A unit of the Mesoproterozoic Apple Creek Formation of the Lemhi Range previously
was correlated with part of the lower subunit of the Mesoproterozoic Yellowjacket
Formation in the Salmon River Mountains. Strata currently assigned to the middle
subunit of the Yellowjacket Formation lie conformably above the Apple Creek unit in the
Salmon River Mountains, and are here renamed the banded siltite unit and reassigned to
the Apple Creek Formation. Almost all of the banded siltite unit is preserved within the
Salmon River Mountains, where it grades upward into clastic rocks that currently are
assigned to the upper subunit of the Yellowjacket Formation and that here are reassigned
to the Gunsight Formation.
The banded siltite unit of the Apple Creek Formation is composed of a turbidite
sequence, as recognized by previous workers. Uppermost strata of the unit were
reworked by currents, possibly storm generated, and adjusted to a high water content by
developing abundant soft-sediment deformation features. Basal strata of the overlying
Gunsight Formation in the Salmon River Mountains display abundant hummocky
crossbeds, storm-generated features deposited below fair-weather wave base, that are
conformable above the storm-reworked deposits. The hummocky crossbedded strata
grade upward into marine shoreface strata deposited above fair-weather wave base, which
in turn are succeeded by fluvial strata. Hummocky and shoreface strata are absent from
the Gunsight Formation in the Lemhi Range. The major thickness of the Gunsight
Formation in both the Salmon River Mountains and the Lemhi Range is composed of
fluvial rocks, transitional in the upper part into marine rocks of the Swauger Formation.
The fluvial strata are mainly characterized by stacked sheets of metasandstone and coarse
siltite; they are interpreted as deposits of braided rivers.
The Poison Creek thrust fault of the Lemhi Range extends northwestward through the
study area in the east-central part of the Salmon River Mountains. The Apple Creek and
Gunsight Formations on the southwest side of the thrust fault were transported to the
northeast as part of the Poison Creek thrust sheet. A segment of the thrust fault within
the Gunsight Formation in the Salmon River Mountains subsequently underwent normal
displacement. Along this segment, lower Gunsight strata on the southwest were
juxtaposed against upper Gunsight and Swauger strata on the northeast.