Genesis of lamellae in sandy soils: A case study in a semi-arid region in NE-Brazil

The occurrence of lamellae in soils and sediments has direct implications for agricultural production, and for stratigraphic and geomorphological studies. Although lamellae have been studied for more than a century, their origin remains controversial. The aim of this study was to identify the origin and formation mechanisms of lamellae in autochthonous soils developed from metamorphic rocks in a semi-arid area of Northeast Brazil. As such, the spatial organization of soil profiles was evaluated in a topolithosequence, through structural analysis of the pedological coverage for subsequent soil sampling. Morphological, chemical, physical, sedimentological, mineralogical, and micromorphological analyses were carried out on the studied slope and in greater detail in four soil profiles. The results indicated that clay particles predominantly smaller than 0.2 mu m are eluviated in a sandy soil matrix, infilling voids and coating quartz grains, which are organized in layers of densely packed fine sand, impeding clay translocation and helping form lamellae. Although the micromorphological properties of the lamellae demonstrate their pedogenic origin, the contribution of strips of polycrystalline quartz constitutes a petrogenic component of the illuviation process. Therefore, the genesis of the lamellae is better described based on a petro-pedogenic origin. We suggest a restructuring of the concept of petro-pedogenic origins, including the structural influence of metamorphic rocks, as illuviated clay deposition occurs in accordance with a physical barrier provoked by the increase and organization of fine sand particles, distributed horizontally and in parallel, arising from the disintegration of polycrystalline quartz grains, which indicates geological control in the genesis of lamellae, corresponding to petrogenic evidence. Furthermore, the morphological distinction of the lamellae seems to be more associated with the fine sand content (<0.25 mm), and its packing with larger grains of quartz in the soils.

Automated summary

The study investigated the origin and formation mechanisms of lamellae in autochthonous soils developed from metamorphic rocks in Northeast Brazil. The results showed that clay particles smaller than 0.2 μm are eluviated in a sandy soil matrix, infilling voids and coating quartz grains, forming densely packed layers of fine sand and impediment of clay translocation, indicating a petro-pedogenic origin of the lamellae. The genesis of the lamellae is associated with illuviated clay deposition occurring with a physical barrier due to the increase and organization of fine sand particles, indicating a geological control in the genesis of lamellae.