Lower Carboniferous shallow marine sequences from the Central Alborz Basin, Iran (north-eastern margin of Gondwana): sedimentology, biostratigraphy and rock– magnetic studies
[en] The Lower Carboniferous Mobarak Formation records the development of a carbonate platform on the
southern Paleo-Tethyan passive margin. This pervasive carbonate factory was deposited following the
opening of the Paleo-Tethys Ocean into the Alborz Basin along the northern margin of Gondwana. The
depositional facies range from the most proximal to the most distal and include coastal environments,
inner ramp peritidal facies, peloidal to crinoidal shoals, storm to fair-weather influenced mid-ramps,
proximal to distal shell beds and low energy outer ramps. Storms clearly played a dominant role in the
distribution of skeletal and non-skeletal shoals on the carbonate ramp. Sedimentological analyses
complemented with foraminiferal biostratigraphy reveal four events which are interpreted to represent
the principal factors controlling carbonate platform evolution in the Alborz Basin during the Lower
Carboniferous: 1) A transgression linked to global temperature rise in the Early Tournaisian (Middle
Hastarain) resulted in the formation of thick-bedded argillaceous limestones. 2) the Hastarian–Ivorian
boundary glaciation phase, 3) Upper Ivorian–Lower Viséan? tectonic block faulting. 4) the Viséan-
Serpukhovian glaciation phase. The three foraminiferal assemblages encountered in the Tournaisian
interval of the Mobarak Formation are restricted to specific periods within the Ivorian. The occurrence
of specific foraminiferal taxa in Alborz is tightly governed by transgressions and migration of North
Paleo–Tethyan biotic elements as response to the thermal period.
We also combined proxies for ambient paleothermometry in addition to the indications for arid
conditions and arid conditions and the presence of foraminiferal taxa with a North Paleo-Tethyan
affinity in the Lower Carboniferous Mobarak Formation to suggest a paleo-position for the Alborz Basin
at lower latitudes than approximately 45 –50 southern paleolatitude reported thus far.
Magnetic susceptibility ( in ) was measured and compared with facies from the same sample. There is a
clear link between in and facies, and the average in values are higher for distal facies than for proximal
ones. The in profile of Lower Carboniferous carbonate sequence reflects stratigraphic variations in
response to relative sea level changes and detrital input. In the context of the sequence stratigraphic
framework, the average in values for lowstand and transgressive system tracts deposits are higher than
for the highstand system tracts deposits. The clear link between in and facies points to at least partly
preserved primary in signal, related to detrital inputs. In respect to the hysteresis measurements the
in 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).
Lower Carboniferous shallow marine sequences from the Central Alborz Basin, Iran (north-eastern margin of Gondwana): sedimentology, biostratigraphy and rock– magnetic studies