Facies and sedimentation processes in the alluvial fan deposits of the lower reach of Zayandeh river, southeast of Isfahan

Four facies associations can be differentiated in the modern deposits of the lower reach of the Zayandeh river drainage basin in the south-east of Esfahan.They are debris-flow-dominated alluvial fan, stream-flow-dominated alluvial fan, lake and overbank. The first includes matrix-supported gravel facies (Gms) and massive muddy sand facies (Sms). The second consists of unstratified to crudely stratified facies (Gm), plannar cross-bedded facies (Gp), through cross-bedded facies (Gt), and clast-supported gravel/conglomerate and trough cross-bedded facies (St). The third comprises red to purple massive mud facies (Fm) and yellow to greenish mud, interlayering with sand layers facies (Fsc). The forth includes matrix-free massive sand/sandstone facies (Sm) and laminated khaki mud facies (Fl).The quantitative summary of these facies emphasizes a periodic deposition in a semi-arid setting and ephemeral rainy conditions of deposition with dominance of low energy. Fluctuation in rainfall caused periodic change of the base level, which were reflected both in lacustrine deposits and in the deposits produced by the alluvial systems, particularly in the distal fan.