Variations in the flow dynamics of a pyroclastic density current revealed by the anisotropy of magnetic susceptibility in the Cimino ignimbrite, northern Lazio, Italy.

The anisotropy of magnetic susceptibility (AMS) of an ignimbrite reflects the preferred orientation of paramagnetic (i.e., biotite) and ferromagnetic (i.e., magnetite) grains. Alignment of these grains may occur during transport and deposition by the pyroclastic density current, and thus their orientation is related to the flow direction at the time of deposition. A detailed study of the orientation and the shape of the AMS ellipsoid at various levels within a deposit therefore reveals the history of flow behaviour and deposition at that location. The Cimino ignimbrite surrounds the Monte Cimino silicic dome complex, in northern Lazio, Italy. The ignimbrite contains abundant biotite crystal fragments as well as magnetite and fine-grained Fe-Ti oxides in the ash-size fraction of the matrix. Eighty-nine oriented samples were collected from the lower 60 cm of the ignimbrite at a locality approximately 11 km from the suspected vent site, defining four levels in a vertical face, at 15 cm spacing. AMS data from these samples show very tight clustering of the three main axes of AMS ellipsoid within the lower 15 cm, and increasing dispersal upwards. This increased scatter may reflect decreasing shear force or non-laminar flow within the pyroclastic density current as deposition progressed, and palaeotopography exerted less control on the flow behaviour. The AMS signature of the Cimino ignimbrite is generally oblate in character, rather than prolate. Because of this, flow direction is interpreted from the inclination of the oblate field, expressed as the minimum axis of the ellipsoid, or Xmin. Systematic shifts in Xmin were observed in the lower 60 cm of the Cimino ignimbrite, from 072.5º at 0-15 cm above the base, to 064º at 15-30 cm, 311º at 30-45 cm, and 335º at 45-60 cm above the base. Additional measurements at 2, 4, and 8 m above the base show increased dispersion and dramatic changes in Xmin. This variability clearly represents incremental deposition or progressive aggradation of the ignimbrite and it shows a meandering of the pyroclastic density current as the ignimbrite was deposited and palaeotopography was buried.