UNRAVELING THE BONNEVILLE FAILURE AND ITS AFTERMATH: SOUTHEAST IDAHO

New observations require major revision of events and locations of thresholds in the NE Great Basin during the catastrophic Bonneville flood (~14.5 14C BP) and subsequent Lake Provo. 1) A south-flowing marsh in Round Valley in northern Cache Valley preserves a Provo-age meander belt that held a large north-flowing river. 2) Analysis of gravity data, present topography, shorelines, and stream gradients reveals transverse bedrock sills and relict drainage divides at the north and south ends of this meander belt. These sills, at Swan Lake and Clifton, ID, controlled an earlier and a later, lower, level of Lake Provo. 3) The Red Rock Pass area generated at most 7 km2 of flood-modified landslide deposits. 4) The Marsh Creek produced a pediment that overlies Tertiary Salt Lake Formation and formed the original sill for Lake Bonneville. The pediment preserves relict sapping landforms in gullies below the highstand of Lake Bonneville and none above that altitude. 5) The flood produced two channels just N of Red Rock Pass. The base of the curving eastern channel is higher than the straighter western channel, and 6) The Provo delta of the Bear River filled northern Cache Valley from east to west (16 km). These new data change the sequence of events. The highstand of Lake Bonneville submerged lowlands of northern Cache Valley. The outburst of Lake Bonneville began north of Red Rock Pass, near Zenda, ID, when piping through the Marsh Creek pediment and underlying Salt Lake Formation undermined the sill along a sapping line. Landsliding across a narrow reach at Red Rock Pass may have shifted the flood's outflow entirely to its western channel. The breach and related scouring lowered the lake to a sill near 4775±10' at the NE end of Swan Lake horst, about 10 km SSE of Red Rock Pass. The Swan Lake sill held during the early part of Lake Provo. Later, Lake Provo dropped to a lower sill, near Clifton, ID (4745±5'), another 10 km farther south, and created the meandering section of its northward-outflow river in Round Valley. Most of the topset of the Provo delta of the Bear River formed then. Landslides near Red Rock Pass did not control the levels of Lake Provo. A dam produced by an alluvial fan of Marsh Creek, isostasy, and/or faulting may explain the Holocene return to southward flow between Red Rock Pass and the Clifton sill.