Centimeter-scale sedimentary structures in a lacustrine delta front, northern China: Ripples or Froude supercritical-flow bedforms?

Sedimentary strata are a significant record of the Earth and planetary history, and accurate recognition of sedimentary structures and their link to environmental conditions is a key component in deciphering past surface processes. Centimeter-scale sedimentary structures are commonly acknowledged as ripple cross-laminations, but here we document ambiguous centimeter-scale structures from a lacustrine delta front in northern China, that at careful look do not seem to fit with the known ripple cross-lamination criteria. The here documented sedimentary structures range from scour-and-fill, irregular lenses with structureless or low- and high-angle, up- and downstream dipping, concave and convex laminations. These centimeter-scale sedimentary structures thus considerably differ from ripple cross-laminations in their outer shape, internal organization, and morphometric parameters. Detailed comparison of these centimeter-scale structures with Froude supercritical-flow structures suggests that they were likely produced by Froude supercritical flows. Such centimeter-scale supercritical structures are not unique in the Bantanzi delta, as they have been also documented in a variety of settings ranging from rivers to deepwater turbidites. In light of this finding, we expand the Froude supercritical-flow sedimentary structures to centimeter scale, and advocate caution in interpreting centimeter-scale sedimentary structures axiomatically as ripple laminations.