USING ELECTRICAL RESISTIVITY SURVEYS TO DETERMINE THE STRATIGRAPHIC POSITION OF THE UPPER TRIASSIC PLACERIAS QUARRY, EAST-CENTRAL ARIZONA

Since the 1920s, thousands of Upper Triassic bones from an array of fossil animals have been excavated from the Placerias quarry ~10 km south of St. Johns. Due to the region's low relief and lack of exposure, the stratigraphic position of this fossil-rich, mudstone-dominated interval is unclear. Competing hypotheses based on the exposed facies are that it is either relatively high, in the Blue Mesa Member of the Petrified Forest Formation, or much (50+ m) lower, near the base of the Bluewater Creek Formation. Determining the actual stratigraphic position requires knowing the depth to the coarser-grained sandstones and conglomerates of the underlying Shinarump Formation (or, possibly, the Moenkopi Formation).

The near surface geophysical technique direct current electrical resistivity (ER) can be used to detect changes in lithology based on variations in the subsurface electrical resistivity. Typically, mudrocks have very low resistivities (e.g. ~1-100 Ωm), while sands/gravels have much higher (~10-10,000 Ωm) resistivity values, making ER surveying a logical technique for determining the depth to the Shinarump Fm. To determine the stratigraphic position of the Placerias quarry, we conducted ER surveys at two locations. The first surveys were conducted near US-180 and acted as a control. Because electrical resistivities of geologic units can vary widely, the goal of the control surveys was to obtain the electrical signature of the Shinarump Fm, which is at a known, shallow (<10 m) depth there. The ER values associated with the Shinarump Fm ranged from 10-350 Ωm, with values consistently > 50 Ωm at the depth of Shinarump Fm subcrops. The second set of transects were located at the Placerias quarry. The ER values there were between 5-65 Ωm (and are consistently ≤ 30 Ωm at registered depths), and we found no indication of lithologic change beneath the Placerias quarry up to 50 m depth. The Placerias quarry ER data show uniformly undisturbed, essentially horizontal strata. The low resistivity values obtained from the Placerias quarry surveys suggest a consistent 50 m+ thick mudstone-dominated interval below the quarry . Our results therefore indicate that the higher stratigraphic position, in the Blue Mesa Member, is most likely correct, and may shorten the stratigraphic ranges of taxa known from the Placerias quarry.