Indicators of propagation direction and relative depth in clastic injectites: Implications for laminar versus turbulent flow processes

Clastic injectites are widely recognized in deep-water stratigraphic successions, although their sediment transport processes, propagation direction, and depth of injection are poorly constrained. Understanding how they form is important, as injectites are increasingly being recognised as significant components of sedimentary basin-fills, yet are not predicted by standard sedimentary facies models. Here, analysis of features on the margins of exhumed clastic sills and dykes, and clasts within them, enables their genesis to be determined. A diverse array of diagnostic structures is found on the margins of injectites in the Karoo Basin, South Africa, where the net direction of injection and position of the parent sand are well constrained. Injectite margin features include mudstone clast-rich surfaces, planar or smooth surfaces, blistered surfaces, and parallel and plumose ridged surfaces. Combined, these features are critical in distinguishing injected sands, where injectites are strata-concordant, from those of primary deposition. All features are indicative of propagation through brittle, very fine-grained sediments, at depths where the applied shear stress is at least four times the tensile strength of the host rock. Additionally, the presence of parallel ridges, plumose ridges, and steps allows local fracture propagation to be constrained, and in turn injection direction. The features described provide evidence that sands were injected at considerable depth in closed fractures with limited capacity for flow dilution and turbulence enhancement. Calculated Reynolds numbers, lack of erosion at injectite walls, and the presence of mud clasts at the top and base of sills, indicate that many flows were likely fully laminar during injection. The sedimentary features of these confined, relatively deep, laminar flow-induced injectites are very different from injectites that reach the surface and produce extrudites. Surface-linked injectites are associated with open conduits where a greater fraction of carrier fluid to particles can be accommodated, enabling highly turbulent, lower-concentration flows.