HR: 0800h
AN: OS21A-1198    [Abstracts]
TI: Distribution Patterns of Glaucony From Lower Cenomanian Deposits of Northern France
AU: * Amorosi, A
EM: amorosi@geomin.unibo.it
AF: University of Bologna, Dipartimento di Scienze della Terra e Geologico-Ambientali, Via Zamboni 67, Bologna, 40127 Italy
AU: Ferrari, V
EM: valpruz@libero.it
AF: University of Bologna, Dipartimento di Scienze della Terra e Geologico-Ambientali, Via Zamboni 67, Bologna, 40127 Italy
AU: Sammartino, I
EM: irene@geomin.unibo.it
AF: University of Bologna, Dipartimento di Scienze della Terra e Geologico-Ambientali, Via Zamboni 67, Bologna, 40127 Italy
AU: Tateo, F
EM: tateo@igg.cnr.it
AF: Istituto di Geoscienze e Georisorse - CNR, Via Giotto 1, Padova, 35137 Italy
AB: Vertical and lateral distribution of autochthonous glaucony from a stratigraphically condensed interval of Early Cenomanian age in Northern France is shown through detailed sedimentological investigation of five sections (from south to north: Cap de La HŠve, Cauville, Octeville, St. Jouin, and Fecamp), extending along spectacularly exposed cliffs. The condensed interval is up to 3m thick and includes 70-96% glaucony in the southern outcrops. To the north (St. Jouin and Fecamp), the thickness of the condensed interval decreases to about 1m, and glaucony content to 40-60%. Vertical distribution of glaucony invariably shows a decreasing upward tendency, generally with maxima in the basal 50 cm. Geochemical and mineralogical analyses of 53 samples from the condensed horizon enable precise characterization of glaucony maturity, through determination of K2O content and the distance "d" between (001) and (020) peaks. X-ray diffraction analyses reveal patterns characteristic of well-ordered, highly-evolved glauconite, with sharp and narrow (001) reflections. The distance "d" is equal to 10,8 cm in 50 samples. The K2O content varies in a consistent manner, showing values narrowly constrained between 7% and 8%. These data allow classification of the green grains as evolved glaucony, suggesting a depositional hiatus of several hundreds of ka. Upsection, thinner Lower to Middle Cenomanian glaucony-bearing horizons are recorded. These differ from the major condensed horizon by the lower glaucony content (less than 15%) and lower maturity of the green grains (K2O = 6-7%; "d" = 10.9-11.2 cm). These major differences are laterally persistent, suggesting a lower degree of stratigraphic condensation (a few tens of ka), and providing the basis for basinwide correlations. The glauconitic minerals do not show significant changes in terms of maturity throughout the entire condensed interval, even when they represent less than 10% of the sand fraction. This suggests a geochemically open system in early diagenetic phases, resulting in homogeneous glaucony composition over thicknesses much higher than previously thought.
DE: 3675 Sedimentary petrology
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting