Morphologically induced density lag formation on bedforms and biogenic structures in aeolian sands

In a variety of sand-dominated depositional settings, an increase in near-surface wind velocity typically culminates with the formation of a lag deposit, such as an increase in coarse fraction, accumulation of shell fragments, or a concentration of heavy minerals (density > 2.9 g/cm(3)). This study presents examples from six coastal dune sites where prominent heavy-mineral concentrations (HMCs) are confined to the crests of aeolain ripples and marginal ridges of biogenic structures. In settings with low background fraction of heavy minerals (2-5%), micro-topographic highs (<1 cm) are sufficient for the formation of a density lag. In a backshore area of Assateague Island (Maryland, USA), a clear increase in heavy-mineral content (mostly magnetite) was observed in less than 24 h on asymmetrical ripples and vertebrate tracks. Surface peals reveal a dramatic increase in magnetite content within the uppermost laminae. The microtopography is proposed as a trigger of HMC formation due to a combination of factors, including: (1) localized increase in near-surface turbulence, and (2) high density of incident grain paths over the bed-form crest, compared to the slipface. Along Revere Beach (Massachusetts, USA), a clear increase in garnet fraction over step-like (aeolian ramp prevention) structure is likely related to a larger-scale increase in near-surface velocity field. Where deflation lags mimic the general outlines of the topographic anomalies, they can be used to identify analogous structures in the geologic record. The properties of many heavy minerals lend them to examination by geophysical techniques, such as magnetic susceptibility and high-frequency (>500 MHz) georadar imaging. (C) 2011 Elsevier B.V. All rights reserved.