Clarifying stages of alluvial fan evolution along the Sfakian piedmont, southern Crete: New evidence from analysis of post-incisive soils and OSL dating

Abstract

Analysis of fan sediments and post-incisive soils was combined with luminescence dating to re-assess Nemec and Postma's [Nemec, W., Postma, G., 1993. Quaternary alluvial fans in southwestern Crete: sedimentation processes and geomorphic evolution. In: Marzo, M., Puigdefábregas, C. (Eds.), Alluvial Sedimentation. Special Publication of the International Association of Sedimentologists, vol. 17, pp. 235–276] model of fan evolution on the Sfakian piedmont, southern Crete. Field mapping supports the assertion that sedimentation occurred in three developmental stages. Stage 1 sediments comprise angular debris flows forming small cone-like deposits; stage 2 fluvial gravels form large, relatively steep streamflow-dominated telescopic fans; and stage 3 sediments consist of coarse sieve-type alluvium, localised mudflows and hyperconcentrated flow deposits. Irrespective of gradient, fan surfaces are capped by post-incisive soils that form a chronosequence comprising remnant chromic luvisols. The most developed profiles, highest redness rating, and greatest concentrations of Fe d and magnetic minerals are associated with soils formed on stage 1 surfaces. The stage 2 and 3 soils record progressively lower redness rating, Fe d, and magnetic values, indicating that the stage 1 soils and fan surfaces formed first, followed by stage 2 and 3 soils and fan surfaces. Nanofossil data strongly suggest that stage 1 sedimentation commenced no earlier than the Early Pleistocene. Optically stimulated luminescence (OSL) results suggest that sedimentation responsible for stage 2 surfaces occurred between Marine Isotope Stage (MIS) 6 and MIS 2, while archaeological data indicate that stage 3 sedimentation is of Holocene age. The re-investigation of fan sediments and morphology corroborates the sedimentary and morphological elements of Nemec and Postma's model. The soil data support the model's assumptions that sedimentation was broadly synchronous across the piedmont and the locus of deposition progressively shifted away from the range-front zone. OSL dating suggests that previous age estimates assigned to fan stages 1 and 2 are too old. Climate appears to exert a fundamental control over fan development, with sedimentation occurring during cold stages and cold stage-interglacial transitions. Tectonic activity provides the relief required for fan development and controls fan incision. Local uplift resulted in variable rates of incision that culminated in differential fan segmentation across the piedmont.