Cutouts and rolls
Definition and formation: Cutouts are areas where a coal seam is truncated by noncoal rock (generally sandstone, but not uncommonly fine-grained rock). Rolls are areas where a noncoal rock (generally sandstone) cuts through overlying roof rock to the top of the coal, causing the coal to bend or bow down beneath it, without a loss of seam height.
Many cutouts and roof rolls identified by miners are related to ancient river or tidal channels, termed paleochannels. The most common channels are filled with sandstone, but channels can also be filled with interbedded sandstone and shale, or shale. Above the Herrin (W. Ky. No. 11) coal, rolls and cutouts may be composed of marine limestone, possibly representing marine channels. Sandstone paleochannels may be 10 to 100 feet thick and hundreds to thousands of feet wide (Hylbert, 1974; McCabe and Pascoe, 1979; Moebs, 1981; Kertis, 1985; Ingram and Chase, 1987). Channels that formed at the same time as the peat swamp that formed the coal are called syndepositional channels. Channels that formed after the peat (future coal) accumulated are termed post-depositional channels. Paleochannels commonly have a sharp, scoured basal contact that truncates underlying rock strata. The scour is commonly overlain by coal, siderite, shale or rock pebbles.
Discontinuities and obstacles: Paleochannels can be major obstacles to mining. Where paleochannels truncate, or cut out, the coal seam, they may block further development. Both syndepositional and post-depositional channels can cut out a coal seam. Syndepositional channels are commonly associated with splits or partings in the coal bed, and the splits and partings commonly thicken or increase in number toward the channel. Post-depositional channels are not associated with trends of partings or splits in the coal seam.
Changes in floor elevation, in which the floor initially rises and then falls near a paleochannel, are called floor rolls, dips, and swags (McCabe and Pascoe, 1979; Kertis, 1985; Ingram and Chase, 1987). Some dips in floor elevation may be steep and difficult to mine through. Compaction-related faulting is also common beneath paleochannels (both rolls and cutouts), leading to small offsets in the underlying coal bed, generally downstepping toward the cutout (McCabe and Pascoe, 1979; Nelson, 1983; Ingram and Chase, 1987; Greb and Popp, 1999). Where multiple, narrow roof rolls or cut outs occur beneath a broader sandstone in the roof, the coal may rise and fall many times beneath those irregularities.
Changes in coal thickness are common near sandstone cutouts. In many cases, coal thickness increases dramatically for short distances adjacent to paleochannels before being cut out (McCabe and Pascoe, 1979; Kertis, 1985; Ingram and Chase, 1987). The rapid or local increase in thickness is generally caused by parts of the cutout or thinned coal being faulted or thrust along slip planes onto itself.
Oil, natural gas, and water can migrate into permeable paleochannel sandstones, leading to oil-, gas-, and water-charged sandstones in mine roofs just above coal beds in rolls, and cutting into the coal bed in cut-out situations. Sandstones in the roof of the Lower Elkhorn coal and its equivalents in eastern Kentucky have been saturated with heavy oil in several localities. Water can infiltrate into surrounding moisture-sensitive rocks, causing deterioration (Molinda and Mark, 2010). In extreme cases, encountering sandstone aquifers can cause mine flooding, requiring continuous pumping. At least one subsurface mine in western Kentucky had to be closed because of excessive water infiltration from a channel sandstone that had cut down to the top of the mined seam.
In some cases, coals become mineralized (calcite veins, etc.) beneath or adjacent to paleochannels. The sulfur content of coals may also increase or decrease near paleochannels because of past fluid migration that led to deposition or removal of sulfates from the seam (Gluskoter and Simon, 1968; Nelson, 1983; Rimmer and others, 1985; Hower and others, 1991).
Floor heave and squeezes can occur along paleochannels in coals with thick underclays or weak floors during mining (McCabe and Pascoe, 1979; Keim and Miller, 1999). Floor heaving results in a reduced height in the mine rooms, impeding movement of equipment and airflow.
Potential roof-fall hazards: Roof falls lateral to cutout channels, or beneath rolls in the roof, are safety concerns and can impede mining. In some instances, channels can be mined through to get to the other side; in other cases, this is not practical (Horne and others, 1978; Donaldson, 1979; McCabe and Pascoe, 1979; Moebs, 1981; Nelson, 1983; Ingram and Chase, 1987; Greb, 1991). Slickensides, joints, and fractures are common in shales cut by sandstone (and limestone) channels (McCulloch and others, 1975; McCabe and Pascoe, 1979; Hylbert, 1984; Kertis, 1985; Ingram and Chase, 1987). The contact of shales with the overlying sandstone is almost always slickensided and commonly separates from beneath the sandstone. Where sandstones rest within a few feet of the roof, pockets of deformed and slickensided shale can occur beneath irregularities in the base of the sandstone. Because sandstone roof rolls and cutouts redistribute loading stresses on pillars, they are commonly associated with cutter-style fractures, which propagate upward from the rib along entries (Hylbert, 1974, 1984; Kertis, 1985; Hill, 1986). Also, the rocks within the paleochannel fill may contain bedding weaknesses as described for weak sandstone. Roof-fall hazards are particularly problematic where sandstone rolls and cutouts are superimposed on other weak roof conditions such as coal riders and claystone, stackrock, or weak shale, which are all common beneath and lateral to paleochannels.
Roof falls in paleochannel-influenced roof can also be associated with paleoslumps, which are discussed separately.
Trends: Cutouts truncate the coal and are usually readily identified when encountered. In underground mines, however, roof falls along cutout margins may cause abandonment of an entry prior to actually encountering the cutout. Roof rolls are also readily identifiable underground, but often a downward dip in the coal seam, or dip in the immediate roof shales, may be the only hint of the overlying sandstone or siltstone paleochannel, which is causing the roll in the roof. The paleochannels which cause most rolls and cutouts have linear to broadly sinuous paths, similar to modern-day river and tidal channels. If encountered or detected from core in advance of mining, or from mapping in adjacent mines (by using old mine maps), a trend can be projected in advance of mining, and miners can be warned of channel potential (Hylbert, 1974; Horne and others, 1978; McCabe and Pascoe, 1979; Nelson, 1983; Ingram and Chase, 1987; Greb 1991, 1992). On old mine maps, cutouts and rolls are sometimes specifically noted. In many cases, however, cutouts and rolls are not specifically noted, but the mine terminated along elongate or slightly sinuous trends of roof falls or “bad top” that weren’t beneath valleys or weren’t oriented along regional stress directions. These mine terminations sometimes mark the margin of a paleochannel. Trends are easier to define and project for sandstone paleochannels cutting through shale than shale-filled channels, which can be difficult to detect in core and roof.
Paleochannels above or cutting through the Fire Clay coal in eastern Kentucky have east-west to northwest-southeast trends (Greb and others, 1999, 2002). In western Kentucky, north-south trends are known in the Henderson paleochannel through the Springfield coal bed (Beard and Williamson, 1989). Also, in western Kentucky, limestone-filled paleochannels have been documented in linear, parallel trends, suggesting control by structural mechanisms (Mathis, 1983).
Known Kentucky occurrences: Paleochannels can occur in any coal bed. Published cross sections of roadcuts along major eastern Kentucky highways show examples of sandstone channels, rolls, and cutouts above most eastern Kentucky coal beds (see, for example, Horne and others, 1978; Chesnut, 1991). Published reports of channels above eastern Kentucky coals include examples above the Vancleve coal (Eble and others, 2016), Blue Gem coal (Greb, 1991), Lower Elkhorn (Pond Creek, Imboden) coal (Hylbert, 1980; Greb and others, 1999), Lower Elkhorn (Pond Creek) coal (Hylbert, 1980; Greb and Popp, 1999; Greb and Weisenfluh, 1999), Upper Elkhorn No. 1 (Harlan) coal (Hylbert, 1980), Upper Elkhorn No. 3 (Jellico, Darby) coal (Hylbert, 1980; Greb, 1991), Fire Clay coal (Cobb and others, 1981; Andrews and others, 1994; Greb and others, 1999; Greb, 2002, 2003; Greb and Weisenfluh 1996), Hazard No. 8 coal (Greb and Cobb, 1989; Greb, 1991), and Stockton coal (Hylbert, 1980). In eastern Kentucky, paleochannels are not commonly mapped basinwide, so are more likely to be encountered without advance knowledge.
In the major mined coals of western Kentucky, a few large, continuous channels are known to occur in the Springfield (W. Ky. No. 9), Herrin (W. Ky. No. 11), and Baker (W. Ky. No. 13) coal beds, so they can be planned for. Local limestone rolls and cutouts have also been reported above the Herrin (W. Ky. No. 11) coal and are described in Mathis (1982) and Nelson (1983).
Planning and mitigation: Detecting paleochannels (cutouts and rolls) in advance of mining requires data from core and surrounding mines. Individual channels are commonly only hundreds of feet wide (although larger channels are known), and can be missed by exploration drilling (see, for example, Nelson, 1981). Lateral changes in rock type and coal thickness can sometimes provide clues of proximity to channel conditions, however (as illustrated in the diagram above). Individual cores with unusually thick or thinned coal in specific locations relative to surrounding areas may indicate a channel and cutout, especially if the roof is composed of thick sandstone. Lateral changes in roof lithology from shale to sandstone in core or outcrop may indicate an approaching channel and the possibility of rolls in the roof or lateral cutouts of the bed. Examination of old mine maps in surrounding mine areas can also aid in locating paleochannels. If cutouts and rolls are noted on maps in nearby mines, those trends should be projected into any potential mine area. Similarly, continuous curvilinear mine terminations on adjacent mine maps may indicate the trend of a possible paleochannel and should be planned for in case they continue into the planned mine area. Cutouts and rolls are generally curvilinear; however, some very straight channels have been observed above the Herrin coal in western Kentucky. Additional coring is often needed to define the width and trend of cutouts in advance of mining.
Mapping the marginal zone and strike of large slickenside planes can aid in projecting the paleochannel trend in advance of mining. Larger slickensides and fractures in shales beneath and along sandstone (or limestone) roof rolls and cutouts tend to parallel the elongate or sinuous trend of the roll or cutout (McCulloch and others, 1975; Horne and others, 1978; McCabe and Pascoe, 1979; Hylbert, 1980; 1984; Moebs, 1981; Moebs and Ellenberger, 1982; Nelson, 1983; Ingram and Chase, 1987).
Where entries are parallel to slickensides and fractures related to vertical loading stresses of the overlying roll or cutout, reorienting the entries to intersect rolls, cutouts, and their marginal areas at oblique angles may aid in roof support (Moebs and Ellenberger, 1982; Nelson, 1983; Hylbert, 1984; Hill, 1986; Ingram and Chase, 1987).
Roof support: Sandstone rolls and cutouts that cut underlying shales within 10 feet of the top of a mined coal commonly lead to roof falls in the shales. These channel-margin roof conditions almost always require supplemental support such as longer roof bolts, angled or larger tensioned bolts, steel mats or crossbars, steel sets or cribbing, or truss bolts (McCabe and Pascoe, 1979; Kertis, 1985; Ingram and Chase, 1987). In most cases, bolts must be anchored into stronger zones in the sandstone (or limestone) channel above weakened shale.