Stabilized eolian landforms of the largest quaternary inland eolian system in Brazil

Research on coastal Brazilian Quaternary eolian systems has significantly progressed in recent years. However, few contributions have been published on eolian depositional forms in Brazilian inland areas. Directly associated with the São Francisco River, the Xique-Xique eolian system is the largest quaternary interior dune field in Brazil (8,000 km²). Despite dune reactivation in some sites during the last decades, most Xique-Xique dunes are stabilized. In this study, we characterize dune morphology, vertical stratigraphic sections and sedimentary facies coupled to a dataset of optically stimulated luminescence (OSL). Four geomorphological zones were recognized and mapped: 1 - simple megadunes; 2 - compound dunes; 3 - perched dunes; and 4 - eolian plains. Simple megadunes (SM) occur as well-preserved parabolic dunes with simple elongated or lobate morphologies, and subordinately, nested, digitate, and superimposed dunes also occur. The SM can be recognized on both sides of the São Francisco River, have length of the arms (L) from 1.2 to 16.3 km, the width (W, distance along the transverse line that joins the two most distant points) from 0.9 to 5.7 km, and heights of up to 30 m at their noses. Six sediment samples have been dated by OSL, revealing ages from 26.5±4.4 ka to 4.8±0.9 ka. Compound dunes (CD) consist of well-preserved parabolic landforms comprising morphologies usually en-echelon and nested. They have L of 0.3-2.0 km and W of 0.3-1.4 km, with a predominant L/W ratio <1.7, and up to 50 m high. The ages of the five dated samples span from 14.3±1.5 ka to 5.1±0.7 ka. Perched dunes (PD) are small climbing and cliff-top parabolic dunes that form a narrow and high barrier along the riverbanks on the western margin of the São Francisco River, where they merge to form a dune belt 0.5-1.5 km wide. PD climb other landforms, reaching altitudes up to 160 m higher than the river level and forming stoss side ramps inclined up to 30º towards the river. Two PD samples were dated (14.0±1.0 and 1.2±0.1 ka). Eolian plains (EP) are almost flat stabilized surfaces between the dunes, which may correspond to sand sheets and vegetated deflation areas. They include deflationary eolian features such as kilometers-long trailing ridges. Five samples were dated and showed ages ranging from 16.2±1.1 ka to 6.8±0.6 ka. Sediments of all landforms have similar textural characteristics, fine- to medium-grained, well-sorted, rounded, quartzose unconsolidated sands. The colors of the sands are varied, ranging from dark brown to beige, usually with darker tones in the simple megadunes and eolian plains. A horizon with millimeter-sized charcoal fragments was observed on both sides of the river. Parabolic dunes started to form before 26 ka, but samples of all mapped landforms showed post-LGM ages, revealing a period of strong dune mobility from 15 to 5 ka in the Xique-Xique eolian system.