Lower Ordovician Research Articles

Fossilized embryos are widespread but the record is temporally and taxonomically biased

We report new discoveries of embryos and egg capsules from the Lower Cambrian of Siberia, Middle Cambrian of Australia and Lower Ordovician of North America. Together with existing records, embryos have now been recorded from four of the seven continents. However, the new discoveries highlight secular and systematic biases in the fossil record of embryonic stages. The temporal window within which the embryos and egg capsules are found is of relatively short duration; it ends in the Early Ordovician and is roughly coincident with that of typical "Orsten"-type faunas. The reduced occurrence of such fossils has been attributed to reducing levels of phosphate in marine waters during the early Paleozoic, but may also be owing to the increasing depth of sediment mixing by infaunal metazoans. Furthermore, most records younger than the earliest Cambrian are of a single kind-large eggs and embryos of the priapulid-like scalidophoran Markuelia. We explore alternative explanations for the low taxonomic diversity of embryos recovered thus far, including sampling, size, anatomy, ecology, and environment, concluding that the preponderance of Markuelia embryos is due to its precocious development of cuticle at an embryonic stage, predisposing it to preservation through action as a substrate on which microbially mediated precipitation of authigenic calcium phosphate may occur. The fossil record of embryos may be limited to a late Neoproterozoic to early Ordovician snapshot that is subject to dramatic systematic bias. Together, these biases must be considered seriously in attempts to use the fossil record to arbitrate between hypotheses of developmental and life history evolution implicated in the origin of metazoan clades.

Influence of slump folds on tectonic folds: an example from the Lower Ordovician of the Anglo-Brabant Deformation Belt (Belgium)

Although it is generally accepted that buckle folds will not develop in a perfectly planar layer without the presence of some irregularity or perturbation at which the folds initiate, there are very few cases in which individual natural folds can be linked to specific irregularities. Within the Lower Ordovician Abbaye de Villers Formation, Anglo-Brabant Deformation Belt, metre-scale tectonic folds occur, of which the position and, to a certain extent, the geometry appear to be controlled by slump folds and related features. The metre-scale tectonic folds, interpreted as parasitic structures on the limb of a large-scale host fold, occur only within a stratigraphic level affected by slumping. In this level, tectonic antiforms tend to form superimposed on antiformal slump folds and on zones of abrupt, slump-related thickness increase, and tectonic synforms on synformal slump folds and on zones of abrupt thickness decrease. The rather irregular 3D geometry of sedimentary sequences suggests that many more similar cases should exist in which folds can be linked to specific irregularities. However, possibly it is also this abundance of irregularities in sedimentary sequences, in combination with fold and outcrop scale, that makes it difficult to attribute a particular fold to a particular perturbation.

New early Ordovician calcareous foraminifers of the middle Tien Shan

Twelve new species of calcareous foraminifers of the genera Parathurammina, Parathuramminites, Bisphaera, Archaesphaera, and Vicinesphaera from the Lower Ordovician of the Middle Tien Shan are described.

Early Ordovician Triangulodus (Conodonta) from the Honghuayuan Formation of Guizhou, South China

Two new conodont species, Triangulodus, T. bifidus sp. nov. and T. zhiyii sp. nov., are described and illustrated from the Early Ordovician Honghuayuan Formation of Guizhou, South China. Both consist of a septimembrate apparatus having similar M and Sd elements. Triangulodus bifidus is distinguished from all other known species of Triangulodus in displaying bifurcating costae on all seven constituent elements. Although the ranges of both species overlap and extend through most of the unit, T. bifidus is more abundant in the lower part of the Honghuayuan Formation, whereas T. zhiyii is most common in the middle and upper part of the formation in the more comprehensively sampled type section. Both new species are widely distributed in South China, with T. bifidus also reported in the Lower Ordovician of the Tarim Basin.

Lower Ordovician Leetse Formation in the North Estonian Klint area

Sequences of the Leetse Formation were studied in order to find out the conodont biozonation, grain-size distribution, and glauconite mineral properties and their variations along the North Estonian Klint. On the basis of the conodont zonation of the Saka section, combined with earlier data, it was found that the Leetse Formation belongs to the Paroistodus proteus, Prioniodus elegans, and Oepikodus evae zones of various thickness and completeness along the North Estonian Klint. The Pr. elegans Zone of the Saka section was found to be remarkably complete and correlate well with equivalent intervals of sections in the St. Petersburg Region. Poorly lithified glauconitic sandstones of the Leetse Formation vary lithologically vertically as well as laterally. The Leetse Formation in the west has been repeatedly redeposited and is depleted in suspended fraction, while in the east it is more homogeneous and represents continuous shallow-water conditions. Glauconite is represented by a potassium- rich variety (about 9% K2O) and most likely forms at least two discrete modifications. Laterally, glauconite of the Leetse Formation in the western part of the klint contains less K than that of the eastern part, indicating faster sedimentation. The d(001) value of glauconite diminishes upwards in the sequence, indicating acceleration of the sedimentation rate.

The importance of plankton and nekton distributions in Ordovician palaeogeographical reconstructions

AbstractTrilobites and brachiopods are the two main fossil groups that allowed construction of the first palaeogeographical maps for the early Palaeozoic. Together with the bivalves and ostracodes, the benthic elements of these fossil groups have proved to be of great palaeobiogeographical importance. For this reason, these groups are usually considered to be ‘better’ fossils for inferring Ordovician palaeogeography. The present study indicates that planktic and nektic fossil groups should not be neglected in such palaeobiogeographical studies. The plotting on a palaeogeographical reconstruction for the Arenig (Lower Ordovician, – 480 Ma) of some planktic (acritarchs, chitinozoans) and nektic (vertebrates, pelagic trilobites) fossil groups indicates that their distribution appears in part surprisingly similar to that of the benthic trilobite faunas that are considered to display the greatest provincialism. For example, the distribution of the ‘peri-Gondwanan’ acritarch province including Arbusculidium filamentosum, Coryphidium and Striatotheca, and the distribution of the Eremochitina brevis chitinozoan assemblage are almost identical to the palaeogeographical distribution of the Calymenacean-Dalmanitacean trilobite fauna. A review of the different planktic and nektic fossil groups also indicates that it is very important to carefully select ‘good’ palaeogeographical indicators, in most cases from a large number of taxa. It appears that almost all fossil groups include some ‘good’ palaeobiogeographical ‘markers’. Therefore it is important to search for ‘better’ taxa within each fossil group, instead of looking only for the ‘better’ fossil groups as a whole.

40Ar/ 39Ar geochronology of gold mineralization in Brasiliano strike-slip shear zones in the Borborema province, NE Brazil

40Ar/ 39Ar geochronology of muscovite and biotite grains genetically related to gold and Be–Ta–Li pegmatites from the Seridó Belt (Borborema province, NE Brazil) yield well-defined, reliable plateau ages. This information, combined with data about paragenetic and field relationships, reveals Cambro-Ordovician mineralization ages (520 and 500–506 Ma) for the orogenic gold deposits in the Seridó Belt. Biotite ages of 525±2 Ma, which represent the mean weighted results of the incremental heating analysis of six biotite single crystals, record the time of pegmatite emplacement and reactivation of Brasiliano/Pan-African strike-slip shear zones. These results, along with previous structural evolution studies, suggest that shear zones formed during the Brasiliano/Pan-African event were reactivated in the Upper Cambrian–Lower Ordovician. Mineralization occurs late in the history of the orogen.

Parablastoid holdfasts from the Lower Ordovician of South Wales

AbstractProblematical discoid fossils from the Lower Arenig (Moridunian Stage) of the Llangynog Inlier, South Wales, are interpreted as parablastoid holdfasts. They constitute the first record of holdfasts in this rare echinoderm class. The lower surface has a radial ribbed structure, in contrast to the concentric, reticulate structure of the upper surface. The substrate for attachment was not lithified and may instead have been sediment stabilized by microbial mats. An associated pluricolumnal indicates that the parablastoid column was homeomorphic. Copyright © 2005 John Wiley & Sons, Ltd.

Lower Ordovician Sequences of the Ebeta Antiform, the Southern Urals

Lower Ordovician sequences of the Ebeta antiform, a southern extension of the Uraltau zone, were deposited at the conjugation of paleocontinental and paleoceanic sectors of the southern Urals. Four types of sections were formed on opposite sides of cordillera that existed at the margin of the East European paleocontinent on the Preordovician volcanic belt. Sections of the first three types made up the western apron on the uplift that served as a provenance. Lateral and vertical relationships of various sedimentary associations, as well as their variable (in space and time) facies patterns and sedimentation conditions are considered. An important role of redeposition, slumping, and faulting in the apron development has been revealed. The apron fringed the eastern wall of the Sakmara marginal riftogenic basin that arose at the Cambrian-Ordovician boundary. On another side of the marginal uplift in the east, a slightly modified perioceanic environment existed in the Early Ordovician. Sections of another type formed here at the periphery of Uralian paleocean. These sections are characterized by the universal occurrence of ophiolithoclastic olistostrome with fragments of an older oceanic crust.

Origin of the Ostracoda and their maxillopodan and hexapodan affinities

There are Cambrian fossils attributed to the Ostracoda but the extant subclasses Podocopa and Myodocopa do not appear until the Ordovician. At this time the morphologically similar, free-living ancestors of the now sedentary Thecostraca (Ascothoracida, Acrothoracica and Cirripedia) may have still been extant, and from an ecological point of view it seems likely that, by and large, ostracods replaced them. However, living ostracods have an abbreviated, direct development, and some key aspects of their morphology, such as the nature of the maxillary segment and abdomen, are conjectural. Thus the affinities between these and related taxa remain uncertain; e.g., while some contemporary carcinologists place Ostracoda as a taxon coordinate with the Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Malacostraca, others tentatively or unequivocally ally them with the Maxillopoda (generally Mystacocarida, Copepoda, Tantulocarida and Thecostraca, and sometimes Branchiura and Pentastomida). Others, largely involved with fossils, have stretched the definition of the Maxillopoda even further, to the point where it seems even less likely a monophyletic taxon. Until recently cladistic analyses utilizing genetic (largely 18S rDNA) as well traditional morphological characteristics have given confusing results regarding the affinities between these taxa, and an important one suggested the Ostracoda might even be diphyletic. Furthermore, a very recent genetic study utilizing protein encoding genes places a podocopine ostracod among the most primitive of the extant crustaceans (Branchiopoda, Cephalocarida Remipedia and Mystacocarida), and then generally at the base of a lineage leading to the Malacostraca, a lineage giving rise to copepods and cirripeds along the way. This indicates these so-called maxillopodan taxa evolved independently from a malacostracan-like ancestor, and if so they are convergent. And finally, from genetic studies it is not only becoming well documented the Crustacea rather than Myriapoda gave rise to the Hexapoda, but it appears the Hexapoda stem from among the lower rather than the higher crustaceans, possibly even from the Ostracoda. Whether there were terrestrial ostracods at the time hexapods appeared in the Lower Ordovician is unknown, but the modest diversity of terrestrial ostracods today are podocopines which also first appeared in the Lower Ordovician. Thus, if current interpretations of living ostracodan and fossil hexapodan body plans are largely correct, it can be hypothesized the Ostracoda are close to the ancestor of the Hexapoda.

Middle Cambrian pterobranchs and the Question: What is a graptolite?

The presence of distinct fusellar structure is taken as evidence to include a number of fossils from the Middle Cambrian to the Lower Ordovician of North America and Europe with the Pterobranchia. The dome of the pterobranchs and the prosicula of the planktic graptolites are contrasted and evidence is given for the re-assignment of a number of well known dendroid graptolites to the pterobranchs. A non-destructive method is described to reveal fusellar development of delicate hemichordate exoskeletons from shales. Rhabdotubus robustus n. sp. from the Czech Republic and ? Cephalodiscus sp. from the Wheeler Shale of North America are described as new Middle Cambrian pterobranchs.

Skeletal homologies, phylogeny and classification of the earliest asterozoan echinoderms

Synopsis A thorough reappraisal of extant and fossil asterozoan plate systems is used to construct working hypotheses of primary plate homologies for Ordovician Asterozoa. Results indicate that inferomarginal ossicles of asteroids, somasteroids and the primitive ophiuroid Phragmactis are likely to be homologous with edrioasteroid marginals. The intermediate virgals and coverplates of somasteroids are identified as modified actinals and the sub‐laterals of certain stenurids are axial, here termed podial plates, with laterals in these taxa homologous to true lateral plates and adambulacrals of ophiuroids and asteroids, respectively. The nature of the asterozoan mouth frame is reinvestigated as a result of new study of the mouth frames of key taxa, with a reappraisal of existing developmental and symmetry data. It is argued that primitive asterozoan mouth angle plates were entirely ambulacral and probably not spine‐bearing, but that mouth spines evolved in asteroids and ophiuroids either on the existing first ambulacrals or by the superposition of first adambulacrals adoral to first ambulacrals. These new data are used to undertake a cladistic analysis of all known genera of Ordovician asterozoan echinoderms (38 taxa), based on a dataset of 150 morphological characters, rooted on the Middle Cambrian edrioasteroid taxa Stromatocystites and Cambraster. The resulting tree is fully resolved although some deeper branches show poor support. It reveals that Ordovician asteroids form a monophyletic group (defined by the presence of an unpaired axillary inferomarginal, opposing ambulacrals and blocky abutting adambulacrals) and are a sister group to the remainingasterozoans, including the somasteroids. Somasteroids are monophyletic and, with the primitive taxon Phragmactis, are a sister group to the remaining ophiuroids. The class Ophiuroidea is diagnosed by the near‐simultaneous appearance of apomorphies including an interradial disc with highly separated arms and aboral and oral interambulacral longitudinal musculature. A phylogenetic tree constructed for asterozoans based on current stratigraphic occurrence data indicates that at the time of their first appearance, asteroids and ophiuroid body plans were already established. This suggests a significant but unobserved pre‐Ordovician history. Comparison of observed and phylogenetic diversity together with the appearance of relatively young taxa close to the base of the tree point to a poor asterozoan record, particularly in the Lower Ordovician. A revised classification for stem‐group asterozoans is proposed based on well‐supported clades with the new taxon Eopentaroida erected. Mapping of character states on the most parsimonious solution suggests that whilst asteroid morphological evolution was conservative in the Ordovician, ophiuroids underwent a profound morphological diversification. Functionally, these changes are linked to the adoption of carnivory by derived Ordovician ophiuroids. In contrast, asteroids probably remained dominantly deposit‐feeders throughout the period. Character distribution on the cladogram refutes none of the hypotheses of primary plate homology, although it suggests that podial ossicles were developed in two clades independently and that the marginals of eopentaroids may not be homologous to those of other asterozoans and edrioasteroids.

The Lower and Middle Ordovician Cephalopod Faunas of England and Wales

The Lower and Middle Ordovician Cephalopod Faunas of England and Wales

Macroborings (Gastrochaenolites) in Lower Ordovician Hardgrounds of Utah: Sedimentologic, Paleoecologic, and Evolutionary Implications

Other| December 01, 2004 Macroborings (Gastrochaenolites) in Lower Ordovician Hardgrounds of Utah: Sedimentologic, Paleoecologic, and Evolutionary Implications JACOB S. BENNER; JACOB S. BENNER 1Department of Geology, Tufts University, Lane Hall, Medford, MA 02155, jacob.benner@tufts.edu Search for other works by this author on: GSW Google Scholar A.A. EKDALE; A.A. EKDALE 2Department of Geology and Geophysics, University of Utah, 135 South 1460 East, Room WBB 719, Salt Lake City, UT 84112 Search for other works by this author on: GSW Google Scholar JORDI M. DE GIBERT JORDI M. DE GIBERT 3Departament d'Estratigraphia, Paleontologia i Geociències Marines, Universitat de Barcelona, Martí Franquès s/n, 08028 Barcelona, Spain Search for other works by this author on: GSW Google Scholar Author and Article Information JACOB S. BENNER 1Department of Geology, Tufts University, Lane Hall, Medford, MA 02155, jacob.benner@tufts.edu A.A. EKDALE 2Department of Geology and Geophysics, University of Utah, 135 South 1460 East, Room WBB 719, Salt Lake City, UT 84112 JORDI M. DE GIBERT 3Departament d'Estratigraphia, Paleontologia i Geociències Marines, Universitat de Barcelona, Martí Franquès s/n, 08028 Barcelona, Spain Publisher: SEPM Society for Sedimentary Geology Accepted: 27 Apr 2004 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2004) 19 (6): 543–550. https://doi.org/10.1669/0883-1351(2004)019<0543:MGILOH>2.0.CO;2 Article history Accepted: 27 Apr 2004 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JACOB S. BENNER, A.A. EKDALE, JORDI M. DE GIBERT; Macroborings (Gastrochaenolites) in Lower Ordovician Hardgrounds of Utah: Sedimentologic, Paleoecologic, and Evolutionary Implications. PALAIOS 2004;; 19 (6): 543–550. doi: https://doi.org/10.1669/0883-1351(2004)019<0543:MGILOH>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract New evidence of fossil macroborings in the Lower Ordovician (Ibexian) of western Utah demonstrates that the macroboring behavioral strategy was firmly established in the earliest stages of the great Ordovician diversification of the marine biosphere. In Utah, borings were excavated in hardgrounds that had developed on sponge-algal mounds and flat-pebble conglomerates in the Fillmore Formation (Ibexian). The most complete specimens possess a neck up to 1 cm in length that opens into a teardrop-shaped chamber with a maximum diameter of 1 cm. The chamber terminates at a depth of 3–4 cm below the hardground surface. These borings belong to the ichnogenus Gastrochaenolites. The organisms responsible for creating the borings are unknown. Sedimentologically, the effect of boring on hardgrounds was to break them into pebble- and cobble-sized clasts. The endolithic lifestyle represented by the borings may have evolved in response to ecologic pressures such as predation or competition for food resources. The macroborings from the Fillmore Formation represent an innovative strategy that may have resulted in the later development of new body plans and the early establishment of endolithic macroinvertebrates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Early Ordovician foraminifers from the Lava River Section, northwestern Russia

Research Article| January 01, 2004 Early Ordovician foraminifers from the Lava River Section, northwestern Russia Galina P. Nestell; Galina P. Nestell 1Department of Geology, The University of Texas at Arlington, Arlington, TX 76019, USA Earth Crust Institute, St. Petersburg State University, St. Petersburg, Russia e-mail: gnestell@uta.edu Search for other works by this author on: GSW Google Scholar Tatiana Yu. Tolmacheva Tatiana Yu. Tolmacheva 2Department of Stratigraphy and Paleontology, All-Russian Geological Research Institute (VSEGEI), 199106 St. Petersburg, Russia e-mail: tatolm@hotmail.com Search for other works by this author on: GSW Google Scholar Micropaleontology (2004) 50 (3): 253–280. https://doi.org/10.2113/50.3.253 Article history received: 03 Mar 2004 accepted: 14 Jul 2004 first online: 06 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Galina P. Nestell, Tatiana Yu. Tolmacheva; Early Ordovician foraminifers from the Lava River Section, northwestern Russia. Micropaleontology 2004;; 50 (3): 253–280. doi: https://doi.org/10.2113/50.3.253 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyMicropaleontology Search Advanced Search Abstract Recent studies on the biostratigraphy and fauna of the Lower Ordovician successions of the Russian part of Baltoscandia have shown for the first time that agglutinated foraminifers are very numerous in the deposits of the lower part of the Latorp Russian Regional Stage in the vicinity of St. Petersburg, northwestern Russia. These forms were discovered in a classic section exposed on the left side of the Lava River canyon in the northern outskirts of the village of Vassilkovo. The lower part of the Latorp Regional Stage is represented in this section by unlithified sands and clays of the Lakity Member of the basal part of the Leetse Formation and corresponds to the upper part of the Hunneberg Regional Stage of Baltoscandia. The clay beds of the Lakity Member encompass the upper part of the Paroistodus proteus and lower part of the Prioniodus elegans conodont zones and correspond to the Tetragraptus phyllograptoides graptolite zone. Foraminifers from the Lakity Member were collected from strata belonging to both of the above mentioned conodont zones and are represented by monothalamous agglutinated foraminiferal tests of a new species of a new genus Lakites ordovicus n. gen., n. sp., new species of the genus Amphitremoida: A. asperella n. sp., A. laevis n. sp., A. longa n. sp., A. orbicularis n. sp., and A. rugosa n. sp. (family Hippocrepinellidae), a new genus and species Lavella cucumeriformis n. gen., n. sp. of the family Saccamminidae, and one specimen assigned to the genus Arenosiphon? of the family Hippocrepinidae. The finding of representatives of the genus Amphitremoida (emended herein), previously known only from the Middle Ordovician to the Lower Carboniferous (Tournaisian) permits the extension of the range of this genus down into the Lower Ordovician. You do not currently have access to this article.

Aromatic compounds in crude oils and source rocks and their application to oil–source rock correlations in the Tarim basin, NW China

Ten series of aromatic hydrocarbons (biphenyls; naphthalenes; phenanthrenes; anthracenes; retenes; chrysenes; benzoanthracenes; dibenzofurans; fluorenes; and dibenzothiophenes) isolated from Cambrian and Ordovician marine carbonate source rocks in Tabei and Tazhong areas of the Tarim Basin, and Triassic and Jurassic lacustrine mudstones and swamp coals in the Kuche depression have been characterized by GC-MS. The same series of aromatic compounds have been isolated and characterized from crude oils of the Tabei and Tazhong uplift. Results presented in this paper describe the distribution of these aromatic compounds in crude oils and source rocks from the three different kinds of sedimentary environments. Similarities in distributions of the fluorenes and the dibenzothiophenes in the aromatic fractions suggest the Tabei uplift oils are derived predominantly from the Lower Ordovician marine source rocks of Tabei Lunnan area (on the south of Luntai) and the oils from Tazhong uplift are mainly derived from Upper Cambrian, Lower Ordovician, and Middle–Upper Ordovician marine source rocks of Tazhong area.

Billingen (Lower Arenig/Lower Ordovician) acritarchs from the East European Platform and their palaeobiogeographic significance

Billingen (Lower Arenig/Lower Ordovician) sediments of the St. Petersburg region, northwest Russia and the Leba area, northern Poland of the East European Craton yield acritarch assemblages, which are largely homogenous though displaying minor compositional differences that probably reflect a gradient from inner to outer shelf environments. Comparison with coeval acritarch microflora from the Yangtze Platform, South China, shows an overall similarity between Baltoscandian and South Chinese phytoplankton. The widespread uniformity in the fossil microphytoplankton may be related to the extensive global ‘evae’ sea-level transgression, which characterized the Billingen time. This suggests that during the Tremadoc through early Arenig times, acritarch assemblages displayed essentially an undifferentiated cold-water and oceanic character along the whole margin of Perigondwana in the South, as well as on the South Chinese and Baltic platforms, at middle latitudes (Mediterranean oceanic Realm). Despite this overall similarity, however, some typical taxa of the high-latitude Mediterranean Province (Arbusculidium, Coryphidium and Striatotheca) occur in South China, but are absent in Baltica. This discrepancy is explained as caused by differences in climatic and physiographic conditions that prevailed at the two palaeocontinents at this time. The inferred pattern of oceanic circulation during the Lower Ordovician is consistent with the palynological evidence of a prevailing warmer climate in Baltica than in South China, although the two palaeocontinents occupied the same palaeolatitudinal position.

Siliciclastic sedimentation and sequence stratigraphic evolution on a storm-dominated shelf: the Lower Ordovician of the Central Iberian Zone (NE Jaén, Spain)

The Lower Ordovician of the southernmost part of the Central Iberian Zone (southern Spain) is a 1000-m thick, transgressive, largely siliciclastic succession deposited on a shallow epicontinental shelf. Facies analysis reveals lithofacies representing a variety of depositional environments, ranging from foreshore-shoreface (Facies S 0), proximal to distal parts of the shoreface (Facies S 1, S 2), proximal to distal shelf (Facies H 1, H 2, H 3), outer shelfzone or continental slope (Facies M). The vertical arrangement of the facies is characterized by the presence of high-frequency regressive cycles (partial shelf progradational sequences), themselves interrupted by strata resulting from catastrophic phenomena (storms), producing higher order autocycles of transgressive–regressive character. Major regional unconformities and the vertical distribution of the facies delineate successive genetically related units in relation to relative sea-level changes. Two second-order megacycles have been distinguished. The first of them (MC1), Tremadoc?-Lower Arenigian in age, comprises one third-order sequence (Ar-1). The lower part of the second megacycle (MC2), deposited during the Arenig-Llanvirnian, displays a trangressive stratigraphic architecture. Within this megacycle, three third-order sequences are proposed (Ar-2, Ar-3, Lln-1), in which a transgressive systems tract and a highstand systems tract can be distinguished. Facies attributable to lowstand systems tracts are restricted to the base of the Ar-2 sequence, at the boundary between megacycles.

Sedimentary facies, depositional evolution of the Upper Cambrian–Lower Ordovician Santa Rosita formation in northwest Argentina

The Upper Cambrian–Tremadocian Santa Rosita Formation, the lowermost unit of the Santa Victoria Group in northwest Argentina, records part of the infill of a retroarc basin. Sedimentologic, ichnologic, and sequence stratigraphic evidence indicates that this unit represents a complex mosaic of siliciclastic facies that reflects the vertical passage of restricted fluvial and tide-dominated estuarine environments to open-marine settings affected by waves. Twelve facies grouped in two facies associations were recognized. The base of the Santa Rosita Formation represents a sequence boundary formed from a relative sea level fall responsible for the incision of fluvio-estuarine valleys in the underlying Mesón Group, which most likely occurs close to the Middle–Late Cambrian transition. Valley filling, represented by the Tilcara Member, started with deposition of conglomerate and sandstone of the lowstand systems tract in braided fluvial systems restricted to the valley axis. The bulk of valley sedimentation, however, is recorded by estuarine deposits that accumulated during a subsequent transgression and were in turn replaced upward by transgressive marine facies (transgressive systems tract). The maximum flooding is represented by lower offshore, greenish gray shales of the Casa Colorada Member. These strata are overlain by upper offshore to offshore transition deposits that from progradational parasequence sets, which represent a highstand systems tract. Fine-grained deposits of the Casa Colorada Member are replaced upward by forced regressive shoreface sandstone of the Alfarcito Member. A major transgression, recorded at the base of the Rupasca Member, probably occurred during the Early Tremadocian and led to deposition of anoxic black shales in shelf areas. The Rupasca Member is characterized by a series of transgressive–regressive events during the Early Tremadocian and early Late Tremadocian. Deposition culminates in a regressive event recorded by the shoreface sandstone of the Humacha Member during the late Late Tremadocian.

Cambrian-Ordovician shales in the Humber Zone: thermal maturation and source rock potential

ABSTRACT Thermal maturation and potential for hydrocarbon generation and pooling are evaluated in the Upper Cambrian Grosses-Roches Formation and the Lower Ordovician Riviere Ouelle Formation of the Humber Zone in Gaspe Peninsula. Five types of organic matter (OM) analyses are used: petrography of rock thin sections and of OM concentrates, reflectance analysis, programmed pyrolysis, and combustion analysis. All analyses indicate that these successions are overmature with respect to hydrocarbon generation, but do not reach the threshold of metamorphism. Source rocks in these formations are in the dry gas zone, and the previously generated oils are now altered into various types of migrabitumen or native coke. A regular increase in maturation is observed in the Riviere Ouelle Formation (Anse du Crapaud member) from the upper condensate zone to near the end of the dry gas zone. No such gradient is observed in the Grosses-Roches Formation. Our results suggest that the upper member of the Grosses-Roches Formation may have had initial fair hydrocarbon potential (some initial TOC values up to about 2 wt%, assuming Type II kerogen) and the D2 unit of the Anse du Crapaud member may have had initial good hydrocarbon potential (some initial TOC values up to about 3 wt%, assuming Type II kerogen). The OM petrographic composition of these fine-grained potential source rocks shows some similarities with those of the Late Ordovician Macasty (Anticosti Island) and Lower Ordovician Green Point (Western Newfoundland) formations. The OM petrographic composition of the A to B2 units of the Anse du Crapaud member suggests former gas storage. No producible hydrocarbons remain. The recognition of Upper Cambrian to Lower Ordovician oil source rocks down-dip of potential Ordovician dolostone reservoir rocks of the Anticosti and St. Lawrence platforms opens a new perspective for hydrocarbon exploration in eastern Quebec. End_Page 213------------------------