Amorphognathus Ordovicicus Research Articles

The origin and significance of euhedral apatite crystals on conodonts

Crystal overgrowth on fossil remains is well-documented in the literature. Attention has specifically focused on bioapatite (i.e., an apatite of biochemical origin regardless of post-mortem changes) configurations, in order to decipher any possible relation to fossilization/diagenesis. This study investigates the Rare Earth Element (REE) and other High-Field-Strength Element (HFSE) composition of euhedral crystals formed on the surface of conodont elements compared with that of crystal-free surfaces. Euhedral crystals are by definition crystals characterized by sharp faces, developing solids that, for apatite, assume the form of hexagonal prisms, reflecting its crystal symmetry. Late Ordovician (Amorphognathus ordovicicus Zone) conodonts from two localities in Sardinia and the Carnic Alps (Italy) are herein investigated. Conodont elements reveal the occurrence of smooth surfaces and surfaces partially covered with euhedral crystals. Since euhedral crystals did not reasonably grow during the organism's lifetime, the REE and HFSE analysis can provide important insights into the crystal growth process. The experimental results indicated a substantial contribution of diagenetic imprinting for all the analyzed material, although more evident on euhedral crystals that are significantly enriched in middle and, subordinately, in heavy REE with respect to smooth surfaces. The positive correlations between La + Th vs log[ΣREE] and Ce + Th vs log[ΣREE] could support the hypothesis that the neoformed euhedral crystals grew also by depleting the pristine bioapatite of the conodont elements. Nevertheless, the occurrence of two types of apatite cannot be ruled out: euhedral crystals as neoformed products of diagenetic processes and smooth surfaces as remains of the pristine conodont bioapatite after diagenesis.

Lower and Upper Ordovician conodont biostratigraphy and revised lithostratigraphy in the fault and fold zones of the Boothia Uplift, south-western Boothia Peninsula, Nunavut

The strata exposed near Pasley Bay on south-western Boothia Peninsula were previously named the Franklin Strait Formation, spanning Lower Ordovician through early Silurian and dissected by two disconformities. The interpretation of the age and correlation were based on limited fossil evidence. New detailed field investigations and sampling at the Pasley Bay section resulted in a finding of over 1000 identifiable conodont specimens representing 37 species among 28 genera. Five Lower Ordovician and four Upper Ordovician North American standard conodont Zone-equivalent faunas are documented from the section, namely: Lower Ordovician Cordylodus angulatus, Rossodus manitouensis, Acodus deltatus/ Oneotodus costatus, Oepikodus communis, Reutterodus andinus, and Upper Ordovician Belodina confluens, Oulodus velicuspis, Oulodus robustus, and Amorphognathus ordovicicus Zone-equivalent faunas. These faunas enable a new understanding of the age and stratigraphic position of the Franklin Strait Formation: (1) it is now correlated to the Turner Cliffs (upper part of lower member and upper member), Ship Point, Thumb Mountain, Irene Bay, and Allen Bay (lower member) formations straddling Lower to upper Upper Ordovician with the Middle and lower Upper Ordovician missing; (2) it biostratigraphically overlaps with the simultaneously named Netsilik Formation for the strata containing the Cordylodus angulatus, Rossodus manitouensis, Acodus deltatus/Oneotodus costatus, and lower Oepikodus communis Zone-equivalent faunas, and lithostratigraphically for the upper part of lower member and upper member of the Turner Cliffs Formation and lower Ship Point Formation. This study also provides stratigraphic support for the tectonic model that advocates the Boothia Uplift to be a deep-seated, east-dipping thrust block. Additionally, the conodont Colour Alteration Index is used to estimate the increased heat effects close to the fault zones in the environs of the Boothia Uplift.

Uppermost Katian (Ka4, Upper Ordovician) conodonts in South China: Biostratigraphy, biofacies, and paleobiogeography

The conodont fauna from the Daduhe Formation in southwestern Sichuan and northeastern Yunnan, southwestern China, is analyzed and correlated with the uppermost Ordovician Amorphognathus ordovicicus conodont Biozone. This age assignment is further supported by the graptolites from the same sections, indicating a Ka4 Stage Slice, i.e., from the Dicellograptus complexus to Paraorthograptus pacificus graptolite biozones. Two common conodont species of the Upper Ordovician, Hamarodus brevirameus and Scabbardella altipes, are reported for the first time from the upper Katian in South China. The analysis of conodont biofacies indicates that the conodont fauna in the Daduhe Formation belongs to the cool-water Hamarodus brevirameus–Dapsilodus mutatus–Scabbardella altipes Biofacies. This discovery provides additional evidence for the cold tongue model during the Late Ordovician. The distribution of Yaoxianognathus yaoxianensis confirms the close biogeographical ties among South China, North China and Tarim. This distinctive species of eastern Gondwana might have originated in North China and dispersed to Tarim and South China.

Upper Ordovician conodont biostratigraphy and revised lithostratigraphy and geological map, Akpatok Island, Ungava Bay, Nunavut

Stratigraphic units exposed on Akpatok Island, Ungava Bay, Nunavut, were previously recognized as Boas River and Akpatok formations; their biostratigraphic ages and correlations, in particular the stratigraphic position and age of the organic rich “Boas River” Formation, were largely based on limited data. New detailed field observations have recognized three stratigraphic units, namely the Amadjuak, Akpatok, and Foster Bay formations, in which extensive microfossil sampling recovered over 22 000 conodont elements from 66 productive samples from both outcrops and rubble at over 20 localities in four areas. Four Upper Ordovician conodont zones and one unzoned interval are established for the three redefined formations: Belodina confluens and Oulodus velicuspis Interval zones are confined to the exposed Amadjuak Formation and correlated to Edenian and Maysvillian; Amorphognathus ordovicicus – Plegagnathus dartoni Concurrent-range Zone occurs with the Akpatok Formation and correlated to lower Richmondian; Rhipidognathus symmetricus Taxon-range Zone is retained in the Foster Bay Formation and correlated to upper Richmondian; and an “Unzoned Interval” is defined between the last two zones in the lower Foster Bay Formation. The conodont Amorphognathus ordovicicus occurs in both Akpatok and lower Foster Bay formations, and it is also recovered from the bituminous, argillaceous limestone rubble, based on which the stratigraphic position of the previously named “Boas River” Formation is most likely positioned within the lower Foster Bay Formation. The biostratigraphy and lithostratigraphy are combined with geographic information systems (GIS) to establish the thicknesses of the three formations and revise the geologic map of the island.

Hirnantian (Ordovician) through Wenlock (Silurian) conodont biostratigraphy, bioevents, and integration with graptolite biozones, Cape Phillips Formation slope facies, Cornwallis Island, Canadian Arctic Islands

Cornwallis Island in the Canadian Arctic Archipelago provides one of the world’s best areas for establishing an integrated graptolite–conodont biozonation for the Late Ordovician – Silurian given the well-exposed interfingering relationship of the basinal shale and carbonate platform facies. Building on earlier graptolite work, 180 samples were collected from nine sections of the Cape Phillips Formation, of which 118 yielded approximately 7600 conodont elements representing 54 species in 25 genera, including one new genus and species, Mayrodus melchini; the conodonts are well preserved, with minimal thermal alteration (colour alteration index, CAI 1). Nine conodont zones are recognized and defined for this region, namely the Amorphognathus ordovicicus, Distomodus kentuckyensis, Aspelundia expansa, As. fluegeli, and Pterospathodus celloni Interval zones, Pt. a. amorphognathoides Taxon-range Zone, Pt. pennatus procerus and Kockelella ranuliformis Highest-occurrence zones, and K. ortus absidata Taxon-range Zone. An integration of graptolite and conodont zones is documented. Earlier paleontological and isotope geochemical studies have demonstrated the dynamic nature of the Silurian ocean–climate system and identified major faunal turnovers or events. Five of these are recognized in the Cornwallis fauna and related to oceanographic and climate changes, in part referring to recent oxygen isotope data from conodonts from the Cornwallis collections: Hirnantian extinction event, Sandvika Event (late Aeronian, Llandovery), Snipklint Primo Episode (early Telychian, Llandovery), Ireviken Event (late Telychian, Llandovery–Sheinwoodian, Wenlock), and Mulde Event (early Homerian, Wenlock).

Upper Ordovician – Upper Silurian conodont biostratigraphy, Devon Island and southern Ellesmere Island, Canadian Arctic Islands, with implications for regional stratigraphy, eustasy, and thermal maturation

The conodont biostratigraphy for the Upper Ordovician – Upper Silurian carbonate shelf (Irene Bay and Allen Bay formations) and interfingering basinal (Cape Phillips Formation) facies is established for parts of Devon and Ellesmere islands, central Canadian Arctic Islands. Revisions to the interpreted regional stratigraphic relationships and correlations are based on the stratigraphic distribution of the 51 conodont species representing 32 genera, identified from over 5000 well-preserved conodonts recovered from 101 productive samples in nine stratigraphic sections. The six zones recognized are, in ascending order, Amorphognathus ordovicicus Local-Range Zone, Aspelundia fluegeli Interval Zone, Pterospathodus celloni Local-Range Zone, Pt. pennatus procerus Local-Range Zone, Kockelella patula Local-Range Zone, and K. variabilis variabilis – Ozarkodina confluens Concurrent-Range Zone. These provided a more precise dating of the members and formations and, in particular, the range of hiatuses within this stratigraphic succession. The pattern of regional stratigraphy, facies changes, and hiatuses is interpreted as primarily related to the effects of glacioeustasy associated with the terminal Ordovician glaciation and smaller Early Silurian glacial phases, the backstepping of the Silurian shelf margin, and the geodynamic effects of the collision with Laurentia by Baltica to the east and Pearya to the north. Conodont colour alteration index values (CAI 1–6.5) from the nine sections complement earlier graptolite reflectance data in providing regional thermal maturation data of value in hydrocarbon exploration assessments.

Katian (Upper Ordovician) conodonts from Wales

AbstractMiddle and upper Katian conodonts were previously known in the British Isles from relatively small collections obtained from a few localities. The present study is mainly based on 17 samples containing more than 17 000 conodont elements from an approximately 14‐m‐thick succession of the Sholeshook Limestone Formation in a road cut near Whitland, South Wales, that yielded a diverse fauna of more than 40 taxa. It is dominated by representatives of Amorphognathus, Aphelognathus/Plectodina and Eocarniodus along with several coniform taxa. Representatives of Decoriconus, Istorinus and Sagittodontina are reported from the Ordovician of UK for the first time. The fauna is a typical representative of the British Province of the Atlantic Realm and includes a mixture of taxa of North American, Baltoscandic and Mediterranean affinities along with pandemic species. Based on the presence of many elements of Amorphognathus ordovicicus and some morphologically advanced specimens of Amorphognathus superbus, the Sholeshook Limestone Formation is referred to the lower A. ordovicicus Zone. Most of the unit is also coeval with Zone 2 of the Cautleyan Stage in the British regional stage classification, and stage slice Ka3 of the middle Katian Stage in the global stratigraphical classification, an age assignment consistent with data from trilobites, graptolites and chitinozoans. The unusually large collection of M elements of Amorphognathus provides insight into the complex morphological variation in this element of some Katian species of this genus. The Sholeshook conodont fauna is similar to those of the Crûg and Birdshill limestones, but differs in several respects from the slightly older ones from the Caradocian type area in the Welsh Borderland. Although having some species in common, the Sholeshook conodont fauna clearly differs from coeval Baltoscandic faunas and is even more different in composition compared with equivalent North American Midcontinent faunas.

Ordovician conodont biostratigraphy and redefinition of the age of lithostratigraphic units on northeastern Melville Peninsula, Nunavut

Northeastern Melville Peninsula, Nunavut, Canada, preserves the stratigraphic record of the northwestern margin of the Foxe Basin. The Ordovician sequence on the peninsula includes the Lower Ordovician Ship Point Formation and Upper Ordovician Frobisher Bay, Amadjuak, Akpatok, and Foster Bay formations. Their biostratigraphic ages and correlations are poorly understood; in particular it is unclear whether the organic-rich “Boas River Formation” exists on the peninsula. Following extensive sampling of these stratigraphic units, studies of numerous conodont elements from both outcrops and rubble at about 60 localities have established five conodont assemblages through the five lithostratigraphic units on the peninsula. Oepikodus communis – Jumudontus gananda Assemblage in the Ship Point Formation is correlated to the Reutterodus andinus Zone in the uppermost Ibexian, Lower Ordovician. The other four assemblages from the Upper Ordovician are as follows: Appalachignathus delicatulus – Polyplacognathus ramosus – Belodina confluens in the Frobisher Bay Formation correlated to lower B. confluens Zone in the upper Chatfieldian; Belodina confluens – Periodon grandis in the Amadjuak Formation to the upper B. confluens, Oulodus velicuspis, O. robustus, and lower Aphelognathus grandis zones from Edenian to lowest Richmondian; Amorphognathus ordovicicus – Plegagnathus and Rhipidognathus symmetricus – Aphelognathus cf. A. divergens in the Akpatok and Foster Bay formations to the lower and upper Richmondian. The biostratigraphy is combined with geographical information systems (GIS) and Google Earth technologies in estimating the thickness of Paleozoic strata, which reduces the likelihood of “Boas River Formation” existing on Melville Peninsula to minimum.

Rings without a lord? Enigmatic fossils from the lower Palaeozoic of Bohemia and the Carnic Alps

Fossilized ring-like structures with enigmatic function and taxonomic affiliation were recovered for the first time from the Upper Ordovician of the Carnic Alps and the Silurian of Bohemia. These rings, already mentioned as minor constituents in previous conodont studies (e.g. Webers 1966, p. 1; Bischoff 1973, p. 147), were reported from the Palaeozoic of several regions in Europe and North America. Originally considered as inwardly accreted adhering discs of a benthic hyolithelminth worm with a phosphatic tubular projection, they were later reinterpreted in relation to a putative crinoid epibiont or even as possible scyphozoans. Despite a long debate, neither the function of the enigmatic Palaeozoic rings nor their taxonomic affiliation has been fully clarified. The studied material, extracted by a standard technique in use for conodonts, consists of 235 elements from 16 stratigraphic levels in the Plöcken Formation (Carnic Alps, Cellon Section; Amorphognathus ordovicicus Biozone, Hirnantian, Ordovician) and in the Kopanina Formation (Bohemia, Mušlovka Quarry; Polygnathoides siluricus Biozone, Ludfordian, Silurian). To explore whether ring size and shape changed over time, we employed a novel combination of geometric morphometric approaches for outlines with no ‘homologous’ landmarks and showed that only size appreciably varied with an increase of ca. 20%. The emerging data from this study are consistent with the interpretation of the rings as an adhering structure of a benthic organism living on a relatively uniform hard substrate.

Upper Ordovician conodont biostratigraphy and the age of the Collingwood Member, southern Ontario, Canada1Earth Science Sector (ESS) Contribution 20100302.

The Upper Ordovician stratigraphy in southern Ontario represents the clastic foredeep associated with the Appalachian Taconic Orogeny transitioning northwest into coeval carbonate platform facies. Ten measured and sampled sections in both the Collingwood area and on Manitoulin Island, Lake Huron, provide two relatively complete composite sections (277 and 95 m, respectively) through the marine part of the sequence. A total of 100 2 kg samples collected for a conodont biostratigraphic study yielded 77 215 well-preserved specimens. Taxonomic study of the fauna, illustrated herein, identified 34 species representing 22 genera and three taxa in open nomenclature. Taxonomic revisions are made to five species of Pseudobelodina and one of Rhipidognathus ; a new species, Pseudobelodina microdentata , is established. The fauna primarily represents the Midcontinent Province with incursions from the North Atlantic Province primarily in the Collingwood area. Four conodont zones are recognized that help refine the ages for the Upper Ordovician upper Lindsay (Collingwood Member), Blue Mountain, Georgian Bay, and Queenston formations. In particular, the Collingwood Member of the Lindsay Formation, a regionally distributed organic-rich shale of hydrocarbon source rock potential, is demonstrated to lie within the Amorphognathus ordovicicus Zone of North Atlantic Province and the Oulodus robustus Zone of Midcontinent Province and to be early Richmondian age.

A new upper Middle Ordovician–Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden: 1. Stratigraphical review with regional comparisons

A recent drilling at Borenshult near Motala resulted in discovery of the stratigraphically most complete succession through the upper Darriwilian–Rhuddanian interval known in Östergötland. The approximately 70 m long drillcore succession is subdivided into eight formations, the oldest being the late Darriwilian Furudal Limestone and the youngest being the Rhuddanian Motala Formation. Conodonts are used for a detailed biostratigraphic classification of the Borenshult drillcore into three subzones of the Pygodus serra Zone, two subzones of the Pygodus anserinus Zone, and three subzones of the Amorphognathus tvaerensis Zone. The base of the Amorphognathus superbus Zone is taken to be ∼10 m above the Kinnekulle K-bentonite, that of the Amorphognathus ordovicicus just below the Fjäcka Shale, and that of the Ozarkodina hassi Zone at the base of the Middle Member of the Loka Formation. Because of its unique lithology and paleontology and its wide geographic occurrence, this member is formally named herein the Skultorp Member. The previously uncertain stratigraphical position of the internationally known “Borenshult fauna” is shown to correlate with the Skultorp Member. A regional comparison of the Borenshult drillcore succession shows it to be most similar to coeval successions in Västergötland and Dalarna but there are some significant regional differences. The average rate of net rock accumulation during late Darriwilian and Sandbian time is calculated to be ∼3–4 mm/ka.

Late Ordovician conodont biostratigraphy and redefinition of the age of oil shale intervals on Southampton IslandEarth Science Sector Contribution 20100054.

Southampton Island was located on the north margin of the Hudson Bay Basin, one of the largest Paleozoic sedimentary basins in North America. The Upper Ordovician sequence on the island includes the Bad Cache Rapids and Churchill River groups and the Red Head Rapids Formation; the latter contains three oil shale intervals in its lower part, which may have significant potential as hydrocarbon source rocks, but their precise biostratigraphic position remains unclear. Numerous conodont elements from 14 localities allow establishment of four interval zones throughout the Upper Ordovician on Southampton Island: the Belodina confluens and Pseudobelodina v. vulgaris zones in the Bad Cache Rapids Group, which are correlative to the upper Edenian – lowest Richmondian Stage; the Amorphognathus ordovicicus Zone from the uppermost Bad Cache Rapids Group to the top of Churchill River Group to the lower Richmondian Stage; and the Rhipidognathus symmetricus Zone in the Red Head Rapids Formation to the upper Richmondian Stage. The oil shale intervals in the lower Red Head Rapids Formation exposed at Cape Donavan, Sixteen Mile Brook, and Boas River on Southampton Island are correlated with the lower R. symmetricus Zone of the upper Richmondian Stage, not the Maysvillian–Richmondian Stage as previous interpreted. The “Boas River shale” and “Sixteen Mile Brook shale” biostratigraphically and lithostratigraphically equate to the Cape Donovan lower and middle oil shale intervals, respectively. The conodont data and continuous sections across the boundaries between the different lithostratigraphic units have proved the presence of Maysvillian strata on Southampton Island, rather than absence as previously interpreted.

First Late Ordovician conodont fauna in the Betic Cordillera (South Spain): a palaeobiogeographical contribution

Terra Nova, 22, 330–340, 2010 Abstract 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 altipes 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.

Carbonate mud mounds, conglomerates, and sea-level history in the Katian (Upper Ordovician) of central Sweden

The Katian (Upper Ordovician) facies succession of the Siljan district, central Sweden, records some of the most prominent environmental changes in the Ordovician of Baltoscandia. These changes include two separate phases of major sea-level drawdown that were of basinwide and presumably global importance. The first regression and lowstand terminated an entire generation of carbonate mud mounds (the Kullsberg Limestone) and resulted in the formation of polymict carbonate conglomerates (SkAyenlberg Limestone) belonging to the Amorphognathus superbus Zone. New stable isotope data from the Amtjarn quarry shows that this is immediately after the peak of the Guttenberg Carbon Isotope Excursion (GICE), which reaches a delta C-13 peak value at 3.3aEuro degrees in the uppermost Amorphognathus tvaerensis Conodont Zone. A second major regression and sea-level lowstand is manifested by palaeokarst morphologies in the Slandrom Limestone, which formed close in time to the comparably minor Waynesville positive carbon excursion in the basal Amorphognathus ordovicicus Conodont Zone. The widespread exposure associated with this latter lowstand terminated carbonate production in much of the basin, and, during the subsequent flooding, organic-rich, graptolitic shale formed across most of Baltoscandia. The two corresponding sequence boundaries are amalgamated at the top of truncated carbonate mud mounds in the Siljan district, resulting in a pronounced Middle Katian hiatus in the immediate mound areas.

Late Ordovician-Early Silurian conodont biostratigraphy and thermal maturity, Hudson Bay Basin

Abstract Hudson Bay Basin is one of the largest Palaeozoic sedimentary basins in North America. It was explored for hydrocarbon resources, at a reconnaissance scale, more than 30 years ago, although the exploration potential remains largely untested. The lower part of the basin succession comprises approximately 600–1040 m of Upper Ordovician (Bad Cache Rapids and Churchill River groups and Red Head Rapids Formation) and Lower Silurian (Severn River, Ekwan River, Attawapiskat and Kenogami River formations) strata. These formations mainly comprise carbonate rocks consisting of alternating fossiliferous limestone, evaporitic and reefal dolostone, and minor shale. A study of 4500 conodonts from 390 conodont-bearing samples from continuous cores and well cuttings from six exploration wells in the Hudson Bay Lowlands and offshore area has revealed 50 species representing 28 genera. The conodont studies have significantly improved our understanding of the Early Paleozoic geology by providing: 1. clear definition of conodont zones and their stratigraphic ranges. Seven zones are established for the Upper Ordovician–Lower Silurian interval, namely the Belodina confluens, Amorphognathus ordovicicus, Rhipidognathus symmetricus, Ozarkodina elibata, Kockelella? trifurcata and Distomodus staurognathoides interval zones, as well as the Pterospathodus celloni–P. eopennatus Assemblage Zone. 2. precise biostratigraphic control for the different formations. Upper Ordovician formations are dated as late Caradocian–late Ashgillian with the Maysvillian or part of Maysvillian and Gamachian missing: Lower Silurian formations are dated as early Rhuddanian–middle Telychian, Llandovery. 3. recognition of sea-level events based on the stratigraphic distribution and known ecologic partitioning of key conodont species in wells representing a shallow to deep gradient. 4. definition of the position of the Ordovician–Silurian boundary, typically associated with the global hiatus created by the terminal Ordovician glaciation. Most of the conodonts from the six wells studied have a Colour Alteration Index (CAI) value of 1, indicating little alteration of organic matter and that the strata have not reached burial temperatures greater than 80°C. However, slightly higher CAI values are recorded from the deepest part of the wells in the offshore area of the basin, indicating burial temperatures just within the oil window. The Late Ordovician Boas River oil shale, which is inferred to have significant potential as a source rock and occurs at the surface along the northern part of the basin, does not appear to be present in the wells.

The Ordovician Sebree Trough: An oceanic passage to the Midcontinent United States

The Sebree Trough is a relatively narrow, shale-filled sedimentary feature extending for several hundred kilometers across the Middle and Late Ordovician carbonate platform of the Midcontinent United States. The dark graptolitic shales within the trough stand in contrast to the coeval bryozoan-brachiopod-echinoderm– rich limestones on the flanking platforms. We infer from regional stratal patterns, thickness and facies trends, and temporal relations established by biostratigraphy and K-bentonite stratigraphy that the Sebree Trough initially began to develop during late Turinian to early Chatfieldian time (Mohawkian Series) as a linear bathymetric depression situated over the failed late Precambrian–Early Cambrian Reelfoot Rift. Rising sea level and positioning of a subtropical convergence zone along the southern margin of Laurentia caused the rift depression to descend into cool, oxygen-poor, phosphate-rich oceanic waters that entered the southern reaches of the rift from the Iapetus Ocean. The trough apparently formed in a system of epicontinental estuarine circulation marked by a density- stratified water column. Trough formation was accompanied by cessation of carbonate sedimentation, deposition of graptolitic shales, development of hardground omission surfaces, substrate erosion, and local phosphogenesis. The carbonate platforms on either side of the trough are dominated by bryozoan-brachiopod- echinoderm grainstones and packstones that were deposited in zones of mixing where cool, nutrient-rich waters encountered warmer shelf waters. Concurrently, lime mudstone and wackestone were deposited shoreward (northern Illinois, Wisconsin, Iowa, Minnesota, Michigan) in warmer, more tropical shallow seas. Coeval upward growth of the flanking carbonate platforms sustained and enhanced development of the trough shale facies. Five widespread diachronous late Mohawkian and Cincinnatian omission surfaces are present in the carbonate facies of the Midcontinent. These surfaces include sub-Deicke K-bentonite, DS1; top of Black River Limestone, DS2; base and top of the Guttenberg Limestone Member of the Decorah Formation, DS3 and DS4; and top of the Trenton Limestone, DS5. Some of the surfaces correspond to previously described depositional sequence boundaries. All five surfaces, which embody deepening phases on top of highstand-systems tracts, converge in the Sebree Trough, indicating that the trough was a long-lived feature and was the source of eutrophic waters that episodically spread across the adjacent platforms, terminating carbonate production. Late Turinian and early Chatfieldian incipient drowning episodes were followed by a final drowning event that began in the Sebree Trough during the late Chatfieldian ( Climacograptus spiniferus Zone) and reached southernmost Minnesota and other regions far within the platform interior by Richmondian time ( Amorphognathus ordovicicus Zone).

Conodont biostratigraphy of the attenuated Dent Group (upper Ordovician) at Hartley Ground, Broughton in Furness, Cumbria, UK

SUMMARY At Hartley Ground, Broughton in Furness, Cumbria, the Dent Group, a mixed clastic-carbonate succession at the base of the Windermere Supergroup, is attenuated (only 20 m thick) and rests unconformably on the Borrowdale Volcanic Group. This succession includes a debris-flow deposit (Lumholm Member), the age of which was thought previously to be either Ashgill or Llandovery. Five samples from the Hartley Ground succession have yielded an Amorphognathus ordovicicus Biozone conodont fauna, considered to be of mid- to late Rawtheyan (Ashgill) age. The conodont fauna occurs in strata below and within the matrix of the Lumholm Member, although reworking within the latter cannot be completely discounted. Conodont colour alteration index (CAI) values of 5 for all the samples suggest these strata have been heated to temperatures of 300–400°C. At normal continental geothermal gradients this would indicate a minimum depth of burial of approximately 10km. New multi-element reconstructions are proposed for Amorphognathus ordovicicus Branson & Mehl and Birksfeldia circumplicata Orchard; Hamarodus europaeus (Serpagli) is considered to be a member of the family Periodontidae.

Conodont paleontology and biostratigraphy of Ordovician carbonates and petroliferous carbonates on Southampton, Baffin, and Akpatok islands in the eastern Canadian Arctic

Carbonate and petroliferous carbonate units ("oil shales") on Southampton, Baffin, and Akpatok islands have yielded a total of 2277 conodonts, the more biostratigraphically useful of which indicate not all units are correlative. The Boas River "shale", the lower of the two petroliferous units on Southampton Island, overlies the Bad Cache Rapids Group and contains a diverse fauna, including elements of Amorphognathus ordovicicus Branson and Mehl. Previous reports have indicated the presence of Culumbodina penna Sweet, a species whose range only barely overlaps that of A. ordovicicus in the middle Maysvillian. Carbonate beds and bedding-plane surfaces of the higher Red Head Rapids Formation at Sixteen Mile Brook yielded A. ordovicicus faunas containing Aphelognathus cf. A. divergens Sweet. These beds are likely Richmondian, since A. divergens is known elsewhere only from Richmondian strata. A metasicula of "Glyptograptus" hudsoni Jackson, several natural conodont assemblages, and fused enigmatic coniform elements were also found at Sixteen Mile Brook.The petroliferous unit in unnamed strata at Amadjuak Lake on Baffin Island contains Belodina area Sweet, which is indicative of a late Edenian to early Maysvillian age. Conodonts from the petroliferous strata at Jordan River on Baffin Island suggest a Trentonian to early Maysvillian age. The conodonts recovered from unnamed strata on Akpatok Island are not very diagnostic but indicate an age range from Shermanian to Gamachian.

Middle and Late Ordovician conodont faunas and biostratigraphy of graptolitic strata of the Road River Group, northern Yukon Territory

Ordovician conodont faunas from the fine-grained clastic Road River Group in northern Yukon Territory contain a mixture of species from both warm- and cold-water regions. This group in southwestern Northwest Territories also has mixed faunas, whereas conodonts from the carbonate facies are more characteristic of the warm-water regions.Six conodont associations and biozones are identified from the Yukon. Some, such as the early Llanvirn "Cordylodus" horridus – Spinodus spinatus association, contain reworked elements. The Pygodus serra Zone (middle–late Llanvirn) is within the upper P. tentaculatus Zone and well below the G. euglyphus Zone. Genera include Ansella, Periodon, Protopanderodus, Pygodus, and Walliserodus.At Peel River, conodonts are within the P. pacificus Zone. The Blackstone River conodonts occur above the P. pacificus Zone and below the G. persculptus Zone (?) and include Oulodus rohneri, Plectodina florida, and Noixodontus. Amorphognathus ordovicicus, Gamachignathus ensifer, O. ulrichi, and Plectodina tenuis occur in both faunas. These represent Fauna 12, found in late Richmondian strata, rather than the Gamachian Fauna 13 and are assigned to the G. ensifer Zone; both occurrences of G. ensifer are biohorizons. The C.? extraordinarius and G. persculptus zones are not recognized at Peel and Rock rivers and Tetlit Creek.The Ozarkodina n. sp. A – Icriodella sp. B association occurs at Pat Lake between the G. persculptus Zone (?) and the underlying P. pacificus Zone. The Ozarkodina have a Silurian aspect but must be regarded as Ordovician.The Ordovician–Silurian boundary in the Road River Group of the Yukon is best defined using graptolites because of the rare occurrence of conodonts.

Biostratigraphic, paleogeographic, and tectonic implications of Late Ordovician conodonts from the Grog Brook Group, northwestern New Brunswick

Diverse conodont faunas recovered from the Grog Brook Group in northwestern New Brunswick indicate a Late Ordovician, probably Gamachian, age. The conodonts are of mixed provincial affinity including components of the North Atlantic Province (e.g., Hamarodus, Icriodella, Periodon, and Protopanderodus) and taxa representing shallow (e.g., Rhipidognathus) to deeper water environments (e.g., Phragmodus) in the Midcontinent Province. Elements of Amorphognathus ordovicicus Branson and Mehl numerically dominate the faunas that together with sparse representatives of Gamachignathus ensifer McCracken, Nowlan and Barnes suggest a latest Ordovician age. The beds from which the conodont faunas have been recovered are interpreted as distal debris flows that originated at the basin margin and brought Midcontinent Province conodonts down the slope to mix with indigenous North Atlantic Province faunas. The faunas are correlative with those from the Matapedia Group (previously thought to overlie the Grog Brook Group) and their occurrence suggests at least partial lateral equivalence of the two units.The conodonts recovered are only slightly thermally altered. Two alternative hypotheses are proposed to explain the juxtaposition of relatively unaltered conodonts of the Grog Brook Group and highly altered forms from the Matapedia Group. The first suggests that thrust faulting took place in the Late Ordovician – Early Silurian and that this process and later normal faulting account for the unusual distribution of conodont colour alteration. The second possibility is that the strata of the Grog Brook Group in the section examined were deposited on a structural high and overlain by little or no sediment of the Matapedia Group; however, such a structural high must have had access to a source of Midcontinent Province conodonts. Acadian thrusting then brought higher grade Matapedia Group strata into contact with this part of the Grog Brook Group.