An interval through the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) from the Vocontian Basin in southeastern France was examined using integrated sedimentology and ichnology at both macro- and microscopic scales to elucidate subtle bottom water and pore water oxygen changes. This study provides the first comprehensive trace fossil record for the region, confirming a diverse assemblage of ichnogenera and revealing evidence of microscopic reworking even in mudstones previously considered unbioturbated. Relative dissolved oxygen changes were interpreted through temporal fluctuations in trace fossil attributes (diversity, burrow size, and ichnoguilds) to illuminate the dynamic redox history. The onset of the OAE2 marked a rapid decline from pre-event oxic conditions to sustained suboixa. Peak deoxygenation occurred just below the CenomanianTuronian Boundary, marked by oscillations between suboxic and temporarily anoxic conditions. Prior to peak deoxygenation, a significant reoxygenation event (the Plenus Cold Event) resulted in fluctuating oxic and dysoxic conditions. Post-OAE2 recovery was protracted and gradual, transitioning from dysoxic to weakly oxic bottom waters. Integrating these ichnological findings with existing geochemical and micropaleontological data corroborates the interpretation that OAE2 in this region was a dynamic event characterized by fluctuating redox conditions, rather than persistent anoxia. This work underscores the critical value of ichnological analysis in paleoredox studies for resolving subtle changes within the oxic-dysoxic-suboxic spectrum, demonstrating that the absence of macroscopic bioturbation is an unreliable proxy for anoxia.

Conventionally, geologists have regarded mudstones as deposits formed through suspension settling in environments located at the terminus of sediment transport pathways, with the sediment sourced from a mix of detrital inputs into the basin and in situ production within the basin. However, mudstones are sedimentologically enigmatic as they are characterized by intricate small-scale features. Analysing mudstones with the typical techniques used for coarse grained siliciclastics does a disservice to the intricacies of these deposits. Grains, pores, and depositional fabrics within these rocks are not visible in hand sample, and often not even at the petrographic scale. Study of these features, at appropriate scales, can generate valuable insights into the physical and chemical conditions of their deposition. Along with analytical techniques, the conventionally held interpretations of these rocks are out of date. New insights into the origins and composition of grain components reveal significant variability, indicating these deposits are much more complex than traditionally understood. As a result, historical nomenclature and interpretation paradigms have undergone significant revision. However, there is still more research needed to fully address the challenges of mudstone description, classification and interpretation. This paper presents digestible discussions of changes in mudstone paradigms, the most effective practices consistent with modern understandings of mudstones, and considers areas that merit further consideration. Ideas presented herein are aimed at all those interested in mudstones, but is primarily meant for those new to the challenge of conducting mudstone analyses. Herein we recognize several preferred practices that have gained consensus in the literature, these include: (1) clearly defining common historical terms such as ‘clay’, ‘silt’, ‘bed’, and ‘shale’ depending on modern chosen usage; (2) outlining the transportational (i.e., functional) grain size of the deposit, as many constituents may be transported as amalgamated clasts; (3) clearly defining if reported mudstone composition is based on transported or apparent grain size (i.e., individual grain measurements); (4) thin section preparation methods and their integration with other complementary analytical techniques. As well, we discuss: (1) the use of both petrographic trace fossil analysis and microfacies analysis; (2) complex depositional mechanisms, beyond suspension settling, that lead to the accumulation of fine-grained deposits; and, (3) the interaction of several variables involved in accumulating organic-rich deposits. Ultimately, when embarking on mudstone analysis, one must first decide what question they are trying to answer. This will dictate the approach used, and if the focus is on the intricacies of grain size, composition, or depositional fabric.

Sequence stratigraphy in fine-grained successions is often challenging with traditional datasets, and in such intervals, chemostratigraphy is increasingly used to facilitate interpretations. In this study, we present an example from mudstone units of the Horn River Group (Northwest Territories, Canada), which provide a local record of Middle to Late Devonian marine conditions. The dataset comprises geochemical composition data from X-ray fluorescence (XRF), mineralogical data from X-ray diffraction (XRD), wireline logs, total organic carbon (TOC) profiles, and lithological core and thin section descriptions. We first focus on a mudstone core with high-resolution thin-section and geochemical results, enabling the evaluation of chemostratigraphic signatures associated with surfaces and systems tracts in the Horn River Group. By employing chemostratigraphic proxies for terrigenous sediment supply and proportion of biogenic silica, sequence stratigraphic analysis is extended to three cores and five outcrops lacking extensive thin section coverage. Six complete transgressive–regressive (T–R) sequences are identified and correlated in the Horn River Group, comprising higher-resolution cycles in relative sea-level and sediment supply, which are superimposed on previously identified larger-scale sequences. The sequence stratigraphic framework presented illustrates the local balance between relative sea-level rise and sediment supply along the northwestern margin of Laurentia during the Eifelian to the Frasnian. Moreover, this study demonstrates the utility of integrating chemostratigraphic proxies with sedimentological results as a means of sequence stratigraphic interpretation and correlation in mudstone intervals.

This research examines the spatial and geochemical interactions between mat-forming microorganisms and thalassinid shrimp in an intertidal flat situated on the shores of Willapa Bay, Washington, USA. The study serves as a contemporary analog for the relationships between mats and burrowing organisms in deep time. This methodological approach uses (i) field data to investigate sedimentological characteristics along a transect from heavily burrowed to mat-dominated sites, (ii) orthophotogrammetry to analyse and quantify mat extent and shrimp burrow density, (iii) porewater samples collected using Rhizon samplers and inductively coupled plasma mass spectrometry to determine porewater elemental concentrations, (iv) statistical analysis of the position of burrow openings and mat extent and (v) DNA analysis to characterise the microbial community composition of this site. These findings reveal a complex relationship between burrowing organisms and mat-forming microorganisms, suggesting a potential benefit to burrowing organisms in colonising sediment that is spatially proximal to mats, or covered in discontinuous microbial mats. These findings challenge oversimplified interpretations of the AR that are based on a rigid dichotomy between matgrounds and mixgrounds. A key simplification is the depiction of the AR as a sudden, one-way shift from matgrounds to bioturbated sediment. While burrowing organisms contributed to the decline of microbial mats, factors such as sediment supply, hydrodynamics and chemical conditions also play a significant role in their spatial distribution. This work highlights that burrower-mat interactions, both past and present, are more nuanced than previously understood.

Hydrogen sulphide (H2S) is widely acknowledged as a potent respiratory toxin for eukaryotic cells. However, macrofauna have been observed thriving in environments with elevated H2S concentrations. Here we report on a saltwater aquarium hosting a community of invertebrates and inhabited by an epibenthic microbial mat. The aquarium was left undisturbed for the duration of the COVID-19 pandemic stay-at-home order, leading to the development of high concentrations of H2S. Remarkably, the invertebrate community did not collapse. This success offers valuable insights into how invertebrates respond to physiochemical stressors at both individual and community levels. We also observed persistent disequilibrium between H2S and oxygen (O2), exhibiting out-of-phase periodic cycles driven by a simulated solar cycle. During daylight, photosynthetic O2 production increased, resulting in more active behaviour from the metazoan community. Conversely, H2S production peaked during the dark cycle, causing a moribund animal community. Additionally, over time, overall community diversity in the tank decreased, while macrofaunal abundance appeared largely unaffected. Polychaete worms and cnidarians demonstrated resilience to the high-sulphide conditions for the entire duration of the experiment, whereas others experienced gradual declines in abundance until they perished. These findings challenge conventional expectations of eukaryotic tolerance to H2S and underscore the significance of behavioural adaptations in withstanding high-sulphide environments. Our findings provide insights into how primitive metazoans may have survived in sulphidic to euxinic Ediacaran seas. Metazoans, microbial mats, hydrogen sulphide toxicity, Ediacaran

Aratichnus, new ichnogenus, is described from a lower Eocene-aged, tidally reworked sandstone unit (Baronia Formation) near Baronia de Sant Oisme, Spain. Aratichnus n. igen. is a horizontal, epichnial furrow ranging from simpler forms that display repeating straight to gently curved grooves to more complex serial furrows paralleling each other. Two new ichnospecies are described, Aratichnus hordeicollis n. isp. and Aratichnus apricuscampus n. isp., that encompass the range of morphological variability exhibited by the ichnogenus. A. hordeicollis exhibits ordered, repeating shapes of furrows, while A. apricuscampus exhibits furrows which do not repeat in shape and direction. We interpret Aratichnus n. igen. as a trace fossil made by an interface deposit feeder exploiting a patchy food source in an intertidal environment.

Microbial mats have existed for much of Earth's history. They represent some of the earliest evidence of life, are essential in biogeochemical cycles, and played a pivotal role in oxygenating the atmosphere. In addition, benthic microbiota impact sediment properties by enhancing the cohesion and stability of the substratum, a process known as ‘biostabilization’, which affects sediment dynamics and rheology. A substantial body of research has focused on experimentally quantifying biostabilization in siliciclastic sediments. This review compiles and synthesizes these studies in order to facilitate comparison of results. They, in turn, are compared with; (1) the Shields' diagram, (2) shear stress values in shallow marine environments, and (3) occurrences of microbially induced sedimentary structures in the marine stratigraphic record. The findings reveal significant variability in outcomes, with increases in the Shields' Parameter ranging from 0.1 to 4 orders of magnitude. They also demonstrate that high-energy hydrodynamic conditions, such as those above fairweather wave base, inhibit microbial colonization. Additionally, the review briefly discusses two applications of the data: (1) refining models of the Great Oxidation Event, and (2) evaluating microbial biostabilization as a response to increased coastal erosion driven by climate change.

The Middle to Late Devonian was characterized by the widespread deposition of organic-rich mudstone units and successive biotic crises and anoxic events in the marine realm, the cause of which remains debated and requires constraints from associated marine conditions. This study provides an example of the marine nitrogen cycle throughout the late Eifelian to middle Frasnian anoxic pulses. We present new and previously published organic whole-rock N (δ15Nbulk) and carbon (δ13Corg) isotopic datasets from organic-rich mudstone units of the Horn River Group (Canol and Hare Indian Formations) and overlying Imperial Formation in the Central Mackenzie Valley, Northwest Territories, Canada. In the ConocoPhillips Mirror Lake N-20 core, δ13Corg ranges from −31.0 ‰ to −24.3 ‰ with δ15Nbulk from −3.8 ‰ to +1.9 ‰, whereas the Husky Little Bear N-09 core is characterized by δ13Corg from −31.0 ‰ to −27.2 ‰ and δ15Nbulk from −2.0 ‰ to +5.9 ‰. The N isotopic signatures near 0 ‰ and a lack of δ15Nbulk – δ13Corg relationship are characteristic of N2 fixation by primary producers. Regular oscillations in δ15Nbulk are interpreted as the product of episodic, mild oxygenation events. Together, our δ13Corg and δ15Nbulk results suggest that locally, N2 fixation was the dominant source of N for primary producers in the late Eifelian to middle Frasnian, despite fluctuations in δ13Corg and global marine paleoredox. These findings contribute to our understanding of the nitrogen speciation and bioavailability associated with anoxic events, biotic crises, and widespread organic carbon burial in the Eifelian to Frasnian oceans.

The Hare Indian Formation (HIF) is a late Eifelian to Givetian organic-rich mudstone constituting the lower portion of the Horn River Group (HRG), which has been minimally scrutinized in the literature. This paper proposes depositional environments and a sequence stratigraphic framework for the HIF. Using composition data collected via energy-dispersive X-ray fluorescence, geochemical proxies inform detrital input, silica source, and paleoredox conditions. Cross-plots and chemostratigraphic profiles of detritally sourced Al, Ti, and K and redox-sensitive Mo and V inform depositional and stratigraphic constraints. Silica proportions vary, indicating that sediment was derived from detrital and biogenic sources. Al, Ti, and K distributions increase upwards, showing increased continentally sourced minerals. Redox-sensitive metals are highest in the Bluefish Member (BM), suggesting intermittent euxinia. Based on the presence of continental and pelagic sediments, the sedimentary environment is interpreted as proximal- to mid-shelf. These proxies guide systems tract interpretations. Si and redox-sensitive metal concentrations peak higher in the BM, accompanied by lowered concentrations of Al, Ti, and K, suggesting a maximum flooding surface. At the top of the Prohibition and Bell Creek members, redox-sensitive enrichments are lower with higher concentrations of Al, Ti, and K, suggesting a maximum regressive surface. Transgression occurred during the initial deposition of the BM, followed by regression for the remainder of the HIF. The sedimentology of the HIF can be difficult to decipher; the use of chemostratigraphy supports its geological history (including sedimentation trends and a local record of relative sea level) using methods that may be applied to other fine-grained successions.

The characterization of porosity is an essential step in the evaluation of resource-bearing porous media. Here, we focus on the Devonian Hare Indian and Canol Formations, two potential unconventional mudstone reservoirs, in a core from the Horn River Group of the Central Mackenzie Valley, Northwest Territories, Canada. By combining bulk porosity, low-pressure N2 adsorption, and scanning electron microscopy (SEM) results with composition and lithofacies datasets, we assess the porosity in these successions to understand pore types, size, distribution, degree of connectivity, and controls and predictors of porosity. Mineral matrix pores (interparticle and intraparticle), organic matter pores, and lithofacies-dependant natural fractures are present. All pore types display limited connectivity in two dimensions. Mineralogy is the most significant control on porosity with trends in porosity present among lithofacies. No relationship is observed between porosity and total organic carbon (TOC), suggesting that mineral matrix pores, rather than organic matter pores, are dominant in this unit. We compare these results to other mudstone reservoirs in North America and show that the Bluefish Member (Hare Indian Formation) and the Canol Formation are characterized by comparable bulk porosity, lower N2 mesopore volume, and higher quartz content relative to the other units considered. In contrast, compared to the other unconventional reservoir examples, the Bell Creek Member of the Hare Indian Formation exhibits lower quartz and higher clay content, average bulk porosity, and lower N2 pore volume. The results collectively suggest that high quartz and low clay content are the best predictors of porosity in the Horn River Group. Natural fractures may serve as flow pathways to induced fractures; however, these units lack the network of interconnected organic matter pores that can be present in other successions.

The Hare Indian and Canol Formations, which are part of the Horn River Group in the Northwest Territories, Canada, primarily consist of organic-rich mudstones deposited during the Middle to Late Devonian. The formations were previously considered to represent marine basin fill accumulated in an oxygen-starved distal shelf setting, evidenced by the organic-rich character, pyrite content, and lack of macro-scale bioturbation. The depositional model, paleo‑oxygenation interpretations, and methods of organic carbon preservation presented in this study are in contrast to previous assumptions of the Horn River Group mudstones. Detailed petrographic, sedimentological, and ichnological analyses were carried out on thin sections taken from several cored Horn River Group intervals. These organic-rich mudstone units contain eight distinct microfacies, representing four main sedimentation processes acting on an interpreted distal shelf setting: (1) pelagic suspension settling, (2) plug-like sediment-gravity flows, (3) surge and surge-like low-density turbidity currents, and (4) debrites. Pelagic suspension-settling dominated in distal, quiet waters out of the reach of persistent storm influence. Debrites, plug-like flows, and low-density turbidite processes represent a continuum, wherein storm influence was the dominant driver in sediment delivery. Several morphologically distinct, microscopic biogenic-sedimentary structures (i.e. ichnofossils) have been identified throughout the mudstone intervals, indicative of sediment pore waters that were at least periodically partially oxygenated. Evaluation of total organic carbon content against bioturbation and microfacies interpretation suggest that persistent anoxia was not the dominant factor in organic carbon preservation, but rather a result of a combination of heightened sedimentation and burial rates and possible amplified rates of primary productivity. The results of this study may be applied to evaluations of other organic rich mudstones to enhance paleo-depositional interpretations.

Sequence stratigraphy is commonly used to understand basin history and the distribution of conventional reservoir facies. Establishing a sequence stratigraphic framework in organic-rich mudstone successions is challenging because macroscale sedimentological and petrophysical variations can be subtle, while biostratigraphic and seismic data may be unavailable or of limited use. For these reasons, it is becoming increasingly common for chemostratigraphic profiles to be integrated with other datasets to facilitate sequence stratigraphic interpretation. This paper summarizes the whole-rock inorganic geochemical proxies relevant to sequence stratigraphic analysis in fine-grained, organic-rich marine units and reviews studies that have incorporated chemostratigraphic trends for sequence stratigraphy. This synthesis demonstrates that chemostratigraphic datasets are useful in identification of transgressive-regressive cycles, allowing for a preliminary summary of the chemostratigraphic characteristics of the maximum flooding surface, maximum regressive surface, transgressive systems tract, and regressive systems tract to be established based on existing work. A preliminary synthesis of the chemostratigraphic characteristics of the highstand systems tract is also possible for highstand systems tracts recognized using other criteria. However, a chemostratigraphic means of identifying the correlative conformity and basal surface of forced regression in order to subdivide the regressive systems tract into the lowstand systems tract, falling-stage systems tract, and highstand systems has not yet been demonstrated. Further work is also required in order to establish the differences in the chemostratigraphic signature of surfaces and systems tract depending on the depositional setting. Chemostratigraphic proxies are an emergent and promising tool for the identification of cyclicity in organic-rich mudstone intervals, which will become increasingly useful as further research is conducted on the topic.

Sequence stratigraphy is useful for understanding basin history and to facilitate regional correlation of reservoir intervals in sedimentary successions (Catuneanu et al., 2009). However, establishing a stratigraphic framework for mudstone intervals can be challenging because often the datasets traditionally used in coarser grained successions (e.g. geophysical, biostratigraphic, petrophysical, or sedimentological datasets) are unavailable or of limited use (Ratcliffe et al., 2012; Pearce et al., 2005). As a result, many studies are incorporating chemostratigraphy when studying fine-grained successions, meaning that whole-rock geochemistry is used for characterization and correlation of stratigraphic units (Ratcliffe et al., 2010; Pearce et al., 1999). Furthermore, chemostratigraphic proxies are being employed to interpret sequence stratigraphic cycles (e.g. Turner et al., 2016; Sano et al., 2013). This project presents data from the Middle to Late Devonian Canol Formation, an organic-rich mudstone present in the Northwest Territories, to demonstrate how elemental proxies can be used to establish a sequence stratigraphic framework.

The McMurray Formation comprises fluvial, estuary, delta and embayment deposits, for which the ichnology is well studied. Workers have recognized the dominantly brackish-water character of trace-fossil assemblages and those studies have contributed heavily to the interpretation of the deposit. This study considers the significance of lower McMurray Formation strata that contain a continental trace fossil assemblage.The studied outcrops comprise a coarse-grained sandstone channel that overlies and crosscuts large-scale, unidirectionally-dipping, interbedded, fine-grained sandstone and minor siltstone (i.e. Inclined Heterolithic Stratification referred to henceforth as IHS). Both lithofacies are overlain by a pedogenically altered clayey siltstone. The coarse-grained channel contains oriented wood clasts, and lacks bioturbation. The cross-cut IHS is devoid of large wood clasts and is commonly bioturbated. The IHS locally displays beds of granular sandstone that are lithologically similar to the channel sandstones.The IHS-associated trace fossil assemblage is composed of a range of forms that hitherto now, have not been formally reported from the McMurray Formation. Trace fossils that contain irregular back-fill and burrow diameters, as well as varying orientations, are assigned to a range of adhesive meniscate backfilled burrows, including the ichnogenus Naktodemasis and Taenidium. Collectively, these trace fossils are associated with insect larvae and are taken as indicators of freshwater sedimentation. Other trace fossils, such as Siphonichnus and Cylindrichnus, are interpreted to be associated with low-salinity waters: as such the outcrops most likely represent sedimentation in the innermost estuary, with the coarse-grained channel representing a cross-cutting fluvial deposit.The fresh-water and low-salinity trace fossil assemblage has not previously been described in the McMurray Formation. Importantly, the presence of continental and brackish-water trace fossils together offers a perspective of this McMurray locale. Lithologically, the fluvial and estuary deposits are clearly discernible.