Surface karst geomorphology in the Jahani salt extrusion, Zagros Mountains, Iran

Abstract

This work analyses the superlative salt karst developed on Jahani salt extrusion (ca. 70 km², 900 m in local relief). This active diapir is expressed as a salt fountain comprising a summit dome above the feeding vent and laterally spreading salt glaciers (i.e., namakiers) moving at rates of the order of cm/yr. The salt extrusion hosts the first documented and mapped salt karst poljes, developed at the foot of the steep rock salt slopes of the summit dome by differential suballuvial dissolution and expansion by rim dissolution. A cartographic inventory of 6489 sinkholes has allowed to characterise morphometrically the differences between the juvenile and mature sinkhole landscapes developed in the proximal and distal sectors of a namakier, respectively. Sinkholes developed in the recently expelled salt at the proximal sector have an average length four times smaller (23 m versus 83 m) and a density three times higher (600 versus 200 sinkholes/km²), reflecting the variable impact of expansion and coalescence processes. The Firuzabad River has trimmed the northern namakier, generating a 6 km long and > 400 m high salt escarpment, likely the largest on Earth. The dynamics of the escarpment is governed by the antagonistic roles of salt flow and rapid erosion by fluvial undermining, rock falls and dissolutional removal of the rock salt debris, causing severe hydrochemical degradation of the river waters. Exceptional halite rimstones occur in a marginal stream largely fed by permanent brine springs.