Cleats in coal form during coalification. In some cases, the orientation of cleats is similar to the modern stress field; in others, it is a paleo-stress field indicator, and more aligned with tectonic structures that formed closer to the time of coalification. In the central Appalachian Basin (Kulander and Dean, 1993) cleat orientation changes slightly across parts of West Virginia from east-west to northeast-southwest. Kulander and Dean (1993) noted that within broad, fault-bound areas, cleats tend to be similar, a situation they termed "domains." Hence, cleat orientations are slightly different north of the hinge-line on the northern margin of the Rome Trough (a deep-seated structure) than south of the fault. Engelder and Whitaker (2006) suggested that the overall east–northeast-striking orientation is coincidentally parallel to the modern stress field in some areas, but actually records the paleostress field at the time of cleat formation. Similarly, in southeastern Virginia and eastern Tennessee, where there is significant tectonic folding and thrust faulting, cleat orientation tends to be related to major geologic structures (Millici and Gathright, 1985). Similar investigations in southern Indiana documented at least two distinctly different and cross-cutting cleat sets: one inferred to have formed early in coalification and the other formed later with an orientation similar to the modern-day stress field (Solano-Acosta and others, 2007).

 

 

 

 

 

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