Applying ichnology in the sequence stratigraphy of a storm-dominated mixed carbonate/siliciclastic shelf: Geirud Formation (Upper Devonian), Central Alborz, Iran

Abstract

In storm-dominated shelves, examining transit and dissipation times during transgressive and regressive conditions is crucial for understanding factors shaping tempestites, biotic activity, and trace fossils preservation. The highly bioturbated, storm-dominated mixed siliciclastic-carbonate shelf of the Famenian-Frasnian Geirud Formation in Central Alborz provides a valuable record for documenting ichnological changes within a high-resolution sequence stratigraphic framework. Three third-order depositional sequences (DS1-DS3) have been identified, evolving from incised valleys (lowstand) to transgressive incised-valley-fill successions (LST and early TST), and then to shoreface-offshore complex to shelf during late TST and HST. A medium to large grained bioclastic grain-dominated carbonate facies expands throughout the TST and HST of DS3 due to basin slope reduction, siliciclastic input by rivers, and climatic changes. System tracts in DS1-DS3, along with facies evolution, stratal surfaces, and stacking patterns, are identified based on ichnofabrics patterns. In DS1 and DS2, the vertical replacement of ichnofaunas, from brackish ichnofabrics with opportunistic tracemakers to fully marine ichnofabrics, differentiates open marine systems from estuarine valley fill. In fully marine settings, during TST phases, the reduction of environmental stresses and slower sedimentation rates or longer periods of sedimentary stasis can erase or alter tempestites due to extended transit times and short dissipation times. This creates a larger window for colonization, promoting a mix of opportunistic and resident fair-weather communities. In late TST phases, longer transit times favor deep-tier trace fossils and complex feeding strategies. Conversely, HST phases are characterized by high sediment supply and aggradation, short transit times, and long dissipation times. Early HST phases, with alternating storm and fair-weather conditions, promote sophisticated feeding strategies and complex trace systems. In late HST phases, storms and rapid sedimentation create conditions favorable for opportunistic communities with high growth rates and short generation times, resulting in ichnofabrics dominated by infaunal suspension-feeding organisms. These findings highlight the importance of sedimentation rates, transit and dissipation times, and colonization windows in understanding trace fossils preservation, tempestites, and the organism strategies in response to sea level changes.