Early Middle Jurassic magnetostratigraphy from Iberian sections

Abstract New magnetostratigraphic data are presented from four well-dated Iberian sections (Fuentelsaz, Cerro Méndez, Sierra de Lúgar and Puerto Escaño, from the Iberian Ranges and the Betic Cordillera) covering the Toarcian–Aalenian and Bajocian–Bathonian boundaries. In addition to a viscous component with the direction of the present-day field, two magnetisation components could be isolated by thermal demagnetisation: a secondary component of always normal polarity and intermediate unblocking temperature (350°–475 °C) and a high unblocking temperature component (up to 575 °C) that exhibited normal and reversed polarities that is considered the characteristic remanent magnetisation of primary origin. The magnetostratigraphic pattern for the Toarcian–Aalenian boundary investigated at Fuentelsaz section (currently the Global Boundary Stratotype Section and Point for this boundary) correlates well with the obtained from Cerro Méndez 1 section (Betic Cordillera). The transition between both stages took place during a normal magnetozone preceded by a reverse magnetozone located at Lower Aalensis–Upper Meneghinii Zones (the end of the Toarcian). The Bajocian-Bathonian stages are characterized by a high frequency of reversals. For the boundary 3 pairs of magnetozones have been identified from Puerto Escaño and Sierra de Lúgar sections (Betic Cordillera). The end of Bajocian (Upper Parkinsoni Zone) is clearly related to a magnetozone of reversed polarity that precedes the normal magnetozone where the boundary is located. Within the Lower Bathonian Zigzag Zone at least two additional magnetozones have been identified in this work. A critical review of magnetostratigraphic data from Iberian sections allow to propose a composite geomagnetic polarity time scale for the Lower-Middle Jurassic based on Iberian sections.