Search

The distribution of conventional subichnofacies in deep‐water fan systems (i.e. Ophiomorpha rudis–Paleodictyon–Nereites‐subichnofacies) depends on the presence of fully turbulent non‐cohesive flows such as low‐density turbidites and high‐density turbidites. However, changes in flow state between laminar and turbulent conditions can affect the distribution style of conventional subichnofacies. This study deals with the effects of environmental or ecological stressors as a result of hybrid sediment gravity flows (hybrid event beds) on distribution and composition of trace fossils in the deep‐marine depositional system of the Ordovician Ghelli Formation in the north‐eastern Alborz Range. The spatial distribution and sedimentological character of hybrid sediment gravity flows, in particular their abundance in the down‐dip parts of lobe‐related facies associations, are interpreted as products of longitudinal and lateral transformation from turbidity currents to debris flows promoted by incorporation of significant claystone and mudstone clasts into the flow by seafloor erosion and flow bulking. Seven trace‐fossil assemblages were recognized in the channel and lobe‐related facies associations. The observed spatial distribution of trace‐fossil assemblages in the interpreted frontal and lateral lobe fringes, somewhat contrasts with conventional subichnofacies distribution models. The development of cohesive flows and rapid emplacement of turbidite, banded and debrite beds could be accompanied by fluctuations in physico‐chemical conditions and, hence, the time available for colonization and the lifestyle of producers. This study suggests that, compared to the archetypal models of the subichnofacies in down‐dip parts of lobe‐related facies associations, ichnodiversity and density of bioturbation are drastically reduced, and trace fossils are limited to simple, shallow‐tier feeding strategies. Commonly, the presence of cohesive flows may have forced the producers in the Paleodictyon and Nereites‐subichnofacies to migrate into lateral lobe fringe with more participation of low‐density turbidites. This study shows that, in addition to sedimentological and architectural criteria, ichnological criteria can also be used to distinguish between frontal and lateral lobe‐fringe deposits in unconfined systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study integrates ichnological and sedimentological data to interpret depositional environments of the carbonate sediments of the Tirgan Formation (Lower Cretaceous) in the eastern Kopet-Dagh Basin, north-east Iran. Lithofacies analysis shows that these sediments were deposited in inner ramp, middle ramp and offshore (outer ramp) environments. Five ichnoassemblages are identified in the sediments that consist of Thalassinoides, Thalassinoides–Rhizocorallium, Planolites–Rhizocorallium, Arenicolites–Diplocraterion, and Arenicolites. Th, Th-Rh and Pl-Rh with low diversity and abundance of the trace fossils formed during waning phase of storms in a predominantly medium to high-energy hydrodynamic regime. High sedimentation rate and mobile substrate condition featuring a shallow-marine setting. Ar–Di ichnoassemblage, consisting of horizontal and vertical traces of deposit and suspension feeders, respectively, portray two different phases. A predominantly high energy phase with instable substrate is displayed by the vertical traces, while a minor omission phase, associated with a decrease in sedimentation rate or non-deposition, is indicated by the horizontal structures. Arenicolites ichnoassemblage with low bioturbation index and low ichnodiversity is related to a semi-sheltered area of lagoon environments with periodically marine water circulation. The study of the ichnological attributes in the studied successions indicates the presence of a shallowing up-ward trend in the storm‒tide-dominated ramp sequence. Ichnoassemblage development is largely controlled by depositional and ecological conditions, e.g., the stability of substrate, hydrodynamic regime (wave and tide), and food abundance, which altogether control the substrate colonization. Based on an integrated ichnological and sedimentological approach, we characterize the depositional environment, deciphering allogenic and autogenic environmental controls on the trace fossil distribution on a passive margin depositional setting. [ABSTRACT FROM AUTHOR]

The Mississippian successions of the Alborz tectonostratigraphic zone have been studied in several sections (Dozdehban, Naserabad, Shahmirzad, and Mighan) extending from north to southeast. The sedimentological data indicate three main facies associations (FA): (FA1) a basin to distal outer ramp of low‐energy, low‐oxygen environment with low bioturbation and less developed in situ fossils; (FA2) a middle ramp facies accompanied by storm‐induced sediments (tempestites), echinoderm‐dominated mud mounds, and high fragmentation of skeletal components; and (FA3) an inner ramp facies, represented by sand shoal, and lagoonal/back‐barrier, mainly characterized by the dominance of oolitic, bioclast, and intraclast grainstones. The sedimentary features imply the formation of the succession on a broad flat‐topped, high‐to‐moderate‐energy shelfal carbonate ramp which had been open to the N/NE, the downslope direction, as revealed by local palaeoenvironmental reconstructions. Analysis of several proxies indicates a subtropical palaeoclimate evidenced by (a) widespread tempestite facies and occurrence of storm‐depositional processes, (b) diverse and abundant pellet‐forming thalassinoideans and burrowing echinoids trace fossil assemblages, (c) geochemical proxy indicating a palaeotemperature of 23°–25°, and (d) a rich subtropical heterozoan fossil association. The data provided here imply the palaeolatitudinal position of the Alborz Mississippian ramp is 30–40°S along the southern margin of the Palaeotethys. [ABSTRACT FROM AUTHOR]