Ichnology of the Aptian Louiza Marls Formation and the palaeoenvironmental significance of omission surfaces in the Ouled-Naïl Mountains, Algeria

The upper Aptian Louiza Marls Formation, exposed at its type locality in the Ouled-Naïl Mountains, Algeria, preserves an exceptional and previously undocumented ichnoassociation. The ichnological record reveals a moderately diverse ichnoassociation comprising sixteen ichnotaxa representing softground, firmground, and hardground colonisation suites. Integrated sedimentological and ichnological analyses indicate deposition across a spectrum of environments, ranging from a middle-ramp open marine setting to inner-ramp semi-restricted lagoonal and peritidal (subtidal to tidal-flat) environments. Each sub-environment is characterised by distinct ichnoassociations reflecting specific ecological conditions and substrate properties. The lower part of the succession, interpreted as a middle-ramp open marine setting, is dominated by softground ichnotaxa attributable to the Cruziana ichnofacies. In contrast, higher stratigraphic intervals are characterised by substrate-controlled ichnofacies, notably the Glossifungites and Trypanites ichnofacies, which record the development of omission (discontinuity) surfaces within the carbonate succession. These omission surfaces occur at several stratigraphic levels and are predominantly expressed by firmground burrows of the Glossifungites ichnofacies, especially Balanoglossites triadicus and well-developed Balanoglossites ichnofabric, locally associated with Thalassinoides and subordinate ichnotaxa. A distinct omission surface, identified in the lower part of the succession, is represented by hardground colonisation and bioerosion structures of the Trypanites ichnofacies, including Rogerella isp. and Gastrochaenolites aff. ornatus. This surface overlies several metres of Cruziana ichnofacies deposits and marks a significant shift in substrate conditions and colonisation dynamics. The associated borings indicate colonisation of fully lithified substrates, with Gastrochaenolites suggesting very shallow-water conditions compared to co-occurring ichnotaxa. Overall, the succession records a clear shallowing-upward trend, reflecting a regressive phase driven by combined eustatic and tectonic controls, possibly enhanced by progradational fluvial input. This evolution promoted the recurrent development of omission surfaces linked to relative sea-level fall and subsequent transgressive reworking.