The Tapeats Sandstone is the lowest Cambrian layer in the Grand Canyon in Arizona. It has been interpreted as beach, estuarine, and shallow marine coarse sand deposits, representing the initial stages of a slow transgression over a highly weathered and eroded, pre-vegetated epicratonic surface. In the basal Tapeats, two distinct bedforms occur: (1) hyperconcentrated laminar bedforms deposited by high-velocity hyperconcentrated currents and (2) sandy debris flows in high-density turbulent flow. The high-velocity hyperconcentrated currents predominated, but were occasionally interrupted or overlaid by cascades of breccia that initiated short-lived, high-density turbulent flow. Both reflect extremely rapid deposition rather than tidal reworking on a passive margin. Rheologically plastic flows are distinct from fluidal flows. The structure of high-density turbidity currents and hyperconcentrated flood flows are discussed, showing how the hyperconcentrated laminar bedforms and sandy debris flows could have been produced. Hydrological interpretation indicates that bedforms were produced by very rapid deposition in continuous currents over an extremely large area without regard for minor paleotopography, suggesting the rapid transgression of the Tapeats Sandstone in a massive flooding event.