Did evaporite cements and infiltrated silts assist preservation of reptile tracks in Permian desert sediments?

Many Permian desert tracks are found in formations dominated by rather homogeneous aeolian quartz arenites. This raises questions around how they got preserved. Here we test the hypothesis that strong palaeoenvironmental controls affect style and quality of footprint preservation in Permo-Triassic desert settings. To answer this, several examples of tracks and trackways from Moray, Scotland, are described in the context of their host sedimentary successions. We then discuss petrographic clues in the specific track-bearing layers with regard to taphonomy. Two key sections were logged and sampled at Hopeman Beach: (i) Hopeman Coastal Section A, being a site from which tracks have previously been recovered; and (ii) Hopeman Coastal Section B, a section still exhibiting several in-situ tracks. Tracks were also examined on the surfaces of metre-scale quarried blocks within Clashach Quarry. Logging was also undertaken at quarries in Quarrelwood near Elgin. Collected samples were examined optically and with a scanning electron microscope. Hopeman Coastal Section A exhibits convolute bedding best interpreted as dewatering structures; a pustular bed that could be linked to growth of evaporite crystals impinging on a sediment-binding microbial mat; adhesion ripples formed by dry, wind-blown sand sticking to a wet or damp surface; and laterally continuous pebble layers that are the result of ephemeral sheet floods. The oscillation-rippled layer from which NMS footprint specimen G.1997.60.1 was extracted exhibits a halite cement and petrographic evidence for re-worked halite, and these rippled sediments were most likely deposited in an interdunal lake. Hopeman Coastal Section B similarly exhibits metre-scale planar cross beds and occasional coarser-grained lag deposits that are consistent with aeolian dunes that were episodically inundated by sheet floods. Samples containing halite and lesser amounts of gypsum or anhydrite were collected from the same layer as the in-situ Hopeman Coastal Section B tracks. Metre-scale planar cross-bedded quartz arenites of Cutties Hillock quarry were clearly deposited in an aeolian dune setting. Some sands in the Cutties Hillock Sandstone with scoured bases were aeolian sediments that were reworked by fluvial processes. We conclude that this study demonstrates three different modes of track preservation in the Permian Moray area: (i) indentation of near-surface layers constituted by particles of fine silt that in many cases had infiltrated between sand grains of aeolian dunes; (ii) trackways in sediments deposited around the margins of lakes in the interdunes, with early cementation by evaporites, noting that in the studied cases the halite cement might have helped preservation of the tracks in the sense of long-term fossilisation, but probably not anatomical preservation (i.e. quality of fidelity); and (iii) indentation of clays that had been deposited in some interdunal lakes.