Significance of Geologic and Biostratigraphic Relations Between the Overlap Assemblage and Havallah Sequence, Southern Shoshone Range, Nevada

ABSTRACT
New geologic mapping of the Upper Paleozoic stratigraphic
section of the southern Shoshone Range shows that it consists of
rocks of the overlap assemblage and Havallah sequence. The basal
unit of the overlap assemblage in the southern Shoshone Range is a
conglomeratic unit here assigned to the Middle Pennsylvanian
Battle Formation. These strata overlie the Devonian Slaven Chert
and older Paleozoic units in a profound angular unconformity and
consist of interlayered marine sediment-gravity-flow units composed
dominantly of channelized conglomerate and thin-bedded siltstones.
A calcareous sandstone and siltstone (CSS) unit overlies the
Battle Formation on a sharp contact and consists of shelfal clastic
strata with minor limestone that yields late Middle and Late
Pennsylvanian conodonts as well as older redeposited conodonts.
The CSS unit grades upward into a siltstone and argillite (SA) unit,
which consists of thin-bedded outer shelf and slope deposits that
yield Permian (Leonardian to Guadalupian) radiolarians. Although
having some similarities to the Antler sequence of the overlap
assemblage in the Battle Mountain area, we designate the southern
Shoshone Range succession as the Cedars sequence because of the
marine depositional environment for the Battle Formation, the
absence of a limestone unit comparable to the Pennsylvanian and
Permian Antler Peak Limestone, and the shaly character of the upper
two units of the sequence. Rocks of a fourth upper Paleozoic unit, a
chert and argillite (CA) unit, consist of lenticular packages of
siliceous argillite, black chert, and argillaceous red, green, and
brown chert that yield Mississippian, Pennsylvanian, and Early
Permian radiolarians. The CA unit displays intraformational olderon-
younger relations indicative of contractional deformation not
seen in the underlying Cedars sequence. Although the substrate for
the CA unit is not exposed, we conclude that the CA unit is
allochthonous and composes part of the Havallah sequence of the
Golconda allochthon, which was thrust eastward on the Golconda
thrust onto the overlap assemblage in the latest Permian and (or) earliest
Triassic.
Although belonging to two lithotectonic units, the overlap
assemblage and Havallah sequence (Golconda allochthon), the CSS
and SA units of the Cedars sequence both consist dominantly of
fine-grained clastic strata that are sufficiently similar to classic
lithologies of the Havallah sequence that they were both included in
that unit by earlier workers. Our paleogeographic reconstruction of
late Paleozoic strata restores the Cedars sequence to a position in a
subsiding fan delta transitional to the basin in which deposits of the
Havallah sequence accumulated and outboard of the more proximal
depositional environments present in the Antler sequence. On the
basis of these relations, we suggest that overlap assemblage strata
may be present locally in fine-grained strata mapped as Havallah
sequence on regional geologic maps and that the basal thrust of the
Golconda allochthon, the Golconda thrust, may lie at a position
within, instead of at the base of, the Havallah sequence in these
areas. The tectonic position of such strata has significance for estimates
of the amount of displacement for the Golconda allochthon
and for models for mineral exploration in north-central Nevada.