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Volcanic Control of Sedimentation in Debris-Flow Dominated Aprons, Oligocene-Miocene Mount Dutton Formation (Southwestern Utah)

Beth A. Palmer, Tony W. Walton

Volcanism controlled facies-tract position and development of vertical sequences within the Mount Dutton Formation, an accumulation of coalescing lahar-dominated aprons associated with dacitic volcanoes. The lateral facies tract is divided into five facies assemblages. Assemblage I occupies the proximal position and consists of thin, channel-filling deposits of clast-supported diamictite. Assemblage I represents deposition in the upper, channelized part of the apron. Assemblage II occupies the area of maximum deposition and consists of thick lobes of matrix-supported diamictite. Assemblage II grades into assemblages III and IV, consisting of progressively thinner deposits of matrix-supported diamictic and fluvial beds, representing progressive reworking by weak lahars in he distal reaches of the apron. Assemblage V consists predominantly of large, thick chaotic deposits of debris-avalanche origin. Deposits are up to 100 m thick proximally, thinning distally to only 10 m. Texturally, the deposits are either clast- or matrix-supported, contain clasts to 15 m, and display a hummocky surface. Major shifts in facies-tract position resulted primarily from changes in eruption intensity and development of vents in different areas within the volcanic center, causing complex interfingering of the facies assemblages. In addition, emplacement of the debris avalanches forced sedimentation to shift from established patterns.

Organized vertical sequences within the Mount Dutton are present only in assemblage II. There, sequences consist of thick, basal beds containing large clasts overlain by thinner beds containing smaller clasts. The resulting record is a poorly organized, thinning-upward sequence. This sequence records geologically instantaneous influx of sediment during volcanic activity, causing perturbation of the apron system, followed by reworking as the system regained equilibrium. The sequence developed from such a volcanic cycle bears no resemblance to the organized sequences common in other fan environments.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.