Variations of the trace fossil Zoophycos with respect to paleoenvironment and sequence stratigraphy in the Mississippian Mobarak Formation, northern Iran

Abstract

Zoophycos in the Mississippian Mobarak Formation (Central Alborz, Iran) shows its potential in interpreting depositional settings, ecological features and sequence stratigraphy. Zoophycos A (ZoA), Zoophycos B (ZoB), Zoophycos/Phycosiphon (Zo/Phy) and Phycosiphon (Phy) ichnocoenoses are identified in the middle and outer ramp facies of the Mobarak Formation on the basis of shape, ichnodensity, ichnocomplexity degree and size of Zoophycos and other trace fossils. Zoophycos portrays a proximal-to-distal trend in the storm-dominated ramp sequence. In deeper parts of the ramp with long-term stable conditions and organic enrichment, Zoophycos producers selected equilibrium strategies (K-selected), whereas into the shallower parts they opted for opportunistic behavior (r-selected) due to short-term stable conditions with low food flux. The feeding behavior proposed in this study is based on general morphology as well as mode of sediment exploiting, and includes deposit feeding and the caching model. Moreover, this study confirms that the distribution pattern of the Paleozoic Zoophycos producers within a sequence stratigraphic framework that was totally different from that of the Mesozoic and Cenozoic. During the late transgressive systems tract (TST) and early highstand systems tract (HST), the excess of benthic food and the environmental stability, together with the prevalence of dysaerobic conditions, caused intense burrowing by the Zoophycos producer in proximal outer ramp to distal middle ramp. Its ethology, paleoecologic strategies and taphonomic signatures suggest that erosionally-exhumed, dewatered muddy beds (firmground) were not able to provide appropriate conditions for deposit-feeding Zoophycos producers as a consequence of their lower environmental stability. Therefore, Zoophycos as a standard component of the resident fair-weather community cannot be a good representative of the distal expression of the Glossifungites ichnofacies.