Integrated magnetobiochronology of the Early/Middle Eocene transition at Agost (Spain): Implications for defining the Ypresian/Lutetian boundary stratotype

Larrasoana, J.C., Gonzalvo, C., Molina, E., Monechi, S., Ortiz, S., Tori, F. & Tosquella, J. 2008: Integrated magnetobiochronology of the Early/Middle Eocene transition at Agost (Spain): Implications for defining the Ypresian/Lutetian boundary stratotype. Lethaia, vol. 41, pp. 395-415 In this paper, we present an integrated study of a 115-m-thick section that spans the Ypresian/Lutetian boundary at Agost (Betic Cordillera, SE Spain). Our study includes magnetostratigraphic results and biostratigraphic and palaeoenvironmental data derived from planktic foraminifera, small and larger benthic foraminifera, and calcareous nannofossils. Our results demonstrate that the Agost section is continuous and spans from Zones P9 to P12 (E7 to E10), Zones CP11 to CP14a (NP13 to NP16), Zones SBZ11 to SBZ15, and Chrons C22n to C19r. The first occurrence (FO) of H. nuttalli (base of P10) and the FO of G. nuttalli (base of E8) are found within Chron C20r, at a much younger age (3-5 Myr) than previously considered in standard calibration schemes. Similarly, the boundary between SBZ12 and SBZ13 is located within Chron C21n, also at a younger age than previously considered. On the contrary, the FO of B. inflatus (base of CP12b) is found within Chron C21r, which conforms to the magnetostratigraphically calibrated age of ca. 48 Ma (middle part of C21r) considered in standard calibration schemes. These results corroborate earlier studies and indicate that all the events that have been proposed to mark the Ypresian/Lutetian boundary appear at different stratigraphic intervals and have different ages. Based on our results from Agost and on data from other sections elsewhere, we suggest that the Ypresian/Lutetian boundary might be approximated by the FO of B. inflatus (base of CP12b). The Agost section might be considered as a potential candidate to locate the Global Stratotype Section and Point (GSSP) of the base of the Lutetian Stage, because it includes all the events that might be selected as marker events for the Ypresian/Lutetian boundary and it fulfils most of the geological, biostratigraphic and infrastructure requirements demanded for definition of a GSSP.