Layering in fallout deposits of explosive eruptions. Learning from the 1991 Hudson eruption in Patagonia

SCASSO, R.A.; CAREY, S.N.; KRATZMANN, D.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The 1991 explosive eruptions of Hudson volcano in southern Chile produced about 3 km3 (dense rock equivalent) of basaltic and trachyandesitic pyroclastic material mostly deposited as a multilayered tephra. The paroxysmal phase (Aug 12-15) involved three separate events with a maximum ~25-km-high eruption column. Tephra from this paroxysmal phase of the eruption was directed southeast in an elongate plume. Fall deposits extended to the Malvinas Islands ~1,500 km to the south in a narrow band almost 370 km wide that covered ~100,000 km2 of southern Patagonia. The plume associated with the paroxysmal phase produced a multilayered deposit composed of alternating layers of fine ash and pumice lapilli. The stratigraphy from the paroxysmal phase of the 1991 eruption is quite complex, and many well-defined fall units are traceable up to 50 km from source. About twenty-two well-defined beds up to coarse lapilli in size are present 30 km SE from the vent. Nine ash layers are present in Los Antiguos, 120 km east southeast from the vent. Stratigraphic complexity within Hudson fall deposits was produced by three concurrent factors. First, the three eruptive pulses that occurred during the paroxysmal phase of the eruption. Second, the development of a “wandering plume” that repeatedly crosses the main dispersal axis as a result of a changing wind direction. Third, wind reworking of ash after the eruption. The first two factors developed graded layers composed of lapilli and ash; the third resulted in better sorting and lamination of fine pumice or ash layers. Recent simulations using the lagrangian ash tracking model PUFF demonstrate that changes in the wind direction during an eruption can result in dramatic changes in the direction of the plume. The resultant ‘wandering plume’ may impact the grain size being deposited from fine ash that typifies the edges of the plume to the pumice lapilli that dominates the center of the plume. This depositional regime could produce a multilayered deposit composed of alternating fine and coarse units, similar to the base of the Hudson stratigraphy. The shifting plume in the earlier stages of the phase II eruption has also produced a much wider overall deposit than would be expected from a plume with a relatively fixed transport direction. Hudson’s case helps to understanding layering in ancient fallout deposits.