Meandering flow of a pyroclastic density current documented by the Anisotropy of Magnetic Susceptibility (AMS): the case of the Pleistocene Peperino Tipico ignimbrite (Monte Cimino Volcanic Complex, Central Italy)

A pattern of shifting current direction is revealed by the anisotropy of magnetic susceptibility (AMS) in the quartz latite ignimbrite associated with the Pleistocene Cimini dome complex in central Italy. High-field AMS documents that both paramagnetic and ferrimagnetic minerals contribute to the magnetic fabric of the ignimbrite. The results of low field AMS studies suggest that the pyroclastic density current changed both direction and character during deposition of each flow unit that makes up the ignimbrite. The meandering behaviour of the currents can be observed on the scale of 15-cm increments in the fine-grained basal ‘layer 2a’ of one flow unit in an area lacking significant palaeotopography. AMS data from 1-, 2-, and 4-metre increments through the flow units also show complex changes in flow direction throughout the deposition of each unit studied. The most closely aligned AMS fabrics occur near the bases of flow units, and dispersionincreases upward. This trend may represent decreased shear between the flow and the substrate or increased turbulence in the upper depositional zone of the pyroclastic density current, or both, suggesting a progressively waning current (in terms of capacity or particle concentration). The phenomena observed in this study have broad implications for the general methodology of AMS studies. Detailed vertical profiles made up of several individual sample sites within 1-metre thickness intervals can reveal small-scale current flow variations that may not be evident if sampling is done more sparsely.