Lower Carboniferous ramp sedimentation of the Central Alborz Basin, northern Iran: integrated sedimentological and rock–magnetic studies

Abstract

AbstractThe Lower Carboniferous Mobarak Formation in the Alborz Basin (northern Iran) was deposited along the northeastern margin of Gondwana in a carbonate ramp setting. This paper focuses on the Tournaisian stratigraphic interval of this formation that crops out at the Jaban section in the southwestern Central Alborz Basin. The following facies associations, representing different ramp palaeoenvironments, have been identified: (1) mudstone–wackestone outer-ramp facies; (2) crinoidal to skeletal grainstone–packstone mid-ramp facies; (3) peloidal to crinoidal grainstone–packstone inner-ramp facies; and (4) coastal facies, which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs. This ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non-skeletal shoals in the distal mid-ramp to the most proximal inner-ramp areas. The development of the skeletal to non-skeletal shoals along the sea side of the ramp formed a semi-enclosed lagoon sensitive to the influence of both high tides and storm surges. The magnetic susceptibility (χin) of all the samples was measured and compared with that of the facies from which the sample was taken. There is a clear link between χin and the facies; the average χin values were higher for the distal facies than for the proximal facies. The χin profile of this Lower Carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials. In the context of the sequence stratigraphic framework, the average χin values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits. The clear link between χin and facies indicates at least a partly preserved primary χin signal related to the detrital inputs. However, to obtain a better understanding of the nature and origin of the minerals carrying the χin, we performed hysteresis measurements on selected samples. It appears that the χin signal is mainly carried by low-coercivity ferromagnetic minerals such as magnetite, with a mixture of relatively coarse grains (detrital fraction) and ultra-fine grains (probably formed during diagenesis).