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2 2006

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Cyclostratigraphic analysis of Upper Jurassic marly formations (Bure, eastern Paris Basin): comparison of different tools, record of sedimentary gaps and implications for regional correlations

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Huret E. , Galbrun B. , Collin P.-Y. , Boulila S. , Hinnov L. , Elion P. , Ravenne C.

tom Vol. 4, no. 1

173-174

This study integrates research conducted by ANDRA at the Underground Research Laboratory of Bure (Meuse, France) to investigate the feasibility of a deep geological waste repository in clay for high-level and long-lived intermediate-level radioactive waste. The aim of this study is to detect possible sedimentary gaps, by comparison of different tools (well logging/laboratory measurements) in the Upper Callovian – Lower Oxfordian homogeneous marly formation where the laboratory is located, and to estimate the duration of stages and biostratigraphic zonation by comparison with a basin sequence from the southeast of France that is presumed to have accumulated continuously. The search for hiatuses was made using a high resolution cyclostratigraphic approach based on the study of magnetic susceptibility (MS) and gamma ray (GR) fluctuations. Four ANDRA boreholes (EST 342, EST 322, EST 103 and EST 312) oriented on a SW-NE transect (40 km) were analyzed. Magnetic susceptibility measurements were made on core samples with a Bartington Instruments MS2E1 sensor every 4 cm. Gamma Ray Measurements are realised by well logging (measurement interval: 15.2 cm). Sedimentological data suggest that variations of the clay content influence the long term evolution of MS and GR. The high frequency variations in MS and GR were subjected to spectral analysis. Composite cycles of 0.5, 1 and 2.5 m thickness were recognized on the basis of frequency ratio and correspond to the frequency ratio of orbital Milankovitch cycles. The duration of the Mariae ammonite Zone in the Paris Basin was estimated by counting the cycles, to be between 2.4 and 2.6 My š0.3 ky (borehole EST 322 and EST 103). The amplitude spectrum shows sedimentation rate variations in particular in the Lower Oxfordian and interruptions in cycle evolution correlated to sequential limits. We interpret these zones as short condensed levels. The same methods were applied to sections from the southeast of France (Aspres-sur-Buëch, Thuoux), and the Mariae ammonite Zone estimated to be between 2.3 and 2.8 My of age. By comparison of this biostratigraphic zone between the two studied regions, sedimentary hiatuses of a few hundreds of thousands of years appear to affect the Oxfordian marly formation of the Paris Basin. MS and GR records as laboratory measurements and well logging provide an excellent means for regional stratigraphic correlations, and are especially promising in high resolution cyclostratigraphic studies. Hiatus detection and estimation of their durations can also be considered by this approach.

Micropalaeontologic, geochemical and cyclostratigraphic approach for the timing of the Early Toarcian Oceanic Anoxic Event in the Paris Basin (GPF-Sancerre borehole)

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Clemence M. , Huret E. , Bartolini A. , Galbrun B. , Gardin S. , Hinnov L. , Beaumont V.

tom Vol. 4, no. 1

154-155

The marine sedimentary record of the Toarcian exhibits evidence for a perturbation of the global carbon cycle associated with high burial of organic matter, known as the Early Toarcian Oceanic Anoxic Event (OAE). It is accompanied by climate warming, elevated rates of marine faunal extinction and short-lived, strong negative isotope excursion in both oceanic and terrestrial reservoirs. The timing and the pattern of the negative shift in 13C are critical for understanding the possible mechanism of this isotopic event, and the nature and the origin of the Early Toarcian OAE. To improve our understanding of the palaeoenvironmental background of the Early Toarcian OAE, we have investigated the sedimentary record of the GPF-Sancerre borehole from the southern Paris Basin (Cher, France) by integrating geochemical analyses (13Corg, TOC and CaCO3) with synecological analyses of benthic foraminifers and calcareous nannofossils; and assessing the duration of the 13C excursion by cyclostratigraphic analysis using magnetic susceptibility (MS) and CaCO3 data. Our results indicate progressive environmental deterioration from Domerian/Toarcian to a paroxysm coincident with the Early Toarcian OAE (highest TOC values and negative excursion of 13Corg). This deterioration is marked by a high fertility period that precedes anoxic conditions. The OAE coincides with a major crisis in the benthos and with a decrease of calcareous nannofossils. Following this major dysoxic episode, the water column is characterized by a succession of alternating suboxic and stagnation phases that correlates well with positive values of 13Corg. These results attest of a highly perturbed environment, characterized by the presence of opportunist species both in the benthos and nannoplankton communities. To quantify the timing of these events, high resolution cyclostratigraphic analysis is applied to MS and CaCO3 data (sampling interval – 2 cm). Cycles of ca. 0.5, 1 and 2.5 m are observed. The calculation of cycle frequency ratios matches that of the Milankovitch orbital cycles. The duration of the negative isotope excursion can be estimated by counting cycles to 120,000š40,000 yrs. The diminution of cycle thicknesses at the Domerian/Toarcian boundary (353-358 m interval) reflects a decrease of sedimentation rate. We interpreted this result as an evidence for a potentially condensed level. This multi proxies approach is innovative and promising to better understand the water column dynamic during the OAE (nannofossil and foraminifer association), shows that the OAE is a multiphase event (beginnings, acme and recovery) and estimates brief events (<200,000 yrs), follows the sedimentation rate evolution and highlights potential hiatuses (condensed zones).