Ichnology of the Cenomanian–Turonian boundary event in the southern Tethyan margin (Khanguet Grouz section, Ouled Nail Range, Algeria)

• Published in: Arabian journal of geosciences 2022, Vol.15 (12), Article 1150
• Main Authors: Naimi, Mohammed Nadir, Cherif, Amine, Mahboubi, Chikh Younes, Benyoucef, Madani
• Format: Article
• Language: English
• Subjects: Anoxia; Benthos; Bioturbation; Bivalvia; Chondrites; Earth and Environmental Science; Earth science; Earth Sciences; Ecological distribution; Ecological niches; Ethology; Fossils; Marine molluscs; Mollusks; Niches; Original Paper; Oxic conditions; Oxygenation; Pore water; Shales; Shellfish; Shells; Substrates; Trace fossils
• ISSN: 1866-7511; 1866-7538
• DOI: 10.1007/s12517-022-10420-y

The Cenomanian–Turonian boundary event (C/TBE) is studied in the southern Tethyan margin from an ichnological point of view for the first time. The ichnotaxa Chondrites isp., Pilichnus isp., Planolites isp., Ptychoplasma isp., ?Thalassinoides isp. and ?Trichichnus isp. are reported from the Cenomanian–Turonian succession of the Khanguet Grouz section (Ouled Nail Range, Algerian Saharan Atlas), subdivided into five informal units. The pre-event deposits are characterised by high abundance of burrows attributed to Planolites and Ptychoplasma , associated with bivalve and gastropod shells, which indicate that the ecological niche was occupied by different organisms with variable ethology. The beginning of the C/TBE is marked by an important change bioturbation intensity as well as ichnodiversity as revealed by the decrease of trace fossils, by either their generalised scarcity or their total absence. The recorded traces from the C/TBE deposits are Chondrites , Pilichnus , Planolites , ?Thalassinoides and ?Trichichnus , indicating stressful conditions, which is supported also by the joint presence of dwarfed forms of bivalves. The C/TBE black shales are laminated and largely unburrowed to sparsely burrowed, suggesting episodes of improved palaeoenvironmental conditions during which Chondrites , Pilichnus and ?Trichichnus producers could colonise the substrate in unfavourable, dysoxic to anoxic benthic conditions. However, these black shales are intercalated by different facies characterising oxic conditions, as indicated by the presence of fauna (i.e. inoceramid bivalves). The post-event (or recovery) phase is characterised by high bioturbation intensity, with abundant Planolites and other undetermined burrows. As evidenced in the northern Tethyan margin, the important decrease in ichnoabundance and ichnodiversity is related to a significant decrease in the oxygenation of pore water. Moreover, the ‘Oceanic Anoxic Event’ (‘OAE-2’)-ichnoassemblage of the Khanguet Grouz section is similar to those found in the Rio Fardes section (Betic Cordillera, Spain), in comparison with the other Tethyan sections.