Ignimbrite depositional facies: the anatomy of a pyroclastic flow

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Abstract
A model is presented for the depositional regimes in a pyroclastic flow, to explain the origin of facies in the Taupo ignimbrite and to compare these with other published examples. In this model, a pyroclastic flow consists of a head, a body and a tail. The head, which is where fluidization caused by air ingestion occurs, generates layer 1 deposits. On lithological grounds these are divided into layers 1(P) which is fines-depleted and rich in pumice, and 1(H) which is fines-depleted and rich in lithics and crystals. Layer 1(P) represents material thrown forwards from the flow head and is termed the jetted deposits. Layer 1(H) represents material sedimented from within the flow head and is termed the ground layer. The body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction; these parts generate the layer 2 deposits which include the valley-pond ignimbrite and its associated ignimbrite veneer deposit. Localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee-side lenses behind obstacles. At its outer limits all of the flow on interfluves is affected by air-ingestion fluidization, producing the distant facies which combines features of both layer 1 and 2 deposits. One important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow.