Exotic facies episodes of a carbonate platform: implications for middle and late Cambrian ecosystems and impact of bioturbation in the Alborz Basin, Iran

Abstract

The sedimentological and ichnological data of the Mila Group in the Alborz Basin, northern Iran indicate that the appearance of exotic facies and resurgence of microbialites resulted from the development of extraordinary palaeoceanographic conditions with widespread environmental stress and enhanced precipitation of CaCO3. Inhospitable environmental conditions in the Mila Group platform led to a considerable reduction in ichnodiversity, bioturbation intensity, depth and size of burrows, which led to periods of dominating calcimicrobe ecology. The development of exotic, or anachronistic facies started around the Cambrian Stage 4-Wuliuan boundary, probably simultaneous with the SPICE event whereas the Furongian exotic facies period probably took place simultaneous with the HERB event. Biotic and physico-chemical changes in the ocean during these events may have led as well to the virtual absence of the mixed layer and infaunal ecospace utilisation in the deposits of the Mila Group. Nevertheless, the local presence of metazoan-microbial build-ups that established between the two suggested exotic facies developments indicates that these periods started at the Cambrian Stage 4-Wuliuan boundary interval, but a fluctuation of abnormal palaeoceanographic conditions and ambient normal marine conditions took place during the Miaolingian and Furongian in the Alborz Basin as a representative of northern Gondwanan margin.

Late Cambrian climate change recorded by a shift from an arid carbonate platform to a storm‐dominated cool‐water platform at the Gondwana margin (Alborz Zone, Iran)

“…The lack or paucity of infaunal ecospace utilization leading to the virtual absence of the bioturbated mixed layer has been regarded the cause of development of ribbon rocks as their development is promoted after mass extinction events (see Lee et al, 2021 and Bayet‐Goll et al, 2022 for further information). Diagenesis (e.g., Munnecke & Samtleben, 1996; Westphal, 2006) and microcyclicity of environmental conditions (e.g., Einsele & Ricken, 1991) have also been discussed as possible causes.…”

“…Similarly,Thompson (2003) recognized a general negative δ 13 C excursion from +0.7 to À2.7‰ in the Conococheague Formation attributed to the HERB event.These examples support the view that the deposition of late Furongian ribbon rocks and associated facies featuring repeatedly unstable depositional environments represent a global episode, which correlates with the HERB event. This also may have led to the occurrences of exotic (anachronistic) facies with an unusual predominance of microbialites (including ribbon carbonates) in Earth's history as discussed inBayet-Goll et al (2022). Based on that study, unusual or extraordinary sedimentary facies of the Cambrian, which are characterized by the post-Phanerozoic revival of microbialites, referred to in their study to as 'exotic facies', signify periods of reduction in bioturbation intensities, which may demonstrate specific palaeoceanographic conditions that might have been coincident with the SPICE and HERB events.…”