Depositional environment and Ichnology (Nereites ichnofacies) of the Late Devonian Sahara region (SW Algeria)

摘要

The late Famennian strata of the Marhouma outcrop, located in the Saoura (Algerian Sahara), consist of two formations: the Argiles de Marhouma Formation and the overlying GrSs de Marhouma Formation. The former is characterized primarily by fine-grained turbidite successions, intersperse with griotte limestones bearing a pelagic fauna. There is a wide variety of trace fossils in the lower part of the formation: Chondrites, Helminthopsis, Ichnyspica, Megagrapton, Neonereites, Nereites irregularis, Nereites jacksoni, Nereites missouriensis, Palaeophycus, Paleodictyon, Phycodes, Planolites beverleyensis, Protopaleodictyon submontanum, and Thalassinoides suevicus. The hydrodynamics of the depositional environment as well as the associated trace fossils indicate the Nereites ichnofacies. The depositional conditions indicate a basinal environment in a mesodistal area. The upper coarser formation is composed of two members. The lower member consists of alternating turbidites, green clay and thin sandstone, with slumps, debrites, olistoliths, and synsedimentary faults. There are some visible fossil traces that may point to the Zoophycos ichnofacies. It marks the slope deposits of a submarine deep-sea fan that completes the Devonian megasequence. The upper member of the second formation starts with a transgressive muddy deltaic system that formed during platform construction. It is followed by iterative sequences of muddy and sandstones layers deposited on a storm-dominated platform that emerged at the beginning of early Tournaisian time. The distributions of fossil traces and sediments are indicative of bathymetric fluctuations. A major fluctuation, probably due to tectonic instability, is recorded as early as in the basal part of the Argiles de Marhouma Formation, which indicates the beginning of the Eovariscan phase.