The fragmentation of Pangaea as a consequence of the opening of the Atlantic Ocean is documented in the Alpine–Mediterranean region by the onset of widespread pelagic sedimentation1. Shallow-water sediments were replaced by mainly pelagic limestones in the Early Jurassic period, radiolarian cherts in the Middle–Late Jurassic period, and again pelagic limestones in the Late Jurassic–Cretaceous period1. During initial extension, basin subsidence below the carbonate compensation depth (CCD) is thought to have triggered the transition from Early Jurassic limestones to Middle–Late Jurassic radiolarites1. It has been proposed that the transition from radiolarites to limestones in the Late Jurassic period was due to an increase in calcareous nannoplankton abundance when the CCD was depressed below the ocean floor1. But in modern oceans, sediments below the CCD are not necessarily radiolaritic. Here we present palaeomagnetic samples from the Jurassic–Cretaceous pelagic succession exposed in the Lombardian basin, Italy. On the basis of an analysis of our palaeolatitudinal data in a broader palaeogeographic context, we propose an alternative explanation for the above facies tripartition. We suggest that the Lombardian basin drifted initially towards, and subsequently away from, a near-equatorial upwelling zone of high biosiliceous productivity. Our tectonic model for the genesis of radiolarites adds an essential horizontal plate motion component to explanations involving only vertical variations of CCD relative to the ocean floor. It may explain the deposition of radiolarites throughout the Mediterranean and Middle Eastern region during the Jurassic period.
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E. Dallanave is thanked for assistance in the field and laboratory analysis. Comments by D. Bernoulli and J. Hagstrum greatly improved the manuscript.
The authors declare that they have no competing financial interests.
Thermal decay of three-component isothermal remanent magnetization (IRM) and Curie balance experiments. (PDF 368 kb)
Orthogonal projection plots of thermal demagnetization data. (PDF 23 kb)
Thermal demagnetization data of Colle Sogno samples. (PDF 140 kb)
Characteristic component directions extracted by means of principal component analysis from thermal demagnetization data listed in Supplementary Table S1. (PDF 49 kb)
Site mean characteristic component directions and associated paleomagnetic poles from Colle Sogno and the literature. (PDF 77 kb)
Legends to accompany the above Supplementary Figures and Supplementary Tables. (DOC 28 kb)
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