituminous collotelinite and corpogelinite with liptinite. P-197 bottom block 04; scale = 50 μm. C) High volatile C bituminous telinite and transitions in semifusinite (sf) reflectance. P-197 middle block 03; scale = 50 μm. D) Collotelinite in the form of a stem flanked by cutinite. Some of the macerals, particularly at the top, show signs of a disrupted structure without accompanying oxidation. P-197 middle block 04; scale = 50 μm.
A Horiba Jobin-Yvon LabRaman spectrometer interfaced to an Olympus microscope was used. Spectra were obtained using the 632.8 nm emission line of HeNe laser (20 mW). The objective lens (× 100) of the microscope was used to focus the laser beam on the sample and also to collect the scattered radiation. 3. Basic petrology Ambient-rank vitrinite (Fig. 2) of the Herrin coal, as defined by the coal rank in the nearby mines, ranges from high volatile C bituminous (Millport core; Hower et al., 1987) to high volatile B bituminous (de Wet et al., 1997; O'Keefe et al., 2008) to high volatile A bituminous (Hower and Williams, 2001). The forms of the ambient-rank vitrinite at those sites correspond to vitrinite textures defined by ICCP (1998). Aside from fusinite and semifusinite in the latter, unaltered coal, probable fire-derived fusinite and semifusinite can be found in the carbonate cement (Fig. 3a) and in the altered vitrinite (Fig. 3b). Fusinite also occurs in a type developed after the vitrinite maceral corpogelinite (Fig. 3c–d). Macrinite (Fig. 4a) occurs in forms similar to the types described by Hower et al. (2009, 2011), Hower and Ruppert (2011), and O'Keefe and Hower (2011). A rare weathered form (Fig. 4b) may not be macrinite, but rather a high-reflectance form of vitrinite with some resemblance to weathered forms described by Wagner (2007). Secretinite occurs in the classic (Hower et al., 2008a; ICCP, 2001) solid and vesiculated forms (Fig. 5a, b). A secretinite variety with a brecciated inner core and a
more solid rim, although exhibiting some fractures, was observed (Fig. 5c). Secretinite with desiccation cracks is common (Fig. 5d, e), including forms with subtle boundaries with the surrounding degraded fusinite and semifusinite (Fig. 5f). Among the maceral textures associated with degradation are thickened cell walls in fusinite and semifusinite (Fig. 6 a–d). The divisions between the primary formation of fusinite by fire (Bustin and Guo, 1999; Evans, 1929; Guo and Bustin, 1998; McParland et al., 2007; Petersen, 1998; Scott, 1989, 2000, 2002; Scott and Glasspool, 2005, 2006, 2007; Scott and Jones, 1994; Scott et al., 2000; Stach, 1927; Winston, 1993) and inertinite formed by the degradation of wood, as discussed by Hower et al. (2009, 2011), can be diffuse. In the latter case (as seen in Fig. 6 a–d), one possible pathway of maceral development is partial fungal or bacterial degradation followed by conflagration. Coke (Fig. 7), a rare maceral texture in these samples, is formed after the vitrinite has reached bituminous rank. At least superficially, some of the maceral textures resemble ‘pseudovitrinite.’ As defined by Benedict et al. (1968), with further studies by Thompson et al. (1966) and Thompson (2000), pseudovitrinite is marked by (1) higher reflectance than other vitrinite in the same coal, (2) slitted structures, (3) unusual fracture patterns, (4) remnant cell structure, and (5) paucity or absence of pyritic inclusions. The origin of pseudovitrinite seems to be related to (i) pre-coalification oxidation of the woody material, leading to pseudovitrinite-like features (Benedict et al., 1968) (Mackowsky (in discussion by Smith (1980)) suggested that pseudovitrinite may have developed as a result of
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Fig. 3. Fusinite and semifusinite. A) Fusinite and semifusinite in carbonate cement. Vitrinite on right has a higher reflectance than the ambient high volatile C bituminous reflectance of vitrinite in the region. 7873 11; scale = 25 μm. B) Fusinite and semifusinite in carbonate and altered vitrinite. Vitrinite has a higher reflectance than the ambient high volatile C bituminous reflectance of vitrinite in the region. 7867 01; scale = 25 μm. C) Fusinite after collogelinite. 7875 14; scale = 25 μm. D) Fusinite after collogelinite. P-197 top block 11; scale = 50 μm.
primary oxidation during the biochemical phase of coalification), or (ii) post-coalification coal oxidation forming slitted pseudovitrinite as an oxidation product of vitrinite that is intermediate, in terms of degree of oxidation, between vitrinite and oxyvitrinite (Kaegi, 1985), or (iii) thermal influence at a certain coalification stages (Kruszewska, 1998). Benedict and Berry (1964) conducted controlled oxidation experiments, producing material with the same microscopic features as pseudovitrinite. Clark et al. (1984), Kaegi (1985), Diessel and Gammidge (1998), Gurba and Ward (1998), Mastalerz and Drobniak (2005), and Hower et al. (2008b) have made more recent studies of the origin and properties of the maceral.
In the brecciated “Ragged Edge” Herrin coal, pseudovitrinite-like macerals (Fig. 8) are commonly highly slitted (dominantly by sigmoidal-slits parallel or oblique to bedding plane), with higher reflectance than the parental vitrinite. Among the pseudovitrinite characteristics noted by Thompson et al. (1966), certainly features 2–5 (above) are found in the Ragged Edge Herrin coal. Slitted structures, albeit more abundant than in the eastern Kentucky Pennsylvanian coals described by Thompson et al. (1966), are common in the Ragged Edge coals (Fig. 8 a–f) and remnant cell structures are also found (descriptions by de Wet et al., 1997; Hower et al., 1987). The reflectances of vitrinite vary widely. The reflectance of the ‘normal’ vitrinite
Fig. 4. Macrinite. A) High-reflectance coprolite macrinite, possibly after coprolites, in high-reflectance vitrinite/calcite matrix. 7867 08; scale = 25 μm. B) Oxidized/weathered macrinite. 7875 16; scale = 25 μm.
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Fig. 5. Secretinite. A) Round, solid secretinite with fusinite. 7872 12; scale = 25 μm. B) Vesiculated secretinite with round to oblate secretinite. 7872 20; scale = 25 μm. C) Round secretinite with solid rim and fractured center with fusinite in carbonate. 7874 06; scale = 25 μm. D) Oblate secretinite or collogelinite with desiccation cracks with carbonate. 7873 06; scale = 25 μm. E) Oblate secretinite or collogelinite with desiccation cracks with carbonate. 7875 06; scale = 25 μm. F) Oblate secretinite or collogelinite with desiccation cracks; gradation with degraded fusinite is visible. 7875 12; scale = 25 μm.
in the nearby mines and in the unaltered coal clasts is within the high volatile bituminous rank range; for example, 0.55% Rmax in the Hopkins County section studied by Hower et al. (1987). Fig. 8b is a good example of the variation in reflectance, with semi-anthracite/ anthracite-reflectance vitrinite-like macerals adjacent to a liptinite band. As seen in Table 1 (the location key to the reflectance values is provided in Appendix A), a number of vitrinite particles exceed the ambient 0.55% Rmax in the Millport (Hower et al., 1987, study) samples, with values up to 4.63% Rmax reported. The bright, vitrinite-like macerals, while having inertinite-level reflectances, albeit lower than some unequivocal fusinite in the same sample, do not have textures that comfortably fit within the ICCP (2001) inertinite definitions. Several maceral forms test the limits of the ICCP (1998, 2001) vitrinite and inertinite definitions (Fig. 9). Parts of Fig. 9a and b have areas in which the maceral structures suggest devolatilization. Aside from the forms shown in Fig. 7, few macerals suggest coking. Fig. 9f has fusinized and semifusinized cell walls with possible macrinite in the cell interiors. Portions of Fig. 9c, g, and h suggest fungal damage to the vitrinite, but the evidence is not entirely definitive.
Among the mineral evidence for heating are mineral assemblages in the de Wet et al. (1997) cores. Aside from the common Ca–Mg–Fe– Mn carbonates, the sphalerite/pyrite/carbonate assemblage in the Herrin coal in their core P-197 (Fig. 10) resembles assemblages found by Hower et al. (1983, 2001) in mineralized coals in fault zones in the region. 4. Discussion Summarizing arguments in Hower et al. (1987) concerning the origin of the Millport section: • Wood- and herbaceous-derived material was deposited on a swamp located near the marine margin. • In turn, water level variations, due to marine transgressions and flooding from rivers, were responsible for: (i) periodic oxidation of collotelinite precursors, developing pseudovitrinite-like structures; (ii) periodic ripping of the peat and oxidation during transport, preserving the peat/lignite humic textures; followed by re-deposition, and calcite cementation.
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Fig. 6. Degraded fusinite & semifusinite. A) Thickened cell walls in semifusinite suggesting degradation. 7867 09; scale 25 μm. B) Thickened cell walls in fusinite suggesting degradation. 7871 03; scale 25 μm. C) Thickened cell walls in fusinite suggesting degradation. 7874 10; scale 25 μm. D) Thickened cell walls in fusinite suggesting degradation. 7875 18; scale 25 μm.
Each time sea level rose and flooded the swamp, a high-energy environment and varying oxidation potential contributed to coal brecciation. In some periods, the sea level was high enough to form marine carbonates, for example, the marine-carbonate parting in Millport
Block 18 and carbonate-cemented coal breccias, both discussed in Section 4.1, below. The emphasis in the following discussion is on heating by transient thermal fluids. The concept of alteration and preservation in acidic brine, which may also be expected at the intersection of mire and marine waters, could be considered an alternative hypothesis to heating, but will not be discussed further in this paper. 1
4.1. Evidence for heating? When considering thermal-fluid metamorphism of the coal, the way these fluids came in contact with coal must also be addressed. After a “brecciation event”, as described by de Wet et al. (1997), hot fluid flow from deeper levels could have been channelized through high-permeability conduits (such as fracture sets, or individual fractures). If certain macerals or microlithotypes, such as sporinite- and cutinite-rich clarites, were subject to selective devolatilization, then permeability and fluid fluxes would have been greatest and parallel to coal layering, increasing permeability, and thus causing more flow to focus into the reacting area. There is a general trend for increased Rmax of vitrinite and apparent vitrinite macerals towards the bottom of the Millport quadrangle borehole. However, some very high-Rmax particles do occur above the parting in Block 18, possibly an indication of the heterogeneity of fluid flow within the brecciated section. It is also possible that the brecciation and concomitant faulting juxtaposed coal particles of
Fig. 7. Fine carbon resembling a fine borderline isotropic/anisotropic low-temperature coke. 7868 07; scale = 50 μm.
1 Acidification “reduces bacterial activity, and the plant remains are well preserved” (Taylor et al., 1998, pp. 32). In one application of this principle, this is a pickling process applied in the preservation of certain vegetables and of seafood (ceviche). Taylor (1926, 1927, 1928) applied similar principles in his discussion of base exchange as a means of maceral alteration.
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Fig. 8. High reflectance vitrinite with low-rank and desiccation textures. A) Variations in brightness of vitrinite/vitrinite-derived macerals, some with signs of internal fractures/ desiccation cracks. 7866 04; scale = 25 μm. B) Vitrinite layers alternating with darker macerals with inertinite textures and even darker liptinite band. For reference, the vitrinite has reflectances in the semi-anthracite to low anthracite reflectance range. This is in contrast to the high volatile C bituminous reflectance 0.55% Rmax for the coal mined near this site and for unaltered coal clasts in the breccia and the higher reflectance of fusinite in the breccia (ca. 4.5% Rmax). 7867 04; scale = 25 μm. C) Desiccation cracks in high-reflectance vitrinite. 7872 17; scale = 25 μm. D) Desiccation cracks in high-reflectance vitrinite. 7874 13; scale = 25 μm. E) Desiccation cracks in high-reflectance vitrinite. 7874 09; scale = 25 μm.
considerably different degrees of metamorphism. Delicate botanical structures present as fusinite can be largely preserved, despite the brecciation, albeit with some signs of shearing (Fig. 11). Certain structures do bear some resemblance to the heated remains of liptinites. Relict sporinite structures can be seen in Fig. 12. In this case, a void area with the shape of a trilete spore contains 100-nm carbon spheroids. If the void is actually a relict sporinite, the carbons could be the pyrolysis product of the liptinite. Similar carbon spheroids were found in relict cutinite structures.
Liptinite survives, however, in close association with high-reflectance vitrinite, generally in the higher portions of the brecciated coal (Figs. 8b, 13). Rare coke structures (possibly, but not definitively, Fig. 7) and low reflectance anisotropy were found in the brecciated Herrin coal, which could explained by the low rank of the coal at the time of alteration, since one of the most important factors affecting the nature of thermally altered carbonaceous matter is the rank or level of maturity at the time of intrusion. It is also
Table 1 Maximum reflectance of vitrinite and vitrinite-derived macerals in selected samples. Measure location
Collotelinitic
Above B18P parting Middle of B18 beneath B18P Middle of B10, less affect area Bottom of B10 Bottom of B9, less affect area B9 between less affect area and fold
0.55 0.76 0.93 0.99 0.88 0.98
Fused
Fusinite
0.94 0.95 0.87 0.94
2.03 2.74 1.72
Middle of B9, reflectance sequence at chevron fold Bottom of B9, beneath chevron fold Between B6P1 and B6P2 partings Between B6P2 and B6P3 partings Top of B2 bellow B6P3 parting Bottom of B2
1.30 1.23 1.32 1.38 1.81
1.25 1.35 1.27 1.46 1.65
2.05 2.41 1.97 2.38 2.64
High reflectance collotelinitic
High reflectance fused
High reflectance fusinite
2.41 2.81
2.16 2.51
3.59 3.72
4.63
3.23
Collotelinitic material — high reflectance smooth structureless layers like collotelinite. Fused material — high reflectance layers with rounded edges, distorted pores, and relict spore holes. Fusinite — inertinite maceral as defined in ICCP (2001).
3.51 3.08
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Fig. 9. Bizarre structures. A) Desiccation cracks in high-reflectance vitrinite with structures resembling devolatilization vacuoles. 7873 04; scale = 25 μm. B) Remnant of rootlet (?) filled with carbonate surrounded by high-reflectance vitrinite with desiccation cracks. 7874 12; scale = 25 μm. C) Mixed high-reflectance vitrinite and inertinite textures. 7873 07; scale = 25 μm. D) Mixed high-reflectance vitrinite and inertinite textures with corpogelinite with granular interior. 7872 08; scale = 25 μm. E) Mixed high-reflectance vitrinite and inertinite textures with corpogelinites with granular and solid interiors, possibly including macrinite. 7872 08; scale = 25 μm. F) Corpogelinite and secretinite in inertinite breccia. 7868 05; scale = 50 μm. G) Vitrinite and/or inertinite textures with possible fungal contributions to the degradation. 7868 01; scale = 50 μm. H) Vitrinite and/or inertinite textures with possible fungal contributions to the degradation. 7867 12; scale = 25 μm.
possible that the alteration temperatures were not high enough to form structures associated with natural coke, but Hower and Lloyd (1999), among others, have described thermoplastic alteration
of coal at lower temperatures than those associated with natural cokes (Brooks and Taylor, 1966; Mastalerz et al., 2009; Stewart et al., 2005).
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study of the transition from natural coke to unaltered coal, noted less pyrite in the coke than in the normal coal at about 1.2-dike widths from the ca. 10-m-thick intrusion, so, it is possible to see a heating-induced loss of sulfides in coal. More common is the carbonate mineralization pervasive throughout the brecciated sections. The overall carbonate composition for the Millport core noted in the Introduction (MgO-/Mn-rich carbonate in bottom five sections to MgO-poor/lesser-Mn in top five sections) is a bulk composition. On a finer scale, there are differences within the carbonates phases, as shown in Fig. 14 for a portion of Millport Block 2. In this case, the carbonate rhomb (Z1) has Mg > Ca ≫ Fe while the matrix (Z2) has Ca > Mg ≫ Fe (but more Fe than Z1). Millport Block 10 has examples of Mn enrichment in Ca > Mg > Fe carbonates (Fig. 15). Other carbonates in Millport Block 10 have little or no Fe and/or Mg. The parting in Millport Block 18 was analyzed in detail. The severely mylonitized ca. 1 cm-thick parting is intercalated between brecciated coal and is composed of b1 mm layers of calcite intercalated with thin layers of clay. At the top and at the bottom of the parting the contacts with brecciated coal are cemented by Ca-rich carbonate and by Mg–Ca carbonate, respectively (Fig. 16). The characteristic features of this parting are as follows: Fig. 10. Sphalerite (sp), pyrite (py), and carbonate (c) cementing coal-clast breccia (core P-197 from de Wet et al., 1997). Scale = 50 μm.
4.2. Mineralization Mineralization, such as the emplacements of sphalerite, as in de Wet et al.'s (1997) P-197 core (Fig. 10), can lead to an increase in reflectance. Hower et al. (1983) noted increased rank, high volatile A bituminous vs. the ambient high volatile C bituminous, attributed to thermal fluids associated with sphalerite mineralization in coal in a Union County, Kentucky, borehole (see also Hower and Gayer, 2002). Among the specimens observed in all of the Ragged Edge studies, the P-197 sphalerite is an anomaly. Considering the amount of pyrite in the ‘normal’ Herrin coal (for example, Frankie and Hower (1987) discussed pyrite in mine sections of the Herrin within a short distance of the Millport Ragged Edge section), pyrite in the Ragged Edge coals is much less abundant. Stewart et al. (2005), in their
(i) Fragmented calcite layers of ca. 50 μm, with some having pyrite layers or more finely dispersed pyrite; (ii) Rounded Ca-phosphate nodules; (iii) Rounded pyrite nodules with a clay core; (iv) Shells, surrounded and filled by clay, between thin, Mg-free calcite layers (of ca. 50 μm) occurring in narrow band of ca. 0.5 cm in the middle of the parting (Fig. 17); and (v) Ca-phosphate nodules and fragments of calcite layers in the clay associated with the shells. The absence of Mg-rich carbonate in the parting suggests that it was not permeable to the Mg–Ca fluids responsible for the emplacement of the carbonate matrix in the coal breccia. Fragments of rock with characteristics similar to the Millport Block 18 parting, in particular the rounded Ca-phosphate grains, were found several cm above and below the parting. The fragment seen in Fig. 18, however, while in the proximity of the latter parting, does not have the P of the parting and does have quartz grains, not found in the parting. The
Fig. 11. Fusinite in clay layer in Block 2 (Transect2; B2 block; ×60; SEM BSE mode). The “Chevron pattern” zone shows some deformation and shearing, which continues to the right, not evident in the fusinite above the latter zones.
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Fig. 12. Micrographs A (BSE mode) and B (SE mode) show one area were spore holes are abundantly represented. The cross section “trilete spore” shaped hole was selected for analysis (micrograph C; SE mode). After 100,000× magnification of the hole (micrographs D–F; SE mode), ca. 100 nm carbon spheroids are visible. Backscattered electron mode and EDS analysis confirmed that these spheroids are made of carbon.
carbonate vein (Fig. 18, top right) exhibits the compositional heterogeneity observed in other veins. The Mg–Ca fluid path can be traced between two partings in Millport Block 6 (Fig. 19). The Mg–Ca-enriched fluid suspended an illitic clay (with a small Ti signal in the EDS spectra) in the upper parting. The lower parting contains a mineral with a composition suggesting glaucophane. The latter parting acted as a barrier to the migration of the Mg–Ca-enriched fluids (Fig. 20). Pathways of fluid migration can be followed in Millport Block 2 (Fig. 21), with the coal fragments tracing the flow paths through the breccia. As described above, distinct compositions of carbonate minerals are found in the matrix (Fig. 22), with a central zone (Z1) with Ca > Mg ≫ Fe flanking zoned dolomites (Z2) with Mg > Ca (with trace Fe) (see also Fig. 23).
As noted above, a major brecciation evidence described in the strata overlying the Herrin coal (de Wet et al., 1997) formed conduits for the flow of fluids, possibly the fluids responsible for the fracture fill and carbonate cementing of the brecciated Herrin coal. A hypothetical brecciation scenario may be exemplified by considering the Cambro-Ordovician Upper Knox Group (UKG) underlying the entire state of Kentucky. Montanez (1994) described a sequence of episodes (paleokarsting, dolomitization, stylolitization, dissolution, fracturing, and brecciation) occurring in the UKG from the Lower Ordovician up to the Late Pennsylvanian–Permian that could have generated collapse breccias in the overlying formations, including the Herrin coal seam. Conduits could then channel hot fluids from deeper levels of the basin to the upper levels, causing thermal metamorphism in the coal. Considering fluid migration as the source of heat is not
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2002; Hower et al., 1983, 2001). As with the Hower et al. (1983, 2001) sites, this study demonstrates the superimposition of a brecciation event(s) and hot fluid migration on a more local scale than the regional settings discussed by Hower and Gayer (2002). 4.3. Problematic features Returning to the problematical features enumerated in the Introduction, we will use the evidence presented to consider answers to the problems raised.
Fig. 13. Blue-light image of sporinite from Block 18.
extraordinary. This was suggested (Daniels et al., 1990; Harrison et al., 2004; Hower and Gayer, 2002; Oliver, 1986, 1992; Ruppert et al., 2010) for the nearby Appalachian and Ouachita basins, due to conduits opening during the Alleghanian and Ouachita orogenies, as being responsible for high coal rank found in the Appalachian basin. Of particular importance to this study, coal metamorphism and mineralization within the coalfield appear to be related to the mineralization of the Illinois–Kentucky Fluorspar District, immediately to the southwest of the Western Kentucky coalfield (Hower and Gayer,
4.3.1. The general lack of pyrite, at least in comparison with typical western Kentucky coals Both options noted, those being heating-induced oxidation and encasement of the coal breccias prior to the formation of secondary pyrite, seem to be possible causes of the lack of pyrite. The apparent loss of pyrite in heating, as in the case of intrusions into coal (Stewart et al., 2005), is a viable mechanism, but we do not have conclusive evidence for the dominance of either mechanism. 4.3.2. Juxtaposition of heated/oxidized and normal coal and the narrow zone of heating/oxidation Within the coalfield, with the exception of the Gil-30 borehole coals (Hower et al., 1983) and the faulted and mineralized coal studied by Hower et al. (2001) (both in Union County), we know of no other mineralized coals other than the Herrin coals examined here. In the Ragged Edge Herrin brecciated coal, the coincidence of extensive vein carbonate mineralization accompanied by sphalerite, albeit rare, and the metamorphism of the coal corresponds to the trends at the other two sites. In addition, the presence of coke within the Ragged Edge coals is an indicator of heating.
Fig. 14. Scanning-electron microscopy back-scattered electron image of carbonate vein in Millport Block 2. Z1 and Z2 spectra correspond to areas marked on image.
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Fig. 15. Millport Block 10. Z1: Ca–Mg (C–O) (Fe–Mn) spectra, possibly ferroan dolomite. Z2: Ca (C–O) (Fe–Mg–Mn) spectra, calcite. Z3: similar to Z2.
As noted in the Introduction, the zone of brecciation is narrow, as shown in Fig. 24, reproduced from de Wet et al. (1997). Only one borehole, P-181, has a complete, albeit thin, section of oxidized coal breccias. The P-197 section is the perhaps the most complex of any in our studies of the Ragged Edge, passing from normal coal at the base, to unoxidized breccias, to oxidized breccias, and terminating in a fusain lithotype (Fig. 24 and de Wet et al.'s (1997) Figure 8). A thin inertinite or oxidized vitrinite breccia is found within P-193 (Fig. 24 and de Wet et al.'s (1997) Figure 8). In the case of the boreholes in their study, brecciation had to be independent of mineralization; the breccias providing a ready path for the mineralizing fluids, but not necessarily an open path for pervasive heating and mineralization of the entire breccia. The relatively narrow zone of mineralization, as known from coring and mining in at least three of the locations (de Wet et al., 1997; Hower et al., 1987; O'Keefe et al., 2008), may also serve as a clue to the both the limited vertical and lateral extents of the mineralization. If the mineralization was a near-surface phenomenon, then cooling would have been rapid, limiting the extent of the associated heating. The maceral textures, seemingly frozen in the early stages of coal development, if not still in the peat stage, further suggest that the mineralization and consequent coalification was a near-surface process. All four sites in the western part of the Western Kentucky coalfield are close to the Central Faults (Fig. 1), which in turn, originate on the boundary of the extensively faulted and mineralized Fluorspar District. The mineralization in the latter region is imprecisely dated, bracketed as post-Early or Middle Pennsylvanian to pre-Late Cretaceous (Trace and Amos, 1984).
fit in the conventional ICCP (1998, 2001) vitrinite and inertinite definitions. Within a narrow zone at the marine margin of the coalbed, the coal is brecciated and cemented by carbonate. Some of the brecciated clasts have been metamorphosed, with the some clasts having anthracite-rank vitrinite, an increase from the ambient high volatile C bituminous coal rank in the region. In addition to the increase in rank, textures more common in low-rank coals were preserved in the clasts, suggesting that the metamorphism took soon after deposition of the peat. Mineral and major element distributions suggest that the metamorphism was caused by a narrowly restricted influx of hydrothermal minerals. Based on the bulk analyses, the Millport (Hower et al., 1987) section was shown to have a MgO-/Mn-rich carbonate cement in bottom half of the coal and a MgO-poor/lesser-Mn carbonate in top five sections. Among a variety of Ca/Mg/Fe/Mn-carbonate associations, fine-scale analyses, for example, demonstrate that carbonate rhombs with Mg > Ca ≫ Fe are found in the Ca > Mg ≫ Fe matrix. The brecciation at the marine margin provided the pathways for the influx of hydrothermal fluids. Un-metamorphosed breccias, such as in parts of the Nebo cores (de Wet et al., 1997), indicate that mineralization was independent of brecciation. The breccias provided an open path for the mineralization and metamorphism, but mineralization and metamorphism were not universal throughout the breccias. The narrow vertical and lateral extents of mineralization and metamorphism suggest that the process was a near-surface process, with the rapid cooling limiting the extent of metamorphism.
5. Summary
The samples for the first study of the Ragged Edge of the Herrin coalbed were logged in on 25 August 1983. The results of that study were presented at the first meeting of The Society for Organic Petrology in 1984 and published as part of the collected papers of that
The Ragged Edge of the Herrin (Western Kentucky No. 11) coal bed has a variety of maceral textures and assemblages that do not
Acknowledgments
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conference as Hower et al. (1987). In the 30 years of investigating this sequence throughout the Western Kentucky coalfield, we have been fortunate to be able to collaborate with a number of coal companies including Island Creek Coal and Pyro Coal, both defunct, and Alliance Coal. Our collaborators include Eric Trinkle (now Delaware Department of Natural Resources), Anne Graese (Illinois), Gary Neuder (Louisiana), Carol and Andy de Wet (Franklin & Marshall College), Sean Brennan (US Geological Survey), Steve Moshier (Wheaton College), Anne Raymond (Texas A&M), John Popp (Natural Resource Partners), Mike Shultz (EOG Resources), and Sue Rimmer (Southern Illinois University, Carbondale). Mike McClure (currently with Marshall Miller & Associates) supplied the core samples for the de Wet et al. (1997) study when he was employed by Island Creek Coal Co. The Portuguese authors would like to thank FEDER through the Program COMPETE, FCT project “PEst-OE/CTE/UI0039/201-UI 39” and Ciência 2007 Program.
Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.coal.2013.05.001.
References
Fig. 16. Parting in Millport Block 18 (Transect2; B18 block; ×60; SEM BSE mode). (A) Top of parting in contact with brecciated coal (Bc), and fragment of B18P (Fg); (B) middle of parting; and (C) bottom of parting in contact with brecciated coal. Note: each area has a height of 3.8 mm and the gap between lines and areas is 3.8 mm.
Austin, S.A., 1979. Depositional Environment of the Kentucky No. 12 Coal Bed (Middle Pennsylvanian) of Western Kentucky, with Special Reference to the Origin of Coal Lithotypes. (Ph.D. thesis) The Pennsylvania State University, University Park (390 pp.). Benedict, L.G., Berry, W.F., 1964. Recognition and Measurement of Coal Oxidation. Bituminous Coal Research, Inc., Monroeville, PA. Benedict, L.G., Thompson, R.R., Shigo III, J.J., Aikman, R.P., 1968. Pseudovitrinite in Appalachian coking coals. Fuel 47, 125–143. Brooks, J.D., Taylor, G.H., 1966. Development of order in the formation of coke. In: Given, P.H. (Ed.), Coal Science: American Chemical Society Advances in Chemistry, 55, pp. 549–560. Bustin, R.M., Guo, Y., 1999. Abrupt changes (jumps) in reflectance values and chemical compositions of artificial charcoals and inertinite in coals. International Journal of Coal Geology 38, 237–260. Clark, C.P., Freeman, G.B., Hower, J.C., 1984. Non-matrix corrected organic sulfur determination by energy dispersive X-ray spectroscopy for western Kentucky coals and residues. Scanning Electron Microscopy 1984/II, 537–545. Daniels, E.J., Altaner, S.P., Marshak, S., Eggleston, J.R., 1990. Hydrothermal alteration in anthracite from eastern Pennsylvania: Implications for mechanisms of anthracite formation. Geology 18, 247–250. de Wet, C., Moshier, S., Hower, J.C., Rimmer, S., 1991. Deposition and diagenesis of a marineswamp margin: the Providence Limestone and adjacent coals, western Kentucky. In: Lomando, A., Harris, P. (Eds.), Mixed Carbonate–Siliciclastic Sequences: Dallas, Texas, Society of Economic Paleontologists and Mineralogists Core Workshop, 15, pp. 169–203. de Wet, C.B., Moshier, S.O., Hower, J.C., de Wet, A.P., Brennan, S., Helfrich, C.T., Raymond, A.L., 1997. Disrupted coal and carbonate facies within two Pennsylvanian cyclothems, Southern Illinois Basin, USA. Geological Society of America Bulletin 109, 1231–1248. Diessel, C.F.K., Gammidge, L., 1998. Isometamorphic variations in the reflectance and fluorescence of vitrinite—a key to depositional environment. International Journal of Coal Geology 36, 167–222. Evans, W.P., 1929. The formation of fusain from a comparatively recent angiosperm. New Zealand Journal of Science and Technology 11, 262–268. Frankie, K.A., Hower, J.C., 1987. Variation in pyrite size, form, and microlithotype association in the Springfield (No. 9) and Herrin (No. 11) coals, western Kentucky. International Journal of Coal Geology 7, 349–364. Guo, Y., Bustin, R.M., 1998. FTIR spectroscopy and reflectance of modern charcoals and fungal decayed woods: implications for studies of inertinite in coals. International Journal of Coal Geology 37, 29–53. Gurba, L.W., Ward, C.R., 1998. Vitrinite reflectance anomalies in the high-volatile bituminous coals of the Gunnedah basin, New South Wales, Australia. International Journal of Coal Geology 36, 111–140. Harrison, M.J., Marshak, S., Onasch, C.M., 2004. Stratigraphic control of hot fluids on anthracitization, Lackawanna synclinorium, Pennsylvania. Tectonophysics 378, 85–103. Hower, J.C., Gayer, R., 2002. Mechanisms of coal metamorphism: case studies from Paleozoic coal fields. International Journal of Coal Geology 50, 215–245. Hower, J.C., Lloyd, W.G., 1999. Petrographic observations of Gieseler semi-cokes from high volatile bituminous coals. Fuel 78, 445–451. Hower, J.C., Ruppert, L.F., 2011. Splint coals of the Central Appalachians: petrographic and geochemical facies of the Peach Orchard No. 3 Split coal bed, southern Magoffin County, Kentucky. International Journal of Coal Geology 85, 268–273. Hower, J.C., Williams, D.A., 2001. Further examination of the ragged edge of the Herrin Coal Bed, Webster County, Western Kentucky Coal Field. International Journal of Coal Geology 46, 145–155.
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Fig. 17. Parting in Millport Block 18. (A) Micrograph with a general view of aquatic fauna hosting zone (×60; SEM BSE mode). Includes severe mylonitization pattern, calcite layers (Ca) with layered (PyL) or dispersed pyrite, clay (Cl), Ca-phosphate nodules (Pn), and aquatic fauna shell (Afs); (B) magnification of micrograph “A” and location of EDS analyses (×200; SEM BSE mode); (B Z1-4) EDS spectrum of made in areas Z1 to Z4 marked in micrograph B. Z5 is not presented since it is pyrite spectra.
Hower, J.C., Fiene, F.L., Wild, G.D., Helfrich, C.T., 1983. Coal metamorphism in the upper portion of the Pennsylvanian Sturgis Formation in western Kentucky. Geological Society of America Bulletin 94, 1475–1481. Hower, J.C., Trinkle, E.J., Graese, A.M., Neuder, G.L., 1987. Ragged edge of the Herrin (No. 11) coal, western Kentucky. International Journal of Coal Geology 7, 1–20. Hower, J.C., Williams, D.A., Eble, C.F., Sakulpitakphon, T., Moecher, D.P., 2001. Brecciated and mineralized coals in Union County, Western Kentucky coal field. International Journal of Coal Geology 47, 223–234. Hower, J.C., O'Keefe, J.M.K., Eble, C.F., 2008a. Tales from a Distant Swamp: petrological and paleobotanical clues for the origin of the sand coal lithotype (Mississippian, Valley Fields, Virginia). International Journal of Coal Geology 75, 119–126. Hower, J.C., Trinkle, E.J., Raione, R., 2008b. Vickers microhardness of telovitrinite and pseudovitrinite from high volatile bituminous Kentucky coals. International Journal of Coal Geology 75, 76–80.
Hower, J.C., O'Keefe, J.M.K., Watt, M.A., Pratt, T.J., Eble, C.F., Stucker, J.D., Richardson, A.R., Kostova, I.J., 2009. Notes on the origin of inertinite macerals in coals: observations on the importance of fungi in the origin of macrinite. International Journal of Coal Geology 80, 135–143. Hower, J.C., O'Keefe, J.M.K., Eble, C.F., Raymond, A., Valentim, B., Volk, T.J., Richardson, A.R., Satterwhite, A.B., Hatch, R.S., Stucker, J.D., Watt, M.A., 2011. Notes on the origin of inertinite macerals in coal: evidence for fungal and arthropod transformations of degraded macerals. International Journal of Coal Geology 86, 231–240. International Committee for Coal and Organic Petrology, 1998. The new vitrinite classification (ICCP System 1994). Fuel 77, 349–358. International Committee for Coal and Organic Petrology, 2001. New inertinite classification (ICCP system 1994). Fuel 80, 459–471. Kaegi, D.D., 1985. On the identification and origin of pseudovitrinite. International Journal of Coal Geology 4, 309–319.
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Fig. 18. On the right is a quartz-bearing clay fragment. Unlike the parting shown in Fig. 17, this fragment has no P. The composition of the zones is as follows. Z4: Ca ≫ Mg-carbonate filling cell voids; Z5 and Z8: Quartz; Z6: Calcite nodule; Z7: Al–Si clay; Z9: Ca ≫ Mg-carbonate vein; and. Z10: Ca > Mg-carbonate vein. The inset, enlarged on the top left, shows a carbonate vein with area Z11 having a Ca > Mg ≫ Fe composition with traces of Al, Si, and S. Area Z12 has a Ca ≫ Mg > Fe composition with traces of Al, Si, and S.
Kruszewska, K.J., 1998. The reactivity of pseudovitrinite in some coals. Fuel 77, 1655–1661. Mastalerz, M., Drobniak, A., 2005. Optical properties of pseudovitrinite; implications for its origin. International Journal of Coal Geology 62, 250–258. Mastalerz, M., Drobniak, A., Schimmelmann, A., 2009. Changes in optical properties, chemistry, and micropore and mesopore characteristics of bituminous coal at the contact with dikes in the Illinois basin. International Journal of Coal Geology 77, 310–319. McParland, L.C., Collinson, M.E., Scott, A.C., Steart, D.C., Grassineau, N.V., Gibbons, S.J., 2007. Ferns and fires: experimental charring of ferns compared to wood and implications for paleobiology, paleoecology, coal petrology, and isotope geochemistry. Palaios 22, 528–538. Montanez, I.P., 1994. Late diagenetic dolomitization of lower Ordovician, upper Knox carbonates: a record of the hydrodynamic evolution of the southern Appalachian basin. American Association of Petroleum Geologists Bulletin 78, 1210–1239. Neuder, G.L., 1984. Controls on Peat Accumulation in the No. 9 to No. 13 Interval in Western Kentucky. (MS thesis) University of Kentucky, Lexington (78 pp.). O'Keefe, J.M.K., Hower, J.C., 2001. Revisiting Coos Bay, Oregon: a re-examination of funginite–huminite relationships in Eocene subbituminous coals. International Journal of Coal Geology 85, 34–42. O'Keefe, J.M.K., Shultz, M.G., Rimmer, S.M., Hower, J.C., Popp, J.T., 2008. Paradise (and Herrin) lost: marginal depositional settings of the Herrin and Paradise Coals, Western Kentucky Coalfield. International Journal of Coal Geology 75, 144–156. Oliver, J., 1986. Fluids expelled tectonically from orogenic belts: their role in hydrocarbon migration and other geologic phenomena. Geology 14, 99–102. Oliver, J., 1992. The spots and stains of plate tectonics. Earth-Science Reviews 32, 77–106. Petersen, H.I., 1998. Morphology, formation and palaeo-environmental implications of naturally formed char particles in coals and carbonaceous mudstones. Fuel 77, 1177–1183. Ruppert, L.F., Hower, J.C., Ryder, R.T., Levine, J.R., Trippi, M.H., Grady, W.C., 2010. Geologic controls on thermal maturity patterns in Pennsylvanian coal-bearing rocks in the Appalachian basin. International Journal of Coal Geology 81, 169–181. Scott, A.C., 1989. Observations on the nature and origin of fusain. International Journal of Coal Geology 12, 443–475. Scott, A.C., 2000. The pre-Quaternary history of fire. Palaeogeography, Palaeoclimatology, Palaeoecology 164, 281–329. Scott, A.C., 2002. Coal petrology and the origin of coal macerals: a way ahead? International Journal of Coal Geology 50, 119–134. Scott, A.C., Glasspool, I.J., 2005. Charcoal reflectance as a proxy for the emplacement temperature of pyroclastic flow deposits. Geology 33, 589–592.
Scott, A.C., Glasspool, I.J., 2006. The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration. Proceedings of the National Academy of Sciences of the United States of America 103, 10861–10865. Scott, A.C., Glasspool, I.J., 2007. Observations and experiments on the origin and formation of inertinite group macerals. International Journal of Coal Geology 70, 53–66. Scott, A.C., Jones, T.P., 1994. The nature and influence of fire in Carboniferous ecosystems. Palaeogeography, Palaeoclimatology, Palaeoecology 106, 91–112. Scott, A.C., Cripps, J.A., Collinson, M.E., Nichols, G.J., 2000. The taphonomy of charcoal following a recent heathland fire and some implications for the interpretation of fossil charcoal deposits. Palaeogeography, Palaeoclimatology, Palaeoecology 164, 1–31. Smith, A.H.V., 1980. An appraisal of the work carried out on pseudovitrinite by the I.C.C.P. and the direction of future work. Proceedings, 28th Annual Meeting, International Committee for Coal Petrology. Stach, E., 1927. The origin of fusain. Gluckauf 63, 759. Stewart, A.K., Massey, M., Padgett, P.L., Rimmer, S.M., Hower, J.C., 2005. Influence of a basic intrusion on the vitrinite reflectance and chemistry of the Springfield (No. 5) coal, Harrisburg, IL. International Journal of Coal Geology 63, 58–67. Taylor, E.M., 1926. Base exchange and its bearing on the origin of coal. Fuel 5, 195–202. Taylor, E.M., 1927. The decomposing of vegetable matter under soils containing calcium and sodium as replaceable bases. Fuel 6, 359–367. Taylor, E.M., 1928. Base exchange and the formation of coal. Fuel 7, 230–238. Taylor, G.H., Teichmüller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic Petrology. Gebrüder Borntraeger, Berlin (704 pp.). Thompson, R.R., 2000. History of applied coal petrology in the United States III. Contributions to applied coal and coke petrology at the Bethlehem Steel Corporation. International Journal of Coal Geology 42, 115–128. Thompson, R.R., Shigo, J.J.I.I.I., Benedict, L.G., Aikman, R.P., 1966. The use of coal petrography at Bethlehem Steel Corporation. Blast Furnace and Steel Plant, September 1966, pp. 1–8. Trace, R.D., Amos, D.H., 1984. Stratigraphy and structure of the Western Kentucky Fluorspar District. U.S. Geological Survey Professional Paper 1151-D (46 pp.). Wagner, N.J., 2007. The abnormal condition analysis used to characterize weathered discard coals. International Journal of Coal Geology 72, 177–186. Winston, R.B., 1993. Reassessment of the evidence for primary fusinite and degradofusinite. Organic Geochemistry 20, 209–221.
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Fig. 19. Path of the Mg–Ca-enriched fluid flow between Millport Block 6 partings. The lower parting is seen on the collage.
Fig. 20. Mg–Ca carbonate vein at the base of parting in Millport Block 6.
Fig. 21. Mg–Ca-enriched fluid path through coal breccias in Millport Block 2.
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Fig. 22. Mg–Ca carbonate vein in Millport Block 2 with two distinct compositions: (Z1) Mg b Ca at light gray areas under SEM BSE mode and (Z2) Mg > Ca at darker gray areas under SEM BSE mode with zoned dolomites.
Fig. 23. Zoned dolomite in same Millport Block 2 vein shown in Fig. 21.
Fig. 24. Idealized lateral and vertical lithologic transitions in the Herrin coal in the de Wet et al. (1997) study area. Figure reproduced from de Wet et al. (1997).