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43907-B8
Refining the Ordovician Time Scale: An Integrated Biostratigraphic Approach
The most fundamental tool for studying Earth system history is the geologic time scale. Modern studies on climate change, the evolution and diversity of life, geochemical cycles, geodynamical processes, and other aspects of the Earth system increasingly rely on precise, time-calibrated data. The primary objective of this research is to integrate conodont and graptolite biostratigraphies in order to refine the Middle and Late Ordovician time scale. We have developed a model that uses sequence stratigraphy as a predictive guide for locating conodonts within graptolite-rich black shale sequences.
It has been our experience that biostratigraphically useful graptolites, conodonts, and chitinozoans are often clustered on black shale horizons that we interpret as parasequence-top flooding surfaces. These surfaces represent fossil accumulation horizons where the number of specimens per millimeter of sediment is greatly increased, a well-known phenomenon at marine flooding surfaces. We think that conodonts, which are extremely dense, were concentrated at the flooding surfaces as a “lag” deposit that remains in place as bottom currents wash the lighter sediments away.
In the summers of 2006 and 2007 we used our model to locate fossil-rich horizons and collect samples from 4 well known, important graptolite localities Trail Creek, Idaho; Black Knob Ridge, Oklahoma; Hartfell Score, southern Scotland; and ; the island of Bornholm, Denmark. We discovered conodonts on bedding plane surfaces at all 4 localities. The general finds are summarized below.
1) Trail Creek, Idaho: We collected samples from 4 sections in the Trail Creek Region. Graptolite samples from the Trail Creek Road Section indicate a biostratigraphic age of Didymograptus protobifidus to Isograptus victoriae victoriae in the Australasian graptolite succession. These collections also uniformly yielded a rich conodont fauna that includes Oepikodus sp. cf. O. communis, Periodon flabellum, Protopanderodus sp., Paroistodus sp. cf. P. proteus, Drepanodus sp. cf. D. arcuatus. The conodonts are long ranging species whose ranges agree with the graptolite biostratigraphy. At the Little Fall Creek Section, we collected Cardiograptus morsus to Archiclimacograptus riddellensis Zone graptolites. Shale surfaces at 39.2 meters contained the conodonts Periodon flabellum and Drepanodus sp. cf. D. arcuatus. Our uppermost collection at 201.5 meters yielded the conodonts Drepanodus sp., Periodon sp. cf. P. aculeatus, and the first recorded bedding plane assemblage of Pygodus serra.
2) Black Knob Ridge, Oklahoma: This section is the Global Stratotype Section and Point for the base of the Katian Stage of the Upper Ordovician Series, contains a continuous graptolite succession across the Climacograptus bicornis – Diplacanthograptus caudatus graptolite zonal boundary, and also yields biostratigraphically important conodonts. We recovered well-preserved conodonts on the shale bedding planes across the Womble-Bigfork boundary at BKR that are typical of the North Atlantic Fauna. The uppermost Womble Shale contains an abundant, low diversity conodont fauna that includes elements of Amorphogathus tvaerensis and Icriodella cf. I. superba. The presence of I. cf. I. superba with A. tvaerensis demonstrates that the uppermost Womble at BKR is within the B. alobatus Subzone of the A. tvaerensis Zone. The uppermost Womble Shale at BKR also contains Periodon grandis, Drepanoistodus suberectus, Scabbardella altipes, Oistodus sp., and Panderodus sp. The conodont fauna from lowermost Bigfork Chert at BKR consists of A. tvaerensis, Periodon grandis, Protopanderodus sp. cf. P. liripipus, Drepanoistodus suberectus, Scabbardella altipes, Phragmodus sp., and Panderodus sp. Of interest is the occurrence of 2 specimens of Amorphognathus sp. cf. A. superbus approximately 5.7 meters above the base of the Bigfork Chert. The conodonts from BKR suggest that the Climacograptus bicornis - Diplacanthograptus caudatus zonal boundary is located in the B. alobatus subzone of the Amorphognathus tvaerensis conodont zone.
3) Hartfell Score, southern Scotland: The Hartfell Shale at Hartfell Score contains a continuous succession of graptolites from the Climacograptus bicornis through the Pleurograptus linearis zones. We found numerous conodonts with graptolites in samples from the C. bicornis and D. clingani zones. Specimens of Amorphogathus tvaerensis were abundant in the upper C. bicornis Zone and specimens of Amorphognathus superbus occurred in the upper D. clingani Zone. These finds suggest a graptolite/conodont zonal correlation consistent with that at Black Knob Ridge.
4) Island of Bornholm, Denmark: About 9 meters of graptolitiferous black shale crops out at the Læså rivulet, Vasagård, Bornholm. The graptolite succession spans the Diplograptus foliaceus, Dicranograptus clingani, and Pleurograptus linearis zones. We made detailed collections of graptolites and located conodonts on shale surfaces in the field. These collections are still being worked on.
Generally, the graptolite/conodont co-occurrences recorded in our initial work confirms the previous zonal ties established by integrating platform and basinal successions. We have also collected a large number of bedding plane assemblages including apparatuses or partial apparatuses from Periodon grandis, Scabbardella altipes, Plectodina aculeata, Oepikodus sp. cf. O. communis, and Pygodus serra. Future investigations include examining collections from the Kalpin area of Tarim, western China, and additional fieldwork in New York, Oklahoma, and Idaho.
