Cambrian reef complexes and pelmatozoan-rhynchonelliformean meadows from the Alborz Mountains, northern Iran: A reassessment of the Miaolingian–Furongian paucity of metazoan reefs paradigm

The Alborz Mountains in northern Iran represent one of the few exceptions to the worldwide decline in Miaolingian–Furongian (Cambrian) carbonate abundance and reefal volume, likely related to the arrival of northeastern Gondwana to subtropical latitudes. This Gondwana margin recorded the stepwise nucleation of microbial and shelly carbonate factories, lithostratigraphically recognized as members of the Lalun and Mila formations. The late Cambrian Epoch 2 to Miaolingian interval displays the development of oncoid-pisoid accumulations and microbial build-ups, with laminated and clotted textures, in open-sea and protected (back-barrier) settings. In contrast, the late Miaolingian and Furongian interval recorded a generalized establishment of pelmatozoan-rhynchonelliformean meadows, locally affected by horst-and-graben topographies. Hanging-wall blocks offered some protection from high-energy, shallow-water conditions, which favoured the growth of anthaspidellid-microbial reef complexes. The primary porosity recorded in the pelmatozoan-rhynchonelliformean shell beds, and the associated microboring and micritization of skeletal walls, favoured the episodic development of carbonate and iron oxyhydroxide hardgrounds. Such a singularity leads to the development of a more comprehensive global model linked to Miaolingian–Furongian progressive declines in ⁸⁷Sr/⁸⁶Sr ratios, estimated extent of carbonate platforms and reefal volume and atmospheric O₂ (punctuated by a sharp increase related to the SPICE event), coeval with relative increases in seawater calcite saturation. In some subtropical areas, such as in the Alborz margin of Gondwana, the re-occurrence of metazoan frame-builders took place in late Miaolingian times, and their subsequent reefal success was primarily controlled by tectonic activity and the nucleation of hardgrounds in highly porous pelmatozoan-rhynchonelliformean shell beds.