Sedimentation of the Late Triassic Higham Grit in a South SaskatchewanlPlatte River-Type Braided Stream Complex, Southeastern ldaho and Western Wyoming

Abstract

The Late Tnassic Higham Gnt m southeastern ldaho and western H$aming IS comprised pmdommantly of coarse to medrum- grarned sandstone and pebble conglomerate wrth mrnor mudstone Lithofacies present include: Massive to crudely horizon- tally bedded pebbly conglomerate (Gm), trough crossbedded sandstone (St), planar crossbedded sandstone (Sp), horizon- tally stratified sandstone (Sh), ripple crosslaminated sandstone (Sr), and finely laminated sandstone and mudstone (FI). Deposition occurredm a South Saskatchewan/Platte River-type braided fluvialcomplex wrth the development of longitudinal bars and gravel lags (Gm), straightcrested transverse bars (Sp), and sinuouscrested transverse bars and dunes (St). Perrodic, high-velocity flow resulted m development of upper flow regime plane beds (Sh). Minor episodes of flood plain rnundation produced werbank deposits (FI). Application of the South Saskatchewan and Platte River brarded stream models to the Higham Gnt is m good agreement with provenance studres (Schmitt and Hazen, in preparation) which postulate a sediment source area in the Ancestral Rocky Mountains uplift, a distance of 500-700 km to the southeast Sediment transport distances in both the South Saskatchewan and Platte braided fluvial systems closely approximate this distance.