Alat's mysterious "donuts"

A3 - Distinctly-recognizable composite key horizons consisting of 2 to 3 Calcareous beds, which are 5 to 20 cm thick, intensely bioturbated and alternating with thin silts and clays (Fig. 10 a, e, f). These calcareous beds exhibit peculiar ring-like structures that are preserved as concave circular depressions on the surface of the bedding plane (Fig. 10 b). These impressions are casts of an oolite-rich sand that can be massive or faintly laminated, and subordinately contain Cypria ostracods and fish spines. Their mineral composition is mainly calcitic or dolomitic with secondary barite and phyllosilicates.
On overturned beds these casts look like "donuts" and this term is used to designate this basin-wide key horizon. Sometimes donuts overlap (Fig. 10 c, d). We estimate average density of 50 donuts per 100sq meters to greater occurrence as in the outcrop of fig 10 f.
To our knowledge they cannot be equated to any ring-like structures described in the literature and their ambiguous features make it problematic to relate them either to just biological or physical processes. The donuts of the Alat Formations will be the subject of a paper in progress.
Preliminary hypothesis on their origin take into account their similarities with nests dug by some fish species in the mud flats along the margins of the East Africa rift valley lakes (Fleibel 1987). In the same environment flamingos build nests as small circular mounds.
Donuts may be also compared to casts of corms and bulbs of plants living in the marshes or swamps. Corn rootlets may be matched with some finger-like casts that are found associated with the donuts.
Affinities with ring structures produced by impact or gas seepages (Aiello et al. 2001) are less convincing.