The Chefar El Ahmar Formation (late Emsian-Frasnian Ib) in the Marhouma section (Saoura, Algerian Sahara): insights from ichnology and sedimentology

摘要

This contribution reports an ichnological study of the well-exposed strata belonging to the Chefar El Ahmar Formation, located in the Saoura Valley (Ougarta Range, Northwestern Algerian Sahara). This unit consists mainly of irregularly alternating shale and limestone with some intercalated sandstone. The data reflect hemipelagic to pelagic carbonate ramp character, episodically interrupted by internal waves in an extra-tempestite context. The ichnological analysis suggests a low bioturbation intensity. Eight ichnotaxa documented represent the Cruziana ichnofacies: Alcyonidiopsis, ?Balanoglossites, Chondrites, Nereites missouriensis, Neonereites biserialis, Palaeophycus, Thalassinoides and Zoophycos, distributed in an irregularly manner dominated by a Zoophycos-Chondrites assemblage in the lower part (units "a", "b", "c" and bottom part of unit "d"), a Zoophycos-Chondrites-Alcyonidiopsis-?Balanoglossites-Nereites missouriensis-Thalassinoides assemblage in the middle part (unit "d" and lower part of unit "e") and a Chondrites-Alcyonidiopsis-Nereites missouriensis-Neonereites biserialis-Palaeophycus assemblage in the upper part (major part of unit "e" and unit "g"). The distribution of these trace fossils frame the major bioevents represented in this section. They are more frequent above and below the bioevent levels while rare to absent in the crisis beds themselves. The control was evidently provided by palaeoenvironmental conditions (including bathymetry, oxygenation and nutrient supply), as a result of the general deepening of the northern Gondwana carbonate platform from the late Emsian to the early Frasnian, due to the major transgression during the Middle Devonian.