Abstract
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 recognized 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 dikes, 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.
Highlights
Clastic injectites have been documented in many sedimentary environments
We address the following fundamental questions: (1) Can injection propagation direction be determined using margin structures?; (2) Can injection depth be estimated?; and (3) What flow processes occur during injection? These questions support a discussion on sand injectite emplacement mechanisms, including the current debate on laminar versus turbulent flow and how this controls differences in injectite geometries and surface features
If margin structures occur due to fracturing, in the absence of any later reworking by the intruded flows, the morphology and distribution of structures on injectite margins can be used to infer the properties of the host rock and sediment, and their interaction, at the time
Summary
Clastic injectites have been documented in many sedimentary environments (see Hurst et al, 2011; Ross et al, 2011, and references therein). Reflection seismic data can help to constrain the large-scale architecture and in some cases the propagation direction of injection complexes (Hurst et al, 2003; Huuse et al, 2004; Cartwright et al, 2008; Vigorito et al, 2008; Szarawarska et al, 2010; Jackson et al, 2011), but flow direction and relative depth of formation are hard to interpret, even with the addition of core and outcrop analogues. Despite their importance, many of the underlying formation processes remain poorly understood, such as the mode of propagation and nature of sediment transport processes within these conduits.
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