Resumen de Tesis Doctoral: Magnetoestratigrafia de l'Oligocè del sector sud-oriental de la Conca de l'Ebre: implicacions magnetobiocronològiques i seqüencials

Abstract

The magnetostratigraphic study of terrestrial, late Eocene to earliest Miocene, alluvial and shallow lacustrine sequences in the SE Ebro Basin, combined with its well-documented fossil mammal record, provides a well-resoluted chronology and has allowed to establish a complete Oligocene magnetic polarity stratigraphy. We present the lithostratigraphic, biostratigraphic and magnetostratigraphic cross-correlation of 11 thick, continuous and correlatable sections throughout the stratigraphic successions in the study area. More than 1500 standard specimens were stepwise thermally demagnetized following standard procedures and VGP latitudes used to establish the magneticstratigraphy. Magnetic record of the distinct lithologies is highly variable, specially concerning the stability of the primary component. The composite magnetic polarity sequence is based on 560 paleomagnetic sites and the VGP data display several reversals defining a continuous and complete sequence from Chron C15n to Chron 6Cn.2n, from 34.8 to 23.7 Ma (late Eocene-early Miocene). In addition, a few short, normal and reverse polarity events that correlate to cryptochrons in the GPTS are found in the studied successions (i.e. In the C11n.1r and the C8n.1r). Other short subchrons, not recorded in the GPTS also occur (i.e. in the C15n or C12n reversed subchrons or in the C8r normal subchron). Although the resolution is still inadequate to unambiguously define them, our results favour the interpretation that these low-amplitude magnetic anomalies represent short-polarity change intervals rather than geomagnetic intensity fluctuations. The relatively high sedimentation rate in the studied alluvial and lacustrine successions (11 to 26 cm/kyear) has thus favoured the record and preservation of such short subchrons, some of them still not reported in the oceanic record. The magnetic polarity zonation of the different sections has been correlated to the GPTS using the distinct sequences and pattern polarity reversals, as well as by considering the fossil mammal assemblages. We report for the first time a continuous and complete magnetostratigraphy of the Oligocene in a terrestrial basin in Europe. A regional biostratigraphic framework based on fossil mammals is proposed as a first approach to define non-marine chronostratigraphic units. More over the accurate magnetochronology now established enables to date more precisely the late Eocene-early Oligocene palae-ofloristic and palaeofaunistic changes in southwestern Europe datet at 34.4 Ma (lower part of C13r), which were coeval and likely related to the ongoing global palaeoclimatic and palaeoceanographic changes caused by high latitude cooling.