Search

This study provides a taxonomic treatment and comparison of lower Gorstian (Silurian) radiolarians recovered from two sections of the Cape Phillips Formation in the Canadian Arctic that accumulated in two different paleoenvironmental settings. Twilight Creek is more basinal, located ~100 km from the paleo-shelf margin, whereas Snowblind Creek is located within 1 km of the paleo-shelf break, on Cornwallis Island. The fauna, like other material from the Cape Phillips Formation, is extremely well preserved and was recovered from four samples at two localities, all from the Lobograptus progenitor graptolite Zone, an interval that has few published studies regarding radiolarians. A total of 28 species are recognized, of which two are new and described herein (Fusalfanus bilateralis n. sp. and Pseudospongoprunumparvispina n. sp.), belonging to the Haplotaeniatidae, Inaniguttidae, 'Sponguridae', Ceratoikiscidae, Entactiniidae, Palaeoscenidiidae, and Secuicollactidae. Based on these new data, the stratigraphic ranges of some taxa are extended. The species concepts of several closely related inaniguttid species are re-evaluated. As a result, Inanihella tarangulica Nazarov and Ormiston, 1984 is transferred to the genus Fusalfanus Furutani, 1990 based on cortical shell structure, and Inanihella duroacus, Inanihella legiuncula, and Inanihella perarmata are synonymized under Fusalfanus tarangulica sensu lato. Aciferopylorum admirandum is transferred to Fusalfanus and considered a junior synonym of Fusalfanus osobudaniensis. The comparative analysis of taxonomic richness and composition reveals that the more distal sample from Bathurst Island exhibits a slight, but statistically significant, higher alpha diversity at the species rank than the more proximal basin/platform samples from Snowblind Creek. Biodiversity indices at the genus rank produced mixed results, indicating that differences between sites are at best slight. There is also a strong taxonomic separation between the fine and coarse size radiolarian fractions recovered during the sieving of each sample. The siliceous sponge spicule assemblages from these faunas show an inverse diversity relationship to the radiolarians in that Snowblind Creek contains greater alpha diversity than Twilight Creek. This study provides the first documentation of facies-controls in Silurian radiolarian diversity and is useful in evaluating the role of taxa used in biostratigraphy. L'etude presente le traitement taxonomique et la comparaison de radiolaires du Gorstien (Silurien) inferieur recuperes de deux sections de la Formation de Cape Phillips, dans l'Arctique canadien, accumulees dans deux contextes paleoenvironnementaux differents. Twilight Creek s'apparente plus a un milieu de bassin situe a ~100 km de la bordure de la paleoplateforme, alors que Snowblind Creek est situe a moins de 1 km de la bordure de cette derniere, dans l'ile Cornwallis. La faune, comme d'autre materiau de la Formation de Cape Phillips, est extremement bien preservee et a ete recuperee de quatre echantillons provenant de deux localites, tous de la zone a graptolite Lobograptus progenitor, un intervalle caracterise par peu d'etudes publiees sur les radiolaires. Un total de 28 especes sont reconnues, dont deux nouvelles decrites dans la presente etude (Fusalfanus bilateralis n. sp. et Pseudospongoprunum parvispina n. sp.), appartenant aux Haploteniatides, Inaniguttides, <>, Ceratoikiscides, Entactiniides, Paleoscenidiides et Secuicollactides. A la lumiere de ces nouvelles donnees, les repartitions stratigraphiques de certains taxons sont elargies. Le concept d'espece pour plusieurs especes d'inaniguttides etroitement reliees est reevalue et, en consequence, Inanihella tarangulica Nazarov and Ormiston, 1984 est transfere au genre Fusalfanus Furutani, 1990 a la lumiere de la structure corticale de sa coquille, et Inanihella duroacus, Inanihella legiuncula et Inanihella perarmata sont etablis comme des synonymes de Fusalfanus tarangulica sensu lato. Aciferopylorum admirandum est transfere a Fusalfanus et considere comme etant un synonyme junior de Fusalfanus osobudaniensis. L'analyse comparative de la richesse et de la composition taxonomiques revele que l'echantillon plus distal de l'ile Bathurst presente une diversite alpha legerement, bien que significativement, plus forte au rang de l'espece que les echantillons plus proximaux de bassin/plateforme de Snowblind Creek. Les indices de biodiversite au rang du genre produisent des resultats mitiges, indiquant que les differences entre localites sont, au mieux, minimes. Il y a egalement une separation taxonomique marquee entre les fractions fines et grossieres de radiolaires produites par le criblage de chacun des echantillons. Les assemblages de spicules d'eponges siliceux de ces faunes presentent une relation de diversite inverse a celle des radiolaires, en cela que l'assemblage de Snowblind Creek contient une plus grande diversite alpha que celui de Twilight Creek. Cette etude fournit la premiere documentation d'un controle de la diversite de radiolaires siluriens par le facies et est utile pour l'evaluation du role des taxons utilises en biostratigraphie. [Traduit par la Redaction], Introduction Since their appearance in the lower Cambrian (Pouille et al. 2011), Polycystine radiolarians diversified during the entire Phanerozoic (De Wever et al. 2001); they thus have the potential to [...]

Upper Ordovician radiolarians are described from limestone within the Malongulli Formation in the Cliefden Caves area using revised systematics. Eight genera and 12 species are described, including the following four new species: Protoceratoikiscum crossingi, Haplotaeniatum ovatum, Haplotaeniatum prolatum, and Borisella dunhilli, and two new combinations, Borisella subulata and Haplotaeniatum spongium. All samples are dominated by Kalimnasphaera maculosa, B. subulata, and Palaeoephippium octaramosum with lesser abundances of Haplotaeniaum spp. and Inanigutta complanata, and rare occurrences of Protoceratoikicsum spp. The rare species Etymalbaillella yennienii is reported for the first time outside of China. The Malongulli Formation is upper Eastonian through lower Bolindian (Katian global stage) based on graptolite and conodont biostratigraphy. From a stratigraphic standpoint, the limestone intervals sampled appear to represent two or possibly three stratigraphically separate intervals within the Malongulli Formation. Conodonts recovered from Coppermine Creek, Angullong/Belubula, and Gleesons Creek localities occur in the lower and middle parts of the formation and fall within one conodont zone, the O. velicuspis Zone. The stratigraphic position of the limestone at the Sugarloaf Creek locality is higher, possibly as high as lower Bolindian, based on proximity to the top of the formation where Bolindian graptolites have been recovered. Despite the different stratigraphic positions of the limestones, the radiolarians contained within are essentially the same fauna, barring some differences in relative abundance in Haplotaeniatum and Palaeoephippiwn. The Malongulli fauna is discussed with respect to other Upper Ordovician faunas, most notably those from siliceous facies in the eastern Lachlan Orogen and from the Hanson Creek Formation of Nevada, U.S. The radiolarian species occur across different depositional environments and from a range of geographically widespread localities, which bodes well for them being recognized as biostratigraphically important faunal markers with correlation potential within the Katian.

Taxa belonging to the Ceratoikiscidae are described in detail from two measured sections of Member C of the Cape Phillips Formation exposed along Rookery Creek, Cornwallis Island, Arctic Canada. The sections encompass uppermost Sheinwoodian (top lower Wenlock) to the base of the Gorstian (base Ludlow) and are dated by graptolites assigned to the Cyrtograptus perneri-Monograptus opimus Zone (upper Sheinwoodian), the Cyrtograptus lundgreni Zone (lower Homerian), and the Colonograptus praedeubeli-Colonograptus deubeli Zone (upper Homerian). Radiolarians are found in great abundance throughout the lower Homerian, allowing for the establishment of the first appearance of the genus Helenifore Ormiston and Nazarov. Helenifore is emended to accommodate the Silurian species H. quadrispina n. sp. and H. speciosus (Furutani 1990). One new genus, Kappaforma, including the new species, K. insecta, is described, as is the new species H. quadrispina. Ceratoikiscum octapleura Renz and C. armiger Furutani are synonymized with 62 lenoides Renz, and H. planus Umeda is synonymized with H. speciosus.

Integrated sequence stratigraphic, biostratigraphic, and chemostratigraphic analyses of three stratigraphic sections in central Nevada indicate that Late Ordovician glaciation-induced sea-level fall produced diachronous, stepwise faunal turnover in graptolites, conodonts, chitinozoans, and radiolarians, and also triggered a strong, but transient, positive [[Delta].sup.13]C excursion. This pattern is very different from that described for most mass extinction events.

Early Devonian (Lochkovian; eurekaensis Zone) conodonts occur in discontinuous limestone beds in the Caballos Novaculite at five localities in the northwestern half of the Marathon uplift, west Texas. Similar conodont faunas at all five localities indicate that the limestone beds lie at one biostratigraphic horizon within the Caballos. The upper novaculite member directly overlies the limestone horizon at one locality, giving the upper novaculite a maximum age of Lochkovian. The limestone beds are dominantly skeletal calcarenites that represent shallow-water carbonate material transported into a deeper water setting by gravity processes. Restriction of the limestones to the northwestern margin of the uplift and provenance of reworked clasts and redeposited Ordovician and Silurian conodonts suggest a North American shelf source and are evidence of the close proximity of some strata exposed in the Marathon uplift to North America in the Early Devonian. Icriodus gravesi n. sp. is described from the limestone fauna.

A regionally extensive Early Mississippian hiatus is recognized at the transition from preflysch to flysch rocks in the Ouachita system of the southern mid-continental United States. It is recognized only in sections where detailed biostratigraphic control exists, and it is coeval with a regional unconformity in the Ouachita foreland throughout parts of Texas, New Mexico, and Oklahoma. Widespread Early Mississippian erosion and nondeposition is attributed to a reconfiguration in basin geometry caused by the onset of Ouachita orogenic activity. Crustal flexure resulting from the impending collision would have initiated erosional processes throughout the foreland, while simultaneously altering bottom-current pathways in the basin and allowing for a prolonged period of sedimentary bypass. In the Late Mississippian, increased constriction may have eventually cut off transequatorial bottom-water circulation, allowing deposition to resume on the shelf and in the basin. An important tectonic implication of this study is that Ouachita deformation started in the Late Devonian to earliest Mississippian, at least 10 m.y. before the onset of flysch deposition.

An unusually fossiliferous sedimentary sequence is exposed near Scarborough, Tobago, West Indies. It occurs within the Tobago Volcanic Group above an undifferentiated sequence of volcaniclastic breccia and lava and includes volcanogenic sedimentary rocks at the base of the Bacolet Formation. An ammonite-radiolarian assemblage has been recovered from this sedimentary interval in the Tobago Volcanic Group. Ammonites from the Spring Gardens quarry are moderately evolute and have unbranched ribs; they are probably juveniles of Mojsisoviczia, of middle Albian age. Another ammonite from a stratigraphically higher locality (i.e., volcanogenic sedimentary rocks at the base of the Bacolet Formation) is identified as Manuaniceras decsernae Young, of early late Albian age. Radiolarian assemblages from intercalations of black siliceous argillite of the Tobago Volcanic Group are characterized by abundant Archaeospongoprunum spp. and the common occurrence of Pseudodictyomitra pentacolaensis, Dictyomitra montisserei, Thanarla brouweri, and Pantanellium ventosum. Also present in lesser abundances are other members of the family Archaeodictyomitridae, Xitus, Napora durhami, and Sciadiocapsa speciosa. This assemblage correlates with the Romanus subzone of O'Dogherty (1994) and Kozurium zingulai zone of Pessagno (1977), both of which are dated as early to middle Albian. These newly reported fossils indicate an Albian age for deposition of the Tobago Volcanic Group. Earlier published [sup.40]Ar/[sup.39]Ar radiometric ages from the group are consistent with the stratigraphic age assignment of the fossils and thereby suggest that volcanism associated with the Tobago Volcanic Group began ca. 105 Ma. Possible correlative, accreted fragments of the Mesozoic oceanic-arc of the southern Caribbean include the Curacao Lava Formation, the lower part of the Washikemba Formation (Bonaire), the Tiara Formation of north-central Venezuela, and possibly the subsurface Mejillones complex of the Carupano basin. The Mesozoic rocks of Tobago consist of two distinct, oceanic-arc rock sequences. The older North Coast Schist ([is greater than or equal to] 120 Ma) exhibits polyphase deformation and lower greenschist facies metamorphism. The structurally overlying Tobago Volcanic Group is not penetratively deformed or folded; it has undergone prehnite-pumpellyite facies metamorphic conditions and has been tilted to moderate dips by various brittle faults. We interpret the hiatus that separates these oceanic-arc rock sequences as representing a significant event in the evolution of the Great Arc of the Caribbean. Consequently, we propose that the commonly postulated reversal in subductionzone polarity of the Great Arc occurred prior to deposition of the Tobago Volcanic Group (ca. 105 Ma), and that older, more highly deformed and metamorphosed oceanic-arc sequences (e.g., North Coast Schist) served as a basement for younger sequences or as wall rock for intrusive elements of younger magmatic suites of the Great Arc. Keywords: Caribbean area, Tobago, Cretaceous, ammonites, radiolarians.

Current knowledge of early Paleozoic radiolarians permits the recognition of faunal assemblages and zones that constitute a biozonation. This new zonation utilizes robust and relatively common, easily recognized morphotypes. It will facilitate rapid acquisition of previously unavailable age constraints for marine siliceous sedimentary successions in hitherto poorly understood early Paleozoic orogenic collages. Keywords: Paleozoic, radiolarian, biozonation.