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1 2006

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

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

Volumina Jurassica

2006

Vol. 4, no. 1

172-173

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).