Bedform climbing in theory and nature

TLDR

Bedforms that migrate upward during deposition transfer sediment to underlying strata, causing their cross‑sectional area or number to decline unless external sediment compensates, and the resulting cross‑stratified bed thickness equals the area reduction divided by migration distance. Sediment deposited on the lee slope and not eroded by subsequent troughs remains as a cross‑stratified bed. The derived equations account for the observed downstream size reduction of tidal sand waves in St Andrew Bay and transverse aeolian dunes on the Oregon coast, and they estimate that the Navajo, De Chelly, and Entrada Sandstone dunes had mean heights ranging from several tens to several hundreds of metres.

Abstract

ABSTRACT Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross‐stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross‐sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two‐dimensional bedforms, the mean thickness of cross‐stratified beds is equal to the decrease in bedform cross‐sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross‐strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.