Gastrochaenolites ornatus Kelly and Bromley and other trace fossils on a Middle Miocene omission surface from the Aures Massif, Algeria

Abstract

Trace fossils provide detailed palaeoenvironmental and palaeoecological information of both ancient and modern sedimentary systems. During middle Miocene times the Aures Massif located in the northeastern part of Algeria, was affected by, at least, one marine transgression. The latter led to the installation of a carbonate platform, which is placed, for the first time, in a Mediterranean context. In the Rhassira basin, the Middle Miocene marine succession is characterised by carbonate platform deposits dominated by rhodolith beds, typical of those known throughout the Mediterranean area. This succession can be divided into many units separated by discontinuities interpreted here as omission surfaces. The Djebel Arhane section shows two omission surfaces characterised by a pre-omission suite (firmground) represented by Balanoglossites burrows for the first surface and Gastrochaenolites ornatus burrows/borings for the second one, and an omission suite (hardground) as evidenced by the bioerosive structures Trypanites and Caulostrepsis, in both surfaces, respectively. Gastrochaenolites ornatus traces were formed and preserved in firm, compact, semi-lithified and fine-grained substrates (firm- to hardground), indicating the Glossifungites ichnofacies. They show bioglyphs which have been formed during contraction of the posterior adductor muscles. These suggest that their tracemakers were represented by suspension-feeding bivalves, most probably Pholadidae or Mytilidae, which rotated during penetration. The fill of these traces is composed of marine deposits related to a transgressive lag. The omission suite is divided into two ichnocoenoses: (i) pre-lithification burrows/borings, and (ii) post-lithification borings. This is the first report of the ichnotaxon G. ornatus from Algeria.