Impact of early Eocene (Ypresian) warming events on ichnological assemblage of the Naredi Formation, western Kutch (Kachchh) Basin of Gujarat, India

The Early Eocene warm climatic episodes have a profound impact on the biotic assemblages. This study presents the distinct changes in trace-fossil signature within the Early Eocene Naredi Formation that consists of a marginal-marine mixed siliciclastic-carbonate succession. The foraminiferal assemblages earlier assigned the formation to the Shallow Benthic Zones (SBZ) 8-11, which incorporates the Eocene Thermal Maxima 2, Eocene Thermal Maxima 3, and Early Eocene Climatic Optimum. The lower part of the succession comprising intercalation of green and red shales is characteristically unbioturbated with sparse bioerosion attributed to the Trypanites Ichnofacies restricted within the nodular horizons. The concretion layer immediately below the uppermost nodule horizon contains a localized remnant of a cohesive-debris-flow deposit characterized by a monospecific firmground Thalassinoides suite of the Glossifungites Ichnofacies. Glauconite-rich green shales, corresponding to the poorly fossiliferous SBZ 8 and barren SBZ 9/10 (?), represent oxygen-depleted bottomwater conditions, inhospitable to infauna. The hyperstressed to barren ichnological suites point to a eutrophic restricted-marine engulfment or lagoonal setting during ETM 2 and ETM 3. The upper part of the limestone-shale intercalated interval corresponding to the SBZ 11 was developed in the oligotrophic shallow open-marine condition during EECO. It is richer in trace fossils solely at the carbonate packstone intervals. The three intermittent packstone beds manifest three different ichnofabrics, viz., Cylindrichnus-Nummipera, Nummipera-Cylindrichnus, and monospecific Thalassinoides, which represent the depauperate expressions of Cruziana Ichnofacies. Recurrences of bioturbated and bioeroded intervals in an otherwise ichno-barren succession mark the repeated flooding events within a transgressive systems tract followed by a highstand systems tract. The environmentally stressed character of rarely preserved ichnofossil assemblages, which ameliorate upward, manifest the repercussions of warm climatic events previously well documented in the formation. The infaunal colonization records a gradual reestablishment of the endobenthic niche after the Mesozoic-to-Cenozoic transition in the basin.