Morphologic Evidence of Lamellae Forming Directly

from Thin, Clayey Bedding Planes in a Dune

Randall J. Schaetzl

An excavation in a sand dune in southern Michigan revealed numerous, thin (< 3 mm) clay-enriched strata between otherwise weakly stratified, clean, fine sand deposits. The parallelism of both the sandy and clayey strata suggest that they have direct geologic origins, having been deposited during formation of the dune. Although the clay-enriched strata and the intervening sandy layers are both within the sand textural class, the former contained between 3.0% and 5.4% clay, and the latter < 1.8% clay. In areas of preferred leaching, such as below vertical root traces, the clayey strata thin out and become absent, or bend downward, become wavy and contorted and take on the appearance of typical pedogenic lamellae. These observations support a theory of lamellae genesis that is both pedogenic and geogenic. This sedimentary strata may be pedogenically transformed into features that would normally be described as (entirely pedogenic) lamellae. Thus, many thin lamellae in Pleistocene-aged soils may simply be deformed and pedogenically altered sedimentary strata. This conclusion has implications for the nature and cyclicity of clay deposition in environments where thin lamellae are common in sandy soils.

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