Cyclostratigraphic studies were conducted on two marine sections in France and Italy for astronomical time calibration of Lower Jurassic and for improvement of the global chronostratigraphic framework. High resolution magnetic susceptibility (MS) analysis was carried out on the Sancerre borehole located in the Paris Basin (Cher, France), where the Lower Jurassic (Sinemurian to Aalenian) is represented by more than 300 m of clay-rich marine deposits.
The recovery of this interval is almost 100%. Biostratigraphic data are based ammonite zones and calcareous nannofossils. MS measurements were made directly on the core with a Bartington Instrument MS2E1 sensor every 2 to 4 centimeters.
The long-term MS variations along the core correlate strongly with lithological changes, and reflect major second and third order transgressive-regressive cycles. Decreases in MS represent regressive phases and increases represent transgressive phases. A second section, Colle di Sogno, 220 meter thick, Upper Pliensbachian-Lower Bajocian basinal carbonate sequence from the Lombardy Prealps (Italy) was also studied.
Cyclostratigraphic analysis was conducted with MS measurements (sample interval: 2 to 4 cm), carbonate content and lithologically coded series. Biostratigraphic data are based on ammonites and calcareous nannofossils.
Cyclostratigraphic comparison between the two sections reveals a number of similarities. At Sancerre, high frequency MS cycles in the Domerian show behavior indicative of orbital forcing by the precession index; in the overlying Toarcian, the cyclicity reflects obliquity-dominant forcing. This pattern change is also observed in the Italian section at Colle di Sogno.
To understand these changes in a high resolution context, time-frequency analysis was conducted to compare the recorded Milankovitch cycles with the theoretical orbital parameters, and to study the forcing changes in the cycles along the sections. In both cases, the fundamental change from precession- to obliquity-dominant forcing, is intimately associated with the black shales in both sequences representing the well-known Early Toarcian Oceanic Anoxic Event.
