SEDIMENT CORRELATION IN THE BIG LOST TROUGH, EASTERN SNAKE RIVER PLAIN AND IDAHO NATIONAL LABORATORY, IDAHO

We analyzed sediment in cores NRF 15 and USGS 142 from the northern part of the Big Lost Trough (BLT) at the Idaho National Laboratory (INL). Although most of the upper portions of these cores are basalt, sediment interbeds at 524 ft in NRF 15 and 732 and 837 ft in USGS 142 are thick enough for grain size and petrologic analysis. Estimated ages, based on paleomagnetic signatures of the basalt, suggest that the interbeds are between 884 ka and 1045 ka. We correlate three sediment interbeds between the two boreholes. Each interbed consists of clay that grades upward to coarse silt and sand. Through grain size and visual inspection of the core, we interpret these each to represent a shallow lake that shallows upward into deltaic sands and windblown loess.

Three depositional environments can be interpreted from the grain size data in each of these upward fining interbeds. The lower part of each interbed is clay dominated and fine-skewed with average grain-size of 6 to 8 phi. This interval is interpreted as a shallow lake deposit. Each interbed then coarsens upward to a coarse-skewed silty sand, interpreted as deltaic and or windblown sediment. Upper portions of interbeds in NRF 15 contain bimodal grain size distributions with peaks at 2 and 8 phi; this is thought to be loess.

Point counting of sands reveals the sands are volcanic lithic arenites (58% the grains are lithics, and of those 63% are volcanic lithics with 54% felsitic volcanic grains). These sands are interpreted to represent the paleo-Big Lost River. The detrital zircons in the sandy interbeds at 840 and 780 feet in USGS 142 resemble previous samples from the Big Lost River. The zircons have a strong Challis volcanic-age peak at 48 Ma, and a strong Neoproterozoic age peak at 675 Ma that represents granitic rocks intruded into the Pioneer Mountains core complex.