<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

The ichnological importance and interest of the Geological Museum of Lisbon collections: Cladichnus lusitanicum in continental facies from the Lower Cretaceous of the Lusitanian Basin (Portugal)

The revision of the classic collections of trace fossils housed in the Geological Museum of the former Geological Survey of Portugal is bringing new information to well established ichnogenera. Through the revision of the type material of Taenidium lusitanicum Heer, 1881 and other specimens housed in the Geological Museum of Lisbon and the National Museum of Natural History and Science we reinstate the ichnospecies Cladichnus lusitanicum (Heer). This is the only ichnospecies of Cladichnus occurring in environment typical from the Scoyenia ichnofacies in otherwise typical turbidite forms

Understanding along-strike variability in controlling mechanisms of paleoenvironmental conditions and stratigraphic architecture. Ordovician successions in the Alborz Mountains of Iran at the northern Gondwana margin

Changes in stacking patterns of facies, stratal architecture and shoreline trajectory along depositional strike are mainly dependent on along-strike differences in factors such as physical processes (rivers, waves, tides), tectonics, eustatic changes, sediment supply, and local physiography. The lower-upper Ordovician successions (Lashkerak Formation) deposited in a Peri-Gondwana shallow continental shelf have been interpreted using the sequence stratigraphic concepts. This approach allowed to highlight main differences along-strike in the co-existing depositional systems (coastal plains, estuaries, deltas, and shoreline to shelf systems) in the Alborz basin along the northern edge of Gondwana. On the basis of sedimentary and ichnological characteristics, the siliciclastic successions are divided into four facies associations: (i) Wave-dominated estuarine, (ii) Wave-dominated shoreface-offshore complex, (iii) Mixed fluvial (F)- and wave (W)-influenced delta (Fw and Wf), and (iv) Tide-dominated, fluvial-influenced delta (Tf). Six third-order depositional sequences (DS1-DS6) were recognized, each of them are bounded by unconformities and pronounced shifts in the facies. Along depositional strike (NE/SW- a distance of c. 150 km), significant intrabasinal changes in sediment supply produced evident lateral changes in the shoreline trajectory and stratal architecture, of the DS1-DS3 (Tremadocian-Floian), may be attributed to differences in physical processes (rivers, waves, tides activity) and topographic irregularities derive probably by different rates of subsidence in the basin. During the deposition of the DS1-DS3, a significant along-strike differences in the composition of ichnological suites and the lifestyle of benthonic organisms, dependent on the relative supremacy of wave, storm, river, and tidal processes. Sedimentary supply and processes are the main controlling factors for the stratigraphic architecture rather than the tectonic activity between Tremadocian and Floian. While during the Darriwilian-Katian interval (DS4-DS6), the tectonic activity predominates with the beginning of extensional phases related to the rifting in the Alborz basin causing the activation of syn-depositional normal faults and the development of block-faulted craton-ward margin. Typically, differential subsidence due to extensional tectonic activity causes the uplift of tectonic blocks west of the Alborz Basin into the emerging areas, on one hand, and simultaneous deepening-upward stacking patterns (i.e., increased subsidence rates) in the hanging-wall depocenters of east of the Alborz Basin, on the other hand. This outcrop-based study suggests that the identification of genetically related depositional units in a larger paleogeographic context is fundamental for understanding along-strike variability in controlling mechanisms of the spatial distribution and stratigraphic architecture of coexisting and linked depositional systems

The Penha Garcia Ichnological Park at Naturtejo UNESCO Global Geopark (Portugal): a Geotourism Destination in the Footprint of the Great Ordovician Biodiversification Event

Penha Garcia Ichnological Park is the most important fossil site and one of the most significant geosites of international relevance, in the territory of Naturtejo UNESCO Global Geopark. This municipal protected area is a key site for the study of trace fossil diversity at high paleolatitudes in the early stages of the Great Ordovician Biodiversification Event. Most famous among 21 ichnotaxa is Cruziana, interpreted as foraging burrows of trilobites, present in high abundance and excellent preservation. They are among the largest fossils found anywhere in the Lower Paleozoic, not uncommonly several meters long and over 20 cm wide, covering large bedding planes almost completely. The birthplace of Naturtejo Geopark is now a nature school, one of the most visited outdoor sites by schools in Portugal. It also became a reference for geotourism in this country because of the Fossil Trail that allows visitors to access a natural fossil art gallery in the Variscan-folded cliffs of the Ponsul Gorge. The Fossil Home provides the interpretation of the local geoheritage, and additional attractions are the Templar castle and church viewpoints, the climbing school, the secular watermills, the views over the lake in the heart of the syncline, and the Pego river pool. This is a major geotourism destination in the center of Portugal. The in situ conservation of fossils in this ‘outside museum’ is a challenging subject, which is being monitored using 3D high-resolution modeling of the whole gorge and the main fossil sites there to an mm-scale precision

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