Clay Mineral Stratigraphy of a 3^<rd> Order Depositional Sequence : Paleoenvironmental Reconstruction and Relations to Orbital Cycles(Clays in Geology,Proceedings of the 13^<th> International Clay Conference)

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Clay mineral assemblages have been widely used in multidisciplinary stratigraphical studies. This paper presents the results obtained in the study of a Lower Cretaceous depositional sequence from the Lusitanian Basin (Portugal). The Upper Hauterivian to Lower Barremian section at Guincho Fort (30km West of Lisbon) has been studied in a detailed bed-by-bed sampling corresponding to a total of 85 samples. The mineralogical composition of bulk sample and clay fractions (<2μm) has been determined on non-orientated and orientated specimens by X-ray diffraction. Multivariate statistical methods were used to establish the relationships existing within a large set of variables, which included results obtained from other traditional methods of stratigraphical analysis (paleontology, sedimentology, facies analysis, geochemistry). Clay mineral assemblages have contributed to the paleoenvironmental and paleogeographical reconstruction of the studied interval and have improved the sequence stratigraphic interpretation and positioning of sequence boundaries and other sequential surfaces (transgressive and flooding surfaces). Finally, it has been possible to establish relations of the overall results to the sedimentary record of orbital pattern variations.