Pre-emergent construction of a lacustrine basaltic volcano,Pahvant Butte,Utah (USA) |
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| Authors: | James D L White |
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| Institution: | (1) Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand Fax: +64 3 479–7527 e-mail: jwhite@gandalf.otago.ac.nz, NZ |
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| Abstract: | The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced
a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes
active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are
fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1
lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification
upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely
by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded
tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The
former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter
from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from
phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies
near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity
flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally
breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and
(b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous
uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization
of mound tephra. Intermittent, explosive magma–water interactions occurred from the outset of the Pahvant eruption, with condensation,
entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition
of the mound lithofacies.
Received: 10 October 1995 / Accepted: 18 April 1996 |
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| Keywords: | Surtseyan volcanism Subaqueous volcanism Hydrovolcanism Sediment-gravity flow Sideromelane Basaltic volcanism |
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