The interplay of environmental constraints and bioturbation on matground development along the marine depositional profile during the Ordovician Radiation

This study documents the distribution of matgrounds in a wide variety of environments recorded in the Ordovician Lashkerak and Ghelli Formations in the Alborz Mountains of northern Iran in order to evaluate controls on their distribution along the marine depositional profile. Detailed facies analysis allowed differentiating three groups of facies associations in the Lower to Upper Ordovician deposits of the Lashkerak formation: (i) estuarine system; (ii) wave-dominated shoreface-offshore complex; and (iii) mixed river- and wave-influenced deltaic system. The Middle to Upper Ordovician deposits of the Ghelli formation are divided into two groups of facies associations: (i) tide-influenced deltaic succession and (ii) deep-water fan system. Microbially induced sedimentary structures (MISS) are present in deposits formed in the central estuarine basin (Lashkerak formation) and in proximal lobes and lobe fringes of deep–water turbidite fans (Ghelli formation). On the contrary, MISS are absent in deposits from the wave-dominated shoreface-offshore complex, river- and tide-dominated deltas, and various subenvironments of the incised wave-dominated estuary (i.e., bayhead delta and estuary mouth) and the deep-marine turbidite fan system (i.e., turbidite channel, slope, and outer lobe). The lack of evidence of mat-building microorganisms in the deltaic systems may have resulted from two factors: (1) high physico-chemical stressors caused by river-induced processes, and (2) increase in degree of sediment disturbance, biodiffusion, and bioirrigation by burrowing organisms. Formation of microbial mats in the wave-dominated shoreface-offshore complex was inhibited by the activity of an abundant and diverse infauna capable of reworking the sediment. Our analysis shows that the spatial distribution of microbial mats was controlled by an interplay of environmental factors and innovations in animal-substrate interactions, mostly expressed by secular changes in bioturbation. This study supports the notion that the agronomic revolution was diachronic, with marginal-marine and deep-sea ecosystems lagging behind shallow-marine settings.