EVOLUTION OF LATEST CRETACEOUS EOCENE NONMARINE DEPOSYSTEMS IN THE AXHANDLE PIGGYBACK BASIN OF CENTRAL UTAH

The Axhandle basin of central Utah is a piggyback basin produced by di sruption of the proximal Sevier foreland basin by the Gunnison thrust system during Late Cretaceous time. Alluvial and lacustrine fill of th e basin, represented by the North Horn Formation, ranges in age from 7 5 to 51 Ma. The eastern basin margin is defined by a system of west-ve rging thrust faults that were active episodically during North Horn de position. Along this margin, well-exposed strata, 10 km wide and 1.2 k m thick, displays striking changes in adjacent depositional facies, se diment provenance, and sediment dispersal directions. Two-dimensional mapping and precise paleomagnetic dating of these strata provide evide nce for deciphering the evolution of and controls upon deposition in t he basin. Sediment accumulation rates in the eastern part of the basin alternated between times of rapid aggradation and times of incision o r nondeposition with a cyclicity of 5-8 m.y. This alternation was caus ed by the interaction of flexural lithospheric subsidence and uplift o f the basin by the underlying Gunnison thrust system. Competition betw een the relative rates of these processes also determined the nature o f depositional facies in the basin, such that active uplift of the bas in margin did not correspond to a unique facies type. Uplift of the ba sin margin on thrust structures of the Gunnison system is associated w ith two different types of sedimentologic response: (1) channel-belt i ncision during and after the formation of well-drained, mature paleoso ls on interfluves; and (2) deposition of lacustrine, palustrine, and p oorly drained flood-plain sediment. The former response probably resul ted from rapid uplift of the basin during times of low rates of region al subsidence, whereas the latter resulted from basin uplift during ti mes of greater regional subsidence. Lack of correspondence between bas in-margin uplift and load-induced subsidence indicates that motion on the Gunnison thrust and loading by out-of-sequence thrusting within th e Sevier orogenic belt were out of phase. Channel deposits that mark t he position of major fluvial systems are stacked vertically within the basin. North of this stacked channel system, fluvial channels are rar e, and deposits of the basin are dominated by overbank and lacustrine facies associations. Fluvial incision of the eastern basin margin appa rently fixed the position at which rivers flowed across the basin-boun ding 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 th e 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.