TRACES IN DARK SAND: GEOPHYSICAL ANALYSIS OF BURIED VERTEBRATE TRACKS

In coarse-grained media, recognition of vertebrate tracks in plan-view and cross-section has been a challenge, especially in dry unconsolidated sands. In many coastal and aeolian environments, distinct lithological anomalies, such as heavy-mineral concentrations (HMCs), form as a density lag during periods of storms and deflation by wind. Prior to their burial by background quartz-rich sand, HMCs serve as tracking surfaces and often accentuate vertebrate and large invertebrate tracks due to their darker color. During post-storm periods or intervals of non-deposition, surficial heavy-mineral concentrations may. In cross-sectional view, even thin HMCs (< 3 mm) are useful in delineating the morphology of biogenic structures. Because many HMCs are enriched in magnetite and paramagnetic minerals, low-field magnetic susceptibility (MS) method was tested to assess its potential in detecting and mapping buried vertebrate (cervid) tracks and smaller structures in the laboratory. With millimeter-scale sensitivity, bulk MS readings decreased markedly from magnetite-enriched near-surface (1-2 mm deep) laminae to quartz-rich track fill, dropping from >400 to <20 (x10^-5 SI), respectively. Both 2D and 3D surveys of 2-10-cm-long structures helped constrain key track parameters, including their location, lateral extent, depth, and morphology. For deeply buried HMC-defined tracks (>2 cm deep), a high-frequency ground-penetrating radar (GPR) revealed buried structures as prominent subsurface reflections, which are the result of sharp dielectric contrast between the enriched and background sands. This study demonstrates the potential of using the GPR electromagnetic pulses in the frequency range of 800-2300 MHz for detecting and mapping buried biogenic structures. Future field and laboratory experiments will focus on correlating the degree of heavy mineral enrichment with the magnitude of MS response and the amplitude of the GPR signal, in order to improve the detection of buried biogenic structures.