Discussion: “Mud dispersal across a Cretaceous prodelta: Storm-generated, wave-enhanced sediment gravity flows inferred from mudstone microtexture and microfacies” by Plint (2014), Sedimentology 61, 609–647

This discussion aims to comment on aspects in a recent paper by Plint (2014) where mud transport during deposition of prodeltaic strata of the Cretaceous Dunvegan Formation is examined. The author has made considerable effort to look at the grain scale fabric of these rocks, and by adding data from drill core also made a good argument that the fabrics of outcrop samples were not fundamentally altered by weathering processes. The author concludes that a considerable portion of the clays in these mudstones did not arrive in discrete flakes or in a flocculated state, but as aggregates that were at least in part transported in bedload. The presumed aggregates are similar in size to the quartz silt grains that they are associated with, and are in essence their hydraulic equivalents. There are, however, issues with the proposed origins of the various types of mud aggregates that are described. Although these concerns may seem subtle to those that are not engaged in mudstone studies, they are nonetheless vital beyond this particular field of inquiry. Because mudstones and shales comprise at least twothirds of the sedimentary rock record (Schieber, 1998), any misunderstanding about the processes that control their deposition have an adverse impact on our understanding of the rock record overall. A good grasp of depositional processes is also of economic importance, because properties like porosity and permeability can be tied back to original depositional fabrics (Schieber, 2011a, 2013). The author specifically proposes: (i) that randomly oriented face–face clay aggregates (2 to 5 lm diameter in size) formed through flocculation in fluid mud; and (ii) that small (5 to 20 lm diameter in size) intraclastic aggregates (termed ‘IAs’ for the remainder of this discussion) were eroded by storms from the seabed. With regard to the first point, the author refers to a paper by Nishida et al. (2013) for support. In that paper, the hypothesis is advanced, on the basis of experiments, that an abundance of dense face–face aggregates (<10 lm in size) is indicative of sea water based fluid muds (10 to 30 g/l clay suspension). However, Nishida et al. (2013) used commercially available ground clays, and face–face aggregates are common in commercial clay products that are mined from compacted or lithified deposits because of face– face relations inherited from compaction. Also, Nishida et al. (2013) oven-dried their samples prior to scanning electron microscope (SEM) examination. Given that modern surface muds have high water contents and porosities (70 to 90 wt% is common, e.g. Schimmelmann et al., 1990) the methodology for sample preparation is critical for making observations that realistically reflect original fabrics (e.g. Bennett et al., 1991). Because surface tension forces enormously distort clay fabrics during sample dehydration, critical point drying has for many years been considered the best method to faithfully maintain particle to particle integrity of wet clay samples for electron microscopic fabric investigations (Bennett et al., 1977; Tovey & Wong, 1978). It is therefore exceedingly unlikely that the samples prepared by Nishida et al. (2013) reflected original depositional fabrics. By extension, from the broader perspective of clay fabric literature, the Nishida et al. (2013) reference does not provide support for the hypothesis that the face–face aggregates observed in the Dunvegan Formation are indicative of deposition from fluid muds. Staying with the issue of fluid muds, it is also doubtful whether aggregates formed in fluid muds would be a mere 2 to 5 lm in size. In multiple settling tube experiments (Schieber, 2011b) that covered the concentration range from highly diluted (<1 g/l) to advanced liquid mud (40 g/l), floccules formed as soon as agita-