Testing high resolution magnetic susceptibility and gamma radiation methods in the Cenomanian-Turonian (Upper Cretaceous) GSSP and near-by coeval section

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Low-field, mass specific magnetic susceptibility (χ) and gamma radiation spectra (GRS) field and laboratory data sets for two Upper Cretaceous marine sedimentary sequences representing the Cenomanian-Turonian (C-T) boundary in the Western Interior Seaway in central Colorado have been measured, sections including the C-T Global boundary Stratotype Section and Point (GSSP) and a ~one kilometer distant co-eval sequence, the S Section. It has previously been established that both χ and GRS data sets are controlled by detrital/eolian fluxes into the marine environment, although the effect of weathering, if any, on these parameters when exposed in outcrop, is not well documented. In addition, these parameters are controlled by differing detrital/eolian components that may be derived from different sources, or be differentially concentrated within the marine system. Of great interest is (1) how well these data sets record climate cycles, (2) how the χ and GRS data are affected by weathering, and (3) how well the two methods support each other. Reported here are the results of a number of field and laboratory experiments designed to evaluate instruments and how χ and GRS data sets co-vary, and to test their usefulness as correlation tools in stratigraphy. Results show that both χ and 40K data sets from laboratory measurement on samples collected in the field, where rocks are relatively fresh, are generally well correlated, respond closely to climate and the corresponding detrital/eolian flux into the marine environment, and can be used for a rough proxy for each other. However, weathering appears to affect the 40K data by removing 40K, thus reducing correlation potential, although χ does not appear to suffer as badly in more weathered sections and good correlations between sections remain. Field GRS measurements produce a broadly smoothed data set that regionally can be very useful for correlation, although potential global correlations are not tested here. Using χ data reported here, and comparing these data to a ~100kyr cyclic, eccentricity (E1) climate model for the Upper Cretaceous at ~93.9Ma, shows that χ and 40K data, measured in the laboratory, can be used to build floating point time scales that produce essentially equivalent results as do other studies using absolute dates. © 2013 Elsevier B.V.

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Palaeogeography, Palaeoclimatology, Palaeoecology

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