Rhizoliths in Lower Pliocene alluvial fan deposits of the Sorbas Basin (Almería, SE Spain)

Abstract Root traces are a valuable tool for paleoenvironmental interpretation and a significant indicator of the potential preservation of paleosol horizons. As such, the analysis of rhizolith-bearing intervals can help resolve high-frequency stratigraphic variability generated at the marine to continental sedimentary interface. This type of stratigraphic complexity is exhibited by the Lower Pliocene sedimentary infill of the intermontane Sorbas Basin (Almeria, SE Spain), where we document a distinctive, ~90-m long and ~ 1–2 m thick continuous horizon of well-preserved, densely spaced rhizoliths, occurring within conglomerate to coarse sandstone deposits along the northern margin of the basin (Moras Member of the Cariatiz Formation). The root traces are locally very abundant, mostly vertical, with very low sinuosity, averaging 10–19 cm long and 0.8–1.9 cm in diameter. The rhizoliths can be classified as root casts, with concentric laminar detrital infill. They are abruptly truncated at the top by a pebble conglomerate bed and overlain by a tabular unit of structureless sand, containing fossils of marine affinity. The rhizoliths occur at the contact between alluvial and marine facies. Facies analysis of the studied interval indicates a retrogradational trend, with upward transition from proximal to intermediate/distal alluvial fan deposits to shallow marine fossiliferous sand. The verticality of the root traces extending through several underlying coarse beds suggests the episode of plant colonization occurred in the presence of a relatively low water table. A sudden pulse of coarse clastic input killed the plants, and was followed by the onset of marine transgression. This case study illustrates an example of rhizolith development and preservation in coarse-grained alluvial fan deposits, and demonstrates the stratigraphic value of root traces as markers of composite discontinuity surfaces, where a surface of subaerial exposure is superimposed by a marine flooding surface.