Salt dome morphology: a quantitative analysis of diapiric process-based classification schemes

Salt domes are bodies of halite-gypsum-anhydrite salt exposed at the Earth’s surface. They typically comprise positive topographic expressions, reaching heights of 10s to 100s m. In plan-view, they display elliptical through to elongated shapes that span metre to km-scale length-width-area-volume dimensions. Salt domes are typically found in drylands, where rainfall-humidity levels are sufficiently low, inhibiting dissolution. Salt is considered to move upwards to the surface by diapiric processes, linked to rock density differences and elevated compressive forces. Thick (>100 m) layers of lacustrine-marine evaporitic sediment are buried to km-scale depths over geological timescales. Salt density = ~2200 kg/m³), becomes buoyant and diapiric due to overburden pressure when buried by >1-3 km thickness of higher density (~2500 kg/m³) sediment cover. Salt movement is further enhanced by tectonics, especially in collisional plate settings. Here, compression provides additional diapiric driving forces, with any resulting folded-faulted strata providing a structural-stratigraphic framework for salt migration pathways. Accordingly, geologists use the term ‘salt diapir’ reflecting the movement-emplacement process, or ‘salt plug’ to reflect its stratigraphic configuration with respect to bounding strata and structures. ‘Salt dome’ is a geomorphological term linked to salt’s surficial geomorphic expression.

Salt dome morphology features widely in geological-geomorphological literature, where dome shape is commonly linked to different stages of diapiric growth. Jahani et al (2007), analysed 43 domes from the Zagros Mountains (Iran), proposing six dome types:  A = buried diapirs, B = high relief active diapirs, C = high relief active diapirs with a salt fountain and glacier, D = like C but with more erosion and without fountain, E = dead and highly eroded diapirs with an empty crater, F = linear diapirs emerging along faults. Although such process-form based classification schemes are well-conceived and likely correct, they surprisingly lack quantitative analytical underpinning of their morphological dimensions. Thus, the purpose of this research is to investigate whether quantitative morphological relationships between salt dome shape and diapiric process-based classification exist.

We focused upon the classic Eastern Fars region studied by Jahani et al (2007) and their 6-fold classification scheme (above). We compiled a spatially expanded database of 67 domes (A = 3%; B = 28%; C = 18%; D = 34%; E = 12%; F = 4%) and their morphological attributes (area [111-0.6km²], relief [1.2-0.7km], length/width ratio [1-6], volume [20- -14km³], hypsometry [curve and integral], slope [mean 33-5°] using 12m TanDEM-X data and satellite imagery for measurement and visualisation purposes. The database was supplemented with geological information sourced from Iranian geological survey maps, including dome relationships to geological units (age/lithology) and tectonic structures (folds/faults).

Results reveal at best a weak qualitative and quantitative relationship between dome morphology and diapir growth and erosion evolution processes. We discuss this finding and suggest that dome location with respect to fold structural configuration as a confinement control provides an improved, and hitherto unexplored, salt dome classification opportunity.

Jahani, S., et al., 2007. The salt diapirs of the eastern Fars Province (Zagros, Iran): A brief outline of their past and present. In Thrust belts and foreland basins. Springer. pp. 289-308.