Geological-archeological study of the Mirk area (Semnan, Iran) using sedimentological and physico-chemical analyzes

Mirak (Semnan, Iran) is a Paleolithic site located in a dry floodplain near the northern edge of the Iranian Central Desert. The pedo-sedimentary successions of Mirak 8 are deposited from late Pleistocene to Late Holocene,. The chronological framework was established using optically stimulated luminescence (OSL) dating. The oldest studied units dating back to ~50 ± 3.0 ka. Within this stratigraphy, two distinct depositional environments are recognizable: Sequence I = alluvial floodplain deposits and Sequence II = calcareous aeolian deposits. The alluvial succession is represented by alternating horizons of poorly pedogenized bodies of loam, intercalated by splay sand interbeddings. The prehistorical remains occur exclusively at distinct limits and it is presumable that some prehistorical layers of Mirak are palimpsests resulted from repeated occupations. After ~1.2 ± 0.2 ka  an at least 3 ka long sedimentary hiatus cuts the sedimentary log of Mirak 8 and about 2m of aeolian assemblages take place over the discontinuity. This shift was clearly reflected in the laboratorial results. Results from XRD, FTIR, SEM and optical microscopy analyses, show very minor differences in mineralogy for the entire record regardless of the type of depositional regime and this we suggests a local and most probably polygenetic source for the deposits under study. Subsequent to deposition, both alluvial and aeolian accumulations were subjected to different extents of pedogenesis and thus developed pedogenic features characteristic of Entisols and calcareous, gypsiferous Aridisols (Bk, By). Varying iron content and different oxygenizing conditions between the two sequences are presumably relatable to higher ground water level and Fe-reducing conditions especially for the units deposited prior to ~28 ± 2 ka BP (OSL) and generally adverse conditions for the Holocene units.