Lower Jurassic magnetostratigraphy at the Breggia Gorge (Ticino, Switzerland) and Alpe Turati (Como, Italy)

Abstract

The Breggia gorge (Ticino, Switzerland) provides a continuous outcrop of Mesozoic limestones covering the time between the Lower Jurassic and Upper Cretaceous. The Jurassic section from the Lower Pliensbachian to Aalenian is well dated by ammonites. The limestones of these stages are a good quality recorder of the polarity of the geomagnetic field during this time interval. More than 500 cores have been drilled along a 120m thick section with a median distance between cores of about 20cm. Detailed thermal and af demagnetization and further rock magnetic studies were used to determine the characteristic NRM direction for each sample. Initial NRM intensities and low field susceptibilities reflect variations in oxidation potential and sedimentation rate during carbonate deposition. The Breggia section covers an estimated 12 Myr of Lower to Middle Jurassic time during which more than 60 distinct reversals of the geomagnetic field are recorded. This corresponds to a comparatively high mean reversal frequency of about 5 reversals Myr-'. The Breggia polarity zonation can be correlated with the nearby section at Alpe Turati (Como, Italy) and comparable polarity patterns are recognized in other biostratigraphically well-dated magnetostratigraphic profiles from Hungary and Umbria (Italy).

Magnetostratigraphy of the Toarcian Ammonitico Rosso limestone at Iznalloz, Spain

“…This rate averages 4 reversals per ammonit.e zone. The Aalenian of the Breggia Gorge section also shows a low reversal rate (Homer and Heller, 1983). During Jurassic times high reversal rates (5 to 10 reversals per ammonite zone) are known for the Bajocian and Bathonian (Steiner et al, 1987), and Pliensbachian (Horner and Heller, 1983).…”

Magnetostratigraphy of the Hettangian Langmoos section (Adnet, Austria): evidence for time-delayed phases of magnetization

“…Recently, considerable effort has been made to expand the reference polarity time scale to the period of time for which there is no sea-floor magnetic anomaly record. Results for the Late-Middle Jurassic (Steiner et al 1986;Steiner, Ogg & Sardoval 1987); Channell et al 1990b), Lower Jurassic (Marton, Marton & Heller, 1980;Horner & Heller 1983;Channell et al 1984;Galbrun, Gabilly & Rasplus 1988;Galbrun et al 1990), Late Triassic (Witte, Kent & O l d n 1991;Molina-Garza et al 1991 ;Galbrun, Crasquin-Soleau & Jaugey 1992;Gallet et al 1992Gallet et al , 1993 Lower Triassic (Steiner et al 1989;Ogg & Steiner 1991) and for the Late Permian (Steiner et al 1989;Haag & Heller 1991) have been obtained. These studies, which provide only a discontinuous magnetic sequence back to the Late Permian, could constrain the changes in the magnetic reversal frequency for these periods (see Gallet et al 1992 for a review).…”

The Neuquén composite section: magnetostratigraphy and biostratigraphy of the marine lower Jurassic from the Neuquén basin (Argentina)

“…In the Pliensbachian, the excellent correlation of the Argentina pattern to that of northern Italy [5] represents the ¢rst veri¢cation of this portion of the MPTS and therefore converts this polarity pattern into a useful time scale. In the Toarcian, the Argentine results con¢rm the paleomagnetic data obtained by Horner and Heller [5] and signi¢cantly improves their part of the MPTS.…”

“…In the Toarcian, Ogg [2] notes that correlations of polarity Zones between the Toarcian sections at Breggia Gorge [5] and Iznalloz [8] were di¤cult to establish, because of stratigraphical discontinuities and regional di¡erences among the sections. Thus, in Breggia Gorge, the Tenuicostatum Zone plus the base of the following Falciferum Zone depict only reversed polarity, whereas in Iznalloz the same interval includes a normal polarity interval as well (Fig.…”

“…The lower part of the Upper Sinemurian, in part controlled by ammonites, depicts a dominant normal polarity, while the uppermost Sinemurian and lowermost Pliensbachian lack magnetostratigraphic data [2,3]. In most of the Pliensbachian^Toarcian interval, polarity ages are assigned on the basis of studies carried out in Breggia Gorge (Switzerland), Alpi Turati (Italy) [5], Thouars^Airvault (France) [7], Iznalloz (Spain) [8] and Bakonycsernye (Hungary) [12]. In Bakonycsernye, a 9 m thick section comprises Ammonitico Rosso facies, in which most of the Pliensbachian (Ibex to Spinatum Standard Zones) was recognized [13].…”