Microbially Induced Sedimentary Structures Research Articles

Evidence of synsedimentary microbial activity and iron deposition in ferruginous crusts of the Late Cenomanian Utrillas Formation (Iberian Basin, central Spain)

Ferruginous sandstones and crusts are prominent sedimentary features throughout the continental (braided)-coastal siliciclastic (estuarine-tidal) wedges of the Late Cenomanian Utrillas Formation in the Iberian Basin. Crust types recognized are: Ferruginous sandy crusts (Fsc) with oxides-oxyhydroxides (hematite and goethite) concentrated on sandstone tops presenting a fibro-radial internal structure reminding organic structures that penetrate different mineral phases, suggesting the existence of bacterial activity in crust development; Ferruginous muddy crusts (Fmc) consisting of wavy, laminated, microbial mats, being composed mainly of hematite. On the other hand, a more dispersed and broader mineralization included as Ferruginous sandstones with iron oxides and oxyhydroxides (hematite and goethite) representing a limited cement phase on these sediments. The presence of microbial remains, ferruginous minerals, Microbially-induced sedimentary structures, microbial laminites and vertebrate tracks preserved due to the presence of biofilms suggest firstly a direct evidence of syn-depositional microbial activity in these sediments; and, secondly, that iron accumulation and ferruginous crusts development occurred immediately after deposition of the host, still soft sediments. Ferruginous crusts cap sedimentary cycles and they represent the gradual development of hard substrate conditions, and the development of a discontinuity surface at the top of the parasequence sets, related to very low sedimentary rates; the overlying sediments record subsequent flooding of underlying shallower environments; crusts are, consequently, interpreted as boundaries for these higher-order cycles in the Iberian Basin.

Early Triassic environmental dynamics and microbial development during the Smithian–Spathian transition (Lower Weber Canyon, Utah, USA)

The Early Triassic biotic recovery following the end-Permian mass extinction is well documented in the Smithian–Spathian Thaynes Group of the western USA basin. This sedimentary succession is commonly interpreted as recording harsh conditions of various shallow marine environments where microbial structures flourished. However, recent studies questioned the relevance of the classical view of long-lasting deleterious post-crisis conditions and suggested a rapid diversification of some marine ecosystems during the Early Triassic. Using field and microfacies analyses, we investigate a well-preserved Early Triassic marine sedimentary succession in Lower Weber Canyon (Utah, USA). The identification of microbial structures and their depositional settings provide insights on factors controlling their morphologies and distribution. The Lower Weber Canyon sediments record the vertical evolution of depositional environments from a middle Smithian microbial and dolosiliciclastic peritidal system to a late Smithian-early Spathian bioclastic, muddy mid ramp. The microbial deposits are interpreted as Microbially Induced Sedimentary Structures (MISS) that developed either (1) in a subtidal mid ramp where microbial wrinkles and chips are associated with megaripples characterizing hydrodynamic conditions of lower flow regime, or (2) in protected areas of inter- to subtidal inner ramp where they formed laminae and domal structures. Integrated with other published data, our investigations highlight that the distribution of these microbial structures was influenced by the combined effects of bathymetry, hydrodynamic conditions, lithology of the substrat physico-chemical characteristics of the depositional environment and by the regional relative sea-level fluctuations. Thus, we suggest that local environmental factors and basin dynamics primarily controlled the modalities of microbial development and preservation during the Early Triassic in the western USA basin.

Primary Sedimentary Structures and MISS in Gulcheru Quartzite along SW Part of Cuddapah Basin

ABSTRACT Sedimentary structures are very useful in paleocurrent analysis and interpretation of siliciclastic shallow marine environment. These interesting sedimentary structures such as parallel bedding, cross bedding, ripples, and mud cracks as well as synaeresis cracks, are best studied in the field. They are formed by a variety of sedimentary processes, including fluid flow, sediment-gravity flow, soft-sediment deformation and biogenic activity. Gulcheru Formation has evidence of interaction of microbial communities with clastic sedimentation during Palaeoproterozoic time. Because of high porosity-permeability of siliciclastics of Gulcheru Formation, palaeontologists do not expect many fossils preserved in such rocks and thus they have been overlooked. Microbially induced sedimentary structures (MISS) are highlighted in the Gulcheru Formation. They are indicative of shallow marine environment. Gulcheru Formation shows alluvial fan to shallow marine shelf environments within a limited thickness of about ∼ 110m, which indicates an alluvial plain coast where alluvial deposits are modified by wave forces.

Trace Fossils from the Brioverian (Ediacaran–Fortunian) in Brittany (NW France)

ABSTRACTThe Ediacaran–Fortunian ichnofauna from Central Brittany (NW France) is revised for the first time since the pioneering work by Lebesconte at the end of the 19th century. The study is based on fossils from the type-localities of the historical Brioverian taxa Montfortia (traces from Montfort-sur-Meu) and Neantia (wrinkle surfaces from Néant-sur-Yvel), and on two new outcrops from Saint-Gonlay. The ichnofossil assemblage includes Helminthoidichnites tenuis, Helminthopsis tenuis, Gordia marina, Palaeophycus tubularis, ?Neonereites uniserialis, and ?Spirodesmos archimedeus. Locally, the grazing traces are associated with wrinkle surfaces considered as microbially induced sedimentary structures (MISS). The sedimentological characteristics of these deposits correspond to a shelf marine environment. Both U-Pb datings on detrital zircon and ichnostratigraphic criteria tend toward an Ediacaran age of the fossiliferous deposits, but we cannot exclude the possibility of a Fortunian age at the present state of knowledge.

X-ray microtomography as a tool for investigating the petrological context of Precambrian cellular remains

Abstract A wide spectrum of tomographic techniques now exists for studying palaeontological specimens, but the suitability of these methods for assessing Earth's oldest prokaryotic life has not been comprehensively investigated. We evaluated the ability of X-ray computed tomography – specifically X-ray microtomography – to reveal the morphology and petrological context of Precambrian microfossils, pseudofossils and biosedimentary structures, all of which are important in the origin and early evolution of life of Earth. The materials tested came from the Pilbara Craton of Western Australia (the 3.49 Ga Dresser Formation, the 3.46 Ga Apex chert and the 3.43 Ga Strelley Pool Formation) and the 1.88 Ga Gunflint Formation of Ontario, Canada. These units chart key developments in palaeobiology. The oldest formations contain profoundly controversial microfossil-like objects and microbially-induced sedimentary structures, whereas definitive prokaryotes are found in the youngest formations. We demonstrate that the imaging of individual microfossils and pseudofossils currently lies at the limits of the capabilities of laboratory-based X-ray microtomography and requires beneficial taphonomy. However, microtomography does provide a good overview of their petrological context at flexible spatial scales, although the quality of the data obtained from mesoscopic microbially-induced sedimentary structures and stromatolites depends largely on their style of preservation. Supplementary material: A zipped Drishti volume for all CT scans, a 0.7z split zip file of one Drishti volume and a HDMI supplementary movie showing digital visualizations for all of the scans are available at https://doi.org/10.5281/zenodo.58161

‘ Intrites ’ from the Ediacaran Longmyndian Supergroup, UK: a new form of microbially-induced sedimentary structure (MISS)

Abstract Simple discoidal impressions are the only evidence of complex life in some Ediacaran and older rocks, but their interpretation is notoriously difficult. We reassessed a puzzling discoidal form from the c. 560 Ma upper Burway Formation of the Ediacaran Longmyndian Supergroup, Shropshire, UK. The structures, previously described as Intrites punctatus Fedonkin, are found on both the bed tops and soles. They vary in morphology from mounds with central depressions to incomplete rings and pairs of short ridges. Examination of the purported Intrites documented from the Longmyndian in cross-section revealed a torus-shaped structure bounded by microbial mat layers and commonly containing white laminae. We interpret the ‘Longmyndian Intrites ’ as a product of microbial trapping, sediment binding and authigenic clay mineral and carbonate precipitation on the flanks of small sediment volcanoes. Subsidence of the ring-like structure into muddy sediments resulted in a torus-shaped microstromatolite. Preferential stromatolitic growth parallel to the prevailing current produced the observed partial rings or parallel ridges and explains their preferential orientation as current alignment. This interpretation of ‘Longmyndian Intrites ’ expands the known variety of microbially-induced sedimentary structures (MISS) and emphasizes the importance of considering microbially-induced structures and abiological processes when interpreting discoidal impressions in ancient rocks.

Ediacaran discs from South America: probable soft-bodied macrofossils unlock the paleogeography of the Clymene Ocean

The origin, affinity and paleoecology of macrofossils of soft-bodied organisms of the terminal Ediacaran Period have been highly debated. Previous discoveries in South America are restricted to small shelly metazoans of the Nama Assemblage. Here we report for the first time the occurrence of discoidal structures from the Upper Ediacaran Cerro Negro Formation, La Providencia Group, Argentina. Specimens are preserved in tabular sandstones with microbially-induced sedimentary structures. Flute marks and linear scours at the base of the sandstone layers indicate deposition under high energy, episodic flows. Stratigraphic, sedimentologic, petrographic and taphonomic analyses indicate that the origin of these structures is not related to abiotic process. Preservational and morphological features, as invagination and the presence of radial grooves, indicate that they resemble typical morphs of the Aspidella plexus. The large number of small-sized individuals and the wide range of size classes with skewed distribution suggest that they lived in high-density communities. The presence of Aspidella in the Cerro Negro Formation would represent the first reliable record of Ediacaran soft-bodied organisms in South America. It also supports the paleogeographic scenario of the Clymene Ocean, in which a shallow sea covered part of the southwest Gondwana at the end of the Ediacaran.

Enigmatic Ediacaran megascopic bedding plane structures on the Sonia Sandstone, Jodhpur Group, Marwar Supergroup, India: seaweed or problematica?

The present study discusses the origin and affinity of enigmatic Ediacaran bedding plane structures reported earlier from the Sonia Sandstone of the Marwar Supergroup (Ediacaran to Lower Cambrian). These forms are conspicuously large, non-carbonaceous and three-dimensionally preserved. Morphologically, these structures are not comparable with any known plant/animal fossil, trace fossil, microbially induced sedimentary structures (MISS) found in the Ediacaran assemblages or any other extant forms and are invariably associated with MISS. Past researches on these fossils did not resolve their affinity, form and function. The present forms have certain features akin to seaweed organization such as thallus, loop pattern within the thallus, variously positioned nodes/swellings, tapering at the end of thallus, branching pattern and rarely occurring holdfast-like features. Systematic description, incorporating detailed morphological features, population size, their relationship with associated sediments and stratigraphic distribution, is based on the study of 54 specimens. Subtidal to intertidal models of depositional sedimentary environment, palaeoecology, biogenicity, affinity, taphonomic changes, association of these forms with MISS and other parameters are also discussed. A succession of plausible interpretations is offered; seaweed is the most likely interpretation. The Marwar Supergroup, thus, unravels some of the hitherto enigmatic forms of the Ediacaran Period (635–541 Ma). Copyright © 2016 John Wiley & Sons, Ltd.

Comparative study of two relatives, MISS and Stromatolites: example from the Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya

Comparison of microbially induced sedimentary structures (MISS) and stromatolitic bearing horizons from the Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya, has been scrutinised to understand the formative processes and controls on MISS and stromatolites in the context of sedimentary facies and response to sea level fluctuations. MISS structures recorded are wrinkle structures, Kinneyia ripples, load casts, domal structures, sand chips, palimpsest and patchy ripples with limited desiccation cracks. Stromatolitic morphotypes recorded are solitary, branching, wavy and domal forms of stromatolites associated with ooids, peloids and fenestral laminae. MISS structures flourished within tidal flats to shallow intertidal while stromatolites mushroomed in environments ranging from tidal to deep subtidal. MISS structures were favoured by resistant substratum, low energy conditions, consistent water supply and low terrigenous input. Stromatolites boomed when the volume of carbonate accumulation exceeded siliciclastic deposition. Fluctuating environmental conditions and sediment budget controlled morphology of stromatolites. Owing to limited siliciclastic input during deposition of dolomudstones (characterizes transgressive systems tract), microbial growth was enhanced. Calcareous shales were deposited over dolomudstones which marks the maximum flooding surface (MFS) indicating the culmination of transgression. Deposition of storm-dominated sandstone-siltstone (FA1), wave-rippled sandstones (FA2), tide-dominated sandstones (FA3), heteroliths (FA4), wackestone-packestone (FA6), boundstone (FA7) and ooid-peloid grainstone (FA8) on top of the MFS reflects initiation of highstand systems tract (HST) which is mainly characterized by stromatolitic horizons, alternation of carbonates and siliciclastics with flourishing microbial activity. Eventually, increased sedimentation in upper part of Kunihar Formation marks late stage of regression.

Microbial mats in the terrestrial Lower Triassic of North China and implications for the Permian–Triassic mass extinction

Evidence for microbial mats has been reported repeatedly from marine Lower Triassic rocks, but scarcely mentioned in post-mass extinction terrestrial facies. Here, we report from the terrestrial Lower Triassic Liujiagou Formation in North China the presence of five kinds of microbially induced sedimentary structures (MISS) or sedimentary surface textures, including “old elephant skin” textures, wrinkle structures, palimpsest ripples, “Manchuriophycus” structures and sand cracks. Terrestrial microbial communities that produced these MISS adapted not only to periodically desiccated conditions, but also to the storm-dominated palaeoenvironments in the Liujiagou Formation. The Permian–Triassic mass extinction (PTME) in North China is marked by the die-off of plants, disappearance of coal beds, extinction of pareiasaurs among tetrapods, decreased bioturbation levels and a dramatic change of sedimentary systems through the Sunjiagou Formation. The Sunjiagou Formation recorded the turnover from an ever-wet to a progressively drier and hotter climate and it spans the PTME in North China. Following this mass extinction, MISS became much more common and widespread, suggesting that the mass extinction provided favourable biological and environmental conditions for the development of the MISS in terrestrial ecosystems, especially the decreased bioturbation intensity and grazing pressure associated with increased temperature and climatic drying. In the upper part of the Liujiagou Formation and overlying Heshanggou Formation, the disappearance of MISS coupled with increased bioturbation might indicate an improvement of terrestrial ecosystems and the beginning of the Triassic biotic recovery. However, as the investigation of MISS in space and time through the geological record is in its early stages, further geobiologic and geochemical studies, as well as high-precision isotopic dating from Permian–Triassic terrestrial successions, are needed to fully reveal the timing and pattern of the Early Triassic terrestrial ecosystem reconstruction.

Microbially induced sedimentary structures in the Paleoproterozoic, upper Huronian Supergroup, Canada

The Paleoproterozoic Gordon Lake and Bar River formations, Huronian Supergroup, contain a variety of sedimentary structures in the Flack Lake area of Ontario, Canada, that have been considered of debatable origin. We identify these structures as microbially induced sedimentary structures (MISS). The preserved MISS are related to microbial mat destruction and decay, and include sand cracks, mat chips, remnant gas domes, pyrite patches, and iron laminae. A biological origin for the fossil structures is supported by their similarities to modern and ancient documented examples of MISS, the sand-dominated nature of the substrate in which they are preserved, and key microtextures identified in thin section. Microtextures include curled, frayed and layered mat chips, carbonaceous laminae, oriented grains, and concentrated heavy minerals. On outcrop scale, the presence of desiccation cracks, flaser and lenticular bedding, and ripples in association with the types of MISS identified in the Gordon Lake Formation support the interpretation of a tidal flat depositional environment. The Gordon Lake Formation contains a greater number and diversity of MISS than the overlying Bar River Formation, as a result of lower energy deposition in the former. The quartz arenite of the Bar River Formation contains fine-grained to pebbly granulestone characterized mainly by tangential and planar cross beds, which is consistent with a tidal channel or sand shoal setting. Although fossil evidence of life is rare in the rocks of the Huronian Supergroup, identification of MISS in the Flack Lake area provides a significant and convincing indication of microbial colonization at the time of deposition.

Proliferation of MISS-related microbial mats following the end-Permian mass extinction in the northern Paleo-Tethys: Evidence from southern Qilianshan region, western China

As a consequence of the end-Permian mass extinction, microbes proliferated in the post-extinction shallow marine ecosystems, in which they grew as various microbially induced sedimentary structures (MISSs) in siliciclastic settings. This paper reports, for the first time, the discovery of abundant MISSs from the lowest Triassic sandstones of shallow-water margin origin in the Zhihema sections of the southern Qilianshan region, West China. The sandstones are characterized by well-developed cross-beddings and ripple marks, and a Claraia-dominated bivalve assemblage of middle-late Griesbachian age. These sedimentary structures, together with the bivalves, suggest a high-energy peritidal zone of a shoreface setting in a clastic shallow sea environment. Seven types of MISSs are recognized and described here: pictograph-like sand cracks/crack-fills, polygonal sand crack-fills, erosional remnants, multidirectional linear grooves, sinuous crack-fills, fusiform sand cracks/crack-fills, and leveled ripple marks. Most of the newly found MISSs are morphologically comparable with their ancient and modern counterparts. Detailed optical microscope and scanning electron microscope (SEM) analyses reveal that thin clayey laminae and filamentous mica grains are aligned parallel to bedding plane, and that the matrix-supported quartz grains, overall, are oriented; both of which are interpreted to indicate biogenic origin. The biogenic origin of these MISSs is reinforced by the presence of copious putative nanoglobules and filamentous biofilm-like organic objects in the interspaces of clay minerals in laminated layers. These nanometer-scale objects are interpreted as bacterial bodies or remains that have been replaced with inorganic minerals upon fossilization. The presence of MISSs on the northern margins of Paleo-Tethys indicates that the post-extinction microbial mats had expanded their distributions from low-latitude to moderate-high latitude regions. Moreover, unlike some previously reported microbial mats that contain very rare body and trace fossils, the southern Qilianshan MISSs were found in association with abundant vertical burrows and bivalves, suggesting that the MISS-forming microbial mats may have served as oases for trace-making organisms and opportunistic bivalves to flourish in shallow-marine habitats immediately after the end-Permian mass extinction.

Microbially induced sedimentary structures from the 1.64 Ga Chuanlinggou Formation, Jixian, North China

The present study documents abundant exceptionally preserved microbially induced sedimentary structures (MISS) from the 1.64Ga Chuanlinggou Formation of the Jixian area, North China Craton. The Chuanlinggou Formation is dominated by black silty shale/mudstone with fine sandstone of the lower part and was likely deposited in a relatively restricted shallow marine environment varying from fair-weather wave action zone to below storm wave action zone. Both wrinkle structures and sand veins are very abundant in the lower and middle parts of the formation. Wrinkled layers are dark colored, densely laminated, and contain floating quartz grains and abundant mica grains, both of which are aligned parallel to bedding plane. Such an arrangement pattern is attributed to the microbe's binding and trapping of MISS. SEM imaging also shows that sticky networks interwoven by filamentous extracellular polymer substances (EPS) and biofilms, with distinct pores, are very pronounced in clay matrix of wrinkled layers, providing direct evidence of microbial activities during the formation of wrinkle structures. In addition, the circularly arranged quartz grains, forming hollow cavities, and vertical channels of quartz grains beneath the laminated layers are unique to the wrinkle structures showing initial forming stage of the sand veins. Sand veins, similar to fills of sand cracks, are usually preserved in silty shales and occasionally occur beneath the winkled surfaces. They were formed due to methane degassing of decay of microbial mats and biofilms. Abundant nano-particles of various EPSs (isolated nanoglobules, paired nanoglobules, dumbbell-shaped nanoglobules, and polyhedron aggregates) are detected in clay matrix close to sand veins, indicating microbial participation in the formation of sand veins. Wide spatial distribution, broadened environmental inhabitation, and extremely high abundance and bed surface density of the MISSs indicate proliferation of MISS-forming microbial mats in Mesoproterozoic times.

Early Jurassic microbial mats—A potential response to reduced biotic activity in the aftermath of the end-Triassic mass extinction event

Wrinkle structures are microbially induced sedimentary structures (MISS) formed by cyanobacteria and are common in pre-Cambrian and Cambrian siltstones and sandstones but are otherwise rare in the Phanerozoic geological record. This paper reports the first discovery of Mesozoic wrinkle structures from Sweden. These are preserved in fine-grained and organic-rich heterolithic strata of the Lower Jurassic (Hettangian) Höganäs Formation in Skåne, southern Sweden. The strata formed in a low-energy, shallow subtidal setting in the marginal parts of the Danish rift-basin. Palynological analyses of fine-grained sandstones hosting the wrinkle structures show that the local terrestrial environment probably consisted of a wetland hosting ferns, cypress and the extinct conifer family Cheirolepidaceae. Palynostratigraphy indicates a Hettangian age, still within the floral recovery phase following the end-Triassic mass extinction event. The finding of wrinkle structures is significant as the presence of microbial mats in the shallow subtidal zone, (in a deeper setting compared to where modern epibenthic microbial mats grow) suggests decreased benthic biodiversity and suppressed grazing in shallow marine environments in the early aftermath of the end-Triassic mass extinction event.

Proliferation of MISS-related microbial mats following the end-Permian mass extinction in terrestrial ecosystems: Evidence from the Lower Triassic of the Yiyang area, Henan Province, North China

Microbially induced sedimentary structures (MISSs) are commonly present in siliciclastic shallow marine settings following the end-Permian mass extinction, but have been rarely reported in the post-extinction terrestrial ecosystems. Here, we present six types of well-preserved MISSs from the upper Sunjiagou Formation and lower Liujiagou Formation of Induan (Early Triassic) age in the Yiyang area, Henan Province, North China. These MISSs include: polygonal sand cracks, worm-like structures, wrinkle structures, sponge pore fabrics, gas domes, and leveled ripple marks. Microanalysis shows that these MISSs are characterized by thin clayey laminae and filamentous mica grains arranged parallel to bedding plane as well as oriented matrix supported quartz grains, which are indicative of biogenic origin. Facies analysis suggests that the MISS-hosting sediments were deposited in a fluvial sedimentary system during the Early Triassic, including lake delta, riverbeds/point bars, and flood plain paleoenvironments. Abundant MISSs from Yiyang indicate that microbes also proliferated in terrestrial ecosystems in the aftermath of the Permian–Triassic (P–Tr) biocrisis, like they behaved in marine ecosystems. Microbial blooms, together with dramatic loss of metazoans, may reflect environmental stress and degradation of terrestrial ecosystems or arid climate immediately after the severe Permian–Triassic ecologic crisis.

Enigmatic spheres from the Upper Triassic Lockatong Formation, Newark Basin of eastern Pennsylvania: evidence for microbial activity in marginal-lacustrine strandline deposits

Microbes within sediments often create films or thick mats that interact with mobile sediment, producing microbially induced sedimentary structures (MISS). Preserved microbes in these sedimentary features are difficult to find, especially in oxygen-rich environments. The study of recent discoveries in the Upper Triassic Norian Lockatong Formation, a dominantly lacustrine facies of the Newark Supergroup strata, has revealed rarely reported structures that are assignable as MISS, namely, sand-cored spheres encapsulated in thin mudstone which range in diameter from the sub-millimetre to millimetre scale. These spheres are embedded in a thin layer of very fine silt and mudstone developed in an interpreted marginal-lacustrine shoreline setting. The cores of the spheres consist of randomly oriented, graded, fine-grained sandstone to siltstone; this eliminates previously proposed physical and biological origins for spheres, leaving a microbial origin. The microbial bound sand balls are localised and generated from a graded bed bound by microbial mats both above and below. The bounded bed was eroded and shaped into spheres during transport. The mats added cohesion by providing extrapolymeric substances which prevented the breakdown of the spheres into individual grains. The mats were rolled into spheres during transportation, thus preserving the microbial bound core. These sand balls add to the catalog of microbial diversity that can be used to increase our understanding of biological systems in lacustrine settings.

New insights into microbially induced sedimentary structures in alkaline hypersaline El Beida Lake, Wadi El Natrun, Egypt

Microbially induced sedimentary structures (MISS) were studied in detail in the alkaline hypersaline El Beida Lake of Wadi El Natrun in the western desert sector of Egypt, based on field observations and sampling performed in 2013 and 2014. Geomorphologically, the lake can be subdivided into three zones, each with characteristic sedimentary and biosedimentary structures. The marginal elevated zone that borders the lake is characterized by thick blocky crusts devoid of microbial mats. The middle–lower supratidal zone has luxuriant microbial mats associated with knotty surfaces, mat cracks and wrinkle structures. A zone of ephemeral shallow pools and channels is characterized by reticulate surfaces, pinnacle mats, sieve-like surfaces, gas domes and mat chips. In the microbial mats, authigenic minerals include thenardite Na2SO4, trona Na3(CO3)(HCO3)•2H2O and halite NaCl. Scanning electron microscopy (SEM) analyses revealed that the minerals are closely associated with the MISS, suggesting some influence of microorganisms on mineral precipitation. Complex interactions between regional hydrological cycles and diagenetic processes imply low preservation potential. MISS signatures of such saline lakes can serve as key analogues for interpreting the geologic record.

The unusual record of Nereites, wrinkle marks and undermat mining trace fossils from the late Silurian–earliest Devonian of central-western margin of Gondwana (Argentina)

Silurian–Devonian deposits of central western Argentina contain one of the most diverse Palaeozoic records of Nereites in Gondwana. Nereites cf. cambrensis, Nereites irregularis?, Nereites jacksoni, Nereites macleayii, Nereites missouriensis and Nereites pugnus are documented in a turbidite-like succession of the San Rafael Block (Río Seco de los Castaños Formation). The succession, historically assumed as deep-sea deposits or interpreted as a deltaic system, also contains abundant microbially induced sedimentary structures (MISS) such as wrinkle marks. Glossy surfaces indicate abundant biofilms, where the fossil record comprises N. macleayii, N. missouriensis, scarce arthropod trackways (Diplichnites?) and undermat-miner structures. Nereites, the pioneer colonizer, is in some cases cross-cut by Dictyodora, while undermat miners are the late bioturbators. The succession is dominated by gravity flow deposits, including storm related, turbidite-like deposits and abundant wrinkle marks. These suggest deposition on the shelf rather than the deep sea. This study shows that microbial mats were not restricted to marginal marine environments during the mid Palaeozoic, and documents one of the most diverse Palaeozoic records of Nereites in western Gondwana. It also contributes to the record of Nereites, Dictyodora, and Zoophycos found, for first time, in association with microbial mat structures in the late Silurian−earliest Devonian, the Nereites ichnofacies together with abundant wrinkle marks.

The ichnogenus Dictyodora from late Silurian deposits of central-western Argentina: Ichnotaxonomy, ethology and ichnostratigrapical perspectives from Gondwana

The association of trace fossils from the Late Silurian Río Seco de los Castaños Formation in central-western Argentina (San Rafael Block) is mainly composed of Dictyodora, including Dictyodora scotica, Dictyodora tenuis and a new ichnospecies named Dictyodora atuelica. The latter shows a tendency to migrate to a deeper level – analogous to Dictyodora liebeana's – which reflects more sophisticated behavioural patterns. The Dictyodora ichnospecies (one of the most diverse assemblages in Gondwana) coexists with Nereites. SEM observations and EDS analyses suggest that the wall structure of Dictyodora could be the result of an activity other than the traditional suggestion of respiration, e.g., feeding. The succession, including some of the levels containing Dictyodora, presents microbially induced sedimentary structures (MISS) that would indicate a photic zone. This discovery favours previous deltaic interpretations instead of the deep-sea fan deposits traditionally suggested for the Siluro-Devonian greywackes of the Cuyania terrane, and particularly for this unit. The abundant Dictyodora, Nereites, Zoophycos and undermat miners ichnofossils, as well as the scarce arthropod trackways, resemble older records in Europe and North America. The studied section is very well age-constrained, and the autochthonous Gondwana ichnofauna permits to cast doubt on previous interpretations, suggesting that after the Ordovician, microbial mats retreated to marginal marine environments and then spread later in the Carboniferous. More importantly, the occurrence of Dictyodora and Nereites in beds that bear abundant wrinkle structures or records of biofilms in some places is challenging with regard to deep-sea interpretations. Dictyodora is probably not a good stratigraphic indicator, and the provincialism suggested for this ichnogenus may be an artefact of the lack of more complete records in Gondwana.

Ancient sedimentary structures in the

Sandstone beds of the <3.7 Ga Gillespie Lake Member on Mars have been interpreted as evidence of an ancient playa lake environment. On Earth, such environments have been sites of colonization by microbial mats from the early Archean to the present time. Terrestrial microbial mats in playa lake environments form microbialites known as microbially induced sedimentary structures (MISS). On Mars, three lithofacies of the Gillespie Lake Member sandstone display centimeter- to meter-scale structures similar in macroscopic morphology to terrestrial MISS that include "erosional remnants and pockets," "mat chips," "roll-ups," "desiccation cracks," and "gas domes." The microbially induced sedimentary-like structures identified in Curiosity rover mission images do not have a random distribution. Rather, they were found to be arranged in spatial associations and temporal successions that indicate they changed over time. On Earth, if such MISS occurred with this type of spatial association and temporal succession, they would be interpreted as having recorded the growth of a microbially dominated ecosystem that thrived in pools that later dried completely: erosional pockets, mat chips, and roll-ups resulted from water eroding an ancient microbial mat-covered sedimentary surface; during the course of subsequent water recess, channels would have cut deep into the microbial mats, leaving erosional remnants behind; desiccation cracks and gas domes would have occurred during a final period of subaerial exposure of the microbial mats. In this paper, the similarities of the macroscopic morphologies, spatial associations, and temporal succession of sedimentary structures on Mars to MISS preserved on Earth has led to the following hypothesis: The sedimentary structures in the <3.7 Ga Gillespie Lake Member on Mars are ancient MISS produced by interactions between microbial mats and their environment. Proposed here is a strategy for detecting, identifying, confirming, and differentiating possible MISS during current and future Mars missions.