New insights from the Choteč Event (early Middle Devonian) in the Marhouma section (Ougarta range, SW Algeria)

The stratigraphic reference section of the Marhouma area (the so-called km 30 section) in the Ougarta Range (North Western Algerian Sahara) provides valuable insights for the Emsian/Eifelian boundary (Lower/Middle Devonian) around the main lithological facies change expressed between the Teferguenite Chefar el Ahmar Formation. This paper presents new data allowing to better constrain and locate regionally the previously established boundary and to locate the Choteč Bioevent. This revision places the boundary below the new record of Icriodus introlevatus Bultynck, 1970 in Bed 3. The ammonoid assemblages place this boundary either at the base of level 25 sensu Göddertz (1987), without excluding placing it a position several meters lower, above the “niveau coralligène of Le Maître (1952)”. The “Choteč” Bioevent is distinguished within a likely polyphase, stepwise local development: i) a facies change well-expressed by a shift from benthic bioclastic packstones to pelagic wackestones (styliolinites); ii) an increasing deepening (thickest marly interval above level 25, Bed 2); iii) the Icriodus introlevatus wackestone Bed 3; iv) a bloom of goniatites (including Pinacites jugleri (Roemer, 1843) and P. eminens Chlupáč and Turek, 1977) in Bed 4, and finally v) a bloom of micromorphic brachiopods in Bed 5, which reflects monospecific mass reproduction due to increased specific trophic levels. This scenario happened in an alternation of marl-limestone layers with a Zoophycus-Chondrites ichnofossil assemblage, interspersed by greenish claystones and calcareous nodule rich in dacryoconarids. The platy shell bed (Bed 5) represented by micromorphic brachiopods represents the peak phase of Choteč Event Interval ranging locally from Bed 4 to Bed 5, and corresponding to Göddertz's level 27.