Flute-like marks and associated structures from the Carboniferous Port Hood Formation of eastern Canada: evidence of secondary origin in association with sediment intrusion

Flutes have been described and interpreted for many years as sole marks, preserved in positive relief on the bases of sandstone beds, and formed on underlying firm mud substrates mainly by corrasion at the sediment/water interface by fluvial, tidal, or turbidity currents. Structures resembling flutes (herein termed flute-like marks, or FLMs) from fluvial sandstone of the Port Hood Formation (mid-Carboniferous) of western Cape Breton Island are present not only on the undersides, but also on the vertical, inclined, stepped, and top surfaces of sandstone beds in contact with mudstone bodies. These occurrences are difficult to reconcile with primary sedimentary processes, even allowing for complex and fortuitous preservation, e.g., by erosion of slumped mudstone bodies or as scoured and overturned mudstone boulders. The presence of associated soft-sediment deformation structures, such as convolute bedding, mudstone diapirs, and mud-injection structures, some of which also have flute-like marks on their surfaces, suggests that these structures are explained more readily by sediment intrusion (rheoplasis) than by current scour and infill. Since flute-like marks have been recognized as positive-relief structures on both upper and inclined mudstone-sandstone interfaces in modern tidal-flat environments and on experimental injection structures, we reemphasize that they should not be regarded as excellent criteria for indicating "way-up" and paleocurrent direction.