Frasnian (Upper Devonian) integrated facies analysis, magnetic susceptibility and sea-level fluctuations in the NW Algerian Sahara

Abstract

Changes in the palaeoenvironment are investigated in two representative Frasnian sections of the NW Algerian Sahara, integrating sedimentology and magnetic susceptibility (MS). The Ben Zireg section is characterized by condensed and ferruginous calcareous deposits; in the South Marhouma section the sedimentation rate is high, dominated by muddy nodular limestones with several hypoxic shale intervals. In both sections, sediments were mostly emplaced on pelagic outer ramps below the limit of storm wave-base, evolving through time from proximal to distal setting. Investigations of the temporal evolution of facies and MS data permit a first estimate of the local sea-level trends in NW Algeria. These trends match the overall long-term rise of sea level recognized worldwide from Frasnian Zone 5 upwards. Noteable positive excursions of the sea-level curve related to the semichatovae transgression, as well as to the late Frasnian transgression prior to the late Kellwasser event, can be established in this area. Although the sharp regression of sea level at the upper Kellwasser level can be confirmed from our data, no particular trend is depicted at the transition of conodont zones (Frasnian Zones 12–13) where the presence of the lower Kellwasser level has not yet been clearly recognized.