Cerro Negro Formation Research Articles

Paleomagnetism of the Ediacaran Avellaneda Formation (Argentina), part I: Paleogeography of the Río de la Plata craton at the dawn of Gondwana

A paleomagnetic and rock magnetic study was carried out in the Ediacaran Avellaneda (∼570 Ma) and Cerro Negro (∼555 Ma) Formations belonging to the La Providencia Group, in the upper part of the Neoproterozoic sedimentary cover of the Tandilia region, in the Río de la Plata craton. The Avellaneda Formation was studied at outcrop level and in three drill cores, yielding a mean characteristic remanence direction of Dec: 21.4°, Inc: 67.1°, α95: 4.2°, k: 23.9, N: 51 and a paleomagnetic pole at 1.0° S, 313.4° E, A95: 5.9°. The Cerro Negro Formation yielded a mean characteristic direction of Dec: 22.0°, Inc: 68.5°, α95: 10.3°, k: 20.8, N: 11 obtained from a single drill core, from which a paleomagnetic pole at 3.6° S, 307.8°E, A95: 16.6° was computed. Rock magnetic data indicates that magnetic remanence is mainly associated with magnetite and hematite. The paleomagnetic information presented here results in a change in the previously accepted Late Ediacaran apparent polar wander path of the Río de la Plata Craton. The newly obtained poles indicate that Río de la Plata Craton experienced a rapid drift from a low latitudes location (ca. 19° S) at ca. 600 Ma to moderately higher latitudes (between 50° and 42°S) from around 580 to 550 Ma.

Paleoenvironment of the Cerro Negro Formation (Aptian, Early Cretaceous) of Snow Island, Antarctic Peninsula.

A study of macro and microfacies, palynoflora and palynofacies of the non-marine Cerro Negro Formation at President Head Peninsula, Snow Island, northwest of the Antarctic Peninsula, was developed. Two assemblages were recognized: Palynofacies assemblage 1 (P1) at the base of the section with a dominance of fern spores and conifer pollen grains, and facies association consisting of a clastic layer, with the predominance of mudstones; and Palynofacies assemblage 2 (P2) at the top of the section, with remarkable abundance of AOM/Pseudoamorphous particles, associated with facies that includes tuffs. The complete section shows in some levels the presence of freshwater algae and translucent phytoclasts. The integrated data characterizes a fluvial-lacustrine environment, what is reinforced by the occurrence of freshwater algae (Botryococcus) in some levels of P1 and P2. We could verify an increase in volcanic activity towards the top of the section that apparently has played an important role in the collapse of the palynoflora. The occurrence of the spore species Muricingulisporis annulatus, Sotasporites elegans, S. triangularis, Foraminisporis wonthaggiensis, and F. asymmetricus in the Cerro Negro Formation allows the correlation with sections in South America and Australia, suggesting an Aptian age for these deposits.

Scratching the discs: evaluating alternative hypotheses for the origin of the Ediacaran discoidal structures from the Cerro Negro Formation, La Providencia Group, Argentina

AbstractIn the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE Argentina), abundant microbially induced sedimentary structures indicate general conditions of substrate biostabilization. Numerous discoidal structures in this succession were previously interpreted as moulds of soft-tissue holdfasts of sessile organisms, within the form genus Aspidella. In this study, we performed a detailed re-analysis of some of these features and discuss two alternative hypotheses to explain their genesis: (1) as the result of soft-sediment deformation and fluid injection structures; and (2) as structures of active animal–sediment interaction (i.e. trace fossils). We show that the dome-shaped discs are internally laminated, with a cylindrical to a funnel-shaped vertical tube at their central region. The presence of these downwards vertical extensions and other intricate internal arrangements cannot be explained under the taphonomic spectrum of discoidal fossils, but shows striking similarities to Intrites-like structures and other sand-volcano-like pseudofossils (e.g. Astropolithon). However, some structures are hard to distinguish from vertical dwelling burrows with funnel-shaped apertures and thick-lined walls, commonly produced by suspension- and detritus-feeding invertebrates (e.g. Skolithos isp., Monocraterion isp. and, less likely, Rosselia isp.). Since reliable age constraints are unavailable, and further investigation concerning other palaeobiological indicators is needed, the most parsimonious hypothesis is that of a structure derived from fluid-escape processes. Our study demonstrates the importance of detailed investigation on discoidal structures in either upper Ediacaran or lower Cambrian strata.

Discs and discord: The paleontological record of Ediacaran discoidal structures in the south American continent

Discoidal sedimentary structures are commonly described in Proterozoic strata, and even more common in Ediacaran to lower Cambrian sedimentary successions. Many abiotic processes are able to produce such circular or discoidal structures in bedding planes, however, their abundance in Ediacaran strata suggests a possible correlation with the evolution and preservation of epibenthic metazoans that emerged at the end of this period. In the South American paleontological record, studies regarding the Ediacaran soft-bodied organisms are meager and restricted to few reports in Brazil, Paraguay, and Argentina. In many cases, such “fossils” were only tentatively characterized in terms of their general morphology and putative taxonomic affinity. Thus, considering the almost absence of work on these enigmatic structures in South America, this paper aims to make a critical analysis on the main occurrences of Ediacaran-Cambrian discoidal structures described in this continent. Based on a detailed review and unpublished data, it was possible to provide a general picture concerning the main paleoenvironmental and sedimentary significance of this structures, as well as on the most promising prospects in terms of the paleontological record of Ediacaran soft-bodied metazoans in South American. In this sense, it was settled that occurrences such as those in the Jaibaras and Itajaí basins should be reassessed in order to establish reliable criteria of biogenicity. In the case of the material from the Sete Lagoas and Tagatiya Guazu formations, it is considered more parsimonious to interpret the discoidal features as resulting from microbial processes. Similarly, the discoidal structures of the Cerro Negro Formation presents a series of internal laminations and textures that resembles those developed by processes of microbial grain binding and trapping suggesting that, at least part of this material, can be related to microbially induced sedimentary structures. Finally, for the ichnologically diversified Puncoviscana and Camaquã basins, two different scenarios were identified. The first presents an ichnological assemblage strongly indicative of lower Paleozoic, and possibly Cambrian affinity. Thus, the discs in association with these traces, should be viewed with caution and interpretations made in light of a Paleozoic context. The second possesses an ichnological association typical of that expected for the Ediacaran-Cambrian transition, and the diversity of discoidal forms can potentially represent imprints of macroorganisms on a microbially bounded substrate, thus deserving a more detailed approach.

Cyathealean Antarctic ferns from the Aptian Cerro Negro Formation: Rafaherbstia nishidai gen. et sp. nov. and associated fertile organs

New remains corresponding to Cyathealean tree ferns are described for the Aptian Cerro Negro Formation at Byers Peninsula, Livingston Island (South Shetland Islands, Antarctica). A new genus and species, Rafaherbstia nishidai, is proposed for several permineralized dictyostelic stems characterized by a heterogeneous pith with a central star-shaped sclerenchyma mass, U-shaped meristeles developing small hooks at the margins of the leaf gap, sclerenchyma sheaths surrounding the vascular strands, and leaf traces omega-shaped and multi parted. The new taxon shows a combination of features that preclude its placement in one of the eight recognized families of Cyatheales, supporting that previous diversity of tree ferns was higher than in modern times. Fragmentary isolated sori with preserved sporangia containing trilete spores, recovered from the sedimentary matrix surrounding the stems, are also described. These new tree ferns increase the diversity of Cyatheales in the Cerro Negro Formation, and support warm climatic conditions in this region during the Aptian.

Marattiaceae synangia from the Lower Cretaceous of Antarctica

Marattiaceous ferns are conspicuous fossil element in upper Paleozoic and lower Mesozoic deposits, but become rarer to almost absent in Cretaceous to Holocene deposits. In this contribution, fragmentary synangia recovered from the Aptian Cerro Negro Formation (Livingston Island, Antarctica) are described. Synangia are bivalvate, containing at least four lachrymiform sporangia per valve. Valves are fused at the base, and connect to the leaf blade by a short peduncle. Sporangia contain more than 400 oval to reniform (probably monolete), smooth to microgranulate spores. The reproductive structures have morphological similarities with extant Marattia and Ptisana, and the apparent absence of labia suggests a closer relationship with the latter genus. The presence of Marattiaceae, along with previously described ferns (e.g. Cyatheales) supports warm climatic conditions during the Aptian in this region of Antarctica.

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

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

Further evidence supporting high diversity of cyathealean tree ferns in the Early Cretaceous of Antarctica

Two new specimens of a cyathealean fern stems collected from the Lower Cretaceous (Aptian) Cerro Negro Formation, which crops out at the South Shetland Islands, Antarctica, are described in detail. The specimens are dictyostelic stems with U-shaped meristeles without sclerenchyma sheaths and petiole bases with numerous, small, vascular strands, and a mantle of adventitious roots. The anatomy of the stems show similarities with Cibotiaceae, but given they are not well preserved, some critical features that would allow a conclusive referral to this family are lacking. Preserved in the surrounding matrix, fragmentary pinnules were found, showing sori with bivalvate indusia, and sporangia with more than 20 spores, attached to an elongate receptacle. The features observed in these fertile remains suggest cyathealean affinities, but are difficult to refer to a particular family. These findings increase the diversity of cyathealean tree ferns in the Cerro Negro Formation, supporting previously suggested warm climatic conditions for this region of Antarctica during the Aptian.

Paleoenvironmental implications of two phosphogenic events in Neoproterozoic sedimentary successions of the Tandilia System, Argentina

Two phosphogenic events of potential economic importance (preserving P2O5 abundances up to 25 and 35%, respectively) are recognized in the Neoproterozoic Tandilia System. The older of the two lies atop a quartz-arkosic facies association of the Villa Mónica Formation. The age of this unit remains controversial, but has previously been considered as Tonian to Cryogenian based on stromatolite assemblages and carbon isotope trends; strontium isotope abundances of <0.7071 currently support an earliest Cryogenian age for the formation. The younger phosphate level lies at the base of the Cerro Negro Formation above a widespread karstic surface in a succession containing Cloudina, so is most-likely to be Ediacaran in age. In both cases, the phosphatic concretionary levels are related to relative sea-level fall and exposure that may be related to glacial eustacy. In order to reconstruct the paleoenvironment associated with the formation of these concretionary phosphates, we combined field observations with thin-section petrography, XRD, SEM and geochemical analyses. Total rare-earth element (REE) contents range from 311 to 1010ppm in concretions from the Villa Mónica Formation and from 290 to 1471ppm those from the Cerro Negro Formation. Villa Mónica concretions reveal no anomalies in Ce abundance, but clearly positive Eu anomalies (ranging from 1 to 1.4). These results suggest reduced conditions in the marine depositional environment. In contrast, Eu anomalies are not recorded in Cerro Negro concretions, while negative Ce anomalies, ranging from −0.14 to −0.18, are noted. These results are consistent with oxic seawater conditions in mixed platform facies. This study suggests that depositional conditions where phosphate was concentrated in the Neoproterozoic Tandilla System were markedly different.

New cyathealean tree fern, Yavanna chimaerica gen. et sp. nov., from the Early Cretaceous of Livingston Island, Antarctica

A new genus and species of cyathealean fern, Yavanna chimaerica gen. et sp. nov., is erected for several permineralized stems recovered from the Early Cretaceous (Aptian) Cerro Negro Formation, which crops out at the South Shetland Islands, Antarctica. The new species is characterized by solenostelic erect stems with medullary bundles traversing the pith, and surrounded by persistent petiole bases and adventitious roots. Proximal petiole bases present a one-parted modified omega-shaped trace, which becomes three-parted distally. The anatomy of the new fern show similarities with both Thyrsopteris elegans Kunze and with the Cyatheaceae s.s., suggesting that it is a representative of an extinct lineage among the Cyatheales.

The La Tinta pole revisited: Paleomagnetism of the Neoproterozoic Sierras Bayas Group (Argentina) and its implications for Gondwana and Rodinia

The Late Ediacaran to Cambrian Sierras Bayas Group (Villa Mónica, Cerro Largo, Olavarría and Loma Negra Formations) and the Cerro Negro Formation, exposed along the Tandilia system in the province of Buenos Aires (Argentina) were revisited and studied paleomagnetically. Our results supersede those of Valencio et al. (1980) for the La Tinta Formation (old stratigraphic name of these units). Three hundred and twenty-eight samples were collected from forty-four sites in gently folded to subhorizontal strata distributed along the whole stratigraphic succession. Detailed paleomagnetic study comprised systematic stepwise demagnetization by both AF and thermal methods, the latter being generally the most effective in isolating the characteristic remanence. Different magnetic components were defined from different units of the succession. Besides a recent, probably viscous, secondary component (component A), the most widespread magnetic remanence (component B) is a dual-polarity post-tectonic secondary remanence. This component, carried by both hematite and magnetite, corresponds to that originally determined by Valencio et al. (1980) and previously interpreted as primary. This component found in all carbonatic rocks of Villa Mónica and Loma Negra Formations as well as in several claystones and siltstones of the Olavarría Formation do not pass conglomerate and regional tilt tests. The mean in situ direction of component B is Dec: 359.8°, Inc: −63.3°, n: 85 samples, k: 24, α95: 3.2° and yields a paleomagnetic pole virtually identical to the previous one of Valencio and colleagues. It also matches those recently determined from secondary magnetizations in carbonatic and clastic Ediacaran units exposed in Uruguay. The pole positions suggest a Late Permian–Triassic age as the more likely for the acquisition of component B and reveal the presence of a widespread remagnetization event that affected very large areas of the Rio de la Plata craton. Despite this widespread event, some clastic units (claystones, marls) apparently escaped remagnetization. A pre-tectonic, dual polarity, mean remanence (Dec: 28.7°, Inc: 56.1°, n: 17 samples, k: 15, α95: 9.5°) was isolated from the latest Ediacaran–Early Cambrian Cerro Negro Formation (component C). In addition, the Ediacaran Olavarria Formation recorded another apparently ancient remanence, although no field test is available. Its direction (component D) is at Dec: 350.9°, Inc: 47.3°, n: 13 samples, k: 37, α95: 7.0°. Siltstones and claystones of the Ediacaran Cerro Largo Formation were carriers of a characteristic remanence (component E) that shows a better directional grouping after bedding correction, although the field test is not statistically significant, and yield a mean corrected direction at: Dec: 73.7°, Inc: −36.6°, n: 11 samples, k: 15, α95: 12.1°. Finally, a purple horizon of marls on top of the Villa Mónica Formation associated with weathering processes before deposition of the Colombo diamictite, was carrier of a characteristic remanence that attained a better grouping after bedding correction, but again with no statistical significance. This direction (component F) was at Dec: 43.4°, Inc: −36.3°, n: 7 samples, k: 45, α95: 9.1°. Components C–F are interpreted as ancient magnetizations associated either to post-depositional or early to late diagenesis. Mean geomagnetic poles computed from these components fall on the apparent polar wander path for the Rio de la Plata craton from around 600 to 520Ma, in a correct stratigraphic order and with ages consistent with the most likely ages (or slightly younger) of the different sampled units. These results confirm the already proposed Ediacaran to Cambrian APWP for the Rio de la Plata craton, indicating that it remained at intermediate to low latitudes during most of the Ediacaran. Comparison with coeval paleomagnetic poles from other cratons indicate that by 575Ma the Rio de la Plata and Congo-Sao Francisco cratons were likely a single plate. It also strongly argues against the generally accepted model that the Rio de la Plata craton was part of the conjugate margin of Eastern Laurentia during the final stages of Rodinia break-up at around 580Ma.

Millerocaulis tekelili sp. nov., a new species of osmundalean fern from the Aptian Cerro Negro Formation (Antarctica)

Vera, E.I. iFirstarticle. Millerocaulis tekelili sp. nov., a new species of osmundalean fern from the Aptian Cerro Negro Formation (Antarctica). Alcheringa, 1–10. ISSN 0311-5518. Ezequiel Ignacio Vera [evera@macn.gov.ar] División Paleobotánica, Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia', Av. Angel Gallardo 470, C1524DJR, Buenos Aires, Argentina. Also affiliated with Área de Paleontología, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón II, Ciudad Universitaria, C1428EGA, Buenos Aires, Argentina. Received 29.10.2010, revised 14.3.2011; accepted 24.3.2011. A new species of the osmundalean fossil morphogenus Millerocaulis Tidwell emend. Vera, Millerocaulis tekelili sp. nov. is defined, based on several permineralized stems recovered from exposures of the Lower Cretaceous Cerro Negro Formation on Livingston Island, South Shetland Islands, Antarctica. This new species is characterized by the presence of an ectophloic–dictyoxylic siphonostele, inner parenchymatic and outer sclerotic cortices, heterogeneous sclerotic ring in the petiole bases, absence of sclerenchyma associated with the petiolar xylem trace, petiolar inner cortex with sclerenchyma strands and stipular wings having a large sclerenchyma bundle and several smaller ones. The presence of non-homogeneous sclerotic rings in the petiole bases allows this new species to be clearly distinguished from other Antarctic Millerocaulis, and suggests that it may represent an intermediate form in the evolutionary lineage leading from Millerocaulis to subgenus Claytosmunda of Osmunda.

The Tandilia System of Argentina as a southern extension of the Río de la Plata craton: an overview

The southernmost outcrops of the Río de la Plata cratonic region are exposed in the Tandilia System in eastern Argentina. The geological evolution comprises mainly an igneous-metamorphic Paleoproterozoic basement named Buenos Aires Complex, which is covered by Neoproterozoic to Early Paleozoic sedimentary units which display subhorizontal bedding. The basement of calc-alkaline signature consists mainly of granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks, and granitoid plutons. Subordinate rock-types include schists, marbles, and dykes of acid and mafic composition. Tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids. The tectonic scenario seems related to juvenile accretion event (2.25–2.12 Ga) along an active continental margin, followed by continental collision (2.1–2.08 Ga) after U–Pb zircon data. The collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks. The tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution. The basement was preserved from younger orogenies such as those of the Brasiliano cycle. After a long paleoweathering process, the Sierras Bayas Group (c. 185 m thick) represents a record of the first Neoproterozoic sedimentary unit (siliciclastic, dolostones, shales, limestones), superposed by Cerro Negro Formation (c. 150–400 m thick, siliciclastics) assigned to Upper Neoproterozoic age. The final sedimentary transgression during Early Paleozoic was the Balcarce Formation (c. 90–450 m thick) deposited over all the mentioned Precambrian units. Based on all the geological background, a tectonic evolution is offered.

Neoproterozoic to Lower Palaeozoic successions of the Tandilia System in Argentina: implication for the palaeotectonic framework of southwest Gondwana

The Cryogenian to Uppermost Ediacaran successions of the Tandilia System (Sierras Bayas Group and Cerro Negro Formation) in central-eastern Argentina is exceptional because of its unmetamorphosed and nearly undeformed character, its sediment provenance trend and the absence of any identified glacial deposit and the deposition of warm water carbonates. We decipher a dramatic change in the basin evolution from small-scale depositional areas during the Neoproterozoic to a larger basin related to an active continental margin throughout the Uppermost Ediacaran. The base of the succession is represented by immature detritus of alkaline composition (Villa Monica Formation), but towards the top of this formation, the material is sorted and reworked, nonetheless still reflecting in its detritus the local rocks. The clastic deposition is interrupted by diagenetic overprinted dolomites. The unconformable overlying quartz-arenitic Ediacaran Cerro Largo Formation reworked the Cryogenian Villa Monica Formation and contains mainly felsic granitic and metamorphic basement material of slightly wider variety, while the dominant alkaline geochemical signature in rocks of the Villa Monica Formation disappears. Based on diagenetic, petrographic and sedimentological data, we can interpret the unconformity representing a longer time of erosion. The Cerro Largo Formation shows a transition to mudstones and the heterolithic facies of the Olavarria Formation. The top of the Sierras Bayas Group is represented by limestones (Loma Negra Formation), which are discordantly overlain by the Uppermost Ediacaran Cerro Negro Formation. The latter displays detrital material derived from a continental arc, mafic and felsic sources. Several arc-related geochemical proxies (Th/Sc 20) are recorded in the sediment detritus, as are syn-depositional pyroclastites. The absence of volcanic material in the underlying rocks allows proposing that the Cerro Negro Formation is related to an active continental margin fringing Gondwana (“Terra Australis Orogen”) as a retro-arc or retro-arc foreland basin.

A new specimen of Millerocaulis (Osmundales: Osmundaceae) from the Cerro Negro Formation (Lower Cretaceous), Antarctica

A new petrified stem of Millerocaulis Erasmus ex Tidwell emend. Vera, collected in the Aptian Cerro Negro Formation outcrops of Livingston Island (Antarctica), is described in detail. This specimen is probably conspecific with Millerocaulis australis (Vera), from the same unit, but it is smaller than the holotype and only known specimen of the latter species. Furthermore, differences between several characteristics of both specimens are observed. A discussion based on biological aspects of this group of plants is given, taking into account the ontogenetic variability. A conservative referral of the new specimen to M. australis is preferred over the proposal of a new morphotaxon for the inclusion of the new materials.

Alienopteris livingstonensis gen. et sp. nov., enigmatic petrified tree fern stem (Cyatheales) from the Aptian Cerro Negro Formation, Antarctica

A new genus and species of Mesozoic fern, Alienopteris livingstonensis gen. et sp. nov., is defined, based on permineralized stems recovered from the Aptian Cerro Negro Formation (Antarctica). This new taxon is characterized by the presence of Y-shaped meristeles in the stem, surrounding an heterogeneous pith, and the presence of complex petiole bases, consisting in abaxial and adaxial archs, similar to the ones present in the Cyatheaceae s.l.. Since no living nor fossil filicalean fern possesses such mixture of features, and taking into account that no leaves and reproductive structures were found, phylogenetic relationships are not clear. Two independent cladistic analysis show clear placement of Alienopteris livingstonensis among the “core tree ferns.”

A new species of Ashicaulis Tidwell (Osmundaceae) from Aptian strata of Livingston Island, Antarctica

A new species of Ashicaulis, A. australis sp. nov., is described in detail. The specimen, collected from the Cerro Negro Formation (Aptian) at Byers Peninsula, Livingston Island (Antarctica), increases the number of fossil representatives of the Osmundaceae in the southernmost continent. The specimen shows a differential state of preservation across the stem, but is good enough to show details of most of the tissues, enabling a study of the anatomy, allowing its comparison with other Ashicaulis species, and justifying its assignment to a new species. Ashicaulis australis is characterized by the presence of parenchymatous pith, sub-exarch protoxylem clusters in the vascular ring, delayed leaf gaps and petiole bases with small patches of sclerenchyma scattered in the cortex, two groups of the same tissue lateral to the trace, and a large patch followed by 6–9 smaller aligned sclerenchymatous groups in the stipular wings.

Chemostratigraphy and diagenetic constraints on Neoproterozoic carbonate successions from the Sierras Bayas Group, Tandilia System, Argentina

Carbon and oxygen isotopic data, combined with a detailed diagenetic study, obtained from undeformed and unmetamorphosed dolomite and limestone from the Neoproterozoic Sierras Bayas Group, Argentina, provide a new record of isotopic stratigraphic variations useful for regional and global correlations. The Sierras Bayas Group is subdivided into the Villa Mónica (sandstones and dolomites), Cerro Largo (siliciclastic rocks) and Loma Negra (limestones) formations, unconformably overlain by the Cerro Negro Formation (siliciclastic rocks with basal marls), and grouped into four depositional sequences bounded by unconformities. The dolomite samples from the Villa Mónica Formation show ranges of δ 13C (PDB) values from − 1.3 to + 2.2‰ and of δ 13C (PDB) values from − 2.1 to − 6.7‰. Elemental data (Mn/Sr, Fe/Sr and Ca/Sr) and stable isotope compositions point to a moderate to significant degree of diagenetic alteration in samples from this formation. The Loma Negra Formation is composed of homogeneous, non-luminescent micritic limestone with δ 13C (PDB) values of samples between + 2.7 and + 4.5‰; δ 13C (PDB) values vary from − 7.1 to − 13.5‰. Selected element ratios using Mn/Sr (< 1.4) and Fe/Sr (< 30) combined with C/O isotope data suggest near to primary carbon isotope signatures in these samples. The Villa Mónica Formation can be broadly constrained between 800 and 900 Ma, based on Rb/Sr radimetric data and stromatolite morphologies. The Loma Negra Formation fits in global δ 13C and 87Sr/ 86Sr trends that suggest an age range of ~ 580 – 590 Ma, which is regionally important as it places constraints on the continuity of Vendian basins in south-western Gondwana. In the same sense, the isotopic signatures of Loma Negra Formation samples along with the preservation of a unique shelly fauna make a direct connection of the Argentinian succession to Vendian intervals in Uruguay (Polanco Formation) and Brazil (Corumbá Group).

Aptian ferns with in situ spores from the South Shetland Islands, Antarctica

Compressions and impressions of fertile fern foliage occur in the Cerro Negro Formation (Early Aptian), South Shetland Islands, Antarctica. Some of them are fronds at least tripinnate, bearing rounded pinnules with sori containing annulate, stalked sporangia. The recovered spores are of the Cyathidites-type, with scattered foveae. These specimens are referred to a new genus and species: Eocyathea remesaliae gen. et sp. nov., confirming the presence of the Cyatheaceae in the area during the Early Cretaceous. Other specimens have elongate and lobed pinnules, usually falcate, bearing one sorus per lobe. Sori with stalked, annulate sporangia bearing spores of the Cyatheacidites-type. These are referred to Sergioa austrina gen. et sp. nov., and point to diversity in the family Lophosoriaceae or related groups during the Cretaceous.

Terrestrial Paleoecology of the Cretaceous (Early Aptian) Cerro Negro Formation, South Shetlands Islands, Antarctica: A Record of Polar Vegetation in a Volcanic Arc Environment

Nothing is known about the community-scale plant ecology of early Cretaceous volcanic environments in the southern high latitudes. The paleoecology of the early Aptian (119–120 Ma) volcaniclastic Cerro Negro Formation of South Shetland Islands, Antarctica, has been investigated to improve understanding of these enigmatic ecosystems. This unit was deposited in an intra-arc setting at the edge of the paleo-polar circle (66°S). Taphonomic analysis of megaflora in seven volcanic facies indicates the existence of a spatially complex vegetation mosaic co-dominated by conifers, bennettites, and ferns. A lower stratigraphic unit was deposited by high-magnitude, low-frequency silicic eruptions, partially reworked by fluvial processes. Following ash falls, shrubby communities of ferns and bennettites dominated for short periods in low-altitude paleovalleys before being replaced by araucarian-podocarp-fern conifer forests. Forests of podocarp conifers and bennettites covered adjacent mid-altitude slopes of active volcanic cones. These were destroyed periodically by hot pyroclastic flows, and their charred remains washed down into paleovalley fluvial systems. Analysis of architectural and phenological data from both major forest communities indicates the predominance of evergreen trees with a canopy height of c. 20–30 m. Growth-ring analysis indicates that, despite occasional catastrophic eruptions, growing conditions were mostly favorable and uniform, although in riparian paleovalley niches flooding locally produced more stressful growing conditions. An upper stratigraphic unit was deposited by low-magnitude, high-frequency basaltic eruptions. Shrubby, fern and bennettite-dominated vegetation, together with local conifer stands, colonized this environment. Growth of more widespread arborescent vegetation in the upper unit was inhibited by high-frequency volcanic disturbances. Bennettites are much more abundant and diverse in the Cerro Negro Formation compared with coeval, non-volcanic sites in the southern high latitudes, indicating that this group may have favored volcanic environments.