Quartz grain microtextures of Holocene sebkhas sediments in southeast Tunisia: Implications for sedimentary processes and depositional environments

Abstract

Detrital quartz grains extracted from sebkha sediments in southeastern Tunisia underwent scanning electron microscopy analysis to identify sediment sources and assess the influence of the saline environment on the grains. Core sediments collected from Sebkha el Melah and Sebkha Mhabeul cover the last 5000 and 2000 BP, respectively. The uppermost Unit III, present in both cores, exhibits two distinct facies based on the mechanical microtextures of its quartz grains. The aeolian facies sediments are characterized by quartz grains with rounded outlines, upturned plates, and crescentic percussion marks. In contrast, the fluvial facies sediments are associated with quartz grains featuring subangular outlines, v-shaped percussion cracks, conchoidal fractures. Observations on the quartz grains of the sebkhas suggest multiple transportation and processing events, indicating long-distance transport and rapid deposition rates. The majority of quartz grains appear to originate from the surrounding terrain, reflecting the dynamic geological history of the region. This study delves into the connection between microtexture variations on quartz grain surfaces and specific historical climatic conditions in the sebkhas. By examining geochemical variations along the cores, facies with elevated salt concentrations corresponding to warmer periods reveal extensively weathered quartz grains. This substantial chemical alteration is evident through microtextures such as oriented etch pits, anastomosed dissolution networks and solution crevasses. The profound dissolution has significantly impacted the quartz lattice, resulting in the decomposition of the grains and the formation of "cauliflower" or "spongy" shapes, erasing prior microtextures. Conversely, during less warm periods, quartz dissolution was less severe, thereby preserving microtextures.

Sand grain surfaces are notably sensitive to both transport processes and variations in the physicochemical environment. In the hypersaline and confined environments of sebkhas in southeastern Tunisia, potent post-sedimentary processes can obliterate and obscure the original microtextures recorded on grains from previous environments due to highly fluctuating physicochemical conditions.