Ordovician matground and mixground ecosystems in shoreface–offshore and barrier-island environments from Central Iran, northern Gondwana

Abstract

The shift of matground ecosystems to bioturbator-free settings was investigated in the Ordovician wave-dominated marine strata of the Shirgesht Formation in Central Iran. Ten ichnofabrics are recognized in shoreface–offshore and barrier-island sedimentary facies, representing a proximal-to-distal depositional trend along the studied profile. In the offshore settings, intensive burrowing on several tiers and bioirrigation, referred to the Thalassinoides (Th), Cruziana–Skolithos–Rosselia (CrSkRo), crowded Trichophycus (CT) and Cruziana–Helminthopsis (CrHe) ichnofabrics, prevented the development of matgrounds. As a result of the Ordovician radiation, progressive ecospace utilization by suspension-, deposit- and detritus-feeders, as represented by the Skolithos (Sk), Rosselia–Skolithos (RoSk), crowded Rosselia (CR) and Conichnus–Skolithos (CoSk) ichnofabrics, is regarded as a key factor for sediment mixing and reworking and thus disruption of the matgrounds in shoreface settings. However, the restriction on microbial growth was reduced in lower-offshore / shelf or tidal-flat and back-barrier settings, where Chondrites–Planolites (ChPl) and Planolites (Pl) ichnofabrics dominate. This restricted microbial growth resulted from physico-chemical stresses on infaunal communities, as shown by the low ichnodiversity, scattered burrowing, absence of complex tiering, and prevalence of simple feeding strategies. This study suggests that following the early Palaeozoic evolutionary radiations, a considerable increase in abundance of detritus-feeders, deposit-feeders, suspension-feeders, predators, vagile bilaterian metazoans and grazers in shallow-water benthic communities caused mat-building microorganisms to migrate into lower-offshore / shelf, tidal-flat and back-barrier settings, where colonization by burrowing organisms was delayed.

The influence of hybrid sediment gravity flows on distribution and composition of trace‐fossil assemblages: Ordovician succession of the north‐eastern Alborz Range of Iran

“…Buatois & Mángano, 2012; Buatois et al ., 2014; Mángano & Buatois, 2017; Gougeon et al ., 2018), were conducive to the restriction of matgrounds to inhospitable environments (for example, deep‐marine and marginal‐marine settings). In the lobe off‐axis and the lateral lobe‐fringe settings, reduced effects of sediment mixing and bioirrigation allowed microbial growth and the establishment of matgrounds (Buatois & Mángano, 2012; Bayet‐Goll et al ., 2021; Bayet‐Goll, 2022).…”