LITHOLOGICAL ANOMALIES IN SANDS: IMPLICATIONS FOR VERTEBRATE ICHNOLOGY

Despite the dynamic nature and coarse texture of sandy substrates, many studies report abundant vertebrate and invertebrate tracks, from Paleozoic sandstones to Holocene depositional environments. Track preservation in sand typically requires moisture or clay laminae, but in mid-high latitudes freezing before burial may also increase substrate strength and resistance to erosion. Recognition of tracks in plan-view and cross-section remains a challenge, especially in unconsolidated sands. In many coastal and aeolian settings, lithological anomalies, such as heavy-mineral concentrations (HMCs), represent density lag formation during episodes of increased wave or wind activity. HMCs not only act as environmental indicators, but also accentuate vertebrate and large invertebrate tracks due to their darker color. Using examples from the U.S. East Coast and Eastern Europe (Baltic Sea coast), this study examines the occurrence of avian and mammalian footprints in beach and aeolian HMCs. During post-storm periods or intervals of non-deposition, surficial heavy-mineral concentrations may serve as distinct tracking surfaces. In cross-section, even thin HMCs (1-3 mm) accentuate the shapes of traces and associated structures, as exemplified by a possible cervid track preserved in 1,100-year-old dune slipfaces at the Baltic Sea site. Since many HMCs are enriched in magnetite, low-field magnetic susceptibility (MS) can be used for in situ analysis of track and undertrack laminae. With sub-centimeter sensitivity, a decrease in MS from magnetite-enriched surface laminae (>50x10^-5 SI) to quartz-rich track fill (<5) helps constrain track parameters. Where HMC thickness is comparable to the resolution of a ground-penetrating radar (GPR) signal, they produce sharp subsurface reflections. This method is useful for detecting deformation features ranging from 20 cm to less than 2 cm using antenna frequencies between 400 MHz and 2.3 GHz, respectively. Some irregular laminations in shallow subsurface images may potentially represent trampled surfaces and caution must be taken in interpreting them as primary sedimentary structures. Future field and experimental research must combine MS and GPR imaging of vertebrate tracks to better understand how lithological anomalies reflect the final track architecture.