Late Ordovician glaciation

regional events at this time, including sea-level fluctuations, tectonic movements, volcanic eruptions, mass extinction, and widespread anoxia. The O-S transitional strata of the Yangtze Platform comprise organic-rich black shales that are an important oil source rock. To explore the evolution of watermass chemistry and its relationship to organic matter accumulation, we conducted an integrated Fe-S-C geochemical study (i.e., Fe-speciation, δ 13 C org , δ 34 S py , pyrite-S and TOC) of the O-S boundary sections at Datianba (Chongqing Municipality) and Shuanghe (Sichuan Province). These sections were located in the restricted inner Yangtze Sea, in contrast to the previously studied Wangjiawan section, which was deposited in a more open setting of the outer Yangtze Sea. In contrast to the well-oxygenated conditions at Wangjiawan, the Datianba and Shuanghe sections record persistently anoxic, and episodically euxinic, conditions during the Hirnantian, which can be explained by the unique paleogeography and restricted hydrography of the inner Yangtze Sea. We propose that the regional Kwangsian Orogeny, which was driven by collision of the Yangtze and Cathaysia blocks, played a key role in basin development, watermass chemistry changes, and accumulation of Wufeng Formation black shales within the Yangtze Sea. The more widespread black shales of the Lower Silurian Lungmachi Formation were linked to the post-Hirnantian global marine transgression.

This paper reports the discovery of Late Ordovician scolecodonts from the Qusaiba-1 core hole, central Saudi Arabia. The collection of about 100 relatively well preserved diagnostic jaws represents one of the richest jawed polychaete faunas from the Gondwanan realm and the first record of scolecodonts from the Arabian Peninsula. Scolecodonts were most diverse and abundant in the Quwarah Member and basal Sarah Formation, corresponding to the Ancyrochitina merga and Tanuchitina elongata chitinozoan zones (latest Katian to early Hirnantian). The polychaete assemblage contains up to 15 apparatus-based species and is dominated by the globally distributed genera Kettnerites, Oenonites and Atraktoprion, and a new genus of probable ramphoprionid affinity. Additionally, Skalenoprion and Kalloprion? are recorded, both for the first time from Gondwana. Characteristic of the assemblage is very low frequency of taxa with placognath-type jaws. Comparing the Qusaiba-1 assemblage with coeval faunas of Baltica and Laurentia revealed that northern Gondwana was more similar to mid-continent Laurentia than Baltica. High proportion of paulinitids, ramphoprionids and atraktoprionids, and scarcity of placognaths is typical of both northern Gondwana and Laurentia. The Baltic faunas, in contrast, had several endemic genera particularly among placognaths, whereas paulinitids and ramphoprionids were rare. This anomalous biogeographic pattern, diverging from that of most other fossil groups, cannot be fully explained without additional first-hand data from the Middle and early Late Ordovician of Gondwana and Laurentia.

Latest Sandbian to early Katian sequences across Laurentia's epicontinental sea exhibit a transition from lithologies characterized as 'warm-water' carbonates to those characterized as 'coolwater'carbonates. This shift occurs across the regionally recognized M4/M5 sequence stratigraphic boundary and has been attributed to climatic cooling and glaciation, basin reorganization and upwelling of open ocean water, and/or increased water turbidity and terrigenous input associated with the Taconic tectophase. Documentation of oxygen isotopic trends across the M4/M5 and through bracketing strata provides a potential means of distinguishing among these alternative scenarios; however, oxygen isotopic records generated to date have failed to settle the debate. This lack of resolution is because δ 18 O records are open to multiple interpretations and potentially confounding factors related to local environmental conditions have not been tested by examining the critical interval in multiple areas and different depositional settings. To begin to address this shortcoming, we present new species-specific and mixed assemblage conodont δ 18 O values in samples spanning the M4/M5 boundary from the Upper Mississippi Valley, Alabama, and Virginia. The new results are combined with previous studies, providing a record of δ 18 O variability across SE Laurentia. The combined dataset allows us to test for regional trends at a resolution not previously available. Our results document a ∼1.5 ‰ decrease in values across Laurentia instead of increasing δ 18 O values across the M4/M5 as predicted in various 'cool-water' scenarios. In short, these results do not support a shift to 'cool-water' conditions as an explanation for changes in early Katian carbonates across the M4/M5.

For more than 150 years, geologic characteristics claimed to be evidence for pre-Pleistocene glaciations have been debated. Advancements in recent decades, in understanding features generated by mainly glacial and mass flow processes, are here reviewed. Detailed studies of data offered in support of pre-Pleistocene glaciations have led to revisions that involve environments of mass movements. Similarities and differences between Quaternary glaciogenic and mass movement features are examined, to provide a more systematic methodology for analysing the origins of more ancient deposits. Analyses and evaluation of data are from a) Quaternary glaciogenic sediments, b) formations which have been assigned to pre-Pleistocene glaciations, and c) formations with comparable features associated with mass movements (and occasionally tectonics). Multiple proxies are assembled to develop correct interpretations of ancient strata. The aim is not per se to reinterpret specific formations and past climate changes, but to enable data to be evaluated using a broader and more inclusive conceptual framework. Regularly occurring pre-Pleistocene features interpreted to be glaciogenic, have often been shown to have few or no Quaternary glaciogenic equivalents. These same features commonly form by sediment gravity flows or other non-glacial processes, which may have led to misinterpretations of ancient deposits. These features include, for example, environmental affinity of fossils, grading, bedding, fabrics, size and appearance of erratics, polished and striated clasts and surfaces ("pavements"), dropstones, and surface microtextures. Recent decades of progress in research relating to glacial and sediment gravity flow processes have resulted in proposals by geologists, based on more detailed field data, more often of an origin by mass movements and tectonism than glaciation. The most coherent data of this review, i.e., appearances of features produced by glaciation, sediment gravity flows and a few other geological processes, are summarized in a Diamict Origin Table.

Throughout the Phanerozoic, δ 18 O of carbonate fossils displays an increasing trend of -8‰ VPDB in the Cambrian to approximately -1‰ VPDB in the modern. This trend has led to three different hypotheses: 1) sea surface temperatures decreased through the Phanerozoic; 2) δ 18 O of seawater rose through the Phanerozoic, or 3) diagenesis has altered older samples more than younger samples. To test these hypotheses, we used clumped isotope thermometry on Late Ordovician brachiopod shells and internal cements from the Cincinnati Arch in North America. The brachiopod shells yield Δ47 temperatures of 25-61°C, and δ 18 OH2O values calculated from Δ47 temperatures and the 18 O paleotemperature equation of provide values of -2.9 to 4.8‰ VSMOW. The quasi-lognormal data distribution suggests partial reordering of some of the shells; however, interpretation of brachiopod temperature data was limited to the modal data (30-35 °C), representing the original temperature distribution. Cement Δ47 temperatures (17-85° C) average higher than those of brachiopod shells, as are calculated δ 18 OH2O values (-5.7 to 4.4‰ VSMOW). Calculated δ 18 OH2O of the brachiopods echo previous studies suggesting that seawater δ 18 O has not changed more than ± 1‰ throughout the Phanerozoic and that the trend seen in the δ 18 O of carbonate fossils is a product of higher temperatures during the early Paleozoic with cooling towards the modern. Clumped isotope analyses of cements, aided by cathodoluminescence microscopy, can provide the progression from sea surface to shallow burial to deep iii burial temperatures. Cements show uniform cathodoluminescence and δ 18 O values mostly similar to those of the best-preserved brachiopod fossils indicating that these cements formed in a marine setting. Based on the burial history of the units, the brachiopods and cements did not reach the threshold of 100 °C necessary to begin reordering clumped carbonate bonds. However, the higher Δ47 temperatures and δ 18 OH2O values and similar δ 18 Ocarb values of cements compared with brachiopods indicate greater solid-state reordering in the cements. The brachiopod shell results support the contention of warm early Paleozoic oceans while the cement results imply that the chemistry of different calcitic materials such as calcites rich in Mn, can have different reordering rates.

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid-pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.

The 2003 rediscovery of the long 'lost' Ozora, Missouri classical conodont locality of Branson & Mehl (1933b), from which the world's first Late Ordovician conodont fauna was described, has made it possible to obtain topotype specimens of the several important conodont species introduced by Branson and Mehl from this site. A taxonomic reassessment of the Ozora conodont fauna indicates that it includes at least 14 multi-element species, eight of which were previously known from this locality. Several specimens of the taxonomically diagnostic M element of the biostratigraphical key species Amorphognathus ordovicicus were recovered. This element was previously not known from the type locality of the species at Ozora. A general review of the evolution of the genus Amorphognathus shows that some elements, particularly the M and Pa elements, exhibit relatively rapid gradual morphological changes that are helpfull for distinguishing a series of biostratigraphically useful species through the Upper Ordovician (Sandbian-Hirnantian) interval. No ancestor of Amorphognathus has yet been identified but currently available data suggest that the genus appeared in the latest Darriwilian or earliest Sandbian and went extinct in the late Hirnantian. None of the Early Silurian platform genera appears to be closely related to Amorphognathus.

27 páginas, 1 figura, 2 tablas, 2 láminas.[EN] Several limestone levels of the “Caliza de Pelmatozoos” were sampled for conodonts in sections of the Cerrón del Hornillo and Valle synclines. The conodont fauna includes: Amorphognathus ordovicicus, A. aff. ordovicicus, Amorphognathus sp., Amorphognathus? sp., Drepanoistodus cf. suberectus, Drepanoistodus? sp., Hamarodus europaeus, Icriodella cf. superba, Istorinus erectus, Panderodus gracilis, Plectodina tenuis?, Sagittodontina robusta, Scabbardella altipes, Scabbardella sp A., Walliserodus amplissimus? y Walliserodus? sp. This association is attributed to the Amorphognathus ordovicus Zone by the presence of the index species, and to the Sagittodontina-Scabbardella Biofacies of the Mediterranean Province of conodonts by the relative abundance of these two taxa. This fauna is close related to coeval associations from several localities of the Iberian Peninsula, except that of the Malaguide Complex, but the presence of Plectodina and Dr...

El estudio para conodontos de numerosos niveles de la “Caliza de Pelmatozoos” en secciones de los sinclinales del Cerron del Hornillo y del Valle ha permitido identificar los taxones: Amorphognathus ordovicicus, A. aff. ordovicicus, Amorphognathus sp., Amorphognathus? sp., Drepanoistodus cf. suberectus, Drepanoistodus? sp., Hamarodus europaeus, Icriodella cf. superba, Istorinus erectus, Panderodus gracilis, Plectodina tenuis?, Sagittodontina robusta, Scabbardella altipes, Scabbardella sp A., Walliserodus amplissimus? y Walliserodus? sp. Esta asociacion, que se adscribe a la Provincia Mediterranea de conodontos, es atribuida a la Zona de Amorphognahus ordovicicus, Kralodvoriense, por la presencia del taxon nominal. Dentro de esta provincia ha sido posible identificar la Biofacies de Sagittodontina-Scabbardella por la abundancia relativa de ambos taxones. Si bien existe una gran similitud entre esta fauna y las de edad equivalente reconocidas en el ambito de dicha provincia, la presen...

Well-preserved siliciclastic domal stromatolites, up to 2 m wide and 1AE5 m high, are found in a 10 to 15 m thick interval within the Late Ordovician Eureka Quartzite of Southern Nevada and Eastern California, USA. These stromatolites appear as either isolated features or patchy clusters that contain more than 70% by volume quartz grains; their association with planar, trough and herringbone cross-bedding suggests that they were formed in an upper shoreface environment with high hydraulic energy. In this environment, sand bars or dunes may have provided localized shelter for initial microbial mat colonization. Biostabilization and early lithification of microbial mats effectively prevented erosion during tidal flushing and storm surges, and the prevalence of translucent quartz sand grains permitted light penetration into the sediment, leading to thick microbial mat accretion and the formation of domal stromatolites. Decimetre-scale to metre-scale stromatolite domes may have served as localized shelter and nucleation sites for further microbial mat colonization, forming patchy stromatolite clusters. Enrichment of iron minerals, including pyrite and hematite, within dark internal laminae of the stromatolites indicates anaerobic mineralization of microbial mats. The occurrence of stromatolites in the Eureka Quartzite provides an example of microbial growth in highly stressed, siliciclastic sedimentary environments, in which microbial communities may have been able to create microenvironments promoting early cementation/lithification essential for the growth and preservation of siliciclastic stromatolites.

With the surge of interest in understanding the evolution of environments and ecosystems on the early Earth, many proxies are being applied to ancient sedimentary rocks. The isotope composition of nitrogen recorded in sedimentary rocks ( 15 N sed) is one of these proxies. Nitrogen isotopes are now routinely used as a tracer of the global and regional marine Nbiogeochemical cycle during the Cenozoic and are increasingly being applied to the ancient rock record, including the Precambrian. The objectives of this review are (i) to articulate guidelines for using  15 N sed as a proxy for the past marine global N-biogeochemical cycle with an emphasis on the Precambrian and (ii) to develop a broad framework for interpreting the Precambrian  15 N sed record. Based on the isotopic pattern displayed by the present day Nbiogeochemical cycle, significant  15 N sed spatial variability is expected for most of the ocean redox structures envisaged for the Precambrian. Furthermore, fundamentally different Ncycling processes may give rise to only subtly different  15 N sed signatures, which themselves may be masked or accentuated by post-depositional processes. Consequently,  15 N sed profiles from individual basins alone are insufficient for inferring behavior of the global nitrogen cycle. Rather,  15 N sed distributions based on data from multiple basins are essential. Furthermore, in order to interpret  15 N sed data from Precambrian sedimentary rocks with more confidence, several avenues of focused research are required. The effects of diagenesis and metamorphism and their manifestation in the  15 N compositions of both bulk and kerogen records need to be better understood. Much more data are required in order to apply statistical approaches to interpreting  15 N sed variability within given geological time intervals. Finally, numerical modeling of the  15 N sed distributions expected from different redox scenarios envisaged for the Precambrian environment is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record. In spite of the intrinsic complexity of the  15 N sed proxy and the great deal of work still required to realize its full potential, the available Precambrian  15 N sed record shows several intuitive features within the context of the inferred evolution of the marine N-biogeochemical cycle. The  15 N sed distributions are roughly similar in shape and amplitude to that of the present ocean, with a change in mode from +1 to +3‰ at ca. 2.7 Ga and transient excursions to 15 N-enriched values at 2.7 and 1.9 Ga. Fundamental shifts in global marine N-cycling, perhaps related to stepwise oxygenation of the surface environment, are inferred, highlighting the potential of nitrogen isotopes to reveal clues about the evolution of early Earth.

Seismic interpretation of depth converted three-dimensional seismic survey from Ewan and Oloye fields; onshore northwestern Niger Delta has helped in the identification of incised valleys (up to 350 m) deep. This study evaluated their morphologies, evolution and the local controls that influence their development. The seismic reflections of the incised valleys are characterized by low-moderate amplitude, variable internal reflections, aggradational, chaotic and progradational-sigmoid reflection patterns overlying by parallel to sub-parallel reflections configuration. The seismic reflection characteristics are probably due to variable sedimentation processes within the valleys which were affected by mass wasting. Asymmetry morphology of the valleys suggests fluvial origin while low sinuosity of the channel may indicate high gradient and high discharge. The occurrence of incised valleys landward of the shelf edge suggests and partially reflects underlying structural control. This might be caused by uplift due to shale diapirism (shelf instability). The magnitude of incision is difficult to explain in terms of sea-level fall alone because ecstaticallydriven sea level variations during the Miocene are generally reported to be less than 100 m. This suggests that the character of fluvial incisions development and depositional facies preserved within the study area is locally controlled by growth faults, rapid relative sea level changes, basin physiography (shelf edge), shelf instability, variation in sediment input, slope collapse (mass flow depositional processes and downward cutting by downslope sediment flow). The incised valleys within the study area probably serve as conduits for sediment transport to the deep water. Seismic delineation of the distribution and morphology of these incised valleys may also provide critical input for reservoir modeling and volumetric analysis.

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid-pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.

The Upper Ordovician glaciogenic deposits are thought to have an important quantity of hydrocarbon across North Africa. Insight is provided about the IV-3 reservoir unit in the In-Adaoui-Ohanet field in the Illizi basin of southeast Algeria. The aim is to: 1) describe the lithofacies; 2) interpret the depositional environment; 3) describe the petrographic characteristics; 4) investigate the petrophysical properties; and 5) perform a biostratigraphic analysis of the unit IV-3 reservoir (Upper Ordovician) in the In-Adaoui-Ohanet field based on the core description and samples collected from the well IA-115. Lithofacies described in the study area are MT1, MT2, MT3, MT6, and MT9. Subglacial tillite is the depositional environment associated with lithofacies MT3, MT6, and MT9. These lithofacies are thought to be deposited medial to a distal fan, whereas lithofacies MT1 and MT2 are thought to be deposited in high-energy flows. Acritarch and Chitinozoan species are the most common biozones described in the study reservoir and they are thought to be affiliated with an Upper Ashgilian stage. Petrographic analysis shows that the study reservoir unit is formed by fine to coarse-grained sandstone. Quartz is considered to be the principal framework mineral (mean, 56.30%). Cementing minerals observed in this reservoir unit are quartz and carbonate. From a compositional point of view, the unit IV-3 reservoir in the In-Adaoui-Ohanet field is predominantly formed by quartz arenites, which are considered to be mature. Additionally, the tectonic setting of these arenites is most likely associated with a passive margin origin. The emphasis throughout this study is on the role of factors such as permeability, carbonate cement, and quartz overgrowth on the reservoir quality of unit IV-3 in the In-Adaoui-Ohanet field. The quality of this reservoir unit is highly influenced by the quartz overgrowth and pores that are plugged by carbonate cement, which dramatically reduces the pore network. An enhanced porosity was observed only in the MT3lithofacies (up to 22.3%). Additionally, from an economic point of view, the study reservoir unit is classified as a tight gas-bearing reservoir.

The early Silurian (Llandovery) Gondwanan South Polar ice sheet experienced episodes of ice retreat and readvance. Marine base level curves constructed for the interval are widely assumed to provide a record of the associated glacioeustasy. In revealing a series of progradational sequences (progrades) bounded by flooding surfaces, recent work on the Type Llandovery succession in mid Wales (UK) has provided an opportunity to test this hypothesis. The grouping of these progrades into three composite sequences underpins the construction of both low order (small amplitude, high frequency) and high order (large amplitude, low frequency) base level movement curves. Revised biostratigraphical datasets for the type succession permit the accurate dating of base level events. The composite sequences record progradational acmes in the acinaces, lower convolutus and upper sedgwickii-halli graptolite biozones. A series of transgressions that postdate the Hirnantian glacial maximum culminated in an upper persculptus Biozone high-stand. Maximum flooding events also occurred during the revolutus and lower sedgwickii biozones, and the base of the early Telychian guerichi Biozone also marked the onset of a pronounced deepening. A review of 62 published datasets, including global and other regional base level curves, records of glacial activity, isotope data, patterns of facies and faunal flux and putative climate models, permits an evaluation of the origins of these local base level events. The concept of a Eustasy Index is introduced and shows that the impacts of global sea level movements can only be demonstrated within narrow 'eustatic windows' coincident with times of ice sheet collapse. At other times, the geometry of Llandovery area progrades reflects their accumulation across a faulted basin margin where, during periods of slow ice sheet advance, epeirogenic processes outstripped sea level movements as the dominant 2 forcing factors. Increased levels of Telychian subsidence at first enhanced and then overwhelmed the influence of glacioeustasy as part of the region's response to the north European Scandian deformation.

The rise and demise of warm-temperate Permian reefs and biostromes reflect the complex geologic history of this dynamic period. Environments suitable for reef-builders were devastated by the Guadalupian/Lopingian crisis, and Lopingian reefs have only been recorded at a small number of localities. The uppermost Permian limestones of the Bellerophon Formation, on the Vojsko Plateau (Slovenia), contain small, lenticular biostromes within a bioclastic wackestone/packstone lithofacies. The major biostrome builders are medium-sized coralline sponges (Demospongea and Calcarea), encrusted by smaller sponges, tube worms, sessile foraminifera, calcareous algae (Archaeolithoporella) and Shamovella (i.e., Tubiphytes), all of which are typically covered by microbial crusts. The biostromes are characteristically composed of bafflestone and bindstone, incorporating sporadic framestone. Narrow belts of floatstone surround the buildups, and sponge debris is also present in lenses within the mud matrix between metazoan bafflestones. The fossils are generally well-preserved, although the fine skeletal microstructure has been partially recrystallized. Sponges are heavily calcified, and ontogenic thickening of the skeleton can be observed in some encrusters. Framboidal pyrite, forming thin films on the inner walls of sponge chambers, suggests the presence of sulphate-reducing bacteria. These microbial symbionts may have enabled the sponges to survive in the anoxic marine environments of the uppermost Permian. The Changshingian sponge biostromes of the Vojsko Plateau represent the westernmost known occurrence of contemporary metazoan boundstones in the Palaeotethys.

A remarkable new fossil horseshoe crab, Lunataspis aurora gen. et sp. nov., from recently discovered Upper Ordovician (c. 445 Ma) shallow marine Konservat-Lagerstätten deposits in Manitoba (Canada), is characterized by fusion of opisthosomal tergites into two sclerites. A broad mesosoma of six or seven fused segments, followed by a narrow metasoma of three reduced segments, represents an advanced transitional condition in the development of the xiphosurid thoracetron. Lunataspis further possesses a large crescentic prosomal shield bearing lateral compound eyes on weak ophthalmic ridges that flank a low cardiac lobe, and a keeled lanceolate telson. Lunataspis is much older than the proposed 'synziphosurine' stem lineage of Carbon-iferous and post-Palaeozoic Xiphosurida, yet is strikingly similar to crown group limuline horseshoe crabs, indicating that major features of the distinctive and highly conserved xiphosurid Bauplan evolved considerably earlier in the Palaeozoic than was previously suspected. In addition to establishing a new temporal benchmark for assessing hypotheses of early chelicerate relationships, the discovery of horseshoe crabs in a Late Ordovician marginal marine setting marks the earliest definitive record of this persistent ecological association.

ABSTRACTIncreasing current interest in sponge fossils includes numerous reports of diverse vermicular and peloidal structures interpreted as keratose sponges in Neoproterozoic to Mesozoic carbonates and in various open marine to peritidal and restricted settings. Reports of their occurrence are fundamental and far-reaching for understanding microfacies and diagenesis where they occur; and fossil biotic assemblages, as well as wider aspects of origins of animals, sponge evolution/ecology and the systemic recovery from mass extinctions. Keratose sponges: 1) have elaborate spongin skeletons but no spicules, thus lack mineral parts and therefore have poor preservation potential so that determining their presence in rocks requires interpretation; and 2) are presented in publications as interpreted fossil structures almost entirely in two-dimensional (thin section) studies, where structures claimed as sponges comprise diverse layered, network, particulate and amalgamated fabrics involving...

In porphyry ore deposit models, the propylitic alteration facies is widely interpreted to be caused by convective circulation of meteoric waters. However, recent field-based and geochemical data suggest that magmatic-derived fluids are likely to contribute to development of the propylitic assemblage. In order to test this hypothesis, we determined the oxygen and hydrogen isotope compositions of propylitic mineral separates (epidote, chlorite, and quartz), selected potassic mineral separates (quartz and magnetite), and quartz-hosted fluid inclusions from around the E48 and E26 deposits in the Northparkes porphyry Cu-Au district, New South Wales, Australia. In addition, the strontium isotope composition of epidote was determined to test for the potential contribution of seawater in the Northparkes system given the postulated island-arc setting and submarine character of some country rocks.Oxygen isotope geothermometry calculations indicate potassic alteration occurred between ~600° an...

Sobre el faldeo nororiental del cerro Negro, en el extremo norte del valle de El Bolsón, en la Cordillera Patagónica de Río Negro, un corte de ruta expone un afloramiento de depósitos glaciarios de aproximadamente 400 metros de longitud. En dicha localidad se observaron una serie de rasgos deformacionales cuya descripción y análisis son objeto de este trabajo. A través de la confección de una serie de secciones y perfiles sedimentarios se interpretaron los depósitos y las estructuras como glaciotectónicas, producto de los esfuerzos que produce el movimiento glaciar, restando una investigación más exhaustiva para descartar su eventual conexión con procesos neotectónicos. Siendo así, la vergencia de las fallas y de los plegamientos observados permitió definir un avance de la masa glaciar desde el norte, siendo un aporte más para estudiar la dirección del flujo de hielo en este sector de los Andes durante el Último Máximo Glaciar. Además, se reconocieron dos pulsos de avance de los glaciares, el primero de los cuales puede no haber guardado expresión geomorfológica tras haber sido sobre cabalgado por el pulso posterior.

Boikothalamia reitner & engeSer, 1985 (J) Marinduqueia yaBe & Sugiyama, 1939 (Ce); this genus has recently been given an alternative assignment to that presented here and classified within the Order Dictyoceratida, Family Vaceletiidae (see discussion, p. 273-275) ?Menathalamia reitner & engeSer, 1985 (Cr) Murguiathalamia reitner & engeSer, 1985 (Cr) Stylothalamia ott, 1967 (P-Cr) Vaceletia picKett, 1982 (Cr-R) this genus has recently been given an alternative assignement to that presented here and classified within the Order Dictyoceratida, Family Vaceletidae (see discussion, p. 273-275). Vascothalamia reitner & engeSer, 1985 (Cr)

Late Ordovician conodonts and brachiopods are described from limestones (previously referred to the Carriers Well Formation) in the Wairuna Formation of the Broken River Province. The conodonts include the new genus and species Molloydenticulus bicostatus. Scabbardella altipes, Besselodus fusus sp. nov. and Protopanderodus liripipus dominate the conodont fauna of 24 species, which is typical of the Protopanderodus biofacies , with two large, robust species of Protopanderodus making up about one-quarter of recovered elements. Associated lingulate brachiopods include species of Acrosaccus, Atansoria, Biernatia, Conotreta, Elliptoglossa, Hisingerella, Nushbiella, Paterula and Scaphelasma , identical with those known from the Malongulli Formation and correlative units of the Macquarie Volcanic Province in central New South Wales. Many of the same conodont species found in allochthonous limestones in the basal Malongulli Formation also occur in the fauna from north Queensland, supporting...

In northwest Argentina, weakly metamorphic clastic and calcareous sedimentary rocks of latest Precambrian to Lower Cambrian age (Puncoviscana Formation and related units) contain an abundant ichnofauna of both chronostratigraphic and paleoenvironmental value. In the western and central Sierras Pampeanas, metasedimentary and metavolcanic rocks are considered to form part of the same geotectonic unit. This "Pampean orogenic cycle" includes geosynclinal sedimentation of latest Precambrian to Lower Cambrian age, as well as magmatism, metamorphism and deformation of Middle to Upper Cambrian age, documented by an angular unconformity below the Upper Cambrian to Devonian rocks of the "Famatinian orogenic cycle". In some of the metamorphic rocks of the Pampean Cycle a pre-Ordovician folding is also distinguished from a later tectonic overprinting. Hence, the concept of a Pampean cycle differs from other concepts of late Precambrian orogenic cycles of South America which are only defined by radiometric ages. The Pampean orogenesis may be compared with the Ross orogenesis of the Transantarctic Mts., the Tyennan orogenesis of Australia and some of the deformation phases of the Damara orogen in Namibia.

Las Sierras Subandinas del noroeste argentino son una faja plegada y corrida que representa la deformación más oriental en los Andes Centrales. La estratigrafía está formada por más de 10 km de sedimentos de edad silúrica a actual. Los sedimentos siluro-devónicos incluyen más de 3.000 metros de sedimentos de plataforma marina somera, dispuestos en tres secuencias granocrecientes. La sedimentación carbonífera-jurásica muestra en la base una fuerte influencia glacial (Grupos Macharetí y Mandiyutí), que evoluciona hacia condiciones más áridas y templadas hacia el techo (Grupos Cuevo y Tacurú). Por último, se deposita una potente secuencia continental en una cuenca de antepaís. El nivel de despegue principal de las Sierras Subandinas se encuentra en las pelitas silúricas e inclina 2°-3° al oeste, y todas las fallas principales se despegan de él. Niveles de despegue intermedios importantes en las pelitas devónicas generan estructuras tipo lift off y el desacople de los niveles estructurales inferior y superior. Las Sierras Subandinas poseen un acortamiento mínimo de 60 km (36%) a la latitud de 22°40'. La deformación comenzó a los aproximadamente 8.5-9 Ma con el levantamiento de la sierra de El Pescado y el backthrust asociado y luego migró en dirección este. Se reconoce un importante estadio de deformación dominado por movimientos fuera de secuencia que comenzó a los 4.5 Ma y está activo en la actualidad. Se proponen dos modelos de acortamiento, para los cuales las tasas de deformación durante el Cuaternario, entre 8 y 11 mm/año, son coincidentes con los resultados de estaciones de GPS del área.

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Striated and faceted clasts found within the diamictites of the San Gaba Ân Formation immediately north of Ayaviri (southeast Peru) are interpreted as evidence for the late Ashgill±Llandovery glaciations in the Peruvian Altiplano, an area where this unit was previously considered not to be deposited. The Lower Paleozoic stratigraphic sequence cropping out in the area includes the Sandia, San Gaba Ân, and Ananea Formations, and thus belongs to the Eastern Cordillera tectonostratigraphic domain, but within the Altiplano morphotectonic region. The marked difference between the Lower Paleozoic sequences of the Altiplano and Eastern Cordillera tectonostratigraphic domains suggests important tectonic shortening along the boundary fault zone during Andean Cenozoic deformation.

Detailed mapping and facies analysis of a thick succession of diamictites of the Upper Ordovician Cancañiri Formation in southern Bolivia has revealed a glacioterrestrial origin for these sediments. The Cancañiri diamictites were deposited during three advances of a temperate, grounded ice sheet. They contain subglacial, englacial, and proglacial outwash sediments that increase in abundance from southeast to northwest. Clast fabrics and deformation features indicate SSE to NNW motion of the ice masses. Components of the diamictites usually display abrasion features such as facets and glacial striae. Provenance studies indicate that the pebbles comprise ~35% of siliciclastic sediments, mainly from the underlying shallow marine Ordovician rocks, 27% of slightly metamorphosed sediments that in part can be attributed to the Precambrian-Cambrian Puncoviscana Formation of northwestern Argentina, and a crystalline basement suite of metamorphic rocks (18%) and magmatic (mainly plutonic) rocks (20%). Due to the absence of typical lithologies, the Brazilian Shield, the Paraguay belt, and the southern Arequipa-Antofalla block could be excluded as possible source areas. The crystalline and metasedimentary clasts display strong affi nities with the Pampean basement in central Argentina. All data consistently suggest that the Cancañiri tillites of southern Bolivia were deposited by a regional, lowlatitude ice sheet that was independent of the main inland ice mass of Gondwana and centered SSE of the study area, in a Neoproterozoic to Cambrian orogenic belt in the area of the present Argentinean Chaco.

Microbial boundstones from Alaska and Russia yield new insights into the paleoecology of Silurian biotas that inhabited stromatolite reefs. These high-energy reefs were built along the Uralian Seaway in the Late Silurian by a diverse suite of microorganisms in association with accessory metazoans, predominantly sphinctozoan sponges. Within the stromatolite framework, three species of small, solitary, sphinctozoans (aphrosalpingids) encrusted a variety of hard substrates, mostly skeletal remains but also microbial laminae and cavity surfaces. Fossils encrusted by the sponges include the problematic hydroid Fistulella, possible stromatoporoids (recrystallized), crinoids, the possible cyanobacterium Ludlovia, corals, and unidentifiable shelly debris. In addition to the ubiquitous microbial laminae, the sponges, Fistulella, and ?stromatoporoids were less commonly encrusted by Ludlovia, Renalcis, or crinoids. Well-developed attachment surfaces, including enlarged holdfasts, allowed the sponges to achieve stability on the seafloor after larvae settled randomly on available hard surfaces. A greater incidence of sponge encrustations on Fistulella than on other organisms indicates that some of the sponges may have enjoyed a commensalistic relationship while attached as juveniles to a living substrate. The sponges' orientation on Fistulella in the sediment suggests that the relationship between the two taxa may have become parasitic, whereby the weight of the sponges caused Fistulella to collapse into the muddy substrate. Recognition of the intimate growth relationships shared by Silurian sphinctozoans, Fistulella, and other organisms expands the fossil record of encrusting sponges, identifies a novel sponge-?hydroid association, and reveals organismal responses to competition for space in mid-Paleozoic microbial reefs.

Striated and faceted clasts found within the diamictites of the San Gaba Ân Formation immediately north of Ayaviri (southeast Peru) are interpreted as evidence for the late Ashgill±Llandovery glaciations in the Peruvian Altiplano, an area where this unit was previously considered not to be deposited. The Lower Paleozoic stratigraphic sequence cropping out in the area includes the Sandia, San Gaba Ân, and Ananea Formations, and thus belongs to the Eastern Cordillera tectonostratigraphic domain, but within the Altiplano morphotectonic region. The marked difference between the Lower Paleozoic sequences of the Altiplano and Eastern Cordillera tectonostratigraphic domains suggests important tectonic shortening along the boundary fault zone during Andean Cenozoic deformation.

Striated and faceted clasts found within the diamictites of the San Gaba Ân Formation immediately north of Ayaviri (southeast Peru) are interpreted as evidence for the late Ashgill±Llandovery glaciations in the Peruvian Altiplano, an area where this unit was previously considered not to be deposited. The Lower Paleozoic stratigraphic sequence cropping out in the area includes the Sandia, San Gaba Ân, and Ananea Formations, and thus belongs to the Eastern Cordillera tectonostratigraphic domain, but within the Altiplano morphotectonic region. The marked difference between the Lower Paleozoic sequences of the Altiplano and Eastern Cordillera tectonostratigraphic domains suggests important tectonic shortening along the boundary fault zone during Andean Cenozoic deformation.

La Cuenca Ferrífera del noroeste argentino (NOA) comprende dos unidades litoestráficas: Formación Zapla (Ordovícico-Silúrico) y Formación Lipeón (Silúrico). Esta última es una secuencia sedimentaria compuesta por areniscas medias oolíticas intercaladas con limolitas micáceas, que fueron depositadas en ambiente marino somero. En estas secuencias se aloja una mineralización de Fe sedimentario tipo Minnette, con 3 Horizontes Ferríferos (I-II-III). En el Distrito Minero Unchimé (provincia de Salta) aflora el Horizonte Ferrífero II (20-45 % Fe), compuesto por minerales ferruginosos con contenidos menores de Mn: chamosita, hematita, limonita y siderita. La hematita y chamosita son dominantes y se encuentran como cemento o recubrimiento de oolitas, las cuales se desarrollan a partir de granos subangulosos de cuarzo o chamosita, fragmentos líticos de conchillas o de oolitas previas. La distribución de algunos elementos trazas indica un origen del Fe a partir de la meteorización continental....

The youngest Ordovician conodont fauna in SW Europe has been found in the Malaguide Complex of the Betic Cordillera, SE Spain. It is also the first Ordovician conodont fauna in the Western Mediterranean Alpine Orogen. The conodont association, attributed to the Hirnantian (upper part of the Amorphognathus ordovicicus Biozone), is characterized by the predominance of Walliserodus amplissimus and Scabbardella alt-ipes and by the absence of Sagittodontina and Istorinus, typical of the Mediterranean Province. This fauna differs markedly from those of the same biozone recorded in the Spanish Variscan Orogen of the Iberian Massif, which are attributed to the Katian. The Malaguide fauna shows, however, striking similarity to faunas of the Carnic Alps and some resemblance to those of the Pyrenees, Northern England and North Wales. These features suggest that Palaeozoic terranes of the Betic Cordillera were located far to the east of their present location and displaced westward during the Alpine Orogeny.

The Paleoproterozoic Dalma Formation, situated on the Singhbhum craton in eastern India, consists almost entirely of mafic to ultramafic rocks that were deposited by effusive volcanic activity and as pyroclastic deposits. The main focus of the present study, however, is on diamictites, consistinglike both other rock typesexclusively of chlorite and actinolitic hornblende, that are intercalated with these volcanic rocks. These diamictites are interpreted as mass-flow deposits. The textures and mineralogical composition of the diamictites suggests a volcanic origin of the material; most probably they are reworked pyroclastic deposits with the same mineralogy as the Dalma lavas. On this basis, it is concluded that the diamictites represent the deposits of one or more volcanic mudflows. They must have formed of material that had already travelled over a considerable distance in the form of grain-by-grain transport, as indicated by the presence of both rounded and distinctly broken clasts, and by the relatively small size of the clasts. Both the grain-by-grain and the mass transport must have occurred following pathways that were covered all over with volcanic material of mafic composition. This transport must have taken place under terrestrial conditions, as no marine or lacustrine clasts or matrix is included. The results of the study suggest that the diamictites within the Dalma Formation cannot be seen as a possible equivalent of Paleoproterozoic glacigenic sediments such as known from Australia and South Africa.

RESUMO: Estudaram-se exemplares de Graptólitos do Silúrico do sinclinal de Buçaco provenientes de colecções de Nery Delgado e de amostragem recente. Foi realizada a actualização taxonómica das colecções antigas. Determinaram-se novas espécies e biozonas. Os novos resultados permitem precisar o Silúrico do Buçaco que tem muitas características estratigráficas e sedimentológicas em comum com o de outras áreas da parte sul da Zona Centro Ibérica.

Plant megafossils resembling Cooksonia caledonica Lang aredescribed from the Kirusillas Formation in southern Bolivia.Faunal and lithological evidence together with field relation-ships advocate a late Silurian (Ludlow) age. Palaeocontinentalreconstructions suggest that the plants grew on the western edgeof Gondwana at 50–60° S and indicate that Cooksonia hadspread to higher latitudes soon after its appearance in the Wenlock.

ABSTRACTIncreasing current interest in sponge fossils includes numerous reports of diverse vermicular and peloidal structures interpreted as keratose sponges in Neoproterozoic to Mesozoic carbonates and in various open marine to peritidal and restricted settings. Reports of their occurrence are fundamental and far-reaching for understanding microfacies and diagenesis where they occur; and fossil biotic assemblages, as well as wider aspects of origins of animals, sponge evolution/ecology and the systemic recovery from mass extinctions. Keratose sponges: 1) have elaborate spongin skeletons but no spicules, thus lack mineral parts and therefore have poor preservation potential so that determining their presence in rocks requires interpretation; and 2) are presented in publications as interpreted fossil structures almost entirely in two-dimensional (thin section) studies, where structures claimed as sponges comprise diverse layered, network, particulate and amalgamated fabrics involving...

Boikothalamia reitner & engeSer, 1985 (J) Marinduqueia yaBe & Sugiyama, 1939 (Ce); this genus has recently been given an alternative assignment to that presented here and classified within the Order Dictyoceratida, Family Vaceletiidae (see discussion, p. 273-275) ?Menathalamia reitner & engeSer, 1985 (Cr) Murguiathalamia reitner & engeSer, 1985 (Cr) Stylothalamia ott, 1967 (P-Cr) Vaceletia picKett, 1982 (Cr-R) this genus has recently been given an alternative assignement to that presented here and classified within the Order Dictyoceratida, Family Vaceletidae (see discussion, p. 273-275). Vascothalamia reitner & engeSer, 1985 (Cr)

Sobre el faldeo nororiental del cerro Negro, en el extremo norte del valle de El Bolsón, en la Cordillera Patagónica de Río Negro, un corte de ruta expone un afloramiento de depósitos glaciarios de aproximadamente 400 metros de longitud. En dicha localidad se observaron una serie de rasgos deformacionales cuya descripción y análisis son objeto de este trabajo. A través de la confección de una serie de secciones y perfiles sedimentarios se interpretaron los depósitos y las estructuras como glaciotectónicas, producto de los esfuerzos que produce el movimiento glaciar, restando una investigación más exhaustiva para descartar su eventual conexión con procesos neotectónicos. Siendo así, la vergencia de las fallas y de los plegamientos observados permitió definir un avance de la masa glaciar desde el norte, siendo un aporte más para estudiar la dirección del flujo de hielo en este sector de los Andes durante el Último Máximo Glaciar. Además, se reconocieron dos pulsos de avance de los gla...

Sobre el faldeo nororiental del cerro Negro, en el extremo norte del valle de El Bolson, en la Cordillera Patagonica de Rio Negro, un corte de ruta expone un afloramiento dedepositos glaciarios de aproximadamente 400 metros de longitud. En dicha localidad se observaron una serie de rasgos deformacionales cuya descripcion y analisis son objeto de este trabajo. A traves de la confeccion de una serie de secciones y perfiles sedimentarios se interpretaron los depositos y las estructuras como glaciotectonicas, producto de los esfuerzos que produce el movimiento glaciar, restando una investigacion mas exhaustiva para descartarsu eventual conexion con procesos neotectonicos. Siendo asi, la vergencia de las fallas y de los plegamientos observados permitio definir un avance de la masa glaciar desde el norte, siendo un aporte maspara estudiar la direccion del flujo de hielo en este sector de los Andes durante el Ultimo Maximo Glaciar. Ademas, se reconocieron dos pulsos de avance de los glacia...

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid-pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.

Detailed mapping and facies analysis of a thick succession of diamictites of the Upper Ordovician Cancañiri Formation in southern Bolivia has revealed a glacioterrestrial origin for these sediments. The Cancañiri diamictites were deposited during three advances of a temperate, grounded ice sheet. They contain subglacial, englacial, and proglacial outwash sediments that increase in abundance from southeast to northwest. Clast fabrics and deformation features indicate SSE to NNW motion of the ice masses. Components of the diamictites usually display abrasion features such as facets and glacial striae. Provenance studies indicate that the pebbles comprise ~35% of siliciclastic sediments, mainly from the underlying shallow marine Ordovician rocks, 27% of slightly metamorphosed sediments that in part can be attributed to the Precambrian-Cambrian Puncoviscana Formation of northwestern Argentina, and a crystalline basement suite of metamorphic rocks (18%) and magmatic (mainly plutonic) rocks (20%). Due to the absence of typical lithologies, the Brazilian Shield, the Paraguay belt, and the southern Arequipa-Antofalla block could be excluded as possible source areas. The crystalline and metasedimentary clasts display strong affi nities with the Pampean basement in central Argentina. All data consistently suggest that the Cancañiri tillites of southern Bolivia were deposited by a regional, lowlatitude ice sheet that was independent of the main inland ice mass of Gondwana and centered SSE of the study area, in a Neoproterozoic to Cambrian orogenic belt in the area of the present Argentinean Chaco.