<i>Thalassinoides horizontalis</i> Myrow, 1995 des siliciclastiques marins peu profonds de l’Ordovicien moyen-supérieur d’Iran (Formation de Lashkerak)

Les Thalassinoides Ehrenberg, 1944 sont des structures bioturbationnelles relativement courantes dans les successions marines carbonatées peu profondes du Paléozoïque précoce. Les références à cet ichno­genres dans les formations siliciclastiques du même âge sont beaucoup plus rares. Dans la formation ordovicienne de Lashkerak, qui se développe dans les montagnes de l’Alborz central, en Iran, Thalassinoides est une trace fossile commune dans les environnements du complexe de la surface littorale dominée par les vagues et de la barre de l’embouchure d’un delta dominé par les cours d’eau. Nous comparons les réseaux de ramification de l’Ordovicien moyen à supérieur de l’unité 2 de la formation de Lashkerak avec l’ichnospèce Thalassinoides horizontalis Myrow, 1995, en soulignant l’orientation presque entièrement parallèle au litage, la ramification régulière et l’absence de constrictions et de renflements. Les halos éodiagénétiques développés à partir de parois tapissées de mucus, ou par changement du tissu sédimentaire d’origine, typiques de cet ichnotaxon et d’autres ichnotaxons de Thalassinoides dans des environnements carbonatés, ne sont pas trouvés dans les grès. Les labyrinthes presque polygonaux de la formation de Lashkerak sont également comparés au Protopaleodictyon aitkeni Morgan, Henderson &amp; Pratt (2019) récemment érigé, considéré comme un graphoglyptide géant à un stade précoce de l’évolution de ces formes dans les environnements marins peu profonds. Les deux traces fossiles sont similaires en termes de morphologie, de taille, de préservation, d’ichnofaciès et de fonction interprétée, ce qui fait de P. aitkeni un synonyme junior de Thalassinoides horizontalis.Thalassinoides Ehrenberg, 1944 are relatively common bioturbational structures in carbonate shallow marine successions from the early Paleozoic. Much rarer is the reference to this ichnogenus in siliciclastic formations from the same age. In the Ordovician Lashkerak Formation cropping out at the Central Alborz mountains, Iran, Thalassinoides is a common trace fossil in wave-dominated shoreface complex and prodelta-mouth bar environments of a fluvial-dominated delta. We compare the Middle-to-Upper Ordovician branching networks of the Unit 2 of the Lashkerak Formation with the ichnospecies Thalassinoides horizontalis Myrow, 1995 emphasizing the almost entire bedding-parallel orientation, regular branching and lack of constrictions and swellings. The eodiagenetic halos developed from mucus-lining walls, or by change of the original sediment fabric, typical of this and other ichnospecies of Thalassinoides in carbonate settings are not found in sandstones. The almost polygonal mazes from the Lashkerak Formation are also compared with the recently erected Protopaleodictyon aitkeni Morgan, Henderson &amp; Pratt (2019), considered as a giant graphoglyptid in an early evolutionary stage of these forms in shallow marine environments. Both trace fossils are similar in morphology, size, preservation, ichnofacies and interpreted function, thus being P. aitkeni a junior synonym of Thalassinoides horizontalis.</p

Ichnofabric analysis of shallow to deep marine Carboniferous sediments, from the southern Paleotethys margin, Alborz Basin (northern Iran): approaching autogenic and allogenic environmental controls

This study integrates ichnological and sedimentological data to interpret depositional environments in the carbonate sediments of the Mobarak Fm. (Lower Carboniferous) in the Alborz Mountains of northern Iran. Seven ichnofabrics including Thalassinoides (Th), Zoophycos‒Phycosiphon (Zo-Ph), Skolithos‒Thalassinoides (Sk-Th), Zoophycos‒Diplocraterion (Zo-Di), Thalassinoides‒Rhizocorallium (Th-Rh), Zoophycos‒Arenicolites (Zo-Ar) and Chondrites‒Phycosiphon (Ch-Ph) have been identified in the studied succession. Th ichnofabric is characterised by the dominance of the deposit-feeding trace fossils, revealing low sedimentation rate and activity of the organisms in a loose substrate. Vertical succession of the Zo-Ph, Sk-Th and Zo-Di ichnofabrics indicates change in the firmness of the substrate. Th-Rh and Zo-Ar ichnofabrics reveal two different energy phases including a lower energy condition with the activity of deposit-feeders and a higher energy phase identified by the activity of suspension-feeders. Ch-Ph ichnofabric is characterised by low diversity and low abundance of trace fossils that were identified in black, limy shale, displaying a long lasting outer ramp setting with low energy, low sedimentation rate, and dysoxic-anoxic conditions of the substrate. Ichnological features of the Mobarak Fm. are controlled by subsidence-inducing changes in the accommodation space of the depositional system dictating variations in paleoenvironmental conditions such as oxygen levels and hydrodynamic energy.Fil: Sharafi, Mahmoud. University Of Hormozgan; IránFil: Rodriguez-Tovar, Francisco J.. Universidad de Granada, Facultad de Ciencias; EspañaFil: Bayet-Goll, Aram. Institute For Advanced Studies In Basic Sciences, Zanjan; IránFil: Richiano, Sebastián Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto Patagónico de Geología y Paleontología; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; Argentin

Depositional Controls on the Ichnology of Ordovician Wave-dominated Marine Facies: New Evidence from the Shirgesht Formation, Central Iran

The Lower Ordovician Shirgesht Formation in central Iran is composed of siliciclastic and carbonate rocks deposited in diverse coastal and marine shelfal environments (tidal flat, lagoon, shoreface, offshore-shelf and carbonate ramp). Five facies associations contain diverse ichnofossil assemblages that show distinct proximal to distal trends formed in a wide range of physical-chemical conditions. The ethological groups of trace fossils in the Shirgesht Formation reflect a gradient of depositional stress conditions across a wave-influenced shoreline and shelf. Deposits of wave-influenced environments make up a significant component of the geological record of shallow marine settings, and the ability to determine paleoenvironments in detail in such successions is critical for reconstruction of depositional histories and sequence-stratigraphic interpretation. The Cruziana ichnofacies of the study shows highly diverse suites that record the establishment of a benthic community under stable conditions and a long-term colonization window. The Skolithos ichnofacies recognized is a low diversity opportunistic ichnocommunity suite that resulted from colonization after tempestite deposition in a stressed environment. The strata record an onshore to offshore replacement of the Cruziana ichnofacies (with abundant feeding traces of deposit-feeders) by the Skolithos ichnofacies (dominated by suspension-feeders and predators). A transitional zone between the two ichnofacies coincides with the offshore-transition/distal lower-surface deposits. The distribution of ichnofacies, the diversity and range of ethological characteristics reflected by the ichnogenera, and the wide range of wave-dominated coastal facies demonstrate the potential to use individual trace fossils and ichnofacies for significantly refined palaeoenvironmental analysis of wave-dominated coastal settings, particularly in Ordovician successions.Fil: Bayet Goll, Aram. Institute for Advanced Studies in Basic Sciences. Department of Earth Sciences; Irán. Ferdowsi University of Mashhad; IránFil: Myrow, Paul M.. Colorado College. Department of Geology; Estados UnidosFil: Aceñolaza, Guillermo Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; ArgentinaFil: Moussavi, Harami, Rez. Ferdowsi University of Mashhad; IránFil: Mahboubi, Asadollah. Ferdowsi University of Mashhad; Irá