Abstract

The Axhandle basin of central Utah is a piggyback basin produced by disruption of the proximal Sevier foreland basin by the Gunnison thrust system during Late Cretaceous time. Alluvial and lacustrine fill of the basin, represented by the North Horn Formation, ranges in age from 75 to 51 Ma. The eastern basin margin is defined by a system of west-verging thrust faults that were active episodically during North Horn deposition. Along this margin, well-exposed strata, 10 km wide and 1.2 km thick, displays striking changes in adjacent depositional facies, sediment provenance, and sediment dispersal directions. Two-dimensional mapping and precise paleomagnetic dating of these strata provide evidence for deciphering the evolution of and controls upon deposition in the basin. Sediment accumulation rates in the eastern part of the basin alternated between times of rapid aggradation and times of incision or nondeposition with a cyclicity of 5–8 m.y. This alternation was caused by the interaction of flexural lithospheric subsidence and uplift of the basin by the underlying Gunnison thrust system. Competition between the relative rates of these processes also determined the nature of depositional facies in the basin, such that active uplift of the basin margin did not correspond to a unique facies type. Uplift of the basin margin on thrust structures of the Gunnison system is associated with two different types of sedimentologic response: (1) channel-belt incision during and after the formation of well-drained, mature paleosols on interfluves; and (2) deposition of lacustrine, palustrine, and poorly drained flood-plain sediment. The former response probably resulted from rapid uplift of the basin during times of low rates of regional subsidence, whereas the latter resulted from basin uplift during times of greater regional subsidence. Lack of correspondence between basin-margin uplift and load-induced subsidence indicates that motion on the Gunnison thrust and loading by out-of-sequence thrusting within the Sevier orogenic belt were out of phase.

Channel deposits that mark the position of major fluvial systems are stacked vertically within the basin. North of this stacked channel system, fluvial channels are rare, and deposits of the basin are dominated by overbank and lacustrine facies associations. Fluvial incision of the eastern basin margin apparently fixed the position at which rivers flowed across the basin-bounding uplift and into the adjacent foreland. The observed concentration of channel deposits within the basin persisted for >24 m.y. This long-term feature may have been caused by pinning of drainage paths either where rivers crossed the western basin margin or at a reentrant in the interior of the thrust belt. This study demonstrates that structural controls may fix the location where major rivers drain from a thrust belt through significant periods of thrust belt evolution.

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