Figure 14. Comparison of the observed distribution of continental sediment on earth today (left) and the distribution of sediment obtained in the illustrative calculation at a time of 220 days (right). The gray regions in the left panel are recent tectonic belts that in many cases contain substantial thicknesses of sediment. The exposed shields correspond to regions with crystalline bedrock largely barren of sediment at the surface. deep ocean floor. Not just any mechanism would lead to the creation, transport, and deposition of such a staggering amount of sediment on the surface of the continents. In this model it is accomplished by tsunamis generated in the deep ocean basins, which then impinge violently upon the continent margins, erode those margins, and carry the resulting sediment particles far inland to be deposited in the continent interiors. Just what other candidate mechanism might achieve this? B. Small picture details But what about the processes at the shorter length scales? In terms of volume, fine-grained sedimentary rocks (shales, mudstones, siltstones, etc.) are the dominant sedimentary rock type, constituting some two-thirds of the sedimentary rock inventory (Potter et al. 2005). Such rocks until fairly recently were thought to have formed in low-energy conditions of offshore and deeper-water environments. However, during the past twenty years a revolution, driven largely by careful laboratory flume experiments, has occurred in understanding how these rocks form (Schieber et al. 2007; Schieber and Southard 2009; Wilson and Schieber 2014; Schieber et al. 2022). These experiments demonstrate consistently that sand-sized floccules of agglomerated clay particles deposit rapidly from bedload transport in laboratory flumes to produce plane-bedded and ripple cross-laminated bedforms. Plane-bedded mud laminae form from bedload transport at current velocity of approximately 0.5 meter per second. Ripple laminated mud deposits at velocity of 0.25 meter per second. There is now growing consensus that most mudstones in fact are dominated by bedforms formed by current deposition of flocculated clays from moving bedload. This applies to prominent North American shales such as the New Albany Shale, Genesee Group shales, (Wilson and Schieber 2014) and Mancos Shale. This research confirmed by field observation not only demonstrates that most mudrocks deposit as sand-sized clay floccules from moving water, but the research further shows that conclusion is true for carbonate sediments as well. The abstract of a paper from ten years ago (Schieber et al. 2013) states this clearly: Flume experiments with fine-grained carbonate particles (< 62.5 mm) show that they form floccules that travel in bedload, form current ripples, and deposit laminated sediments. In order to compare our flume experiments to work on sand transport, we ran flume experiments with medium sand and observed that the velocities at which sand grains started to move and form ripples were in the same range as those where floccules move and form ripples. These ripples are in essence identical to those formed by clay-mineral floccules or sand grains under similar conditions. In light of previous experiments with clay minerals, these results indicate that, of the key controls on mud deposition, flocculation and suspended-sediment concentration are more important than particle mineralogy or water chemistry, and are about as important as bottom shear stress for deposition. Suspensions of carbonate mud show the same pattern of flocculation, ripple formation, and bed accretion as observed previously in experiments with clay-mineral suspensions. The resulting carbonate mud deposits show internal lowangle (2–5 degrees) laminae, and in plan view a pattern of ripple foresets that is identical to rib-and-furrow structure in sandstones. Just as previously assumed for terrigenous muds, there has been a long-standing notion that accumulation of abundant carbonate mud reflects quiescent conditions of offshore and deeper-water environments. These experiments demonstrate unequivocally that carbonate muds can also accumulate in energetic settings. In the sedimentary record of carbonate rocks, interbedded grainstones and lime mudstones may thus not necessarily reflect shifts in depositional energy (or water depth), but alternatively may imply a shift in supplied sediment type. The observations we report suggest that published interpretations of ancient lime muds and derived paleoceanographic conditions may need to be reevaluated. The turbulent transport submodel in MABBUL already includes most of the features required to represent mud deposition and mudrock formation as well as equivalent processes for carbonate rocks. BAUMGARDNER AND NAVARRO Large tsunamis and Flood sediment record 2023 ICC 383
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