Lithostratigraphic analysis of a new stromatolite–thrombolite reef from across the rise of atmospheric oxygen in the Paleoproterozoic Turee Creek Group,Western Australia

This study describes a previously undocumented dolomitic stromatolite–thrombolite reef complex deposited within the upper part (Kazput Formation) of the c. 2.4–2.3 Ga Turee Creek Group, Western Australia, across the rise of atmospheric oxygen. Confused by some as representing a faulted slice of the younger c. 1.8 Ga Duck Creek Dolomite, this study describes the setting and lithostratigraphy of the 350‐m‐thick complex and shows how it differs from its near neighbour. The Kazput reef complex is preserved along 15 km of continuous exposure on the east limb of a faulted, north‐west‐plunging syncline and consists of 5 recognisable facies associations (A–E), which form two part regressions and one transgression. The oldest facies association (A) is characterised by thinly bedded dololutite–dolarenite, with local domical stromatolites. Association B consists of interbedded columnar and stratiform stromatolites deposited under relatively shallow‐water conditions. Association C comprises tightly packed columnar and club‐shaped stromatolites deposited under continuously deepening conditions. Clotted (thrombolite‐like) microbialite, in units up to 40 m thick, dominates Association D, whereas Association E contains bedded dololutite and dolarenite, and some thinly bedded ironstone, shale and black chert units. Carbon and oxygen isotope stratigraphy reveals a narrow range in both δ¹³C_carb values, from ?0.22 to 0.97‰ (VPDB: average = 0.68‰), and δ¹⁸O values, from ?14.8 to ?10.3‰ (VPDB), within the range of elevated fluid temperatures, likely reflecting some isotopic exchange. The Kazput Formation stromatolite–thrombolite reef complex contains features of younger Paleoproterozoic carbonate reefs, yet is 300–500 Ma older than previously described Proterozoic examples worldwide. Significantly, the microbial fabrics are clearly distinct from Archean stromatolitic marine carbonate reefs by way of containing the first appearance of clotted microbialite and large columnar stromatolites with complex branching arrangements. Such structures denote a more complex morphological expression of growth than previously recorded in the geological record and may link to the rise of atmospheric oxygen.     

Facies selectivity of benthic invertebrates in a Permian/Triassic boundary microbialite succession: Implications for the “microbialite refuge” hypothesis

Thrombolite and stromatolite habitats are becoming increasingly recognized as important refuges for invertebrates during Phanerozoic Oceanic Anoxic Events (OAEs); it is posited that oxygenic photosynthesis by cyanobacteria in these microbialites provided a refuge from anoxic conditions (i.e., the “microbialite refuge” hypothesis). Here, we test this hypothesis by investigating the distribution of ~34, 500 benthic invertebrate fossils found in ~100 samples from a microbialite succession that developed following the latest Permian mass extinction event on the Great Bank of Guizhou (South China), representing microbial (stromatolites and thrombolites) and non‐microbial facies. The stromatolites were the least taxonomically diverse facies, and the thrombolites also recorded significantly lower diversities when compared to the non‐microbial facies. Based on the distribution and ornamentation of the bioclasts within the thrombolites and stromatolites, the bioclasts are inferred to have been transported and concentrated in the non‐microbial fabrics, that is, cavities around the microbial framework. Therefore, many of the identified metazoans from the post‐extinction microbialites are not observed to have been living within a microbial mat. Furthermore, the lifestyle of many of the taxa identified from the microbialites was not suited for, or even amenable to, life within a benthic microbial mat. The high diversity of oxygen‐dependent metazoans in the non‐microbial facies on the Great Bank of Guizhou, and inferences from geochemical records, suggests that the microbialites and benthic communities developed in oxygenated environments, which disproves that the microbes were the source of the oxygenation. Instead, we posit that microbialite successions represent a taphonomic window for exceptional preservation of the biota, similar to a Konzentrat‐Lagerstätte, which has allowed for diverse fossil assemblages to be preserved during intervals of poor preservation.     

Geological and trace element evidence for a marine sedimentary environment of deposition and biogenicity of 3.45 Ga stromatolitic carbonates in the Pilbara Craton, and support for a reducing Archaean ocean

Bedded carbonate rocks from the 3.45 Ga Warrawoona Group, Pilbara Craton, contain structures that have been regarded either as the oldest known stromatolites or as abiotic hydrothermal deposits. We present new field and petrological observations and high‐precision REE + Y data from the carbonates in order to test the origin of the deposits. Trace element geochemistry from a number of laminated stromatolitic dolomite samples of the c. 3.40 Ga Strelley Pool Chert conclusively shows that they precipitated from anoxic seawater, probably in a very shallow environment consistent with previous sedimentological observations. Edge‐wise conglomerates in troughs between stromatolites and widespread cross‐stratification provide additional evidence of stromatolite construction, at least partly, from layers of particulate sediment, rather than solely from rigid crusts. Accumulation of particulate sediment on steep stromatolite sides in a high‐energy environment suggests organic binding of the surface. Relative and absolute REE + Y contents are exactly comparable with Late Archaean microbial carbonates of widely agreed biological origin. Ankerite from a unit of bedded ankerite–chert couplets from near the top of the stratigraphically older (3.49 Ga) Dresser Formation, which immediately underlies wrinkly stromatolites with small, broad, low‐amplitude domes, also precipitated from anoxic seawater. The REE + Y data of carbonates from the Strelley Pool Chert and Dresser Formation contrast strongly with those from siderite layers in a jasper–siderite–Fe‐chlorite banded iron‐formation from the base of the Panorama Formation (3.45 Ga), which is clearly hydrothermal in origin. The geochemical results, together with sedimentological data, strongly support: (1) deposition of Dresser Formation and Strelley Pool Chert carbonates from Archaean seawater, in part as particulate carbonate sediment; (2) biogenicity of the stromatolitic carbonates; (3) a reducing Archaean atmosphere; (4) ongoing extensive terrestrial erosion prior to ～3.45 Ga.     

Facies characteristic and paleoenvironmental reconstruction of the Fahliyan Formation,Lower Cretaceous,in the Kuh-e Siah area,Zagros Basin,southern Iran

The Lower Cretaceous Fahliyan Formation, part of the Khami Group, unconformably overlies the Hith Formation and is conformably overlain by the Gadvan Formation in the study area in southern Iran. The Fahliyan Formation is a reservoir rock in Zagros Basin. This formation was investigated by a detailed petrographic analysis in order to clarify the depositional facies and sedimentary environment in the Kuh-e Siah Anticline in Boushehr Province. Petrographic studies led to the recognition of 25 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open-marine in mid-ramp environment. An absence of turbidite deposits, reefal facies, and gradual facies changes indicate that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. These skeletal grains have been studied in order to increase the understanding of their distributions in time and space. A total of ten genera belonging to different groups of calcareous algae and 16 genera of benthic foraminifera are recognized from the Fahliyan Formation at Kuh-e Siah section.     

Formation of micro‐spherulitic barite in association with organic matter within sulfidized stromatolites of the 3.48 billion‐year‐old Dresser Formation,Pilbara Craton

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion‐year‐old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro‐mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro‐mineralization within the sulfidized stromatolites generally forms submicron‐scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro‐spherulites show thin outer shells. Within stromatolites, barite micro‐spherulites are intimately associated with petrographically earliest dolomite and nano‐porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr‐rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro‐spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near‐identical morphology and micro‐texture is known from zones of high bio‐productivity under low‐temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro‐spherulitic barite in the organic matter‐bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well‐preserved, and very ancient microbialites.     

Taphonomy and palaeoecology of Hauterivian–Barremian nerineoid shell beds from the Neuquén Basin,west‐central Argentina

Nerineoid shell beds are described for the first time from Lower Cretaceous deposits of southern South America. These come from carbonates near the top of the Agrio Formation in southern Mendoza Province, west‐central Argentina. To envisage the origin of the nerineoid shell beds, a taphonomic study was carried out, which indicated that these represent within‐habitat time‐averaged, primary sedimentological concentrations with a secondary biogenic imprint related to a relatively high local production of nerineoid shells. The associated palaeoenvironments were studied through a facies analysis of the carbonate succession including the shell beds. The carbonates were deposited in a homoclinal ramp system and depict a shallowing upward trend from mid to inner ramp. The individuals lived and accumulated in oolitic shoals within the inner ramp, in a shallow, well‐lit, high‐energy setting above fair‐weather wave base. Substrate was oxygenated and loose. The nerineoids are shown to belong to one species of the genus Eunerinea, and through the functional morphology of the shells they are tentatively interpreted as infaunal or semi‐infaunal. It is suggested that the recorded monospecific nerineoid shell beds indicate that the palaeoenvironmental conditions may have been favourable for the development of abundant populations of these gastropods in the northern part of the Neuquén Basin during a short time interval in the Hauterivian–Barremian boundary. This could have been related to a brief warming episode, but other factors may have also been involved. □Argentina, Early Cretaceous, Gastropods, nerineoids, Neuquén Basin, shell beds, taphonomy.     

Onshore expansion of benthic communities after the Late Devonian mass extinction

A detailed ichnological analysis of the Upper Devonian–Lower Mississippian Bakken Formation of sub‐surface Saskatchewan and the partially coeval Exshaw Formation of Alberta indicates the presence of an anomalous ichnofacies gradient. The distal Cruziana Ichnofacies, which in rocks of other ages is restricted to lower‐offshore facies, here ranges from this setting to the lower shoreface. No archetypal Cruziana Ichnofacies is present in these deposits. This pattern is interpreted as resulting from the differential effects of the Late Devonian mass extinction in shallow‐water ecosystems. The onshore expansion evidenced by ichnological data is consistent with the pattern displayed by the body‐fossil record, which indicates a re‐invasion of shallow‐water environments by the Palaeozoic evolutionary fauna during the Late Devonian and into the Early Carboniferous. The ichnofauna studied is overwhelmingly dominated by deposit feeders, with suspension feeders being notably absent, further underscoring the importance of trophic type as a selectivity trait during mass extinctions.     

内蒙古东南部开鲁盆地晚白垩世嫩江组孢粉植物群

开鲁盆地白垩系发育,至今已有200余口探井钻遇到上白垩统,其中嫩江组滨浅湖相沉积地层厚度稳定,分布广泛,含较丰富的孢粉化石。通过对嫩江组孢粉化石的系统研究,建立了以Schizaeoisporites-BeaupreaiditesLythraites为代表的孢粉组合。孢粉组合和岩石组合的对比表明,开鲁盆地缺失松辽盆地嫩江组中上部地层。根据沉积特征和孢粉植物群面貌,对古植被、古气候和古环境进行了讨论,认为本区晚白垩世处于东北中生孢粉植物区南缘,孢粉植物群具有过渡性质,反应了偏干的暖温带—亚热带气候特征。     

Early ordovician reefs from Argentina: Stromatoporoid vs stromatolite origin

Summary Late Arenigian biohermal reef mounds and biostromes within the shallow-marine platform facies of the upper San Juan Formation of the Precordillera (Western Argentina) represent a new Early Ordovician reef type. The meter-sized reefs are dominated byZondarella communis n.g. n. sp. The new taxon is characterized by domical, bulbous and laminar morphotypes exhibiting growth layers and thin horizontal and vertical as well as intermingled skeletal elements included within different sets. The fossil maybe compared with stromatolites and stromatoporoids but an interpretation as primitive stromatoporoids is favoured.     

Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups,India

Fossil microbiotas are rare in the early rock record, limiting the type of ecological information extractable from ancient microbialites. In the absence of body fossils, emphasis may instead be given to microbially derived features, such as microbialite growth patterns, microbial mat morphologies, and the presence of fossilized gas bubbles in lithified mats. The metabolic affinity of micro‐organisms associated with phosphatization may reveal important clues to the nature and accretion of apatite‐rich microbialites. Stromatolites from the 1.6 Ga Chitrakoot Formation (Semri Group, Vindhyan Supergroup) in central India contain abundant fossilized bubbles interspersed within fine‐grained in situ‐precipitated apatite mats with average δ¹³C_org indicative of carbon fixation by the Calvin cycle. In addition, the mats hold a synsedimentary fossil biota characteristic of cyanobacterial and rhodophyte morphotypes. Phosphatic oncoid cone‐like stromatolites from the Paleoproterozoic Aravalli Supergroup (Jhamarkotra Formation) comprise abundant mineralized bubbles enmeshed within tufted filamentous mat fabrics. Construction of these tufts is considered to be the result of filamentous bacteria gliding within microbial mats, and as fossilized bubbles within pristine mat laminae can be used as a proxy for oxygenic phototrophy, this provides a strong indication for cyanobacterial activity in the Aravalli mounds. We suggest that the activity of oxygenic phototrophs may have been significant for the formation of apatite in both Vindhyan and Aravalli stromatolites, mainly by concentrating phosphate and creating steep diurnal redox gradients within mat pore spaces, promoting apatite precipitation. The presence in the Indian stromatolites of alternating apatite‐carbonate lamina may result from local variations in pH and oxygen levels caused by photosynthesis–respiration in the mats. Altogether, this study presents new insights into the ecology of ancient phosphatic stromatolites and warrants further exploration into the role of oxygen‐producing biotas in the formation of Paleoproterozoic shallow‐basin phosphorites.     

The Quilon Limestone,Kerala Basin,India: an archive for Miocene Indo‐Pacific seagrass beds

Reuter, M., Piller, W.E., Harzhauser, M., Kroh, A., Rögl, F. & ?ori?, S. 2010: The Quilon Limestone, Kerala Basin, India: an archive for Miocene Indo‐Pacific seagrass beds. Lethaia, Vol. 44, pp. 76–86. The facies of the fossiliferous Quilon Limestone in SW India is described for the first time in detail at the Padappakkara‐type locality. Facies (fossiliferous, micrite‐rich, bioturbated sediment with intercalated sand pockets) and faunal composition (epiphytic foraminifers, seagrass feeding Smaragdia gastropods, bioimmuration of celleporiform bryozoan colonies) indicate a seagrass environment. The large discoidal archaiasin foraminifer Pseudotaberina malabarica, in particular, is considered as a proxy for seagrass communities. Recent seagrasses have their centre of generic richness in the Indo‐Pacific where they cover wide areas in the tidal and shallow sub‐tidal zones. However, their geological record is only fragmentary and their palaeobiogeographic distribution has a big stratigraphical gap in the Miocene Western Indo‐Pacific region. The described nannoplankton flora and planktonic foraminifers from the Quilon Formation demonstrate that the deposition of the studied seagrass bed occurred in nannoplankton biozone NN3. This timing suggests formation during the closure of the Tethyan Seaway. The Quilon Limestone is thus an early Western Indo‐Pacific seagrass bed and an important step in reconstructing the history of seagrass communities. □Quilon Formation, Pseudotaberina malabarica, seagrass facies, Burdigalian, Indo‐Pacific.     

Deep-water stromatolites andFrutexites Maslov from the early and Middle Jurassic of S-Germany and Austria

Summary Despite extensive discussions during the last 20 years stromatolites are still used by many geologists as unequivocal indicators of very shallow-water conditions. We investigated four stratigraphic units from the Lower and Middle Jurassic of southern Germany (Posidonien-Schiefer, Amaltheen-Ton) and of the Northern Calcareous Alps (Adneter Kalk, Klauskalk), which were formerly interpreted as shallow marine sediments by some authors due to the occurrence of stromatolites. Our interpretations of the macro-, micro- and ultrafacies of these sediments are not compatible with shallow-water settings. We therefore propose a deep-marine, aphotic origin of these stromatolites. Former interpretations of the Posidonien-Schiefer as a shallow-water deposit are mainly based on the occurrence of stromatolites. We favour the model of a temporarily stagnant, deep, aphotic basin for these planktonrich sediments. Particles resembling ooids, but lying within mudstones cannot be taken as evidence for shallow agitated water. They either formed within the mud or are allochthonous. The deep-water setting of the red limestone of the Alpine Early and Middle Jurassic is indicated by a lack of platform-typical components like coated grains and phototrophic benthos and by shells of plankton and nekton forming a major part of the sediment. Stromatolites occur on the steep slope of a drowned Rhaetian reef with an estimated relief of 50–100 m and immediately below and within radiolarian limestones, deposited below the aragonite compensation depth (ACD). The aphotic stromatolites show some morphological differences to their shallow water counterparts. In all of our sections they occurred during intervals of reduced sedimentation. They form only thin horizons and probably grew very slowly. Mineralizations by Fe−Mn oxides and phosphate are very common. The presence of a microbial film is evident from binding of sedimentary particles, but the nature of the microbes is not known. Growth habits within the very distinct environments of red limestone and black shales show some common features, but also clear differences. The microproblematicumFrutexites Maslov is a very common component in deep-water stromatolites, but may also itself form small crusts or dendrolites. It occurs in two different forms. Opaque, slender forms with indistinct outlines probably grew within the weakly lithified sediment. Thicker, transparent forms with well defined outlines are found in cavities and probably also grew on the seafloor. Well preserved specimens display an internal fabric of radially arranged fibres of Fe−Mn oxides and calcite. It is suggested that calcite or aragonite were one original mineralogy ofFrutexites, which was later replaced by Fe−Mn oxides or phosphate. It is not certain whetherFrutexites is an organic, biomineralized structure or an inorganic mineralization, but the variable mineralogy and growth forms in different environments point to an organic origin. But even if organic, the occurrence in cryptic habitats and negative phototactic growth-directions make it clear thatFrutexites was not phototrophic.     

A new Early Cretaceous lamniform shark (Chondrichthyes,Neoselachii)

Eoptolamna eccentrolopha gen. et sp. nov. (Chondrichthyes, Lamniformes) from the near coastal upper Barremian Artoles Formation (Early Cretaceous) of Castellote (northwestern Spain) is described on the basis of about 50 isolated teeth. This taxon represents one of the earliest lamniform sharks known to date. We hypothesize that most pre‐Aptian lamniforms belong to an ancient group characterized, amongst others, by a very weak gradient monognathic heterodont dental pattern, and by tearing‐type dentition. There is a nutritive groove in the lingual root protuberance in juveniles of Eoptolamna, which persists in adults. A single pair of symphysial and a pair of upper intermediate teeth might have been present. Consequently, a new family, Eoptolamnidae, is introduced to include the new form, as well as Protolamna and probably Leptostyrax. The Eoptolamnidae represent an ancient family within Lamniformes. The origin of lamniform sharks remains, however, ambiguous despite recent advances. The new Spanish taxon is widespread in the Barremian of north‐eastern Spain, and occurs in a wide range of facies from near‐coastal to lake deposits. This lamniform also occurs in the Lower Cretaceous of northern Africa. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 154 , 278–290.     

New ammonoid taxa from the Lower Cretaceous Giumal Formation of the Tethyan Himalaya (Northern India)

Knowledge of the Early Cretaceous ammonoids of the NW‐Himalayas was poor until recent discoveries. Intense sampling from the Giumal Formation exposed near the village of Chikkim (Spiti Valley, Himachal Pradesh, India) led to the recognition of a new Early Cretaceous ammonoid fauna. The succession consists of arenitic sandstone interbedded with shale that was deposited by turbidity currents on an unstable shelf in the Early Cretaceous. Ammonoids have been obtained only from sandstone beds in the lower one‐third and close to the top of the c. 350‐m‐thick section. Eight new ammonoid taxa (1 genus and 7 species) are described: Sinzovia franki sp. nov. (rare), Giumaliceras giumaliense gen. et sp. nov. (abundant), Giumaliceras bhargavai gen. et sp. nov. (rare), Neocomites (Eristavites) platycostatiformis sp. nov. (rare), Cleoniceras oberhauseri sp. nov. (abundant), Australiceras himalayense sp. nov. (rare) and Deshayesites fuchsi sp. nov. (rare). Sinzovia and Deshayesites are reported for the first time from the Tethyan Himalaya. According to the biostratigraphic relevance of some ammonoid taxa described here, the age of the Giumal Formation can be constrained from Berriasian (Giumaliceras assemblage) to Aptian (Cleoniceras assemblage). The discovery of the new fauna substantiates the significance of the Giumal Formation around Chikkim and facilitates comparison with faunal assemblages from other regions in the Tethys Ocean and beyond.     

Integrated sedimentological,ichnological and sequence stratigraphical studies of the Koti Dhaman Formation (Tal Group), Nigali Dhar Syncline,Lesser Himalaya,India: paleoenvironmental,paleoecological, paleogeographic significance

Abstract

Integrated ichnology, sedimentology and sequence stratigraphy of the Lower Quartzite Member to the Arkosic Sandstone Member of the Koti Dhaman Formation (Cambrian Series 2, Stage 4), Tal Group, Nigali Dhar Syncline, Lesser Himalayan lithotectonic zone are presented. Trilobite traces of Gondwanan affinity i.e., Cruziana salomonis, Cruziana fasciculata, Rusophycus dispar and Rusophycus burjensis are recorded along with Arenicolites isp. and Skolithos isp. from the Lower Quartzite Member. A rich and diverse ichnoassemblage attributed to the Cruziana ichnofacies is described for the first time from the Arkosic Sandstone Member of the same formation. Seven ichnofossil assemblages, i.e., Cruziana-Rusophycus, Planolites-Palaeophycus, Cruziana problematica, Diplichnites, Cochlichnus anguineus, Bergaueria perata and Psammichnites gigas have been recognized in the Lower Quartzite to Arkosic Sandstone members of the Koti Dhaman Formation. Seven sedimentary facies i.e., sandstone–shale facies (FT1), cross-bedded (trough and planar) sandstone (FT2), bedded sandstone facies (FT3), shale facies (FT4), shale–sandstone facies (FT5), shale-rippled sandstone facies (FT6) and planar and trough cross-laminated sandstone (FT7) and four facies associations FA1-FA4 are identified in the Koti Dhaman Formation. The formation contains shallowing upward parasequences of a tidal flat complex. Overall, two major events are recognized: i) the break in sedimentation between the Lower Quartzite Member and the overlying Shale Member probably related to forced-regressive event and ii) the facies shift from FT6 to FT7 of the Arkosic Sandstone Member represents an erosive transgressive event; the surface is interpreted as wave ravinement surface, which also serves as a sequence boundary. Integrated ichnology, sedimentology and sequence stratigraphic studies indicate that the Lower Quartzite Member was deposited in a shallow subtidal sand sheet complex and tidal flat complex; the Shale Member was deposited in a mud flat setting of a tidal flat complex, and the Arkosic Sandstone Member in a mixed-flat (tidal flat complex) to sand sheet complex front and margin (subtidal sand sheet complex). Overall, the lower to middle part of the Koti Dhaman Formation represents a tide-dominated shallow subtidal–intertidal to mud-flat subenvironments of the tidal flat complex. A palaeogeographic reconstruction of lower Cambrian (516–514?Ma) is presented based on the distribution of trilobite traces from the Lesser Himalaya and the Bikaner–Nagaur area of Peninsular India (eastern Gondwana), Egypt, Jordan, Turkey (western Gondwana) and Canada (Avalonia).     

Stromatolite biostromes and associated facies in the late precambrian porsanger dolomite formation of Finnmark,Arctic Norway

The Late Precambrian Porsanger Dolomite Formation, occurring beneath the Varanger tillite in Arctic Norway, consists of various dolomitic lithofacies of subtidal, intertidal and supratidal environments. The lithofacies belong to three facies associations, A, B and C, which are repeated several times in the sequence. Facies association A comprises cryptalgal laminites, dolomicrites and thin-bedded grainstones and flakestones. The environment represented by this facies is broadly intertidal (locally supratidal) flat, with the interbedded carbonate sands representing storm deposits. Facies association B, of shallow subtidal to low intertidal origin, comprises cross-bedded carbonate sands (flakestones, grainstones and oolites) forming units up to 10 m thick. Small stromatolite bioherms (5 m wide, 2 m high) are locally developed within these “high-energy” deposits. Facies association C formed in a subtidal environment consists of laterally extensive (over 20 km) uniformly developed stromatolite biostromes, up to 16 m thick. The biostromes, locally divided by channels filled with grainstones and intraformational conglomerates, are composed of cylindrical and turbinate columnar (SH-V and SH-C) and digitate stromatolites (Gymnosolen, Inseria and Tungussia) in their lower parts. Larger, bulbous (SH-C and LLH-C) and conical (Conophyton) stromatolites occur in the upper parts, as well as the branching conophyte, Jacutophyton.All of the biostromes are always developed above cross-bedded carbonate sands (facies association B). A broadly symmetrical cyclic pattern, A B C B A, of tidal flat deposits (facies association A) passing up into carbonate sands (B), into biostrome (C), overlain by carbonate sands (B) and then tidal flat deposits (A), is repeated four times in the Porsanger Dolomite sequence. The pattern is interpreted in terms of two controls on sedimentation: (1) a slow transgressive phase followed by (2) depositional regression. The former (1) took place either through eustatic sea-level rise or more likely through accelerated subsidence because of tectonic instability and compaction of underlying sediments. This resulted in the sequence: tidal flat sediments, low intertidal/shallow subtidal carbonate sands, subtidal biostrome (A, B, C). Depositional regression through prograding tidal flats, generated the shoaling upward part of the cycle: biostrome, carbonate sands, tidal flat sediments (C, B, A).     

An Updated Review of the Avian Footprint Record from the Yacoraite Formation (Maastrichtian-Danian), Northwestern Argentina

Abstract

The Yacoraite Formation (Maastrichtian–Danian; Balbuena Subgroup; Salta Group), from Northwestern Argentina, represents a shallow epeiric unit which is the result of transgressions in the Andean basin of South America. Herein, we study the avian footprints from the Maimará locality, Jujuy province, and Quebrada del Tapón ichnosite, Salta province. The avian footprints from the Maimará locality is less diverse, made up: cf. Alaripeda isp., Avipeda isp., cf. Gruipeda filiportatis, and cf. Gruipeda isp. The avian track record from the Quebrada del Tapón ichnosite is composed of: cf. Alaripeda isp., Ardeipeda cf. egretta., Gruipeda filiportatis (=Yacoraitichnus avis) and others Gruipeda or compared to this ichnogenus, cf. Uhangrichnus isp., and several indetermined avian footprints. Both ichnological assemblages have tracks assigned to shorebirds or Charadriiformes. The facies distribution of trace fossils reinforces the overall model of a shorebird tracks dominance in moderate to low energy settings. This distribution can be divided into three sub-environments: (1) a moderate to high energy shoreline under wave action, (2) a supratidal body of ephemeral ponds, far away from direct wave influence and (3) a landward position, beyond the ephemeral ponds system. We have included all the ichnoassemblages within the shorebird ichnosubfacies as a subset of the Scoyenia Ichnofacies.     

Ichnology of the Winnipeg Formation,southeast Saskatchewan: a glimpse into the marine infaunal ecology of the Great Ordovician Biodiversification Event

The ichnology of the Middle Ordovician Winnipeg Formation has been analysed based on the study of cores from five wells drilled in southeast Saskatchewan (Canada). Six sedimentary facies, ranging from upper shoreface to lower offshore settings in a shallow‐marine environment, have been characterized. Ichnological attributes are consistent with those in currently proposed models for shallow‐marine wave‐dominated settings, but ichnodiversity is lower than in post‐Palaeozoic settings. Low ichnodiversity in the Winnipeg Formation most likely reflects evolutionary factors rather than environmental controls. Interestingly, low‐energy, distal deposits of the Winnipeg Formation display intense degree of bioturbation, reflecting a well‐developed mixed layer and underscoring the importance of the Great Ordovician Biodiversification Event in terms of sediment mixing.     

Taphonomy of the middle Cambrian (Drumian) Ravens Throat River Lagerstätte,Rockslide Formation,Mackenzie Mountains,Northwest Territories,Canada

The Middle Cambrian (series 3, Drumian, Bolaspidella Biozone) Ravens Throat River Lagerstätte in the Rockslide Formation of the Mackenzie Mountains, northwestern Canada, contains a Burgess Shale‐type biota of similar age to the Wheeler and Marjum formations of Utah. The Rockslide Formation is a unit of deep‐water, mixed carbonate and siliciclastic facies deposited in a slope setting on the present‐day northwestern margin of Laurentia. At the fossil‐bearing locality, the unit is about 175 m thick and the lower part onlaps a fault scarp cutting lower Cambrian sandstones. It consists of a succession of shale, laminated to thin‐bedded lime mudstone, debris‐flow breccias, minor calcareous sandstone, greenish‐coloured calcareous mudstone and dolomitic siltstone, overlain by shallow‐water dolostones of the Broken Skull Formation, which indicates an overall progradational sequence. Two ~1‐m‐thick units of greenish calcareous mudstone in the upper part exhibit soft‐bodied preservation, yielding a biota dominated by bivalved arthropods and macrophytic algae, along with hyoliths and trilobites. It represents a low‐diversity in situ community. Most of the fossils occur in the lower unit, and only the more robust components are preserved. Branching burrows are present under the carapaces of some arthropods, and common millimetre‐sized disruptions of laminae are interpreted as bioturbation. The fossiliferous planar‐laminated calcareous mudstone consists of chlorite, illite, quartz silt, calcite and dolomite and is an anomalous facies in the succession. It was deposited via hemipelagic fallout of a mixture of platform‐derived and terrestrial mud. Geochemical analysis and trace‐element proxies indicate oxic bottom waters that only occasionally might have become dysoxic. Productivity in the water column was dominated by cyanobacteria. Fragments of microbial mats are common as carbonaceous seams. Complete decay of soft tissues was interrupted due to the specific sediment composition, providing support for the role of clay minerals, possibly chlorite, in the taphonomic process.     

An exceptional storm accumulation of nautilids in the Lower Cretaceous of the Neuquén Basin,Argentina

Cichowolski, M., Pazos, P.J., Tunik, M.A. & Aguirre‐Urreta, M.B. 2011: An exceptional storm accumulation of nautilids in the Lower Cretaceous of the Neuquén Basin, Argentina. Lethaia, Vol. 45, pp. 121–138. An exceptional accumulation of nautilid shells of the species Cymatoceras perstriatum (Steuer) is reported and described in the Lower Cretaceous Agrio Formation of the Neuquén Basin (west‐central Argentina). The bed represents a storm deposit in a shallow‐water environment within the mid‐ramp. The evidences of a storm‐related origin of the bed come from the petrographic analysis and taphonomic features of the shells, specially the sedimentary infill pattern. The shells are dispersed in patches within the stratum, without any orientation relative to the bedding plane. It is proposed that the shells were floating after the death of the animals, although most of them have an almost complete living chamber. The presence of some heavily encrusted shells suggests that there is a mixing of specimens with different drift times. A variety of factors related to the origin of such high number of nautilid shells are discussed, including a transgressive stand system tract, the possible existence of a gregarious behaviour, changes in salinity and wind directions. □Lower Cretaceous, Nautilids, Neuquén Basin, shell accumulation, storm deposit.