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Two massive precipitation events of polymetallic ore deposits, encrusted by a mixture of authigenic carbonates, are documented from the Cambrian of the semi-enclosed Baltoscandian Basin. δ34S (‒9.33 to ‒2.08‰) and δ33S (‒4.75 to ‒1.06‰) values from the basal sulphide breccias, sourced from contemporaneous Pb–Zn–Fe-bearing vein stockworks, reflect sulphide derived from both microbial and abiotic sulphate reduction. Submarine metalliferous deposits were triggered by non-buoyant hydrothermal plumes: plumes of buoyant fluid were trapped by water column stratification because their buoyancy with respect to the environment reversed, fluids became heavier than their surroundings and gravitational forces brought them to a halt, spreading out laterally from originating vents and resulting in the lateral dispersion of effluents and sulphide particle settling. Subsequently, polymetallic exhalites were sealed by carbonate crusts displaying three generations of ikaite-to-aragonite palisade crystals, now recrystallized to calcite and subsidiary vaterite. Th of fluid inclusions in early calcite crystals, ranging from 65 to 78 ºC, provide minimum entrapment temperatures for carbonate precipitation and early recrystallization. δ13Ccarb (‒1.1 to + 1.6‰) and δ18Ocarb (‒7.6 to ‒6.5‰) values are higher than those preserved in contemporaneous glendonite concretions (‒8.5 to ‒4.7‰ and ‒12.4 to ‒9.1‰, respectively) embedded in kerogenous shales, the latter related to thermal degradation of organic matter. Hydrothermal discharges graded from highly reduced, acidic, metalliferous, and hot (~ 150 ºC) to slightly alkaline, calcium-rich and warm (< 100 ºC), controlling the precipitation of authigenic carbonates. [ABSTRACT FROM AUTHOR]

Defining the variability and distribution of methane seeps and microbial activity in the aftermath of the Marinoan glaciation is a long-standing challenge in the field of Snowball models. Early diagenetic barite is commonly linked to tepee structures and associated breccias lacking microbial textures and fabrics, giving the impression that chemosynthetic microbes, or at least methane-tolerant microbes, did not participate in the carbonate production of their cap carbonate host. This apparent paradox has been an outstanding question in the lowermost Ediacaran cap carbonates of the Taoudeni Basin, NW Africa. In the Kaarta Mountains of Mali these carbonates exhibit, over short distances (<10 km), sharp facies-related environmental modifications with quiescent-dominated seafloor conditions, episodically interrupted by metre-scale disrupted substrates. The latter comprises fissure and fracture networks, occluded with tabular- and rosette-shaped barite cements, and sealed by decimetre-scale stromatolitic build-ups exhibiting intergrowths with barite needles. The strongly 13C-depleted carbon isotope values of the microbial carbonates (δ13C as low as −43.2 ‰ PDB) suggest the influence of methane, also preserved as fluid inclusions in barite crystals (documented with RAMAN spectroscopy) derived from a gas reservoir below the cap carbonate. T h of other fluid inclusions (Linkam microthermometry), ranging from 174 °C to 222 °C, provides minimum entrapment temperatures for barite precipitation. The microbially induced oxidation of methane and input of Ba-rich fluids were coupled to reduction of sulphate derived from seawater. The Sr/S isotope ratio and barite shape and size point to diagenetic barite crystals. The biomarkers yielded by the cap carbonate reflect a C 29 -dominant steroidal signature characteristic of stigmastanoid algal blooms. Although present-day microbial build-ups related to methane sources commonly occur in deep substrates and under anoxic bottom waters, the cap carbonate of the Kaarta Mountains is representative of shallower substrates, whereas its biomarkers point to deposition under episodic non-oxidising conditions. • Marinoan cap carbonates from Kaarta Mountains display sharp facies-related environmental modifications. • Fracture networks were occluded with barite cements and sealed by stromatolitic buildups. • Strongly 13C-depleted carbon isotope values and methane inclusions point to methane carbon sources. • C29 steroidal signature points to stigmastanoid algal blooms as microbial production [ABSTRACT FROM AUTHOR]

Abstract: The Cambrian Substrate Revolution refers to a substantial and “rapid” change to the nature of marine sedimentary substrates in the early Cambrian and is widely interpreted as a biologically-driven event, a direct response to evolutionary innovations in metazoan burrowing and the development of new shelly faunas. However, abiotic factors such as tectonic and climatic evolution also had the potential to restructure Cambrian substrates, and are here shown to be more plausible drivers of change in the benthic faunas of western Gondwana. The western Mediterranean region underwent a southward drift during Cambrian times, which drove a switch from subtropical carbonates to temperate siliciclastic substrates with short-term episodes of temperate carbonate productivity. As a result, microbial and shelly carbonates disappeared diachronously in a stepwise manner across the lower–middle Cambrian boundary interval. Archaeocyathan-microbial reefs were replaced by chancelloriid-eocrinoid-(spiculate) sponge meadows, in which the stepwise immigration of new echinoderm taxa was primarily controlled by extensional tectonic events, first recorded in rifting settings and later in passive-margin platforms. Availability of new kinds of substrate was thus the primary factor that controlled where and when evolutionary innovations in benthic strategies arose. Examples of this include the early Cambrian colonization of phosphatic hardgrounds and thrombolite crusts by chancelloriids, archaeocyathan and spiculate sponges, and the exploitation by benthos to the increasingly widespread availability of shelly grounds and carbonate firmgrounds by early-diagenetic cementation. A microbial mat/epifaunal antagonistic relationship is demonstrated for echinoderm pelmatozoans based on the non-overlapping palaeogeographic distributions of microbial reefs and mats versus mud-sticker pelmatozoans. Cambrian benthic communities thus evolved in parallel with substrates in response to abiotic factors rather than being the primary drivers of substrate change. [Copyright &y& Elsevier]

In the carbonate-siliciclastic strata of West Gondwana (e.g. in the Montagne Noire, France), the aftermath of the mid Languedocian (mid Cambrian) regression is characterized by a late Languedocian major turnover of trilobite families and a Furongian–early Tremadocian radiation related to the stepwise immigration of trilobite invaders from East Gondwana under persistent transgressive conditions. The scarcity of upper Languedocian fossil accumulations in clayey substrates has inspired the sampling of the palaeogeographically most distal parts of the Iberian Chains (Spain), where diagenetic dissolution of ubiquitous hexactinellid sponge spicules has favoured the formation of siliceous concretions. These have yielded the trilobites Peronopsis cf. insignis, Oidalagnostus trispinifer, Proampyx difformis (= Proampyx aculeatus), Bailiaspis? glabrata (= Holocephalina agrauloides, by ontogeny), Holasaphus cf. centropyge and a paradoxidid gen. et sp. indet. Despite preservation and sampling biases, the identification of this taphonomic window in offshore clayey substrates of West Gondwana allows the recognition of a strong biogeographical link with Baltica, and the correlation of the global Guzhangian Stage and the Solenopleura? brachymetopa Zone of Scandinavia with part of the Mediterranean upper Languedocian Substage. [ABSTRACT FROM PUBLISHER]

Abstract: The so-called Great Atlasian Reef Complex developed during early Cambrian time throughout the Moroccan margin (Souss Basin) of West Gondwana. According to the syndepositional tectonic activity associated with its intracratonic Ediacaran–Cambrian rift, the great reef complex can be subdivided into four major archaeocyathan-microbial reef episodes: (i) The Atdabanian episode is recorded by a SW–NE-trending, 400km long barrier reef that extended across the western Anti-Atlas. It was controlled by large-scale reactivation of an inherited rifting branch, which resulted in the nucleation and growth of linear reef complexes located along its margin. The interplay of block tilting, sharp modifications in accommodation space, and relative sea-level rise led to a composite retrogradational–aggradational reef systems tract, characterized by archaeocyathan-microbial kalyptrate complexes (Tiout Member and Amouslek Formation) that protected stromatolite-dominated, back-barrier environments (lower member of the Igoudine Formation). (ii) The western Anti-Atlas recorded an early Botoman reactivation of the same rifting branch that triggered a lateral migration of frame-building centres of carbonate productivity. As a result, the involved grabens and half-grabens recorded the development of fringing mound complexes (lower Issafen Formation). These occupied some linear intra-platform, deeper depressions capped by marls and shales, whereas laterally equivalent shallower environments recorded the development of patch-reefs and bioherms. (iii) The western Anti-Atlas subsequently recorded a late Botoman interval of tectonic quiescence superimposed to a local interval of progradational pulses. This favoured the nucleation of dispersed archaeocyathan-microbial patch-reefs and bioherms. (iv) A distinct palaeogeographic area is recognized in the southern High Atlas, where the entire Atdabanian–Botoman interval recorded small-scale, synsedimentary block tilting and high rates of volcaniclastic input. As a result, this sector was characterized by the record of microbial and archaeocyathan-microbial patch-reefs and bioherms, preferentially developed on the uplifted parts of tilted blocks. The end of reef development and carbonate productivity in the Souss Basin is related to the progradation of siliciclastic depositional systems (Toyonian regression), considered to have caused the collapse of reef communities throughout West Gondwana. [Copyright &y& Elsevier]

Abstract: The presence of microbial crusts interrupting high-energy clastic sediments in carbonate and mixed (carbonate–siliciclastic) platforms can be considered as distinct indicators of stratigraphic diastems, characterized by either very low rates of deposition or non-deposition. This analysis is based on a microfacies study of the Cambrian Brèche à Micmacca (Atlas Mountains, Morocco), which contains some stratigraphically condensed limestones characterized by: (i) disarticulation and fragmentation of skeletons; (ii) abundance of volcanigenic debris; (iii) rarity of muddy fraction; (iv) dominance of a sessile, epibenthic shelly fauna dominated by echinoderms, chancelloriids and sponge spicules; and (v) a complex diagenetic history due to erosion and reworking of lithified substrates. The Brèche à Micmacca limestones are composite event-concentration, low-relief shoal complexes composed of parautochthonous and allochthonous bioclastic assemblages. These units can be described as ‘hiatal accumulations’ because of the subdivision of the coquinas by minor discontinuities or hiatal surfaces, and the composite, multiple-event nature of the grainstones/packstones. The growth of microbial mats can be used as a record of time interrupting background-sedimentation patterns characterized by the amalgamation of high-energy events. The Brèche à Micmacca was deposited in an environment in which some intervals of non-deposition (or under extremely low-sedimentation rates) alternated with the succession of sedimentary events. The biological response of microbial communities to coat stratigraphic discontinuities can be considered as an integral part of event stratigraphy. They enhance the preservation and identification potential of interruptions in the background sedimentation on substrates devoid of burrowing activity. A distinct event of non-deposition is also recognized, based on the presence of microbial fabrics, at the top of the Brèche à Micmacca. Its microkarst structures reflect a major fall in the relative sea level of a reduced region of the platform resulting in dissolution and colonization of coelobiontic cavities by microbial mats and branching filaments. [Copyright &y& Elsevier]