ARROW CANYON
Southern Nevada’s Arrow
Canyon is developed on the eastern side of the Arrow Canyon Range, which is
northeast of Las Vegas. The canyon has a nice section of Mississippian to
Pennsylvanian to Permian-aged sedimentary rocks. The Mississippian-Pennsylvanian
succession here includes an M-P boundary section representing continuous
deposition. In fact, the official Mississippian-Pennsylvanian boundary
(M-P GSSP) for the entire world is defined at this locality.
The section has long been
studied & is well documented in the geologic literature. It has been
studied intensively by Illinois geologists since the 1950s. It has also
been used as a key reference section by Amoco.
Above: Looking NE. Mouth of
the Narrows portion of Arrow Canyon. The Narrows portion of the canyon
extends toward the left. The tall cliff on the right side of the photo
contains the Mississippian-Pennsylvanian GSSP (global stratotype section and
point).
Stratigraphy of part of Arrow Canyon:
_____________________________
¬Penn.-Perm. boundary
Bird Spring Formation
______________________________ ¬Miss.-Penn. boundary
Indian Springs Formation
______________________________
Battleship Wash Formation
______________________________
Yellowpine Limestone
______________________________
Bullion Limestone
______________________________
Anchor Limestone
______________________________
NARROWS OF
ARROW CANYON
Arrow Canyon’s Narrows are
spectacularly deep & narrow with vertical to subvertical bedrock
walls. The section in the Narrows is a Mississippian-aged carbonate
platform succession.
Above: Looking E. This is the
eastern end of Arrow Canyon’s Narrows.
Above: Looking W. This is
near the eastern end of the Narrows.
Above & below: Looking ~W.
Eastward-dipping beds of the Bullion Limestone exposed in Arrow Canyon’s
Narrows. The Bullion Limestone is part of the upper Monte Cristo Group
(middle to upper Osagean Stage, upper Lower Mississippian).
The photo above shows a
well-defined disconformity in a tilted succession of Mississippian platform
carbonates. This is along the northern side of Arrow Canyon's
Narrows. The lower, light gray unit is the top of the Bullion Limestone
and the upper, brownish-weathering unit is the basal Yellowpine Limestone
(both upper Monte Cristo Group). The Bullion Ls. here is the
uppermost-preserved Osagean Stage (uppermost Lower Mississippian). The
Yellowpine here is the basalmost-preserved Meramecian Stage (basal Middle
Mississippian).
The contact interval is
actually a thin carbonate (condensed zone?) with reddish iron oxidation.
This interval thickens westward into a mappable unit called the Arrowhead
Member.
The Anchor Limestone
is the unit underlying the Bullion Limestone in the Narrows. The Anchor
is part of the lower Osagean Stage (upper Lower Mississippian). The pic
above shows a nice, thin cross-bedded limestone interval in the Anchor
Ls. The photo below shows a fault breccia developed in the Anchor.
MOUTH OF THE
NARROWS,
ARROW CANYON
The eastern end of the Arrow
Canyon’s Narrows has Middle & Upper Mississippian rocks plus some Lower
Pennsylvanian rocks.
Above & below: Looking N, with beds
dipping eastward. Yellowpine Limestone-Battleship Wash Formation
contact. The Yellowpine Limestone is the lighter-gray colored
limestones in the lower half of the photo. Above is the Battleship
Wash Formation, with slightly darker-gray colored limestones.
The upper Yellowpine Ls. and
the Battleship Wash Fm. here are both latest Meramecian in age (latest Middle
Mississippian).
Disconformably overlying the
Battleship Wash Formation is a relatively thin, slope-forming unit called the
Indian Springs Formation (see pic above). The Indian Springs is
Chesterian (Upper Mississippian) & consists of reddish shales with thick
cherty limestone beds (talus blocks in photo above).
The base of the Indian
Springs Formation has some nice Stigmaria roots (see pics below) in the
top surface of the underlying Battleship Wash. This Stigmaria
surface is only present on the southern side of Arrow Canyon.
MISSISSIPPIAN-PENNSYLVANIAN
GSSP CLIFF
As of 1999, the
Mississippian-Pennsylvanian boundary is internationally defined at Arrow
Canyon. The M-P boundary occurs at 7.63 meters above the base of the Bird
Spring Formation, as exposed in the cliff depicted below. The Indian
Springs-Bird Spring contact occurs at a lithology change from reddish shales
below to gray limestones above. The limestones of the Bird Spring are
moderately fossiliferous. The M-P GSSP is defined at the evolutionary
first appearance of the phosphatic microfossil Declinognathodus noduliferus
(Animalia, Chordata, Conodonta, Conodonti, Ozarkodinida, Idiognathodontidae).
Above: James St. John (kneeling)
pointing to the official Mississippian-Pennsylvanian GSSP, accompanied by
geologists Matt Saltzman (standing in foreground) & Mats Eriksson, Katie
Tierney, Amanda Cavin, and Seth Young (background).
EAST
OF THE NARROWS
Above: Looking ~E. The
lower Bird Spring Formation (Morrowan Stage, Lower Pennsylvanian), as exposed
in Arrow Canyon, a little east of the Narrows. The sometimes-cherty
limestone beds here are dipping eastward.
Above: Looking ~W. These
are eastward-dipping limestones of the Bird Spring Formation, which is still
Pennsylvanian in age here.
Above: Looking ~WNW. Upper
Pennsylvanian limestone beds of the Bird Spring Formation, dipping eastward.
Above: Looking ~N.
Fossiliferous limestones of the upper Bird Spring Formation. The
Pennsylvanian-Permian boundary occurs about midway through the section shown in
this photo. The most obvious fossils in the limestones here are fusulinid
foraminifera.
Above: Looking ~E.
Eastward-dipping limestones of the Bird Spring Formation, which is Lower
Permian here.