Penecontemporaneous sandstone dykes, Nonacho Basin (early Proterozoic, Northwest Territories): horizontal injection in vertical, tabular fissures

Sandstone dykes in Proterozoic sedimentary rocks of Nonacho Basin cut only the fine-grained portions of cyclic fluvial, pond, and lacustrine deposits but occur repetitively through hundreds of metres of section. Such restriction to specific beds, in conjunction with erosional truncations of the dykes and association with sand volcanoes, indicates that the dykes are penecontemporaneous. Minimal compaction-related deformation, sharp dyke walls, and mud chips within dykes suggest that the fine-grained host rocks were cohesive before dyke emplacement. In sections parallel to bedding, the dykes are polygonal; normal to bedding, dyke configurations range from tabular to branching to fully developed reticulate networks. The dykes are generally massive, but grain size locally increases towards dyke centers. In contrast to most dykes described in the literature, internal stratification is generally at high angles to dyke walls and at low angles to sill walls.Of the hypotheses considered for dyke emplacement (sedimentary infill, hydroplastic squeezing, fluid injection, and foundering of the flanks of sand volcanoes), fluid injection is favoured as the most likely mechanism. Concurrent mudstone Assuring, sand liquefaction, and injection are attributed to earthquake shocks. Vertical fissures were filled with sand that was injected horizontally, a phenomenon not commonly reported. Widespread stratigraphic distribution of the dykes and an abundance of other soft-sediment deformation structures attest to numerous tectonic disturbances during sedimentation in Nonacho Basin.