Clastic dykes of the Devono—Carboniferous Horton Bluff Formation, Nova Scotia: storm-related structures in shallow lakes

Abstract

Clastic dykes are common in the Late Devonian to Early Carboniferous Horton Bluff Formation of Nova Scotia where they occur in wave-dominated shoreline deposits of lacustrine shoaling-upward cycles. The dykes, typically less than 25 cm wide and 40 cm in vertical extent, project downward into clayshale from broad loads at the base of elongate isolated lenses of hummocky cross-stratified siltstone. Ptygmatic folding of the dykes reflects a shale: siltstone compaction ratio of 2.3 times, and maximum depth of dyke penetration prior to compaction was > 1 m. Dyke orientation parallels the orientation of wave-ripple crests and the elongation direction of the siltstone lenses. Dyke formation was penecontemporaneous with sedimentation. Dyke and load formation was probably initiated in shallow water during storms which promoted sediment failure through cyclic wave loading, microseisms and rapid deposition of HCS-bearing siltstone lenses. Reversed-density gradient and uneven sediment loading from silt lenses probably promoted downward emplacement. Loads reflect ductile failure of surficial water-rich mud, whereas dykes reflect downward injection of silt following brittle failure of underlying, more compacted mud. The depositional slope provided a tensional stress that controlled the fracture direction (orthogonal to the slope) of the failed sediment.