Siliciclastic Environments Research Articles

Sedimentology and ichnology of the Xiaxishancun Formation of Qujing, Yunnan: Earliest Devonian marginal-marine environments and ecosystems in South China

The Silurian–Devonian transition experienced dramatic changes of climates, global carbon cycles and faunas, but detailed documentation of the environments and biotas during this transitional interval in the South China Block is needed. The upper Silurian–Lower Devonian sequence of Qujing, Yunnan, southwestern China, represents one of the rare stratigraphic records of this transition in the South China Block. In this study, the Xiaxishancun Formation of Qujing, a unit generally considered earliest Devonian in age, is studied in terms of sedimentology and ichnology. The results show that the Xiaxishancun Formation typically records the deposition of hyperpycnal flow systems, which have been rarely reported in the Paleozoic. Three facies associations, including mudstone-dominated association, heterolithic association, and sandstone-dominated association, represent different parts of the channels and lobes of hyperpycnal subaqueous deltas on the shelf. Nineteen ichnogenera and 32 ichnospecies were distinguished in the Xiaxishancun Formation, and grouped into two ichnoassemblages, the Lockeia-Ptychoplasma-Oravaichnium ichnoassemblage in hummocky/swaley cross-stratified sandstones, and the Cruziana-Diplichnites-Monomorphichnus ichnoassemblage in thin rippled sandstone beds. The hyperpycnal depositional processes of the Xiaxishancun Formation created nutrient-rich, oxygen-rich, siliciclastic environments that supported diverse benthic ecosystems. Our study provides new insights into the understanding of marginal-marine environments and biotas of South China, immediately following the Silurian–Devonian transition event.

Organic matter enrichment and palaeo-sedimentary environment reconstruction of lacustrine shale in the second member of the Middle Jurassic Qiketai Formation in the Shengbei Sub-sag, Turpan–Hami Basin, NW China

Lacustrine organic-rich shales in the second member of the Jurassic Qiketai Formation (J 2 q 2 ) in the Shengbei Sub-sag, Turpan–Hami Basin, NW China exhibit strong heterogeneity due to frequent alternations of sedimentary environments. The distinct shale environments present in both upper and lower units of J 2 q 2 provide an ideal example for studying the enrichment mechanism of organic matter (OM) under a complex sedimentary background. In this study, petrological, mineralogical, major/trace element and isotopic analyses were used to reconstruct the palaeo-environment and reveal the mechanisms of OM enrichment. The results indicate that the J 2 q 2 shale was deposited in a lacustrine mixed carbonate–siliciclastic environment and the palaeo-environment indicators suggesting an oxic–dysoxic, mildly brackish water condition. Based on a comprehensive comparison of both members of J 2 q 2 , an OM enrichment model is established and the main controlling factor of the formation of the organic-rich shale is elucidated. Under the background of a warm-humid climate, the lower unit of J 2 q 2 was deposited in a deeper and more restricted water body with stronger chemical weathering, resulting in limited terrestrial input compared to that of the upper unit. Inorganic geochemical analysis indicates a higher primary productivity in the lower unit of J 2 q 2 with local fluctuations. High primary productivity and favourable preservation conditions domain the OM enrichment in the study area. Supplementary material: Raw data of carbon and oxygen isotopic analysis, TOC, Rock-Eval pyrolysis, and major and trace element analysis are available at https://doi.org/10.6084/m9.figshare.c.6901480

Selective extinction at the end-Cretaceous and appearance of the modern Decapoda

Abstract Extinction and origination of genera and families of marine Decapoda at the end of the Cretaceous established the modern fauna. Podotrematous crabs suffered high extinction levels, whereas heterotrematous crabs experienced both extinction and radiation. Anomuran decapods exhibited high extinction levels at the generic but not family level. In general, family extinctions predominantly occurred among podotrematous and heterotrematous crabs, which also exhibit overall shorter family ranges. Possible refugia promoted survival of genera and families into the Paleocene, whereas habitation of the Western Interior Seaway ensured high levels of extinction. Areas proximal to the Chicxulub impact site experienced moderate levels of extinction. Habitat specialists preferentially survived the end-Cretaceous extinction in some clades. Heterotrematous crabs recovered rapidly in the Paleocene, especially in the siliciclastic environments of the Americas. Because decapod crustaceans are highly variable in morphology, habitat, and ecology, no one pattern of extinction and recovery can explain the end-Cretaceous decapod extinction event. Rather, a mosaic of responses in various decapod groups led to their differential survival.

Microbialites of modern siliciclastic rock coasts

ABSTRACT Contemporary microbialite formation has been documented on rock coasts in a variety of geomorphic, oceanographic, and climatic settings. Based on a synthesis of these diverse occurrences plus new observations, a generalized model is presented. At each locality microbialite development is associated with discharge of mineralized freshwater in the coastal zone. Microbialite formation in the high intertidal and supratidal zones of rock coasts occurs in a variety of sub-environments (cliff face, shore platform surface, platform surface pools, boulder beach, and sand beach) and forms a variety of laminated rock encrustations and oncoids. Allochthonous microbialites occur on the backshore as breccias of reworked microbialite clasts, oncoids transported from rock pools, and partly encrusted boulders. The microbialite-influenced rock coast is a distinct type of siliciclastic environment that offers potential comparison for ancient microbialite occurrences. It has preservation potential in both transgressive and regressive settings. Potential ancient examples are suggested.

Multi-attribute geomorphological characterization of the Cenozoic deep-water strata in the Exmouth Plateau, Northern Carnarvon Basin: a contribution to the understanding of deep-water mixed carbonate–siliciclastic environments

Abstract The stratigraphic architecture and relationships to the expression of deep-water architectural elements geomorphology are explored for Cenozoic deposits in the Exmouth Plateau Sub-basin in Northern Carnarvon Basin. We used a high-quality 3D seismic survey and an integrated seismic attribute analysis to provide a guide for seismic interpretation of the mixed carbonate–siliciclastic depositional systems focused on four aspects: morphology, lithology, strata thickness and reflector distortion. Three depositional elements are classified into several categories: mass transport deposits (MTDs), channel systems (channel–levee and braided-like) and lobes, whose deposition is closely related to eustatic changes, and can be explained in a sequence stratigraphic framework. The falling stage system tract is characterized by the initial development of MTDs deposited on the basin floor and small erosive channels on the upper slope. Lobes are deposited in lowstand system tracts while highstand system tracts are characterized by the deposition of MTDs and progradational segments. This mixed carbonate–siliciclastic example proved to be sensitive to other variables such as climate (Middle Miocene Climate Optimum) and plate tectonics (northward Australian Plate drift), which affect geomorphological aspects such as the dimensions and preservation of internal characteristics.

The first Middle Ordovician and Gondwanan record of the cincinnaticrinid crinoidOhiocrinus byeongseonin. sp. from South Korea: biostratigraphy, paleobiogeography, and taphonomy

AbstractOhiocrinus byeongseonin. sp. from the Middle Ordovician (Darriwilian) Jigunsan Formation of South Korea in the Sino-Korean (North China) block is the oldest species ofOhiocrinusof the Cincinnaticrinidae and the first record outside Laurentia.O.byeongseoniis characterized by a loosely clockwise-coiled anal sac, isotomous branching throughout arms, long, slender xenomorphic column, and small lichenocrinid-type holdfast. The new species occurs in association with a deep-water siliciclastic environment, unlike the Laurentian species with a shallow-water carbonate environment. The monospecific crinoid assemblage is interpreted as parautochthonous, considering that the crinoids were reworked by relatively weak down-current probably caused by storm but preserved within the environment where they lived. The occurrence ofO.byeongseonipresents a considerable spatiotemporal gap and ecologic disparity in evolution ofOhiocrinusand the Cincinnaticrinidae.UUID:http://zoobank.org/240417a4-8d33-4f45-aedb-afcc07f1ecbb

Stratigraphic evidence of two historical tsunamis on the semi-arid coast of north-central Chile

On September 16, 2015, a Mw 8.3 earthquake struck the north-central Chile coast, triggering a tsunami observed along 500 km of coastline, between Huasco (28.5°S) and San Antonio (33.5°S). This tsunami provided a unique opportunity to examine the nature of tsunami deposits in a semi-arid, siliciclastic environment where stratigraphic and sedimentological records of past tsunamis are difficult to distinguish. To improve our ability to identify such evidence, we targeted one of the few low-energy, organic-rich depositional environments in north-central Chile: Pachingo marsh in Tongoy Bay (30.3°S).We found sedimentary evidence of the 2015 and one previous tsunami as tabular sand sheets. Both deposits are composed of poorly to moderately sorted, gray-brown, fine-to medium-grained sand and are distinct from underlying and overlying organic-rich silt. Both sand beds thin (from ∼20 cm to <1 cm) and fine landward, and show normal grading. The older sand bed is thicker and extends over 125 m further inland than the 2015 tsunami deposit. To model the relative size of the tsunamis that deposited each sand bed, we employed tsunami flow inversion. Our results show that the older sand bed was produced by higher flow speeds and depths than those in 2015. Anthropogenic evidence along with 137Cs and 210Pb dating constrains the age of the older tsunami to the last ∼110 years. We suggest that the older sand bed was deposited by the large tsunami in 1922 CE sourced to the north of our study site. This deposit represents the first geologic evidence of a pre-2015 tsunami along the semi-arid north-central Chile coast and highlights the current and continuing tsunami hazard in the region.

Vauxiids as descendants of archaeocyaths: a hypothesis

Archaeocyaths were one of the major animal groups contributing to the Cambrian Explosion. Despite an overall poriferan appearance of their hypercalcified skeletons, they exhibit a number of peculiar features such as the presence of septa and an absence of spicules. This sets archaeocyaths apart from general trends in early poriferan evolution. Fossils recently discovered from the early Cambrian Guanshan Lagerstätte of South China exhibit features typical for the suborder Archaeocyathina whilst possessing a silicified skeleton associated with carbonaceous material. This rigid fibrous skeletal framework, on the other hand, fits the diagnosis of the supposed ancestral aspiculate demosponges, the family Vauxiidae. By a detailed description of this specimen and a direct morphological comparison between the Archaeocyathina and the Vauxiidae, we suggest that the supposed Cambrian representatives of horny demosponges, that is, vauxiids, probably, were a progeny lineage of archaeocyathines. They survived the extinction of hypercalcified archaeocyaths due to their ecological adaption to siliciclastic environments. The existence of hypercalcified dictyoceratid (‘keratosan’) sponge Vaceletia, well known as a ‘living archaeocyath’, supports such a possibility.

The first record of cornulitids tubeworms from the early Late Ordovician of Spiti, Tethyan Himalaya, India

ABSTRACT Tentaculitoid tubeworms, an extinct invertebrate group, is being reported from the Ordovician strata of Spiti, Tethyan Himalaya, India. External and internal moulds of tentaculitoids are preserved in the lower unit of the Takche Formation which is considered early Late Ordovician in age. The assemblage includes Cornulites cf. sterlingensis, Cornulites zatoni, Cornulites sp., and Tentaculites spp. Cornulites cf. sterlingensis, Cornulites sp., and Cornulites zatoni are recorded and described for the first time from the Ordovician strata of India. Though, the occurrence of genus Tentaculites is previously mentioned from the Early Palaeozoic sequence of Spiti, the detailed documentation is provided herein for the first time from the Ordovician sequence of Spiti, India. The occurrence of solitary-free forms of tentaculitoids from shallow marine settings of Spiti, Tethyan Himalaya, India indicates that unattached free forms might have been more competent in the siliciclastic environment of relatively low palaeolatitude than carbonate platforms of lower palaeolatitudes and siliciclastic environments of the highest palaeolatitudes.

Colonial Corals from the Early Aptian siliciclastic Montlivaltia marls of Jumilla (Murcia, Spain)

Three species of colonial scleractinian corals are reported from Early Aptian sandy marls of the so-called Montlivaltia marls from a section exposed on the SE flank of the Sierra de Sopalmo, south of Jumilla in Murcia, Spain. While solitary corals of the genus Montlivaltia are extremely common in the marls, colonial corals are rare. The three colonial coral species presented here (Cryptocoenia sp. n. aff. C. pygmaea, Holocystis elegans, and Columnocoenia aragonensis) are all common Early Cretaceous species. They range in age from the late Barremian to early Albian with the exception of Columnocoenia aragonensis, which ranges from the Valanginian to Cenomanian. Their geographical distribution is large and all species were found in the central Tethys as well as in the Caribbean province. All species were previously found in siliciclastic environments and while Holocystis elegans occurs primarily in such facies, the other two species occur in pure carbonates as well.

The role of sea-level and climate changes in the assembly of Sri Lankan biodiversity: A perspective from the Miocene Jaffna Limestone

As part of the Western Ghats-Sri Lanka biodiversity hotspot, Sri Lanka harbors a rich and diverse fauna with a considerable number of endemic species, especially in the wet southwestern part of the island. Understanding the origin of this biodiversity requires consideration of the land bridge history between the island and southern Peninsular India. To examine the influence of fluctuating sea levels and climate on island connectivity and isolation during the Miocene, we describe the lithological succession of the Jaffna Limestone in surface outcrops of north(west)ern Sri Lanka and interpret it with respect to palaeoenvironments and sequence stratigraphy. A stack of three marine intervals was identified, which were deposited in seagrass-, coral reef-, and nearshore siliciclastic environments of a land-attached carbonate platform and are separated by caliche and palaeokarst unconformities. They represent high sea-level peaks during the long-term sea-level maximum related to the Middle Miocene Climate Optimum (MMCO). In support of divergence time from Sri Lankan and south Indian species, it is argued that the Palk Isthmus connected Sri Lanka to mainland India prior the MMCO (~17–15 Ma) and after the Middle Miocene Climate Transition (~15–13 Ma), whereas a marine barrier continued to persist during low sea-level stands associated with the Mi2, Mi3a and Mi3 glaciations within this period of time. The early and middle Miocene landbridge connections opened up dispersal opportunities for terrestrial animals. Species migrations via the Palk Isthmus were, however, impacted by a climatic filter, which promoted a high level of ancient endemism on Sri Lanka. Our results indicate that the Sri Lankan sub-center of biodiversity started to evolve in response to increased sea-level and climate variability associated with Antarctic ice dynamics and the initiation of the South Asian monsoon system during the early stage of the current ice house period.

Upper Ordovician (Sandbian–Katian) carbonate outliers in the northern Ottawa–Bonnechere graben (central Canada): records of transgressions and sedimentation patterns in the Laurentian platform interior

Small Ordovician sedimentary outliers, including Brent Crater, within the northern Ottawa–Bonnechere graben are remnants of a once expansive Upper Ordovician sedimentary cover extending across the southern Canadian Shield. Facies successions along with updated macrofossil and conodont biostratigraphy, and isotope (C, O, Sr) chemostratigraphy provide additional insights into the terrestrial-to-marine transformation, carbonate-platform development, and oceanographic communication across the southern Laurentian platform. Four of the outliers document Sandbian shoreline-to-nearshore deposition: near Deux Rivières, Manitou Islands, the upper part of the Brent Crater sedimentary fill, and at nearby Cedar Lake. Marine transgression initially reworked local fine-grained to boulder-rich regolith within high-energy shoreface siliciclastic environments that gave way to low- to high-energy inner carbonate-ramp setting. Continued transgression resulted in more offshore rhythmic and diverse lithofacies successions defining mixed heterozoan, photozoan, and microbial productivity and marine isotope (C, Sr) signatures, but δ13C excursions suggest periods of greater mixing of terrestrial and marine carbon reservoirs. Lower Katian strata are preserved near Lake Nipissing and characterize deepening from high-energy ooid-heterozoan skeletal shoals to deeper water mid-ramp siliciclastics and skeletal carbonates, host to a Cruziana ichnofacies. An upsection decline in δ13C values through this succession may identify deposition during the post-peak decline of the global Guttenberg δ13C excursion. This lithic succession fits well with contemporary expansion of heterozoan skeletal lithofacies across the Laurentian platform, yet the presence of ooids identifies prevailing warm waters within the platform interior during early stages of transgression.

Carbonate collapse and the late Paleozoic ice age marine biodiversity crisis

AbstractThe late Paleozoic ice age (LPIA) was characterized by persistently low diversity of marine invertebrates following a second-order mass extinction. Here, we used a data set of North American (paleotropical) fossil occurrences of brachiopod, bivalve, and coral genera from the Paleobiology Database, combined with lithologic data from Macrostrat, to demonstrate that low diversity was caused by the collapse of carbonate environments during the LPIA. After dividing the data by lithology, low diversity was evident only in carbonate environments, whereas diversity within siliciclastic environments actually increased during the LPIA, after a brief decline in the Serpukhovian (late Mississippian). Diversity patterns closely matched respective changes in the volume of carbonate and siliciclastic rocks. The contrasting patterns observed in the two environments suggest that habitat loss was a direct cause of changes in diversity, because other factors, such as temperature, would have affected genera in both environments. A causal relationship is also supported by the finding that diversity remained high in carbonate refugia (carbonate beds within majority-siliciclastic formations) until the Bashkirian, postdating the onset of icehouse conditions by ∼8 m.y. Our results provide a unifying, mechanistic explanation for the distinctive characteristics of the biotic impact, including its disproportionate effect on the tropical marine invertebrate fauna, prolonged recovery from extinction, low macroevolutionary rates during the recovery interval, and regional differences in its expression.

Data-driven shear wave velocity prediction model for siliciclastic rocks

Theoretical and experimental investigations of elastic wave propagation in sedimentary rocks have shown that shear-wave velocity is strongly correlated to compressional-wave velocity. This is because most of the factors that affect the shear-wave velocity also affect the compressional-wave velocity. In areas where shear-wave velocity logs are not available, empirical relationships based on regression analyses are often used to estimate the shear-wave velocity from the compressional-wave velocity. However, most of the available empirical relationships in the open literature were developed primarily for consolidated rocks and their applications to unconsolidated formations may produce inaccurate estimates. Moreover, in siliciclastic environments, empirical relationships that work well for clean sandstone formations may perform poorly in shale formations and vice versa. In this paper, an attempt is made to develop a new shear-wave prediction model that can be applied to a wide range of lithologies and formation strengths in siliciclastic environments. The model is tested using actual field data from the Niger Delta basin and the Norwegian North Sea. The new model is developed based on establishing a relationship among shear-wave velocity, compressional-wave velocity, and formation bulk density. In general, an excellent agreement is observed between the predicted and measured shear-wave velocity. The statistical analysis shows that the accuracy of the shear-wave velocity prediction can be improved by incorporating the formation bulk density term.

Microbial-mat-associated tephra of the Archean Moodies Group, Barberton Greenstone Belt (BGB), South Africa: Resemblance to potential biostructures and ecological implications

AbstractDocumenting evidence of fossil microbial life on early Earth is made difficult by the paucity of suitable Archean sedimentary rocks, their common metamorphic overprint, the small outcrop areas, and the small size of the objects of interest. Although a large number of putative microfossils dating back as far as 3700 Ma have been described, the syngeneity and biogenicity of many occurrences is debated, and some of the proposed fossils have been found to be either contaminants or abiotic artefacts. The ~3200 Ma Moodies Group of the Barberton Greenstone Belt (BGB), South Africa, contains locally abundant and remarkably well-preserved microbial mats which show indirect evidence of photosynthetic activity. They also contain microstructures which strongly resemble remains of microbial cells. Detailed morphological and geochemical analyses, however, show that these structures mostly represent fragments of volcanic tephra. Our study demonstrates that opaque microstructures within microbial mats can potentially be misidentified as microfossils even when a strict protocol is followed. It also posits the question to which degree volcanic air-borne fertilization contributed to the remarkable growth rate, high mechanical tenacity and wide extent of these oldest tidal microbial mats in siliciclastic environments.

The new formation bulk density predictions for siliciclastic rocks

Accurate determination of the overburden pressure obtained by integrating the formation bulk density from surface to the depth of interest is very critical to pore pressure prediction. When information about the formation bulk density is not available, the current practice is to estimate formation bulk density from compressional-wave velocity using empirical relationships. There is no single formation bulk density prediction model that considers lithologic variation in siliciclastic settings. This imposes severe limitations on the application of the existing empirical relationships to any lithological column that consists of several stratigraphic units. In this paper, attempt is made to develop the new formation bulk density prediction models that can be applied to a wide range of lithologies in siliciclastic environments. The new models are validated using wireline log data acquired from two wells in the tertiary deltaic system of the Niger Delta basin. In the new models, formation bulk density is expressed as a function of compressional-wave velocity and shale volume factor. The accuracy of the new models is quantified using statistical analysis. In general, a good agreement exists between the predicted and measured formation bulk density using the newly proposed models. When compared to the existing models, the new models clearly outperform the most widely used empirical relationships. The new models produce the least root-mean-square errors, normal error distributions and least residual values. Unlike any of the existing empirical relationships, the newly developed models can be applied to clean sands, shaly sands and shales with good accuracy.

Offshore Landslide Hazard Curves From Mapped Landslide Size Distributions

AbstractWe present a method to calculate landslide hazard curves along offshore margins based on size distributions of submarine landslides. The method utilizes 10 different continental margins that were mapped by high‐resolution multibeam sonar with landslide scar areas measured by a consistent Geographic Information System procedure. Statistical tests of several different probability distribution models indicate that the lognormal model is most appropriate for these siliciclastic environments, consistent with an earlier study of the U.S. Atlantic margin (Chaytor et al., 2009, https://doi.org/10.1016/j.margeo.2008.08.007). Parameter estimation is performed using the maximum likelihood technique, and confidence intervals are determined using likelihood profiles. Pairwise comparison of size distributions for the 10 margins indicates that the U.S. Atlantic and Queen Charlotte margins are different than most other margins. These margins represent end‐members, with the U.S. Atlantic margin having the highest mean scar area and the Queen Charlotte margin the lowest. We demonstrate that empirical, offshore landslide hazard curves can be developed from the landslide size distributions, if the duration of mapped landslide activity is known. This study indicates that the shape parameter of the size distribution is similar among all 10 margins, and thus, the shape of the hazard curves is also similar. Significant differences in hazard curves among the margins are therefore related to differences in mean sizes and, potentially, differences in the duration of landslide activity.

Contour current imprints and contourite drifts in the Bahamian archipelago

AbstractNew data collected along the slopes of Little and Great Bahama Bank and the abyssal plain of the Bahama Escarpment provides new insights about contour current‐related erosive structures and associated deposits. The Bahamian slope shows abundant evidence of bottom current activity such as furrows, comet‐like structures, sediment waves and drifts. At a seismic scale, large erosion surfaces and main periods of drift growth resulted from current acceleration related to plate tectonic processes and progressive opening and closure of gateways and long‐term palaeoclimate evolution. At present‐day, erosion features and contourite drifts are either related to relatively shallow currents (&lt;1000 m water depth) or to deep currents (&gt;2500 m water depth). It appears that the carbonate nature of the drifts does not impact the drift morphology at the resolution addressed in the present study. Classical drift morphologies defined in siliciclastic environments are found, such as mounded, plastered and separated drifts. In core, contourite sequences show a bi‐gradational trend that resembles classical contourite sequences in siliciclastic deposits showing a direct relationship with a change in current velocity at the sea floor. However, in a carbonate system the peak in grain size is associated with increased winnowing rather than increased sediment supply as in siliciclastic environments. In addition, the carbonate contourite sequence is usually thinner than in siliciclastics because of lower sediment supply rates. Little Bahama Bank and Great Bahama Bank contourites contain open‐ocean input and slope‐derived debris from glacial episodes. Inner platform, platform edge and open ocean pelagic input characterize the classical periplatform ooze during interglacials. In all studied examples, the drift composition depends on the sea floor topography surrounding the drift location and the type of sediment supply. Carbonate particles are derived from either the slope or the platform in slope and toe of slope drifts, very deep contourites have distant siliciclastic sources of sediment supply. The recent discovery of the importance of a large downslope gravitary system along Bahamian slopes suggests frequent interactions between downslope and along‐slope (contour currents) processes. The interlayering of mass flow deposits and contourites at a seismic scale or the presence of surface structures associated with both contour currents and mass flow processes shows that both processes act at the same location. Finally, contour currents have an important impact on the repartition of deep‐water coral mounds. Currents can actively interact with mounds as a nutrient and oxygen supplier or have a passive interaction, with mounds solely being obstacles orienting erosion and deposition.

Devonian bryozoans from Argentina: new cosmopolitan components of Southwestern Gondwanan basins

AbstractLower to probably Middle Devonian bryozoans from western Argentina are reported. Five genera of trepostomes:Leptotrypa,Leptotrypella,Eridotrypella,Cyphotrypa,Boardmanella, and the cryptostome ?Pseudonematoporaare described and figured. The paleobiogeographical significance of these bryozoans was analyzed in the context of Southern Gondwana during the Devonian. The group was considered as nearly absent from southwestern marine basins. In contrast with highly endemic groups, supporting the recognition of the Malvinokaffric Realm in this region, bryozoans reported are nearly cosmopolitan. The presence of those genera typical of warm-temperate, carbonate settings from low paleolatitudes, such as the Laurentian-EuropeanEridotrypella,Leptotrypa, andLeptotrypella, or the EurasiansBoardmanellaand ?Pseudonematopora, implies that these taxa were euritopic with remarkable capabilities for dispersion to reach high-latitude siliciclastic environments of the Southwestern Gondwanan basins.

Storm-related taphofacies and paleoenvironments of Malvinokaffric assemblages from the Lower/Middle Devonian in southwestern Gondwana

Taphofacies of Malvinokaffric macroinvertebrate-bearing rocks from the uppermost Pragian to lower Givetian in southwestern Gondwana are evaluated. During the Lower/Middle Devonian, the Paraná Basin (southern Brazil) was positioned at high latitudes in southwestern Gondwana (60° to 80°S). Integrated litho- and taphofacies interpretations in siliciclastic environments presented here provide paleoecological and paleoenvironmental insights. The six taphofacies recognized here showed changing depositional conditions in paleoenvironments ranging from shoreface to offshore shelf settings. They vary in recurrence and abundance, but all facies exhibit evidence of storm events. Winter storms are to be expected at latitudes 60° to 80° S under cool climate zone. Storm-related taphofacies recurrence during the latest Pragian to early Givetian points to two agents of sedimentation, direct wave erosion by winter storms and sedimentary processes related to combined flow during deposition (e.g. turbidity currents and oscillatory flow). Winter storms are considered more capable of affecting sediment transport than hurricanes, due to longer duration and larger coverage area in higher latitudes. Therefore, winter storms of high latitudes should generate more persistent currents in space and time. Finally, the taphofacies, lithofacies and high-latitude paleogeographic context of the Paraná Basin suggest the action of storm-influence in the genesis of all Lower/Middle Devonian Malvinokaffric assemblages here recognized.