A provenance investigation of glacial terraces along the Big Lost River, Custer County, Idaho.

This pebble and heavy mineral provenance investigation of glaciofluvial terraces was undertaken to determine both the source and relative contribution of individual canyons, and the downstream rate of mixing of glaciofluvial sediments. In the Copper Basin, Wildhorse Basin, and the North Fork Basin, ten major source areas are defined by pebble lithologies, and eleven populations are identified from the heavy mineral suites. Five source areas were defined in the Copper Basin. Lake Creek Canyon was the source area for terraces on the eastern side of the basin. Terraces on the western side of the basin were sourced from Broad, Bellas, Howell, and Ramey Canyons. In addition, three smaller, localized source areas were identified. They are: (1) Corral Creek, (2) Star Hope and Muldoon Canyons, and (3) Anderson, Smelter, Steve, Charcoal, and Coal Canyons. Two source areas were identified in the Wildhorse Basin. Wildhorse Creek sourced the terraces on the western flank of Wildhorse Canyon while deposits on the eastern side of the valley are sourced from Fall Creek. In the North Fork area the principal sources of glaciofluvial sediments were: (1) the North Fork of the Big Lost River, (2) Summit Creek, (3) Kane Creek, and (4) Phi Kappa Creek. Deposits from each source area are defined by the following distinctive lithologies and heavy minerals: 1. Anderson, Smelter, Steve, Charcoal, and Coal Canyons Lithology: volcanic Heavy Mineral: clinopyroxene 2. Broad, Bellas, Howell, and Ramey Canyons Lithology: pink porphyritic granite Heavy Minerals: biotite and monazite 3. Corral Creek Canyon Lithology: white porphyritic granite Heavy Minerals: opaques and biotite 4. Fall Creek Canyon Lithologies: quartz monzonite and pink porphyritic granite Heavy Minerals: hornblende, monazite, and sphene 5. Kane Canyon Lithology: metamorphic Heavy Minerals: biotite, hornblende, diopside, garnet, and topaz 6. Lake Creek Canyon Lithology: quartz monzonite Heavy Minerals: hornblende and epidote 7. North Fork of the Big Lost River Lithologies: sandstone, black chert, and volcanic Heavy Mineral: opaques 8. Phi Kappa Creek Canyon Lithology: no distinctive lithology Heavy Minerals: spinel and diopside 9. Star Hope and Muldoon Canyons Lithology: clastic Heavy Mineral: opaques 10. Summit Creek Canyon Lithologies: granite and clastic £eavy Mineral: opaques. ,. 11. Wildhorse Canyon Lithology: metamorphic Heavy Minerals: hornblende, diopside, calcite, and garnet In some instances outwash provenance can be used to decipher the glacial history and dynamics of.certain areas. For example, blockage of Star Hope and Muldoon Canyons by Lake Creek and Broad Canyon ice lobes, and the readvances of the Lake Creek and the Fall Creek lobes is supported by outwash provenance data. Aspects consanguineous with deglaciation features are dilution effects by non-glacially derived material, heterogeneous ice marginal drainage patterns, and development of proglacial lakes along Summit Creek and the East Fork of the Big Lost River. Pebbles are valuable primary tools for identification of outwash paths but are. subject to loss by attrition. Heavy mineral provenance is more useful because compositional subtleties are reflected for greater distances and because of minimal mass loss in the sand sized grains. Pedogenic processes affect the calcite, clinopyroxene, and hornblende content of the terrace deposits.