st 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,
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.
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.