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43907-B8
Refining the Ordovician Time Scale: An Integrated Biostratigraphic Approach

Daniel Goldman, University of Dayton and Stephen A. Leslie, James Madison University

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.

In the summer of 2008 we continued our work locating bedding planes that contained graptolites and conodonts in the Trail Creek region of central Idaho. We have also been constructing a graphic correlation composite section and microfossil range chart from 15 boreholes and outcrops in Baltoscandia using the computer program CONOP 9.  The CONOP9 composite solution is being used to examine Middle and Late Ordovician marine microfossil diversity dynamics. The general results are summarized below.

1) Trail Creek, Idaho: We collected samples from 3 sections in the Trail Creek Region. At the Little Fall Creek Section, we collected Cardiograptus morsus to Climacograptus bicornis Zone graptolites. Shale surfaces at 39.2 meters contained the conodonts Periodon flabellum and Drepanodus sp. cf. D. arcuatus. Our collections at 201.5 meters yielded the conodonts Drepanodus sp., Periodon sp. cf. P. aculeatus, and the first recorded bedding plane assemblage of Pygodus serra. Nemagraptus gracilis was collected at 178 and 216.5 meters, and occurred with the conodont species Pygodus anserinus in the latter collection. Collections at 233 meters contained Climacograptus bicornis and Amorphognathus tvaerensis. These collections provide an integrated conodont - graptolite biostratigraphy that generally agrees with previous zonal ties. The Trail Creek Summit section is structurally complex but appears to be nearly continuous across the Late Ordovician. A thick package of fault-bounded, laminated calcisiltites are generally unfossiliferous, but appear lithologically similar to the Middle Ordovician rocks that crop out at Little Fall Creek. Hence, the summit section may contain a more complete sequence of Ordovician strata than previously known. The Trail Creek (creek) section is the type locality for the Paraorthograptus pacificus graptolite Zone. In addition to these latest Katian graptolites, we collected graptolites from the latest Sandbian and Early Katian. Graptolites of Early Katian (Ea1) were not previously known from the Trail Creek region. Thus, a composite of the Phi Kappa Formation in the Trail Creek region represents one of the longest and most complete exposures of Ordovician rocks in the world.

2) Middle and Late Ordovician Biodiversity Dynamics in Marine Microfossils from Baltoscandia - The early Late Ordovician was an interval of significant decline in marine biodiversity that has been variously attributed to sea level, facies, and climatic changes. In the East Baltic area several workers have described a significant diversity decline and faunal turnover in marine microfossils at the Keila-Oandu Stage boundary, an event they called the Oandu Crisis. In order to get a more complete understanding of microfossil diversity dynamics in the Middle and Upper Ordovician rocks of Baltoscandia we used the quantitative correlation method constrained optimization (CONOP9) to construct a composite range chart from the stratigraphic data of 455 chitinozoan, conodont, ostracod, and graptolite species from 14 boreholes and five outcrops in Baltoscandia. We used the CONOP composite as a timescale in which to calculate biodiversity, extinction, and origination rates through the Middle and Late Ordovician. In particular, we were interested in examining faunal dynamics across the late Keila and early Oandu stages, an interval associated with a prominent positive carbonate d¹³C isotope excursion known as the GICE. We divided the CONOP composite into 860,000 year intervals that span the Lasnamagi through Porkuni stages. Our new data show that overall biodiversity increases steadily from the base of the Keila to the middle Rakvere, mainly due to an increase in ostracod diversity. Chitinozoan diversity reaches a zenith in the Late Keila, drops through the Oandu Stage coincident with the GICE, and then exhibits a gradual decline across the rest of the Ordovician. Chitinozoans exhibit constant origination but variable extinction rates and undergo a dramatic diversity decline associated with the GICE event. Conodonts have diversity peaks in the lower Uhaku and lower Kukruse Stages, and then decline gradually through the Late Ordovician. Conodonts exhibit constant extinction and origination rates and their diversity decline is attributable to higher extinction than origination rates. Interestingly, the fossil preservation and recovery rate was highly variable and appears to exert a strong influence on the observed biodiversity pattern.

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