Abstract: The Lower Carboniferous carbonate shallow marine sediments of the Mobarak Formation -Alborz Zone, northern Iran- are interpreted as a deepening-shallowing cycle interpreted as recording changes in relative sea level which correspond to depositional sequences. The field observations and laboratory studies were used to identify nine facies, which can be grouped into four depositional zones: shallow open marine -shaly and marly -A1- and bioclastic mudstones/wackestones -A2- facies-, shoal -crinoidal grainstone, rudstone -A3- and coral framestone -A4- facies-, lagoon -bioclastic peloidal wackestone /packstone -A5-, foraminiferal packstone -A6-, gastropod bioclastic wackestone and packstone -A7- facies- and tidal flat -dolomitized lime mudstone -A8- and oncoidal packstone -A9- facies-. Based on the absence of distinct palaeobathymetric changes, the wide lateral distribution of facies and the depositional system likely represent an eastward-deepening homoclinal ramp. Five 3rd-order depositional sequences were distinguished in the Mobarak Formation. In the Mobarak Formation, four types of shell concentrations can be identified based on sedimentologic, stratigraphic, taphonomic and palaeoecological characteristics. These four types of shell concentrations are found in distinct positions within depositional sequences which are transgressive lag -at the base of transgressive systems tract -TST-, hiatal concentrations, event concentrations and multiple-event concentrations. The features of lag concentrations include distinct basal erosional surface, sorting, high fragmentation and abrasion, disarticulation and convex-up orientation which deposited above the fair-weather wave base in a high-energy inner ramp environment. Concentration at the end of the TST -which overlies by mfs sediments- is a hiatal concentration characterized by high percentages of articulated brachiopods in life position with high rates of bioerosion. This concentration was accumulated during high rates of production of biogenic hard parts and low sedimentation rates, below storm wave base in outer ramp environments. Concentrations occurring in the mfs are in situ event concentrations characterized by autochthonous and highly time-averaging. Besides, they were accumulated during times of the rapid sedimentation in middle ramp above SWB environment. This is approved by their high preservation quality -comparatively high percentage of articulated shells- that the biogenic alteration is the most important taphonomic feature. Multiple-event concentrations at the top of the HST have characteristics such as densely fossiliferous deposits with complete disarticulation, convex-up and high grades of bioerosion, low grades of abrasion in a lower middle to upper outer ramp environments with sporadic high-energy events.

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