Precise timing of the Upper Taghanic Biocrisis, Geneseo Bioevent, in the Middle-Upper Givetian (Middle Devonian) boundary in Northern Spain using biostratigraphic and magnetic susceptibility data sets

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The ability to correlate with high precision among geological successions is critical in evaluating the global coincidence and therefore magnitude and duration of geological events. This is a difficult problem given resolution uncertainties and ambiguities in biostratigraphic data sets. The magnetostratigraphy susceptibility (MSS) method, based on low-field bulk magnetic susceptibility (MS) measurements of closely spaced samples, provides a well-established, abiotic, high-resolution correlation tool in stratigraphy, that when combined with bio-, chemostratigraphic, and other geophysical techniques, offers ways to resolve correlation problems. In addition, such data sets lend themselves to time-series analysis, where developing high-resolution timing of events becomes possible. This paper examines the final episode (equivalent to the Geneseo Bioevent in the Appalachian Basin) of the important Taghanic Biocrisis known from globally distributed Middle Devonian geological sections. The Geneseo Bioevent is important because the extinction of some major fossil groups occurred during this time, while other groups declined significantly, including many benthonic groups. However, the maxima of extinctions does not appear to have occurred everywhere simultaneously. To help resolve timing of this bioevent, results are reported for two Middle Devonian successions from Northern Spain, the S. Huergas de Gordón and Punta Boletos sections, both showing the final Taghanic Biocrisis, Geneseo interval, and correlate these successions to the same interval within an independent, global database. Time-series analyses of the MSS data sets from the Spanish sections was performed using Multi-Taper (MTM) and Fourier Transform (FT) methods, and have extracted well-defined Milankovitch bands from the primary MS data from each section. Using FT and MTM values for eccentricity at ~ 400. kyr and ~ 100. kyr, a floating-point time scale was developed for the two Spanish sections that provides age resolution to ~ 50. kyr for these successions. The results show that the Geneseo Bioevent in Northern Spain occurred within a narrow time window estimated to have lasted for ~ 70. kyr. This time window fall. s within the Polygnathus ansatus (uppermost part)- Ozarkodina semialternans conodont zones, just at the Middle-Upper Givetian boundary of the Middle Devonian in what appears to be the upper, regressive phase of T-R cycle IIa. © 2011 Elsevier B.V.

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Palaeogeography, Palaeoclimatology, Palaeoecology

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