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High-resolution cyclostratigraphic analysis of magnetic susceptibility record from a Kimmeridgian alternating marl-limestone succession (La Méouge, Vocontian Basin, France)

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Boulila S. , Galbrun B. , Collin P.-Y. , Collin P.-Y. , Schnyder J.

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tom Vol. 4, no. 1

149-150

EN

The Kimmeridgian deep-water sediments of La Méouge section (South East Basin of France) show alternating marl-limestone successions. The limestone beds are mudstones or wackstones with benthic foraminifera, radiolarians and filaments which are characteristic of the pelagic realm (Colombié & Strasser 2003). We studied a 43 m thick well exposed interval of mainly Early Kimmeridgian (De Rafélis 2000). Cyclostratigraphic studies of marl-limestone alternations are commonly based on measuring the thickness of beds and bundles and counting their number to extract sedimentary cyclicities. By using high resolutions magnetic susceptibility (MS) measurements, we developed a methodology for detecting and quantifing sedimentary cyclicities recorded in this kind of facies. To perform high-resolution MS measurements, the sampling interval was fixed at nearly 7 cm. All the samples were measured with a Kappabridge KLY-2 susceptometer. The MS variations follow a net cyclic pattern (high and low frequencies). Spectral analysis of MS data show evidence of about all range of Milankovitch cyclicities (precession, obliquity, 100 ky and 400 ky-eccentricity) as demonstrated by the frequencies” ratios methodology and tuning technique. For this latter, we tuned MS data to 405 ky-eccentricity of astronomical solutions (Laskar et al. 2004). Moreover, we found highly significant periods of climatic precession not only in the original MS record but also in the tuned MS. Evolutive harmonic analysis (Maurer et al. 2004) was performed to examine the persistence of the Milankovitch properties throughout the whole section and the possible variations of sedimentation rate. Specifically, the two high peaks, seen in power spectra domain representing the short and the long precession, are characterized in amplitude spectrogram domain by obvious continuous lines, particularly the latter. This means that the precession cycles have much larger amplitudes than the other recognized cycles and are persistent too throughout the entire section. Furthermore, we noted that the long precession cycles show a sudden change in sedimentation rate occurring in the lower part of Lothari ammonite Subzone where the sedimentation rate shifted roughly from 2 to 3 cm/ky. This study proved that MS is a very useful tool for cyclostratigraphic analysis of Mesozoic sections, particularly in alternating marl-limestone successions, and it is also a powerful proxy to detect and estimate possible variations in sedimentation rate.

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Jurassic cyclostratigraphy: recent advances, implications and problems

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Coe A. , Weedon G.

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tom Vol. 4, no. 1

156-157

EN

About 70% of the Jurassic is now covered by floating astronomical timescales based on the recognition of Milankovitch cycles. Astronomical timescales provide the highest resolution robust timescales over the tens of thousands to millions and even tens of millions of year timescales. This presentation will provide a summary of the status of the Jurassic astronomical timescale, including the outstanding problems and possible solutions. The oldest sea-floor magnetic anomaly pattern is Callovian. For earlier stages the scaling for the Geological Timescale 2004 (Gradstein et al. 2004), other than minimum estimates from direct counts of stratigraphic cycles, relied on a combination of c. 20 radiometric dates, the number of ammonite subzones and an assumption that the rate of change of seawater 87Sr/86Sr ratio for the Early Jurassic was linear over intervals of millions of years. However, this assumption has recently been questioned and thus there is an additional need to improve the cyclostratigraphic and radiometric databases. Cyclostratigraphy for much of the Early Jurassic has been completed using sections in England and the Alps. There is no cyclostratigraphy for the Bajocian and Bathonian. For the Late Jurassic, the existence of a sea-floor magnetic anomaly pattern together with recent and ongoing cyclostratigraphic and magnetostratigraphic studies on the same sections in the UK provide the potential to produce a high-resolution integrated timescale for the Callovian to Tithonian (c. 15 Ma duration). However, construction of the Late Jurassic timescale is complex because of the high number of magnetic reversals, the provincialism of the ammonites used for biostratigraphy and lack of agreement on the stages. Weedon et al. (2004) identified regular cycles in the Kimmeridge Clay Formation in England and used these to construct a floating 7.5 Ma astronomical timescale for the latest Oxfordian (as defined in the Tethyan province), Kimmeridgian and most of the Early Tithonian. Comparison of this astronomical timescale with the GTS2004 reveals that the Early Tithonian is c. 1 Ma (25%) longer according to the cyclostratigraphy. This mismatch may be resolved via better correlation of the magnetozones that were defined in France (Tethyan Province) and the Kimmeridge Clay Formation (Boreal Province). For the Oxfordian and Callovian a high-resolution magnetostratigraphy based on sections in the UK has recently been compiled. Work is currently being conducted to produce a floating astronomical timescale using exactly the same exposures.

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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.

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tom Vol. 4, no. 1

173-174

EN

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.

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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.

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tom Vol. 4, no. 1

154-155

EN

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).

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Cyclostratigraphy and chronology of the Domerian in the Paris Basin (GPF-Sancerre borehole, Cher, France)

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Huret E. , Galbrun B. , Clemence M.-E. , Gardin S. , Collin P.-Y. , Rouget I. , Boulila S. , Trouillefou P.

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tom Vol. 4, no. 1

172-173

EN

Marine sedimentary successions recording Milankovitch orbital forcing of paleoclimate variations allowed the orbital tuning of Neogene Time scale (Lourens et al. 2004). For Mesozoic times the most recently calculated astronomical solution is not reliable (Laskar et al. 2004) and the only possibility is to provide “floating chronologies” to estimate the durations of geological processes. Detection of orbitally forced cyclicity patterns from Mesozoic sedimentary sections needs analysis of the right palaeoclimatic proxy which has to be easy in use. Most often in marine sediments magnetic susceptibility (MS), which integrates the concentration of magnetic grains, reflect changes in detrital influx and then it can be used as a palaeoclimatic proxy (Mayer & Appel 1999). Thus MS variations analysis of thick Mesozoic sedimentary successions is probably the easiest paleoclimatic proxy to use, fast and allowing high stratigraphic resolution that would be difficult to obtain with conventional methods. Such methodology was already successfully used for analysis of Jurassic successions (Weedon et al. 1999). We report a cyclostratigraphic analysis of MS record from the Domerian (Upper Pliensbachian, Lower Jurassic) recovered in the GPF-Sancerre borehole (southern Paris Basin, France). The Domerian formation is composed of marly-limestones and marls, and is 50 meters thick. The recovery is almost 100%. The Spinatum and Margaritatus ammonite zones are well recognized, as well as nannofossils and benthic foraminifers datums. High resolution (2 cm spacing) MS measurements were realized with a Bartington MS2-E1 sensor. MS variations are in accordance with lithology: very weak values in marly-limestones, higher values when the clays percentage increase. Long term evolution of MS allow to establish a sequential interpretation. The high frequency variations of MS were the subject of a cyclostratigraphic study by spectral analysis and the realization of amplitude spectrum. Cycles of 0.4, 1, 2 and 6.5 m thick were recognized. Using the methodology of frequency ratios, these cycles are interpreted to record orbital precession, obliquity and eccentricity (100 and 400 ky), respectively. Thus, the duration of the Domerian and the different ammonite zones can be estimated. Moreover, the realization of amplitude spectrum allows to estimate the sedimentation rate evolution and to interpret the cycle evolution breaks as hiatuses. As a matter of facts we observed in the Upper Domerian a significant decrease of cycle’s thickness that we interpreted as a drop of sedimentation rate and probably as a condensed level (Clemence et al. this volume).