Abstract

Submarine fan channel sequences of the Jackfork Formation (Lower Pennsylvanian) at DeGray Dam section in the Ouachita Mountains, Arkansas, contain discrete units (10-75 cm thick) with moderately dipping (25°-40°), sigmoidal imbricate slices. Adjacent units with opposing imbricate slices are common. The sigmoidal structures are similar in geometry to a tectonic feature known as a duplex. A tectonic origin of sigmoidal structures, however, seems unlikely because opposing directions of imbrication in adjacent units would require an unrealistic tectonic movement history for the area. We propose that the Jackfork sigmoidal structures were formed by a process kinematically similar to that responsible for generating duplex structures. Unlike tectonic duplexes, however, the sigmoidal structures were formed by soft-sediment deformation of sand and mud layers as high-energy sediment gravity flows glided over these layers. Sediment gravity flows, responsible for forming the sigmoidal deformation, were probably generated by slumping of adjacent channel walls. Dip direction of sigmoidal slices is perpendicular to channel axes. Thus, recognition of sigmoidal deformation structures may be useful in inferring the trend of channels in ancient submarine fan complexes.

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