Ichnological Analysis Research Articles

Paleoenvironmental conditions and evolution of a muddy turbidite system: an integrated sedimentological and ichnological analysis

Abstract Recent advances in characterization of turbidite-fans have been achieved through various methodologies, including flume-tank experiments, numerical modeling, seismic analysis, and integrated core and outcrop studies. Ichnology has proven valuable for understanding turbidite systems by providing insights into paleoenvironmental conditions such as oxygen levels, nutrient availability, hydrodynamic energy, and sedimentation rates. Since the 1960s, the Nereites ichnofacies has been linked to flysch deposits, characterized by features such as graphoglyptids, shallow-tier ichnotaxa, complex grazing trails, and diverse, but low density, trace fossils. The aim of this study is to combine ichnological and sedimentological data to improve recognition of subenvironments in turbidite systems. Analysis of five sections in a late Miocene, muddy turbidite system in the Tabernas Basin (SE Spain) shows that: 1) the distal fan lobes are characterized by a high diversity and abundance of predepositional trace fossils linked to the Paleodictyon ichnosubfacies, indicating stable and oligotrophic conditions, and 2) the proximal fan lobes by a low diversity and abundance of postdepositional trace fossils with abundant Ophiomorpha rudis ichnosubfacies in areas close to the feeder channel.

Assessing Vertical Bioturbation Intensity from Bedding Planes

Bedding planes hold significant value in ichnological analyses as they improve the recognition of ichnotaxa and yield important insights into the distribution and morphology of trace fossils. However, a significant limitation lies in the challenge of accurately estimating bioturbation intensity from bedding planes. This is in part because current approaches do not establish a connection between bioturbation intensity as seen on bedding planes (plan view) and in cross-section (elevation view). This disconnect complicates the integration of bedding plane assessments into more conventional datasets collected from elevation view. Seventy-seven million Monte Carlo simulations were performed on 11 non-palimpsest Skolithos assemblages, each with varying constraints on burrow length and diameter. These simulations were used to investigate the range and frequency of cross-sectional bioturbation intensities across a spectrum of bedding plane intensities (2-50%). The results show that elevation view bioturbation intensity can be reliably approximated by multiplying plan view bioturbation intensity by the average burrow length relative to the bed thickness, independent of burrow diameter. These findings improve the ichnological analysis of bedding planes for vertical trace fossil assemblages and offer a framework for future research into more complex assemblages.

Ichnological record in gateways and other high-energy deepwater environments: a palaeo- and neoichnological approach

The evolution of gateways determines variations in palaeoenvironmental (ecological and depositional) conditions affecting tracemaker communities, and hence biogenic structures. With respect to palaeogateways, bottom currents and associated deposits (i.e. contourites) may be approached through trace-fossil studies. Ichnological analysis – individual ichnotaxa and changes in ichnofacies – of Late Miocene contourites and associated facies from the Rifian Corridor palaeogateway (Morocco) reveals a major impact of food supply, flow velocity, seafloor heterogeneity and hydrodynamic energy. The fact that Eocene to middle Miocene deep-marine deposits, mainly contourites, from the nearby Indian palaeogateway (Cyprus) show ichnofacies replacement is evidence of increased bottom-current flow velocity associated with sea-level variations. Moreover, ichnofabric changes reflect pelagic, gravitational and bottom-current processes that simultaneously influence sedimentation. Changes in the trace-fossil composition of both areas, and in the shape of burrows, reflect intermittent bottom-current processes that governed contourite deposition. Neoichnological studies complement and validate the trace-fossil record of high-energy deep-sea environments that can occur in palaeogateways. Modern traces ( lebensspuren ) observed on the seafloor show variable features (e.g. diversity, density, morphology) depending on the type of disturbance that high-energy conditions exert on the trace-making benthic fauna. Parameters such as substrate consistency, nutrient availability/distribution and the duration and intensity of energetic events play a major role in determining lebensspuren features. We conclude that an integrative palaeo- and neoichnological approach is a powerful tool when used to improve our knowledge of benthic ecosystems in high-energy deep-sea environments such as gateways.

Small topographical variations controlling trace maker community: Combining palaeo- and neoichnological data at the Porcupine Abyssal Plain

Ichnological research has generally assumed that abyssal plains are dominated by quiescent, homogenous environmental conditions. Thus, deep-sea trace fossil assemblage changes have been usually linked to significant spatial and temporal environmental variations. Here, we conducted a comparative ichnological analysis between a small abyssal hill (50 m elevation) and the surrounding abyssal plain; this modest bathymetric variation is known to generate substantial environmental heterogeneity for the benthic fauna community of the Porcupine Abyssal Plain (c. 4850 m depth), Northeast Atlantic. Based on X-ray data from a 5 × 5 core grid emplaced in two box cores, we compared hill and plain bioturbational sedimentary structures, including trace fossil assemblages (e.g., ichnotaxonomy) and biodeformational structures (e.g., mixed-layer depth). We observed that topographically-enhanced near-bottom currents over the hill likely produce significant changes in depositional dynamics and sediment properties (e.g., grain size, organic matter content and degradation), and control specificities of bioturbational sedimentary structures (e.g., trace fossils, mixed layer attributes such as thickness, mottled background, discrete traces). Palaeoichnological data suggested that the abyssal plain had experienced consistent conditions during the last thousands of years while the abyssal hill recorded improving environmental conditions for the trace maker community. Our results highlight the complexity of the deep-sea environment, demonstrating that small changes in bioturbated sedimentary assemblages appear even within the same box core (m-scale), and that substantial changes can occur due to environmental heterogeneity (e.g., subtle topographic variations) at the local scale (km-scale). Considering the vast global extent of abyssal hill terrain, we suggest that their influence on the bioturbational sedimentary record may be significantly under-appreciated and require more attention in palaeoenvironmental reconstructions.

Depositional, stratigraphic and structural controls on the geometry of Montney Formation reservoirs: Lower Triassic, northeast British Columbia, northwest Alberta

Abstract The Lower Triassic Montney Formation is a world-class unconventional hydrocarbon reservoir that spans the border of northeast British Columbia and northwest Alberta. Integrated sedimentological and ichnological analyses suggest that the Montney Formation was deposited in a predominantly prodeltaic setting that was commonly influenced by both river flood and storm processes. Riverine processes include the local occurrence of normally- and inversely-graded siltstone and sandstone interbeds, interpreted as the record of sediment-laden hyperpycnal flows. Episodic storms are recorded by the local occurrence of hummocky cross-stratified interbeds, as well as normally graded, parallel and wave-ripple laminated beds, which are commonly draped by structureless carbonaceous mudstones. These mudstone drapes represent river-flood-derived fluid muds that collapsed from hypopycnal plumes following the passage of the storms and suggest the intimate linkage between storms and heightened fluvial discharge. The basin in which the Montney Formation was deposited began as three sub-basins, northern, central and southern, each of which experienced different patterns of subsidence and uplift that varied both spatially and temporally during sedimentation. Subsidence of the northern sub-basin was compartmentalized by deep seated faults that provided accommodation for the basal sequence of the Montney Formation, Sequence 1. Subsequent fault movement resulted in reduced accommodation in the northern sub-basin and accelerated subsidence in the central sub-basin, providing accommodation for deposition of Sequence 2, the basal sequence in the central sub-basin. The northern sub-basin remained paleotopographically elevated throughout deposition of Sequence 2 and early Sequence 3 successions, and these younger units did not extend beyond the northern limit of the central sub-basin. During subsequent deposition of the late Sequences 3 and Sequence 4 successions, compartmentalization between the north and central sub-basin was greatly reduced and these intervals were deposited relatively more uniformly along-strike. The unconventional and conventional reservoirs in both the northern and central sub-basins occur at the scale of systems tracts within the four depositional sequences of the Montney Formation. Reconstructing the depositional history of the component systems tracts of Sequence 1 through early Sequence 3 reveals the dynamic interplay between syndepositional tectonism, fluctuating relative sea-level and deltaic depositional processes in controlling the character, geometry and distribution of prolific Montney Formation reservoirs. The recognition of these key geological controls enhances predictability in this valuable resource and may be applicable to other fine-grained reservoirs elsewhere.

Ichnological study of deep marine hyperpycnites: A case study from the Shenhu continental slope (northern South China Sea)

Ichnological analysis plays a pivotal role in the study of sedimentary basins as it records the evolution of paleoenvironments and the associated changes, providing crucial support for sedimentological and stratigraphic interpretations. Yet, rare ichnological studies on deep-marine hyperpycnites have been conducted, resulting in less conclusive ichnological characterization of deep-marine hyperpycnites, compared to pelagites/hemipelagites, turbidites, and contourites. This study conducted detailed investigation of trace fossils distribution within deep-marine hyperpycnites, using non-destructive X-ray computed tomography scanning. The sediment core S19 on the upper Shenhu slope in the northern South China Sea documented deep-marine hyperpycnites facies S3, S2L, and L, possibly representing the proximal to distal hyperpycnal lobe. In this study, hydrodynamic energy and salinity are suggested to be the dominant stress factors within proximal lobes, resulting in less bioturbation and scattered distribution of vertical trace fossils such as Skolithos and Siphonichnus. In the middle lobe, the environment is predominantly characterized by moderate to high hydrodynamic energy, a rich abundance of organic material, and occasional slight salinity fluctuations. These environmental factors have led to the prevalence of trace fossils such as Arenicolites, Conichnus, Nereites, Phycosiphon, Rosselia, Scolicia, and Thalassinoides, which are well-represented in this region of the lobe. The organic material concentrated within the distal lobe to fringe acts as the main stress factor promoting opportunistic organism colonization and the formation of deposit-feeding traces. Therefore, the represented traces within distal hyperpycnites are mainly Nereites and Phycosiphon. This study provides a detailed investigation into trace fossil distribution within deep-marine hyperpycnal lobes, enriching our knowledge of the ichnology of deep-marine hyperpycnites.

Ichnological analysis of the Maastrichtian-Danian interval in the Paraíba Basin: Paleoenvironmental insights from the Itamaracá-1IT-03-PE well

The Gramame and Maria Farinha formations were deposited in the Paraíba Basin during the Maastrichtian-Danian interval, recording the paleoenvironmental evolution during the opening of the Atlantic Ocean. Despite well-investigated, there is still a need to better understand the bioturbation record and the represented ecological conditions during the deposition of these units. This study investigates the ichnofauna of the carbonate facies recorded in the Itamaracá-1IT-03-PE well in the Paraíba Basin, Brazil, focusing on the paleoenvironmental characteristics that influenced this carbonate ramp. The ichnologic analysis allowed to identify environmental stress based on the limited variation in the diversity and complexity. The identified Palaeophycus-Asterosoma and Asterosoma suites suggest low to moderate energy levels, minimal fluctuations in sedimentation rates, and a relatively stress in salinity that persisted for a long period. Variations in δ13C and δ18O values during the Maastrichtian interval corroborates changes in temperature and bioproductivity, reflecting an increase in freshwater influx during this period, which affected paleoenvironmental conditions and trace fossil diversity mainly due to changes in salinity.

Ichnological analysis of glacially-influenced sediments from the late Pleistocene to Holocene on the southeastern Canadian margin: Implications for palaeoclimate and palaeoceanography

The study of trace fossils in glacially-influenced margins provides valuable insights into the changes in palaeoclimate and palaeoceanography during glacial-interglacial periods, by providing biological and ecological data of the benthic environment. Here we present a comprehensive ichnofabric characterisation of a sediment core located on an inter-canyon ridge on the SW Grand Banks Slope off Newfoundland, covering the Late Pleistocene–Holocene period (0–∼23 ka). The core analysis reveals 12 ichnogenera and an escape structure, with Phymatoderma, Tasselia, and Zoophycos being prominent and relatively abundant at specific levels. By examining the distribution of bioturbation content and degree along the core, we establish a strong association between moderate to high bioturbation and cold periods. The Last Glacial Maximum exhibits the highest abundance and diversity of trace fossils, particularly the deposit-feeding Phymatoderma and the deposit-feeding/equilibrium trace Tasselia, indicating benthic adaptation to seasonal food input and relatively high sedimentation rates. Conversely, the warmer deglaciation period shows limited bioturbation. Salinity stress may account for the abrupt decline in bioturbation degree during the early deglaciation. Additionally, the occurrence of large-sized Zoophycos coincides with the Younger Dryas cold event during the last deglaciation. These findings highlight the complexity of palaeoceanographic conditions and benthic environments in proximal glacially-influenced sites compared to distal hemipelagic and pelagic sites. The interplay between down-slope processes (such as turbidity currents and hyperpycnal flows) and along-slope processes (like contour currents) plays a crucial role in shaping the unique sedimentological regime of depositional sites and subsequently affecting the ichnofauna in glacially-influenced margins.

Late Jurassic–Early Cretaceous dinosaur track assemblages from northwestern Hebei Province, China: Implications for paleoenvironment and paleoecology

The relationship between the Middle–Late Jurassic Yanliao Biota and the Early Cretaceous Jehol Biota has long been unresolved due to an approximately 20 Ma “vertebrate fossil gap”. However, a large number of dinosaur tracks have been reported from the Tuchengzi/Houcheng Formation in northern Hebei–western Liaoning, which occupies the stratigraphic position between the Yanliao Biota and the Jehol Biota. This paper presents new discoveries of diverse dinosaur track assemblages from the Upper Jurassic–Lower Cretaceous Houcheng Formation in the Shangyi Basin, northwestern Hebei Province, China. Based on the ichnological analyses, tracks are assigned to the sauropod Parabrontopodus, theropod Grallator, Anchisauripus, Eubrontes, and Therangospodus. The theropod tracks were likely produced by small-sized feathered theropod dinosaurs (Coelurosauria) that thrived in both the Yanliao Biota and the Jehol Biota. These findings imply that these two biotas evolved successively, without evidence of a complete turnover or extinction of biotas. Sedimentological studies of the tracksites reveal their occurrence in diverse sedimentary environments, including braided sandbar, floodplain, and deltaic plain. The coexistence of highly diverse dinosaur tracks with various preservation environments indicates that the living environment during that time was relatively comfortable. The increased diversity of dinosaur tracks in the upper part of the Houcheng Formation closely aligns with the intense volcanic activity of the Zhangjiakou Formation in northwestern Hebei Province. This suggests that volcanic activity likely served a crucial function in the proliferation of the dinosaur fauna in the Shangyi Basin and northern North China during the earliest Cretaceous.

Enigmatic vertebrate swimming trace fossils from the Wapiti Formation, Alberta, Canada, and their implications for paleoenvironmental reconstruction

Vertebrate ichnofossil research, when compared with invertebrate ichnological analyses, tends to put greater emphasis on identifying the trace makers rather than associating the trace fossils with animal behaviors to interpret local paleoenvironmental conditions. In 2011, several sandstone blocks bearing unusual deformational structures were collected from the Wapiti Formation exposed near Red Willow Falls, Alberta, Canada. The sandstone blocks were analyzed for their sedimentary characteristics and the morphology of the enigmatic structures in order to identify their origins and their significance for paleoenvironmental interpretation. The deformational features are interpreted to be the swim tracks of small vertebrates attempting to escape a flash flood event. The sedimentology of the sandstone suggests a rapid increase in water flow in an overbank setting, and the orientation of the traces indicate that the trace maker was initially moving against, and then moving perpendicular to, the current. This is likely the first case of a trace fossil exhibiting clear association between vertebrates and flash flood events. As such, these traces provide an excellent example of the utility of vertebrate trace fossils for assessing organism response to changing environmental conditions, thus facilitating interpretation of local geological events.

A CONSOLIDAÇÃO DA ICNOLOGIA ENQUANTO CIÊNCIA: HISTÓRIA, CONCEITOS E MÉTODOS

The consolidation of Ichnology as a science: history, concepts, and methods. Ichnology, a science bridging Geology and Biology, has undergone different phases since the initial studies on biogenic structures until its scientific consolidation. The approach to Ichnology as a science in this text raises questions about how to demarcate what is considered scientific, while also illustrating how Ichnology is characterized in this field. Thus, it begins with the initial ideas about biogenic structures and the interpretation that these structures had an algal origin, marking the “ Age of Fucoids,” which positioned Ichnology as a branch of Paleontology. With the evolution of knowledge, these interpretations were re-evaluated, and trace fossils were recognized as products of biological activity. Therefore, the current review article aims to analyze the evolution of Ichnology and the establishment of its current paradigms as an independent science. This text also aims to disseminate Ichnology, its main concepts, and methods of study. The discussion of concepts here is intended to provide the tools for ichnological analysis, especially for students entering this field of knowledge. Keywords: Ichnology, epistemology, demarcation of science, ichnofabric.

Evaluating tracemaker recovery after the Cretaceous–Paleogene (K–Pg) boundary event: different biotic responses at the Caravaca section

Trace fossils are an aid to characterize major extinction events, enriching evolutionary paleoecology research. Ichnological analysis at the Cretaceous–Paleogene (K–Pg) marine boundary interval, worldwide, points to a relatively minor disruption in the macrobenthic tracemaker community; that is, trace-fossil assemblages do not change significantly across the K–Pg boundary, showing relatively rapid recovery, locally in just a few years post-impact. To evaluate the incidence of this event and the rapid recovery of the tracemaker community afterwards, the evolution of planktic and benthic groups at the K–Pg boundary interval was analysed in the Caravaca section (Betic Cordillera) based on the integration of available data. In general terms, planktic foraminifera and calcareous nannoplankton dropped in abundance and diversity at the K–Pg boundary, whereas benthic foraminifera did not show significant extinction, but rather a sudden and dramatic decrease in diversity and reorganization. After the K–Pg boundary event, planktic communities exhibit a prolonged delay in recovery—mainly occurring above the dark boundary layer—with respect to benthic foraminifera. The K–Pg boundary event did not induce a severe crisis for the burrowing macroinfauna, as revealed by the similarity between pre- and post-event ichnotaxa, showing a comparatively rapid first colonization phase, less than 2 ky after the event. The record of Zoophycos and Chondrites at the base of the dark boundary layer evidences an opportunistic behaviour of tracemakers and a high independence from oxygen in pore waters and food in the sediment, allowing for the colonization of an overall unfavorable environment.

TAPHONOMIC ANALYSIS OF AN ARTICULATED BALEEN WHALE (CETACEA; MYSTICETI) FROM UPPER MIOCENE INNER SHELF DEPOSITS OF PENÍNSULA VALDÉS, PATAGONIA, ARGENTINA

Abstract Fieldwork at Península Valdés (Chubut, Argentina) in the Puerto Madryn Formation (Late Miocene) resulted in the discovery of a well-preserved, almost fully articulated, baleen whale (Cetacea, Mysticeti). This specimen, one of the most complete balaenid skeletons known from the Neogene of Argentina and worldwide, was the focus of a taphonomic analysis employing a multidisciplinary approach, integrating taxonomic, sedimentological, stratigraphic, and ichnological analyses, with the aim of reconstructing the taphonomic processes and the paleoenvironmental conditions that controlled preservation of the specimen. The skeleton belongs to the family Balaenidae (right whales). It displays a high degree of articulation, moderate pre-burial fragmentation, and relatively high completeness. Our results suggest that after death, the balaenid suffered a brief biostratinomic phase that can be summarized in four stages: (1) death at sea, with initial decomposition and positive buoyancy of the carcass; (2) internal accumulation of putrefaction gases, re-orientation, then gas loss; (3) sinking and deposition in a ventral-up position on the sea floor of the inner shelf; and (4) lateral re-orientation of the postcranial region due to physical and biological processes. The high degree of articulation and association of the skeletal elements, and the presence of both mandibles, indicate no lateral transport on the seabed and excludes refloating of the carcass at any stage. Finally, the data indicate a low-energy shelf environment with normal marine benthic oxygenation and salinity conditions, characterized by a soft bottom and a moderate sedimentation rate. The last, combined with high bioturbation, plus scour-induced self-burial, resulted in rapid burial of the carcass.

Diversity and discrimination of large ornithopods revealed through their tracks (Lower Cretaceous, Spain): A phenetic correlation approach

Large ornithopods were frequent in the alluvial-to-coastal sedimentary systems developed during the Barremian in the Iberian Peninsula. Here, several pes track casts of large ornithopods from the Barremian Camarillas Formation of the southwestern sector of the Maestrazgo Basin (Iberian Basin, Spain) are analyzed. Two morphotypes of tracks from three different size classes are differentiated based on some morphological features, such as length of digits, the width of the “heel” pad impression, and width of the proximal part of digit III. These casts are assigned to Caririchnium isp. Morphotype 1 is represented by small to large-sized tracks, whereas Morphotype 2 by medium-sized tracks. Morphological variation seen among tracks of Morphotype 1 is interpreted to be possibly an ontogenetic variation. On the contrary, the differences seen between Morphotype 1 and 2 cannot be explained only by extramorphological causes, so might represent different trackmakers, especially when taking into account the considerable Barremian ornithopod diversity. A modified phenetic correlation is carried out between several well-preserved casts and the identified feet of Barremian ornithopods from Iberia, in order to infer the putative trackmakers. Morphotype 1 fits with the foot skeletal structure of the large styracosternan Iguanodon, including small-sized tracks that are perfectly accommodated to that of perinates of I. galvensis. Morphotype 2 matches better to the pes skeletal structure of the medium-sized Mantellisaurus. Similar two morphotypes can be also identified in other areas of the Lower Cretaceous of Europe, so, they could be tentatively produced by the two sized groups of styracosternans typical in the Early Cretaceous of Europe. Therefore, the ichnological analysis may allow differentiation of the diversity known through the direct record. In addition, the paleoecological analysis indicates a spatial niche partitioning between two size groups of styracosternans, because large-sized tracks and bones of these dinosaurs are the most common in such sediments.

AVIAN FORAGING ON AN INTERTIDAL MUDFLAT SUCCESSION IN THE EOCENE TANJUNG FORMATION, ASEM ASEM BASIN, SOUTH KALIMANTAN, INDONESIAN BORNEO

Abstract Moderately diverse trace fossil assemblages occur in the Eocene Tambak Member of the Tanjung Formation, in the Asem Asem Basin on the southern coast of South Kalimantan. These assemblages are fundamental for establishing depositional models and paleoecological reconstructions for southern Kalimantan during the Eocene and contribute substantially to the otherwise poorly documented fossil record of birds in Island Southeast Asia. Extensive forest cover has precluded previous ichnological analyses in the study area. The traces discussed herein were discovered in newly exposed outcrops in the basal part of the Wahana Baratama coal mine, on the Kalimantan coast of the Java Sea. The Tambak assemblage includes both vertebrate and invertebrate trace fossils. Invertebrate traces observed in this study include Arenicolites, Cylindrichnus, Diplocraterion, Palaeophycus, Planolites, Psilonichnus, Siphonichnus, Skolithos, Thalassinoides, Taenidium, and Trichichnus. Vertebrate-derived trace fossils include nine avian footprint ichnogenera (Aquatilavipes, Archaeornithipus, Ardeipeda, Aviadactyla, cf. Avipeda, cf. Fuscinapeda, cf. Ludicharadripodiscus, and two unnamed forms). A variety of shallow, circular to cylindrical pits and horizontal, singular to paired horizontal grooves preserved in concave epirelief are interpreted as avian feeding and foraging traces. These traces likely represent the activities of small to medium-sized shorebirds and waterbirds like those of living sandpipers, plovers, cranes, egrets, and herons. The pits and grooves are interpreted as foraging traces and occur interspersed with both avian trackways and invertebrate traces. The trace fossils occur preferentially in heterolithic successions with lenticular to flaser bedding, herringbone ripple stratification, and common reactivation surfaces, indicating that the study interval was deposited in a tidally influenced setting. Avian trackways, desiccation cracks, and common rooting indicate that the succession was prone to both subaqueous inundation and periodic subaerial exposure. We infer that the Tambak mixed vertebrate-invertebrate trace fossil association occurred on channel-margin intertidal flats in a tide-influenced estuarine setting. The occurrence of a moderately diverse avian footprint and foraging trace assemblage in the Tambak Member of the Tanjung Formation illustrates that shorebirds and waterbirds have been using wetlands in what is now Kalimantan for their food resources since at least the late Eocene.

Short-term middle Eocene (Bartonian) paleoenvironmental changes in the sedimentary succession of Olivetta San Michele (NW Italy): the response of shallow-water biota to climate in NW Tethys

This study focuses on the paleontological content of the middle Eocene (Bartonian) carbonate–siliciclastic sediments of the Capo Mortola Calcarenite Formation from Olivetta San Michele (Liguria, Italy). Along the succession, there are significant paleoecological changes triggered by the variation in neritic input as a consequence of tectonic and climatic instability. Among microfossils, nummulitids prevail, followed by orthophragmines, smaller benthic, and planktonic foraminifera, whereas mollusks and ichnofossils are the most abundant macrofossils. The sudden changes in the benthic communities due to the progressive increase in fluvial input are recorded throughout the sedimentary succession. An increase in water turbidity caused stressful conditions for autotrophic taxa, reducing their size and abundance. In contrast, filter feeders became dominant, suggesting an increase in dissolved and suspended nutrients. Ichnological analysis shows environmental fluctuations controlled by the transport of neritic material offshore, thus confirming the general deepening trend of the studied succession. In the upper part of the succession, we recorded an alternation between gravity flows and marly sediments that are interpreted as short-term alternations between low and intense precipitations. The gravity flows yield taxa such as larger benthic foraminifera (LBF), smaller benthic and planktonic foraminifera, mollusks, and corals. In turn, marls display only a few LBF and abundant smaller benthic and planktonic foraminifera. In these intervals, the increase in planktonic foraminifera also suggests a deepening of the carbonate ramp coinciding with a reduction of light that did not favor the development of LBF. These changes are probably related to the climatic dynamics that occurred in the Bartonian in the western Tethys.

Ichnological analysis and paleoenvironmental inferences of Neogene meandering fluvial deposits in Continental Rift of Southeastern Brazil

Ichnological analysis and paleoenvironmental inferences of Neogene meandering fluvial deposits in Continental Rift of Southeastern Brazil

Ichnological analysis of the Kheneg el Aatène Formation (Floian-Dapingian) at its type locality (Saoura Valley, Ougarta Range, southwestern Algeria)

ABSTRACT The Early-Mid Ordovician Kheneg el Aatène Formation of the Ougarta Range is here divided into two members: (i) the Aatène Micaceous Sandstone Member and (ii) the Aatène Quartzite Member. A low diversity trace fossil assemblage is present in the upper part of the second member with abundant Skolithos linearis, Skolithos isp. and Rosselia erecta, common Arenicolites isp. and Diplocraterion isp. and rare Palaeophycus isp. and ?Rusophycus isp. This association is characteristic to the Skolithos ichnofacies and indicates high-energy conditions and opportunistic colonisation within a siliciclastic shallow marine environment. The ichnoassemblage is dominated by vertical traces which occur in great densities forming typical Skolithos pipe-rocks. The pipe-rock ichnofabric is characterised by burrows that are closely spaced and occur at 20 distinct levels. Possible predation has been detected in the same Member due to the presence of ?Rusophycus intersecting worm burrows. High densities of Trypanites are present in two hardgrounds composed of calcareous lithified sandstones developed during transgressive pulses that can be related to the brevis event. Trypanites borings are evidenced for the first time from inorganic hard substrates of the Gondwanan Ordovician. These borings can be related to beginning of the Ordovician Bioerosion Revolution in Gondwana.

Reevaluating the process regime in the Sego Sandstone: Sedimentological and ichnological evidence for an underemphasised fluvial signature

AbstractRocks of coastal to shallow‐marine origin are challenging to interpret owing to the complex interplay of various depositional processes. This study reevaluates the relative roles of fluvial, tidal and wave processes in the Upper Cretaceous Sego Sandstone (and subordinately in the underlying Buck Tongue) of the Book Cliffs, USA, a well‐studied ancient coastal to shallow‐marine succession. Detailed sedimentological and ichnological analyses were used to interpret a previously underemphasised riverine signature, consisting of centimetre‐ to decimetre‐thick alternations of sandstone and heterolithic beds inferred to represent flood–interflood periods of variable river discharge. Recognition of a widespread fluvial‐dominated signature across the studied units better agrees with other sedimentological and regional observations in the study area, such as high sandstone–mudstone ratios, largely unidirectional and seaward‐oriented palaeocurrents, and modelled weak tidal conditions in the basin. When considering all of the sedimentological, ichnological and stratigraphic observations together with its regional depositional context, the Sego Sandstone/Buck Tongue system is better explained using a mixed‐energy but fluvial‐dominated deltaic model. This highlights an historical over‐interpretation of tidal processes and subordinate wave processes in the Sego Sandstone and likely in similar units. The widely used approach that emphasises only certain sedimentary features in discerning the process regime from analysis of rocks of inferred coastal to shallow‐marine origin is unrefined and may therefore underrepresent the actual complexity of these systems.

Early Cretaceous shifting of Zoophycos in the Ouarsenis Mountains (northwestern Algeria)

Early Cretaceous succession of the Oued Fodda Formation in the Ouarsenis Mountains (northwestern Algeria) is mainly composed of marl–limestone alternations, which are subdivided into four informal units (Units 1 to 4), based on distinct lithological, stratonomical, and ichnological features. The ichnological analysis reveals a low diversity of the trace-fossil assemblage, which is exclusively reported from Units 2 and 3. The ichnoassemblage contains six ichnotaxa (Chondrites intricatus, Ophiomorpha isp., Planolites isp., Thalassinoides isp., Zoophycos brianteus, and Zoophycos cauda-galli), among which Zoophycos and Chondrites are the most common elements of the assemblage and occur in distinct mud-rich substrates showing different bioturbation intensities. The development of Zoophycos in the middle part of Unit 2 shows a high degree of bioturbation (bioturbation index (BI) = 4). Zoophycos specimens are of large size, between 45 cm and 75 cm in width, which were interpreted to have formed in a lower offshore environment where the oxygenation amount was optimal, the sedimentation rate was low, and the benthic food was abundant on the seafloor. Toward the upper part of Unit 2, Zoophycos-bearing levels exhibit a less intense degree of bioturbation (BI between 1 and 2) in contrast to Planolites- and Chondrites-bearing levels which have a bioturbation index (BI) between 3 and 4. At these levels, Zoophycos displays relatively small, coiled to U-shaped spreiten, probably in response to stressful and dysoxic conditions prevailing in the water bottom. With improved oxygenation in a quiet lower offshore to shelf margin environment in Unit 3, the benthic organisms recovered, as represented by medium to large size Zoophycos in association with Ophiomorpha and scarce Chondrites burrows, even if the overall bioturbation intensity is very low. The combination of trace-fossil assemblage and lithofacies of the Oued Fodda Formation indicates relatively stable outer shelf environments below the storm wave base, which corresponds classically to the lower offshore to shelf edge environments, and the prevailing palaeoecological conditions are optimal and stressful for the benthic organisms.