Magnetobiochronology of the Izaga section (South Pyrenean foreland basin) and its chronological implications: searching for a potential candidate for the Bartonian GSSP

Here we introduce a new magnetobiostratigraphic section in the Jaca-Pamplona Basin, the Izaga section, which was studied in order to shed light on the chronology of the Lutetian/Bartonian boundary and the possibility of proposing it to host the Bartonian Global Stratotype Section and Point (GSSP). The Izaga section is located in the northern limb of the eastern termination of the Izaga syncline (South Pyrenean foreland basin) and is composed by a total of 1116 m of marine sediments that include the uppermost 450 m of the Jaca Turbidites and ~660 m of the prodeltaic marls of the Larrés (500 m), Urroz (110 m) and Pamplona (56 m) Formations. The uppermost Jaca Turbidites include the youngest South Pyrenean Eocene Carbonate Megabreccia (SPECM) identified in the basin to date, and the uppermost part of the Larrés Formation hosts ferroan dolomitic nodules whose formation can be linked to early diagenetic methanogenesis. Previously published biostratigraphic data based on planktic foraminifers broadly place the Lutetian/Bartonian boundary within the middle part of the Larrés Formation. A total of 173 magnetostratigraphic sites were sampled throughout the succession with an average 6.5 m spacing. Thermal demagnetization reveals the presence of two stable components: 1) a low temperature component that is identified <250ºC and is interpreted as present-day field overprint; and 2) a high-temperature component, that unblocks from 250-300ºC up to 425ºC and is identified as the ChRM. Paleo- and rock-magnetic data point to the dominant contribution of magnetite to the ChRM, although an additional drop in NRM intensity between 300-350ºC suggests the additional contribution by magnetic iron sulphides. To avoid problems with a likely diagenetic origin of magnetic iron sulphides, we established the polarity sequence of the Izaga section by using only ChRM directions associated to magnetite. The local polarity sequence comprises a normal (N1) magnetozone in the uppermost 286 m of the section and a reverse (R1) one spanning its remaining middle and lower parts. The analysis of calcareous nannofossil aseemblages allow the identification of zones CNE14 and CNE15 in the lower/middle and upper part of the succession, respectively. Overall, these new results enable the correlation of R1 and N1 with chrons C18r and C18n.2n, respectively. We have found no evidence for the presence of chron C19n, the proposed marker for the Lutetian/Bartonian boundary, within R1, which indicates that the whole studied section was deposited during the Bartonian and, therefore, has no bearings on the definition of the Bartonian GSSP. Our results also indicate: 1) that sedimentation rates (of >80 cm/kyr) in the South Pyrenean foreland basin increased towards the west; 2) that the SPECM found within the Jaca Turbidites extend the processes of SPECM formation well into the Bartonian; and 3) that the ferroan dolomitic nodules found in the uppermost part of the Larrés Formation can be considered as the sedimentological expression of the Middle Eocene Climate Optimum (MECO) in the basin.