Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
5. (September), Pages 754-762
Concave Sand Grains in Eolian Environments: Evidence, Mechanism, and Modeling
B. T. Werner (1), E. Merino (2)
ABSTRACT
Many eolian sand grains have well-rounded concavities. These concavities
occur in grains of all minerals in a single sample and preferentially at
larger grain sizes, suggesting that they form by mechanical abrasion in
the eolian environment rather than by chemical dissolution. The concavities
develop through abrasion at contacts between near-surface grains in the
bed. The energy for this abrasion is provided by collisions of saltating
grains with the sand bed. As a grain with a concavity cycles through a
range of depths in a ripple, agitation of the bed driven by grain-bed impacts
promotes the formation of a grain-grain contact between that concavity
and a protuberance on a neighboring grain. This in turn focuses further
wear at the contact, thereby circumventing the natural tendency oward grain
sphericity that would arise from random grain orientation. A quantitative
physical model combining theories of grain-bed impacts and of granular
materials with experimental surface erosion rates establishes the plausibility
of this mechanism. Within the model, concavities grow to a limiting size
that is a significant fraction (order 10-30%) of a grain diameter, within
the range of observed concavity sizes.
