Muttoni Giovanni, Erba Elisabetta, Kent Dennis V, Bachtadse Valerian
Department of Earth Sciences, University of Milan, via Mangiagalli 34, I-20133 Milan, Italy.
Nature. 2005 Mar 3;434(7029):59-63. doi: 10.1038/nature03378.
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 sedimentation. 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 period. 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 radiolarites. 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 floor. 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.
在阿尔卑斯 - 地中海地区，随着大西洋的开启，泛大陆的分裂通过广泛的远洋沉积作用开始得到记录。在早侏罗世，浅水沉积物被主要的远洋石灰岩所取代，在中 - 晚侏罗世被放射虫硅质岩所取代，在晚侏罗世 - 白垩纪又被远洋石灰岩所取代。在初始伸展期间，盆地沉降到碳酸盐补偿深度（CCD）以下被认为引发了从早侏罗世石灰岩到中 - 晚侏罗世放射虫硅质岩的转变。有人提出，晚侏罗世从放射虫硅质岩到石灰岩的转变是由于当CCD下降到海底以下时钙质超微浮游生物丰度增加所致。但在现代海洋中，CCD以下的沉积物不一定是放射虫硅质岩。在此，我们展示了来自意大利伦巴第盆地侏罗纪 - 白垩纪远洋层序的古地磁样本。基于在更广泛的古地理背景下对我们古纬度数据的分析，我们对上述岩相三分提出了另一种解释。我们认为，伦巴第盆地最初向一个近赤道的高生物硅质生产力上升流区漂移，随后又远离该区域。我们关于放射虫硅质岩成因的构造模型为仅涉及CCD相对于海底垂直变化的解释增加了一个重要的水平板块运动分量。它可能解释了侏罗纪时期整个地中海和中东地区放射虫硅质岩的沉积。
