Variable expressions of interfluvial sequence boundaries in the Breathitt Group (Pennsylvanian), eastern Kentucky, USA

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Abstract
Interfluvial sequence boundaries are a largely neglected aspect of sequence stratigraphy. They are subtle lowstand exposure surfaces which in marginal and nonmarine environments are often difficult to distinguish from exposure surfaces developed during the transgressive and highstand systems tracts. Three examples of interfluvial sequence boundaries, correlated with incised valley fills along strike, are described from Pennsylvanian delta plain deposits of the Breathitt Group of eastern Kentucky. The most common interfluvial palaeosols in the Breathitt Group are gleys developed under poorly drained conditions. This is in contrast to much of the published literature which suggests that soils on interfluves ought to be freely drained. Previously published geochemical analyses suggest that interfluvial palaeosols within the Breathitt Group are composite, forming initially under freely drained conditions and subsequently becoming gleyed due to a rise in the water table during the late lowstand and early transgressive systems tracts. This is a good criterion for differentiating lowstand palaeosols from those developed during the transgressive and highstand systems tracts. Interfluvial sequence boundaries in the Breathitt Group have low preservation potential, being readily eroded by fluvial activity in the transgressive systems tract, which may totally remove all evidence of their former existence. In addition, if incised valleys overspill their margins during major flood events, interfluves may be represented by aggradational successions comprising thin coal seams, rootlet horizons and mature palaeoforests interbedded with crevasse splay sandstone. In such circumstances the presence of an interfluve can only be determined by the presence of correlative incised valley fills along strike.