Use of high-resolution ichnological and stable isotope data for assessing completeness of a K–P boundary section, Agost, Spain

High-resolution ichnological analysis has been performed at the Agost section (Betic Cordillera, southeast Spain), in order to assess the completeness of the K–P boundary transition below the biostratigraphic resolution (intrasubzone). High-resolution planktonic foraminifera biozonation revealed that the first appearance of the Palaeogene index species does not coincide with the K–P boundary, but is located a few centimetres above, and calibrated ≈ 5.7–6.7 ky later the K–P boundary. To establish the continuity of this first interval of the lowermost Danian (Hedbergella holmdelensis Subzone), and the existence of any hiatus below biostratigraphic resolution, trace-fossil analysis was revealed to be a useful tool. Planolites, Thalassinoides and Alcyonidiopsis structures belonging to the upper tiers, colonizing at or immediately after the K–P boundary event, were studied focusing on the isotopic composition of the filling materials and the genesis of Thalassinoides. Rapid firmground generation and Thalassinoides tracemaker colonization is mainly attributed to cessation of precipitation and/or deposition of calcium carbonate, but an incipient lithification is not disregarded. The δ13C data obtained from trace fossils fit well with those corresponding to: a) the K–P boundary layer (from 1.00‰ to 1.30‰), b) the stratigraphic interval between 2.5 and 5.0 cm above the K–P boundary (between 0.32‰ and 0.47‰), and c) the interval between 7.0 and 11.0 cm above the K–P boundary (between 0.69‰ and 0.79‰), revealing continuity in the colonization from the K–P boundary event to the dark clay deposition above. This macrobenthic colonization occurred continuously during the dark layer deposition from the K–P boundary layer, including the first centimetres, evidencing completeness and the absence of hiatuses below biostratigraphic resolution (intrasubzone) at the Agost section.