Search

The Upper Ordovician Bobcaygeon Formation of southern Ontario is a widespread unit that spans the Sandbian-Katian stage boundary and contains exceptionally preserved invertebrate fossil assemblages, including the famed 'Kirkfield echinoderm fauna.' However, the precise correlation of this interval remains poorly understood. This paper presents new data on high-resolution sequence and carbon isotope chemostratigraphy of the Bobcaygeon interval based on new quarry exposures and introduces refined definitions of unit boundaries based on allostratigraphic criteria. Sequence, chemo-, and biostratigraphic evidence indicate that the Bobcaygeon Formation represents a composite unit as it encompasses a major erosional unconformity. The Coboconk and Kirkfield formations, described in the early 20th century, were merged into a single unit, the Bobcaygeon, out of concern that the original lithostratigraphic divisions would be conflated with biostratigraphic zones of the same names. However, these biostratigraphic zones are no longer favoured, and the lower member of the Bobcaygeon is here elevated again to the status of formation (Coboconk Formation) and represents the uppermost portion of the Sandbian M4 sequence. The middle and upper members of the Bobcaygeon, herein reassigned to the Kirkfield Formation, represent the upper Sandbian to lower Katian M5A and M5B sequences recognized widely in the eastern and central United States. The term Bobcaygeon is retained and elevated to the rank of subgroup. The Kirkfield Formation is divided into three members and contacts are refined, placing a 1-2 m transgressive grainstone at the base of each sequence. These units are correlated with equivalent strata of New York and the Cincinnati Arch. Key words: Upper Ordovician, Kirkfield Formation, Bobcaygeon Formation, sequence stratigraphy, chemostratigraphy. La Formation de Bobcaygeon de l'Ordovicien superieur du sud de l'Ontario est une unite repandue qui chevauche la limite des etages Sandbiens et Katiens et renferme des assemblages de fossiles invertebres remarquablement bien preserves, dont la celebre << faune d'echinodermes de Kirkfield >>. La correlation precise de cet intervalle demeure toutefois mal comprise. L'article presente de nouvelles donnees de haute resolution sur la chimiostratigraphie sequentielle et des isotopes du carbone de l'intervalle de Bobcaygeon provenant de nouveaux affleurements en carriere et propose des definitions plus fines des limites d'unites basees sur des criteres allostratigraphiques. Les observations sequentielles, chimiostratigraphiques et biostratigraphiques indiquent que la Formation de Bobcaygeon represente une unite composite puisqu'elle renferme une importante discordance d'erosion. Les formations de Coboconk et Kirkfield, decrites au debut du [20.sup.e] siecle, avaient ete fusionnees en une seule unite, la Formation de Bobcaygeon, par peur que les divisions lithostratigraphiques originales ne soient confondues avec les zones biostratigraphiques portant les memes noms. L'usage de ces zones biostratigraphiques n'est toutefois plus preconise, et le membre inferieur de l'intervalle de Bobcaygeon est une fois de plus eleve au statut de formation (Formation de Coboconk) et represente la partie sommitale de la sequence sandbienne M4. Les membres intermediaire et superieur de l'intervalle de Bobcaygeon, reaffectes ici a la Formation de Kirkfield, representent les sequences M5A et M5B du Sandbien superieur au Katien inferieur reconnues largement dans l'est et le centre des Etats-Unis. Le terme Bobcaygeon est retenu et eleve au rang de sous-groupe. La Formation de Kirkfield est divisee en trois membres dont les contacts sont definis plus finement, en placant un grainstone transgressif de 1-2 m a la base de chaque sequence. Ces unites sont correlees a des strates equivalentes de l'Etat de New York et du bombement de Cincinnati. [Traduit par la Redaction] Mots-cles : Ordovicien superieur, Formation de Kirkfield, Formation de Bobcaygeon, stratigraphie sequentielle, chimiostratigraphie., Introduction Upper Ordovician sedimentary rocks in the Lake Simcoe area of southern Ontario, Canada (Fig. 1), dominated by shallow-water subtropical carbonates, provide key insights into paleoenvironments, sea level fluctuations, and [...]

Abstract: New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostrati-graphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced [...]

Abstract: The Upper Ordovician (mid-Katian) Kope Formation provides an example of how a detailed study of fine-grained siliciclastic sediment can provide sedimentological insights. The Kope Formation is an exemplar of [...]

Abstract: The 'butter shale' Lagerstatten of the Cincinnati Arch have produced an abundance of articulated trilobites, along with assorted bivalves and cephalopods. These bluish gray shales are rich in clay, [...]

A restudy of the palynology of the Whirlpool Formation and Power Glen Formation in New York (USA) yielded a diverse fossil assemblage with cryptospores, glomalean fungi, acritarchs, chitinozoans, scolecodonts, and [...]

Abstract: Shell-bed development can be a product of complex sedimentological and biological factors. The Upper Ordovician sediments near Cincinnati, Ohio constitute a succession of thinly interbedded shelly carbonates and mudrocks. [...]

The Upper Ordovician (mid-Katian) Kope Formation provides an example of how a detailed study of fine-grained siliciclastic sediment can provide sedimentological insights. The Kope Formation is an exemplar of mixed siliciclastic-carbonate cyclicity; however, most of the sedimentological research to date has focused on the fossiliferous limestone beds. Conversely, this paper provides a detailed investigation of siltstone within 42 metre-scale cycles from 26 locations spanning a large geographic area. The objective of the study is twofold: ( i) to confirm the mineralogical nature of silt-sized sediment. Currently, silt-sized sediment is irregularly defined as being either carbonate or siliciclastic rich. This paper uses X-ray diffraction, thin-section, and elemental analyses to determine the mineralogy of the silt; ( ii) to determine the stratigraphic distribution of siltstone beds. If a distinctive and reoccurring distribution of siltstone can be found, then it can provide additional information about environmental energy changes that led to cyclic fossiliferous limestone formation. This study finds that silt within the Kope Formation is siliciclastic and siltstone, therefore, records periodic minor pulses of coarser terrigenous sediment. Predominantly, metre-scale limestone-shale cycles show an increasing abundance of siltstone beds in the upper half of the shale interval, with a marked decrease just before the capping limestone. This finding supports the idea that limestone formation occurred during slowing sedimentation, perhaps at a time of maximum sea-level rise. Furthermore, decametre-scale fluctuations in siltstone abundance are similar in pattern to previously documented faunal variations, indicating that fauna assemblage might be more controlled by turbidity and not depth. [ABSTRACT FROM AUTHOR]

The 'butter shale' Lagerstätten of the Cincinnati Arch have produced an abundance of articulated trilobites, along with assorted bivalves and cephalopods. These bluish gray shales are rich in clay, poorly calcified, and show vague internal bedding in outcrop. Butter shales form a repetitive motif with similar lithological and paleontological characteristics, suggesting conditions existed that can be explained by the interference between different orders of sequence stratigraphic cyclicity. The characteristics that define butter shales include rarity of coarser interbeds, homogenous, fine grain size, and abundance of burial horizons. The overriding control is siliciclastic sediment supply. During third-order transgressions, sediment supply to the basin is too low to produce thick shale-prone intervals. Conversely, during third-order falling stages, sediment supply is generally too high to favor butter shale deposition. Butter shales formed preferentially during a third-order highstand systems tract, and two subtly different variants resulted from the superimposed effects of higher order cycles. Highstands moderated by small-scale transgressions are characterized by lower background sedimentation and fewer, thinner mud deposition events. Superposition of small-scale sea-level fall on highstands produced increased background sedimentation, higher silt, and patchy fossil occurrences. Juxtaposition of various scaled highstand systems tracts provided the optimal butter shale conditions, characterized by elevated mud influx and frequent episodic burial events, leading to abundant, articulated trilobites and associated fauna. In these scenarios, episodic events provide sufficient mud to smother local faunas and create a soft, fine-grained substrate that prohibited recolonization by taxa adapted to firm substrates. Each scenario differs from the others with respect to sedimentology and faunal composition. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0-1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr ( N = 25). Our results indicate a Floian to early Darriwilian (F3-Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2-Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1-Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostratigraphic synthesis establishes the position of numerous unconformities, the duration of the intervening depositional sequences, and rates of sedimentation. [ABSTRACT FROM AUTHOR]

A restudy of the palynology of the Whirlpool Formation and Power Glen Formation in New York (USA) yielded a diverse fossil assemblage with cryptospores, glomalean fungi, acritarchs, chitinozoans, scolecodonts, and small carbonaceous fossils. These new data, and particularly the presence of the chitinozoan index fossil Hercochitina crickmayi, combined with emerging stable carbon isotope data, suggest a Late Ordovician (Katian or Hirnantian) age for these formations, which is older than their previously suggested Silurian (Rhuddanian) age. [ABSTRACT FROM AUTHOR]