Ichnology, paleosols and palimpsests surfaces in the Tinat Member of the Nuayyim Formation (Unayzah Group), subsurface Saudi Arabia: Trace fossils and genetic stratigraphy in continental settings

The Nuayyim Formation of the Unayzah Group is widespread in subsurface central Saudi Arabia and constitutes a laterally variable stratigraphic unit with a complex mosaic of facies. Sedimentation for the Tinat Member took place majorly in a continental setting dominated by aeolian processes that resulted in dunes, interdunes, ephemeral stream deposits terminating with regional development of paleosols. Facies, ichnofacies and palynological analysis, suggest an arid to semi-arid setting with biogenic activity in the form of continental fauna and plants in deposits associated with pedogenetical alteration in paleosol horizons. Therein, burrowing exhibits elements of the Scoyenia Ichnofacies in beds with sparse to abundant bioturbation. Tectonic uplift and subaerial exposure resulted in the exhumation of previously buried substrates, leading to the formation of burrowed palimpsests surfaces associated with paleosols at the end of Unayzah deposition prior to the onset of the Khuff Formation. Progressive thermal updoming is interpreted to have occurred during the latest portion of the Unayzah Group sedimentation cycle in the Arabian Plate culminating in the regional “Pre-Khuff unconformity” at the top of this megacycle recognized in this study by biogenically-demarcated diastems. Ichnological and sedimentological observations outlined in this study are significant because they: (i) refine existing interpretations of the paleoenvironmental history in the study area; (ii) document diastems marking breaks in sedimentation; and (iii) outline several soil-forming cycles that allowed the establishment of a trace fossil suite with continental affinities, thus demarcating the end of an entire sedimentation cycle. The recognition and delineation of burrowed horizons associated with paleosols provides a tool to understand the tectono-stratigraphic evolution of similar successions in the rock record.