Throughout Earth's history, carbonate factories have been production systems which appeared, declined and re-appeared (in modified form) in function of biotic evolution and systemic stability. ...Reconstructing the history of these intricate systems requires a combined assessment of sedimentological, paleoecological, geochemical and diagenetic criteria. Here, the Ordovician of the western Tarim Basin (Darriwilian to Katian, ramp-platform to basin settings) is used as the case example to test a multiproxy approach, including statistical analysis of proxy data. We identify stratigraphic segmentation of these strata in terms of facies/mineralogy, diagenesis and a first comprehensive data set of component-specific carbon and oxygen isotope values. The published bulk carbon and oxygen isotope data are analysed in the second step to extract marine diagenetic versus burial/meteoric signatures (virtual bulk values, Monte-Carlo simulation). Finally, a synoptic chemo-chronostratigraphic plot for the Ordovician of the western Tarim Basin with segments lasting from tens of Myrs down to several 100 kyrs is proposed. A long-term trend of increasing δ13C values culminates in the early Katian at 3.2 ‰ followed by a steady decrease. A baseline shift, arguably due to the initiation of a bahamite-type factory, blurs the globally reported Guttenberg carbon-isotope excursion (GICE). A medium-term (Darriwilian to earliest Sandbian) negative δ13C isotope excursion coincides with the demise of the Darriwilian sponges-pelmatozoa factory (suecicus Event, term proposed in this study) and a positive δ18O excursion. This event might represent the effects of volcanism/SO2 outgassing during the switch from a passive to an active continental arc. The suecicus Event masquerades the middle Darriwilian carbon-isotope excursion (MDICE).
•Chronostratigraphic architecture (Wheeler diagram) of Ordovician deposits of the western Tarim Basin.•Integrated petrography including the spatial variation in diagenetic sequences.•First comprehensive data set of component-specific carbon and oxygen isotope values.•Statistical methods to qualify published bulk-rock geochemical data.•Synopsis addressing drivers of evolutionary and environmental change.
Cambrian–Ordovician sponge-microbial mounds in the Great Basin of the western USA reveal reef structure and composition immediately prior to the Great Ordovician Biodiversification Event (GOBE). Here ...we describe lithistid sponge-microbial reefs from the upper Cambrian (Furongian, Stage 10) strata of the Arrow Canyon Range, Nevada. The reefs are mound-like structures up to 1 to 2 m high and a few meters wide that consist of an unidentified thin-walled, bowl-shaped anthaspidellid sponge, columnar microstromatolite fabric, and the calcified microbe Angusticellularia. The reefs formed in low-energy, subtidal environments in which lime mud filled spongocoels and inter-reef spaces around undisturbed, in place, thin-walled sponges. The reefs colonized stable substrates provided by oolitic and bioclastic grainstone shoals. The mutually attached lithistid sponges form the main framework of the reefs. These thin-walled and bowl-shaped lithistids most likely were adapted to low-energy environments. Spaces beneath the overhanging sponge walls were filled by microbial carbonates. These include pendent micro-dendritic Angusticellularia attached to dermal sponge surfaces and upward-growing masses of microstromatolites. After death the lithistid spongocoels were mainly filled by micritic sediment that hosted soft-bodied burrowing organisms and keratose-like sponges. These lithistid sponge-microbial reefs, together with an earlier example of late Cambrian (Paibian) dendrolite-lithistid reefs in the same area, characterize skeletal-microbial reefs immediately prior to the GOBE.
•Lithistid sponges formed reef frameworks in the upper Cambrian of Nevada, USA.•Reefs dominated by thin-walled sponges developed under low-energy conditions.•Spaces beneath the overhanging sponge walls were filled by microbial carbonates.•In the Great Basin, gradual reef transition occurred across Cambro-Ordovician boundary.•Regional disparities in reef evolution pattern during early stage of the GOBE are recognized.
Most major phyla, and all skeletonized phyla have their first appearance in the Cambrian. The exception is the Phylum Bryozoa, which first appear in the Early Ordovician (Tremadocian 1b). Bryozoans ...have an excellent fossil record, due in part to their benthic marine habitat with skeletal colonies composed largely of stable, low-Magnesium calcite. These factors provide an unrivaled opportunity to observe patterns and rates of radiation of a new phylum into disparate morpho- ecospace through the Great Ordovician Biodiversification Event (GOBE).
In this study, the colonial growth habits of all known skeletonized Bryozoa (181 species) from the Early and Middle Ordovician are characterized in a new classification scheme based on processes of growth, rather than their end geometry as most traditional classifications of bryozoan growth habits. These fundamental categories are: orientation, dimensions of primary growth, width of colony unit, layers of zooecia, substrate relationships, space utilization, skeleton mineralization, plus sub-categories of orientation based on their geometry.
In the Early and Middle Ordovician there are 85 unique growth habits defined by these eight growth habit characters. By the end of Middle Ordovician, about 4.8% of the morpho-ecospace defined by two subsets of the characters had been occupied by five bryozoan orders (represented by 34 families, 77 genera). When plotted by Ordovician stage time-slices (ca. 2.2 my each), a sharp increase in taxonomic and growth habit occurrence is observed at the Early-Middle Ordovician transition, earlier than in other groups. Individual growth habit character states also show significant changes at this transition. In the Early Ordovician, low bryozoan taxonomic and growth habit richness is due in part to sampling bias and modified search methods are need, however, the overall patterns (Early-Mid. Ordovician transition) observed in this study are robust and not expected to change fundamentally with additional data.
•A process-based classification of growth habits is coded for 181 E. to M. Ordovician bryozoan species.•Bryozoan taxonomic and growth habit richness increase sharply at the E.–M. Ordovician boundary.•Increase of taxonomic richness and variety generally parallel other benthic suspension feeders at this time.•About 4.8% of defined morpho-ecospace is occupied by bryozoans by the end of the M. Ordovician.
A robust stratigraphic framework and a coherent depositional ramp model for the Zitai, Dawan, Meitan and Ningkuo formations of Floian–Darriwilian age (Early–Middle Ordovician) in the Yangtze (Daoba, ...Xiangshuidong, Daling, Gudongkou and Honghuayuan sections) and Jiangnan regions (Nanba section) was created based on lithofacies and major element analysis. Three siliciclastic (LF1–3) and six carbonate (LF4–9) lithofacies are recognized representing sediments that were deposited in mixed siliciclastic and carbonate ramp environment. The intensity of mixed sedimentation and terrigenous input were evaluated using the elemental proxies Intensity of Mixed sedimentation (IM) and Aluminum Accumulation Rate (AlAR), as well as their mean values during certain time intervals. Mixed sediments are most well‐developed along the marginal Yangtze region, strongly impacted by recurrent influx of westerly derived terrigenous materials in response to global eustatic changes and regional tectonic movements, shaping the gently southeast‐dipping morphology. Regular terrigenous influx resulted in periods of enhanced primary productivity on the Yangtze Ramp as evidenced by matching biodiversity peaks in planktonic organisms, i.e., chitinozoans and acritarchs. Brachiopods and other shelly fauna were also able to proliferate as new niches developed along the gently dipping ramp floor with substrate changes. The biodiversification patterns suggest that terrigenous influx controlled in part by regional tectonics played a more important role than previously thought in the development of Great Ordovician Biodiversification Event in South China.
Hybrid depositional systems are created by the interaction of two or more hydrodynamic processes that control facies distribution and their characteristics in terms of sedimentary structures and ...depositional geometry. The interaction of wave and tide both in the geological sedimentary record and modern environments has been rarely described in the literature. Mixed coastal environments are identified by the evidence of wave and tidal structures and are well identified in nearshore environments, while their recognition in lower shoreface–offshore environments lacks direct information from modern settings. Detailed field analyses of 10 stratigraphic sections of the Lower Ordovician succession (Fezouata and Zini formations; Anti‐Atlas, Morocco) have allowed the definition of 14 facies, all grouped in four facies zones belonging to a storm‐dominated, wave‐dominated sedimentary siliciclastic system characterized by symmetrical ripples of various scales. Peculiar sedimentary organization and sedimentary structures are observed: (i) cyclical changes in size of sedimentary structures under fair‐weather or storm‐weather conditions; (ii) decimetre‐deep erosional surfaces in swaley cross‐stratifications; (iii) deep internal erosion within storm deposits; (iv) discontinuous sandstone layers in most depositional environments, and common deposition of sandstones with a limited lateral extension, interpreted to indicate that deposition at all scales (metric to kilometric) is discontinuous; (v) combined flow–oscillation ripples showing aggrading–prograding internal structures alternating with purely aggrading wave ripples; and (vi) foreshore environments characterized by alternating phases of deposition of parallel stratifications, small‐scale and large‐scale ripples and tens of metres‐wide reactivation surfaces. These characteristics of deposition suggest that wave intensity during storm‐weather or fair‐weather conditions was continuously modulated by another controlling factor of the sedimentation: the tide. However, tidal structures are not recognized, because they were probably not preserved due to dominant action of storms and waves. A model of deposition is provided for this wave‐dominated, tide‐modulated sedimentary system recording proximal offshore to intertidal–foreshore environments, but lacking diagnostic tidal structures.
Sponges form a significant component of the diversity of the Tremadocian (Early Ordovician) Fezouata Biota in the Anti-Atlas of Morocco, but are distributed intermittently and have so far received ...only limited attention. New material reveals a high diversity of undescribed taxa (a total of at least 27 species), including numerous representatives of protomonaxonid groups such as the Leptomitidae, Piraniidae and “Choiidae”. Some of these taxa show unusually complex skeletal architecture, and represent derived variations of their lineages relative to those in the Cambrian Burgess Shale-type faunas, although most species are assignable to described families. Reticulosan sponges are rare in the Fezouata Biota, and usually fragmentary. The palaeoecology of the sponge fauna is unusual, with most species known only from single sites (frequently single bedding planes), but are often abundant where they occur. It is very rare for two species of sponge to be found on the same slab, or at precisely the same horizon. With some species of protomonaxonid, particular bedding planes are crowded with individuals of species that are rarely, if ever, found isolated. Only two species (Pirania auraeum Botting, 2007 and an undescribed hazeliid) are found at numerous levels, and these are not known from crowded assemblages; this may relate to differences in reproductive strategy. The dense, usually monospecific populations can best be explained through repeated colonisation events in a frequently hostile environment, rather than representing a range of different stable communities.
The origin of reddish nodular limestone in the Zitai Formation, a deeper-water type of marine red beds (MRBs) distributed along the marginal Yangtze Platform, South China, is enigmatic. Here we focus ...on the red colorations through sedimentological, mineralogical and geochemical analyses, as exemplified by the Zitai Formation at the Xiangshuidong section, Songzi City, Hubei Province and the Daling section, Shitai City, Anhui Province. Limestones studied here can be gray, green or red. The grayish limestones are generally below the reddish and/or greenish limestones, whereas the greenish layers are associated with reddish layers in forms of homogenous, banded or mottled limestone. Hematite that is fine and poorly crystalized could be the coloration mineral for the Zitai MRBs, which is largely caused by increased terrigenous iron, when the bottom seawater was oxic. Except that the irregularly banded and mottled reddish-greenish limestone are of a possible diagenetic origin, the primary greenish limestones are related to relatively reducing conditions compared with reddish limestones, as a result of eustatic changes or other fluctuating environmental factors. During this time interval, the Great Ordovician Biodiversification Event (GOBE) was facilitated by highly oxic seawater indicated by MRBs in South China, Baltica and Sibumasu, but the diachroneity of radiations in South China could be ascribed to increased terrigenous supply, emphasizing that regionally environmental factors might be critical for biological radiations at the early stage of the GOBE.
•Detailed petrological analyses on the deeper-water marine red beds for the first time.•Conducting the elemental analyses on the color-alterations of the red beds.•The implications of Ordovician marine red beds for the GOBE in South China.
Skeletal–microbial–cement reefs are a triple hybrid carbonate that mainly formed during the Pennsylvanian to Mid-Triassic, when a marked increase in microbial carbonate formation coincided with ...extensive precipitation of crystalline crusts on the seafloor. We report a new type of reef-building association from Middle–Upper Ordovician strata of western North China, in which erect thin tubes of tetradiids (coralomorph) are encrusted by the calcimicrobes Renalcis and Angusticellularia and then by a large amount of early marine cement that is presumably high-Mg calcite or aragonite in composition. The resulting meter-scale mound is embedded within intraclastic–bioclastic grainstone, implying high-energy shallow-marine conditions. The thin tetradiid tubes, which would have been unable to physically withstand strong waves and currents, are interpreted to have been consolidated by encrusting calcimicrobes and then by extensive early marine cementation. Tetradiid-bearing reefs have generally been reported from muddy successions; the results of the present study suggest that consolidators were important in reef-building in high-energy environments during the later Ordovician. Considering also the coeval bivalve–sponge–microbial–cement reef reported from the same area and a sponge–microbial–cement reef from Arctic Canada, early marine cementation appears to have been at least locally important in the late Ordovician, similar to the Pennsylvanian through the Mid-Triassic. These triple hybrid carbonates may have formed by a combination of: (1) emergence of newly evolved skeletal reef-builders during the Great Ordovician Biodiversification Event; (2) development of CO2-concentrating mechanisms in calcimicrobes induced by a decrease in atmospheric CO2; and (3) an increase in the calcium saturation state in seawater resulting in extensive abiotic cementation as well as calcification of microbes. All of these factors might have been induced by global cooling throughout the Mid–Late Ordovician.
•The tetradiid–calcimicrobial–cement reef association is a triple hybrid carbonate.•Tetradiids could form reef frameworks with the aid of calcimicrobes and cement.•Global cooling promoted abiotic cementation and the formation of calcimicrobes.
Chitinozoans are a group of biostratigraphically valuable microfossils which appeared in the Tremadocian and diversified during the rest of the Great Ordovician Biodiversification Event, to become ...extinct in the Devonian. The early diversification of chitinozoans on the Baltica palaeocontinent has remained poorly known due to preservation and collecting bias. In this study we document the distribution of chitinozoans through the upper Tremadocian to lower Darriwilian strata in the Jägala waterfall section from northern Estonia, in order to better understand the regional diversity patterns and biogeographic links. Fifty samples were studied from the 7.5 m succession of sandstones, marls and carbonates revealing 47 chitinozoan species and 11 genera. The lowermost productive samples, attributed to the late Tremadocian, represent the earliest rich chitinozoan fauna from Baltica. Combined with previous reports this shows a diversity peak in the Tremadocian with balanced total diversity reaching 17, possibly followed by a decline in the Floian and then a rather gradual increase through the Dapingian and early Darriwilian. The new diversity curve shows more than three times higher values for the Tremadocian and Dapingian than indicated in previous studies, and thus less abrupt radiation in the Darriwilian. The Jägala section provided also new data on the regional biozonal species Conochitina cucumis and Cyathochitina regnelli, which characterize the middle and upper Volkhov and lower Kunda regional stages, respectively. Several other biostratigraphically significant taxa typical of other regions were recovered from the Jägala section, including Conochitina decipiens and Desmochitina bulla. A new short-ranging species Laufeldochitina toilaensis sp. nov. is introduced, having potential to become an index species for middle Dapingian strata. The early Darriwilian assemblage from Jägala includes Rhabdochitina sp. A with up to 2.7 mm long vesicle, making it the largest chitinozoan ever reported.
•Rich assemblage of Early and Middle Ordovician chitinozoans recovered from Baltica.•A peak in regional diversity of chitinozoans occurred in the late Tremadocian.•The main phase of GOBE was more gradual on Baltica than previously known.•The largest chitinozoan with 2.7 mm long vesicle recovered from the Darriwilian.
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