| Abstract
Sediment in cores from drillholes Naval Reactor Facility (NRF) 15 and United States
Geologic Survey (USGS) 142 from the northern part of the Big Lost Trough (BLT) at the Idaho
National Laboratory (INL) document an evolution of facies during Early Pleistocene time.
Although more than 95% of the upper portions of these cores is basalt, sedimentary intervals,
from 520 ft to 595 ft below land surface (BLS) in NRF 15 and from 732 ft to 837 ft BLS in
USGS 142 were analyzed for grain size and petrologic analysis. The large difference in depth
BLS between USGS 142 and NRF 15 is accounted for by variable subsidence across the BLT.
Estimated ages, based on paleomagnetic signatures of the basalt, suggest that the intervals are 884
ka-988 ka. Each interval consists of clay that grades upward to coarse silt and sand. Through
grain size analysis and visual inspection of the core each interval is interpreted to represent a lake
that shallows upward into shoreline sands and loess.
Three depositional environments can be interpreted from the grain size data in each of
these upward coarsening intervals. The lower part of each interval is clay dominated and coarse
skewed with average grain-size of 6 to 8 phi. This interval is interpreted as a shallow lake
deposit. The intervals then coarsen upward to a fine-skewed silty sand, interpreted as shoreline or
eolian sediment. Parts of the upper portions of sedimentary intervals in NRF 15 display bimodal
grain size distributions with peaks at 2 and 8 phi; this sediment is interpreted as loess.
Point counting reveals that sands in the shoreline facies are volcanic lithic arenites (58%
lithics, and of those 63% are volcanic lithics with 54% of the volcanic lithics being felsitic
volcanic grains). These sands are interpreted to reflect transport via the paleo-Big Lost River, and
are most likely sourced from the Challis volcanics, which are primarily dacitic and rhyodacitic in
composition. The detrital zircons in the sandy intervals at 840 and 780 feet in USGS 142
resemble samples previously described from the Big Lost River. The zircon age spectra have an
age peak at 45 Ma that correlates most closely with a Challis volcanic source, and a Neoproterozoic age peak at 675 Ma that correlates with granitic rocks intruded into the Pioneer
Mountains core complex. |