Trace fossils and depositional environments in the Hawaz Formation, Middle Ordovician, western Libya

The ichnology of the Middle Ordovician Hawaz Formation in the Gargaf uplift (W Libya) is here reported. Eleven ichnogenera have been identified in this shallow marine formation: Arthrophycus, Bergaueria, Cruziana, Daedalus, Lockeia, cf. Psammichnites, Planolites, Rusophycus, Skolithos, Teichichnus and Thalassinoides. Their distribution is clearly linked with lithofacies and depositional paleoenvironments. Nearshore to shoreface sandstone facies are characterized by dense, piperock occurrences of Skolithos, which is a characteristic association in high-energy, sediment-shifting, shallow marine settings in the lower Paleozoic. This Skolithos ichnofacies is related to regressive sand belts prograding during high-stand sea level conditions. In contrast, heterolithic facies, most abundant in transgressive intervals, are dominated by horizontal burrows and trails, characteristic of the Cruziana ichnofacies. Higher diversity is achieved in those heterolithics found as forming part of the storm-dominated facies association, while similar facies associated to tidal deposits exhibit a less variety and abundance of traces indicating less favorable ecological conditions.

Research highlights
► Eleven ichnogenera are described from the Middle Ordovician Hawaz Formation.
► Trace fossil distribution is facies controlled with paleoenvironmental significance.
► Skolithos piperocks characterize HST nearshore to shoreface sandstone facies.
► TST heterolithics host various Cruziana ichnofacies assemblages.
► Higher diversity and abundance is seen in storm-dominated than in tidal facies.