The East Range, Northwestern Nevada: A Neglected Key to the Tectonic History of the Region

The East Range occupies a strategic position at the western
edge of the exposed western facies terrane near the Paleozoic continental
margin. There a Cambrian and Ordovician deep-water
sequence including the Preble and Valmy Formations is overlain by
middle to upper Paleozoic strata that are assigned to three separate
stratigraphic sequences on published geologic maps of the range.
The three sequences, which include the Havallah sequence, the
Inskip Formation, and the Harmony Formation, originally were perceived
to be of different ages and lithic composition. The strata
assigned to the Havallah are now known to contain conodonts of
Late Devonian to Permian ages, and fusulinids of Permian age;
those assigned to the Inskip contain conodonts of Early
Mississippian and Permian ages and corals of Mississippian age.
Strata assigned to the Harmony are undated paleontologically. All
three are composed mainly of siliceous deep-water sedimentary
deposits. Lower beds of the Havallah and Inskip totalling hundreds
of meters in thickness are composed of arkosic sandstone and conglomerate
identical to that composing most of the Harmony. The
Havallah and Inskip lie disconformably on the quartzitic, middle
part of the Valmy Formation. The Harmony lies concordantly on distinctive
chert beds at the top of the Valmy Formation. Assuming the
latter contact is disconformable rather than a bedding-parallel fault,
the East Range displays the stratigraphic bases of all three arkosic
map units. The disconformable relation of the Havallah and Inskip
to the Valmy Formation indicates elevation of deep-water deposits,
subaerial erosion down to Middle Ordovician, and in Late Devonian
a return to deep immersion. There is no evidence here of Late
Devonian to Early Mississippian folding. Major folds in the East
Range must be of Jurassic or younger age because Upper Triassic
strata are involved. Evidence of large-scale thrust-faulting is notably
scarce. The existence of the Willow Creek thrust shown on published
maps of the East Range was based on the mistaken belief that
strata below the Valmy Formation are of Triassic age whereas they
are actually of Cambrian and earliest Ordovician age.