Setulfs, modern and ancient: Formative mechanism, preservation bias and palaeoenvironmental implications

Setulfs or inverted flutes, both in growth stage and in fossilized state, are addressed here. This account resolves the long-standing questions about their formative mechanism, depositional prerequisites, and preservation bias, as well as their palaeoenvironmental implications. These structures were observed to grow by accretion of wind deflated sand in the lee of miniscule obstructions, in the littoral-supralittoral transition zone, a little above the high-water mark on top of a partially-emerged bar at Chandipur, eastern India. Pseudopellets of crabs and scarce chips of microbial mat create the obstructions, as shells might also do in the case of other modern analogs reported from essentially similar environmental conditions. Moisture on the beach sand limited the lifting power of the wind, so that even miniscule obstructions could have a well-developed wake zone at their lee side, resulting in localized accumulation of a small ridge of sand extending downwind from the obstruction. Setulfs arrested under microbial mats were also found in Chandipur on the seaward fringe of a shallow marsh encircled by a spit. Like those in growth stage these mat-covered setulfs also had their tapering ends directed landward, as the wind was directed onshore during the period of observation. Fossilized setulfs in swarms are also described here from two Neoproterozoic formations in India, viz., the Sonia Sandstone and the Upper Bhander Sandstone. In both the cases, setulfs are preserved in well-sorted sandstones, and are associated with planar laminae, profound wave ripples, bifurcated parting lineations, infrequent rill marks and local swarms of current crescents on bedsurfaces. Additionally, recorded fossil setulfs are found in association with inversely graded translatent strata, crinkled adhesion laminae, patches of impact ripples and isolated small sets of alternate grain-flow and grain-fall cross-strata of aeolian origin. This suggests preservation in the high littoral-supralittoral zone as in case of all known modern setulf occurrences. The palaeowind direction derived from the ancient setulfs conforms in orientation to that recorded from other aeolian structures in the strata, supporting their wind deflation origin. Although shells and biogenic pellets were altogether absent from these Neoproterozoic rocks, microbial mat curls and fragments had presumably been abundantly present in the Proterozoic high littoral-supralittoral zone to facilitate formation of setulfs. Pertinently many microbial mat-related features are present within both the siliciclastic formations studied. In polished sections some of the studied Proterozoic setulfs reveal the existence of crinkled carbonaceous lamina covers that signify microbial mat growth on top of them. An a priori assumption is that microbial mats must have enhanced the preservation potential of these delicate structures, which would otherwise be extremely unlikely to be preserved. The seeming preference of fossil setulf occurrences from the Proterozoic-Early Palaeozoic time is most likely due to a lack of burrowers and grazers that would devastate the protective microbial mat later in the Phanerozoic. These observations suggest that swarms of setulfs are, in general, wind sculpted, and their presence in the rock record prefers palaeoenvironment interpretration as a high littoral-supralittoral transition zone, and also stands as proxy records of syn-depositional microbial mat growth. (C) 2011 Elsevier B.V. All rights reserved.