HR: 0830h
AN: V51G-0357    [PDF]
TI: Peperite-Fed Andesitic Debris Flows in the Miocene Mehrten Formation, Kirkwood, California, USA
AU: Imai, T
EM: imait@pitt.edu
AF: University of Pittsburgh, 200 SRCC, Pittsburgh, PA 15260 United States
AU: * Skilling, I
EM: skilling@pitt.edu
AF: University of Pittsburgh, 200 SRCC, Pittsburgh, PA 15260 United States
AU: Busby, C
EM: busby@mail.lsit.ucsb.edu
AF: University of California - Santa Barbara, UC Santa Barbara Building 526, Santa Barbara, CA 93106 United States
AB: The Miocene Mehrten Formation is a voluminous predominantly andesitic suite of volcanic rocks erupted along the crest of the Sierra Nevada mountains in northern California. At Kirkwood, California, the Mehrten Formation comprises a sequence of primary volcanic andesitic debris flow deposits, debris avalanche deposits, block-and-ash flow deposits and distal fine-grained rhyolitic ignimbrites, overlying a predominantly reworked sequence of andesitic fluvial debris flow, hyperconcentrated flood and streamflow deposits. The Mehrten Formation in this area is intruded by several andesitic dikes which are both contemporaneous with and postdate the debris-flow deposits. The volcanic debris flow deposits display an unusual internal structure comprising discrete domains of either brittle or ductile-fragmented juvenile blocks in an ash to lapilli-sized matrix. The matrix is locally parallel laminated but where present is often partially destroyed along contacts with juvenile clasts. Some domains of ductile-fragmented clasts are clearly fed from dike-like intrusions, while others display no evidence of feeder bodies. The latter are typically more intimately mingled with the brittle domains. Ductile domains are generally smaller in cross-section ($<$1-50m across) than the brittle domains ($<$1-300m across). The feeder dikes display both fluidal and brittle fragmentation at their margins. These observations suggest that the ductile domains and at least some (or all?) of the blocky domains represent peperite, formed when wet andesitic pyroclastic ash and lapilli deposits on or immediately below the paleosurface was intruded by andesitic magma. Individual juvenile clasts in the peperite range from several millimeters up to half a meter in size. The absence of feeders to many of the peperite domains suggests that the domains were truncated from their feeders and subsequently entrained during downslope flow. This observation also suggests the possibility that intrusion and consequent peperite generation could have initiated and sustained this primary debris flow eruption mechanism. Such a mechanism for the initiation of potentially hazardous debris flows has been largely ignored, though it is not yet clear how applicable this mechanism is to other volcanic debris flow deposits, in the Mehrten Formation or elsewhere.
DE: 8400 VOLCANOLOGY
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting