Facies genesis and dominant hydrological process in Quaternary deposits of colluvial in the semi-arid of Alagoas – a microsedimentological approach

Abstract

Colluvial deposits from the distal, medial, and proximal sectors of colluvial hillslopes in the semi-arid region of Alagoas were studied, with a focus on sandy mud sediments. Although frequently classified in the field as massive, these sediments are hypothesized to correspond to successions of thin, laminated layers formed by overland flow. The aim of the study was to clarify the genesis of the facies and the dominant hydrological processes, particularly for sandy mud. Three representative stratigraphic profiles from each sector were analyzed through field descriptions and grain size analysis, complemented by microsedimentology for sandy mud sediments. The results indicate that all sectors display similar facies successions: laminated sandy mud, massive muddy gravel, and massive silty gravel, with the first predominating. Microsedimentology revealed that the apparent homogeneity observed in the field is superficial, exposing laminations formed by shallow high-density flows originating from overland flow. The galaxy microstructure proved to be a marker of internal turbulence within the flows. High-density overland flow was the predominant hydrological process over the last 15,000 years.