Definition and formation: Concretions are isolated, often oval-shaped rock masses. Concretions are commonly composed of limestone (calcium carbonate), but silica-rich concretions also occur. Limestone concretions are most common in dark gray and black shale. Concretions normally form through accretionary mineral (carbonate or silica) growth. Common limestone concretions are inches to several feet in diameter, although large concretions nearly ten feet in diameter have been reported. Most concretions are oval with their long axis parallel to bedding. The outer surface of the concretion is generally smooth. Concretions commonly contain radial fractures, and sometimes cone-in-cone structures.

Limestone “bosses” are irregular masses of limestone, which have been reported in some Illinois basin (including western Kentucky) coal mines (e.g., Nelson, 1983). They are included here with concretions, because the reported “bosses” tend to be isolated pods of limestone. Limestone bosses tend to be much more irregular in shape than common concretions.

Large limestone concretions are common in dark shales, as in this example from the Magoffin Shale Member,Four Corners Formation in eastern Kentucky.
Limestone boss in underground Herrin coal mine roof. Photograph courtesy of John Popp.

Discontinuities and obstacles: Concretions are not discontinuities or major obstacles.

Potential roof-fall hazards: Concretions are usually a nuisance more than a hazard. Some concretions in black-shale roofs are well cemented with surrounding shales and only pose a roof-fall hazard if the shales deteriorate through exposure, or their location has superimposed joints or fractures, which weaken the surrounding shales. Other concretions are not bound to the surrounding roof rock, however, and easily separate from the roof if undermined. Shales are commonly compacted around concretions, and may also separate from the concretions. Like kettlebottoms, concretions in mine roofs may be up in the roof and hidden from view after mining. Small circular bulges in the roof that develop after bolting may be concretions or kettlebottoms just above the roof line and should be monitored and avoided.

Black shale separating from limestone concretion in roof above the Springfield coal bed, western Kentucky.

Large concretion that fell from between bolts in a dark gray shale roof (from Greb and Cobb, 1987).

Trends: Concretions are generally randomly distributed in dark-shale mine roofs. Where shales coarsen upward, concretions sometimes occur at the vertical transition from shale to siltstone in the roof.

Known Kentucky occurrences: Concretions are documented in marine dark-gray shale roofs in the Eastern Kentucky Coal Field (for example, above the Amburgy and Taylor coal beds) and in dark gray to black shales in all Carbondale Formation coals (for example, above the Springfield and Herrin coal beds) in the Western Kentucky Coal Field. Herrin coal roofs also sometimes exhibit local limestone bosses.

Concretions are common in dark gray to black shales, such as the shale above the Springfield coal bed in western Kentucky.

Planning and mitigation: Concretions are small compared to mine area, and generally cannot be detected in cores. During mining, however, where one concretion is found, more will occur. Also, if limestones (inches to feet thick) are encountered in a core and surrounding cores lack limestones at the same level, then the limestone may be a concretion rather than a limestone bed. Similarly, if a local limestone is encountered in the mine roof during bolting, and not encountered laterally, then the limestone may be a concretion, and more concretions should be expected at that level or rock contact (if it occurs at a change in rock types in the roof).

Roof support: Loose, visible concretions can be removed from the roof or supported in a similar way to kettlebottoms.


Last Modified on 2018-06-22
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