Development of ornamentation on ventifacts: An examination of flow and saltation kinematic mechanisms

Abstract

Ventifacts are rocks that have been shaped through abrasion by wind‐transported sediment. Their surfaces may be ornamented with characteristic microscale features that are replicated and overlain to form complex patterns. Compared to a large body of work on the transport of unconsolidated sediment in which the atmospheric boundary‐layer flow and sand cloud respond to perturbations in the developing topography, no experimental work to this date has addressed morphodynamic feedback at the particle scale in aeolian systems dominated by abrasion. This article reports on a case study in which laser Doppler anemometry was used in wind tunnel experiments to measure air flow and sand transport over a highly ornamented, tabular ventifact from the Taylor Valley, Antarctica. Periodicity characterizing the static microtopography of the ventifact was observed to be strongly imprinted on the kinetics of both the airflow and particle cloud when sampled along a transect of the ventifact aligned with flow in the wind tunnel matching the dominant sand transporting wind direction for the ventifact in situ. The vertical velocity component was especially well synchronized, with changes in elevation on the order of 2–4 mm. The horizontal component of particle velocity demonstrated an offset in the downwind direction associated with the large forward momentum of the saltating sand particles. However, submillimeter relief on the fixed ventifact surface was not sufficient to initiate kinetic feedback, as observed along a transect aligned normal to a series of elongated flute structures and the dominant sand transporting wind direction. The particle‐scale experimental results confirm that (i) abrasion of this ventifact principally occurred under a unimodal wind and sand transport regime of low‐moderate intensity, and (ii) relief arising from weathering, as governed by the ventifact's lithology, likely initiated development of the present‐day ornamentation.