Trace fossils after the KT boundary event from the Agost section, SE Spain

Abstract

Palaeogene trace fossils penetrate the uppermost Cretaceous sediments in the Agost section, Betic Cordillera, SE Spain. Chondrites ?targionii, Zoophycos isp., Planolites isp. A, Planolites isp. B, ?Thalassinoides isp. A, Thalassinoides isp. B, Thalassinoides isp. C, Alcyonidiopsis longobardiae, and Diplocraterion ?parallelum have been identified. A well-developed endobenthic tiering pattern is interpreted. The uppermost tier (A) represents major benthic activity in the shallowest sediments at or just below the seafloor, recorded as bioturbated background (only discrete rare Planolites). The next upper tier (B) displays the highest trace-fossil diversity, with Planolites in the shallow levels, and Thalassinoides and Alcyonidiopsis at slightly deeper ones. The intermediate tier (C) contains randomly distributed Thalassinoides and Alcyonidiopsis, cross-cut by Zoophycos and large and medium-sized Chondrites. The deepest tier (D) contains Zoophycos, cross-cut by medium- and mainly small-sized Chondrites. Two interpretations are proposed: (1) vertical partitioning of a singled multi-tiered community under steady-stable conditions in well-oxygenated water bottom, reflecting gradual changes deep into the sediment, with decreasing oxygen pore water and benthic food, and increasing substrate consistency, and (2) sequential colonization and community replacement, reflecting the work of two successive communities. The first community, mainly represented by Planolites, Alcyonidiopsis and Thalassinoides, was produced in a shallow, oxygenated soft substrate. Probably related to decreasing oxygen content and benthic food availability, this community was replaced by deeply burrowing organisms that produced Zoophycos and Chondrites. This sequential colonization can be determined by changes in environmental parameters related to the Cretaceous–Palaeogene (K–T) event, or may have resulted from normal ecological and early diagenetic processes.