CLAY MINERALS 365 of the Cretaceous The reasons for these particular distributions through time may be related to the availability of suitable formation sites, which may in turn be linked to global climate and the formation/break up of the supercontinents Global climatic shifts, especially in Tertiary and more recent sediments are clearly reflected in the clay minerals formed at the Earth’s surface and consequently deposited in the oceans The global cooling that began at the Eocene– Oligocene boundary and continued intermittently is reflected in the shift from a predominance of smectite to illite, plus minor chlorite and interstratified clays At the same time, falling sea-level associated with global cooling and the build-up in polar ice resulted in the spread of nonmarine basins, hence the widespread freshwater clay deposits of the Oligocene Recent research into clay stratigraphy is beginning to show that where post-formation modification is absent, clay mineral suites can accurately reflect Milankovitch cyclicity in sediments, and hence provide a geologically fine-scale dating tool Changes across stratigraphic boundaries, particularly where they are nonconformable, are not uncommon, and can be useful for dating purposes in nonmarine sediments, while within a single sedimentary unit, clear vertical changes in clay mineral suites in marine shales can be demonstrated to be linked to changes in ocean circulation and faunal changes Note that lateral facies variations can result in lateral clay mineral suite variation, even within shales, and care should be taken when attempting to use clay stratigraphy in this way, especially if the intensity of diagenetic alteration varies between locations (see Sequence Stratigraphy) See Also Analytical Methods: Mineral Analysis Clays, Economic Uses Palaeoclimates Sedimentary Rocks: Clays and Their Diagenesis Sequence Stratigraphy Soils: Modern; Palaeosols Weathering Further Reading Aplin AC, Fleet AJ, and MacQuaker JHS (1999) Muds and mudstones: physical and fluid flow properties In: Aplin AC, Fleet AJ, and MacQuaker JHS (eds.) Muds and Mudstones: Physical and Fluid Flow Properties, Special Publications: 158, pp London: Geological Society Bailey SW (1980) Structures of Layer Silicates In: Brindley GW and Brown G (eds.) Crystal Structures of Clay Minerals and their X ray Identification, Mineralogical Society Monograph No 5, pp 124 London: Mineral ogical Society Chamley H (1989) Clay Sedimentology Berlin: Springer Verlag Colson J, Cojan I, and Thiry M (1998) A hydrogeological model for palygorskite formation in the Danian contin ental facies of the Provence Basin (France) Clay Minerals 33: 333 347 Huggett JM (1992) Petrography, mineralogy and dia genesis of overpressured Tertiary and Late Cretaceous mudrocks from the East Shetland Basin Clay Minerals 27: 487 506 Huggett JM, Gale AS, and Clauer N (2001) The nature and origin of non marine 10A˚ green clays from the late Eocene and Oligocene of the Isle of Wight (Hampshire basin), UK Clay Minerals 36: 447 464 Moore DM and Reynolds RC (1997) X ray Diffraction and the Identification and Analysis of Clay Minerals Oxford: Oxford University Press Merriman RJ (2002) Contrasting clay mineral assemblages in British Lower Palaeozoic slate belts: the influence of geotectonic setting Clay Minerals 37: 207 219 Robert C and Chamley H (1992) Late Eocene Early Oligocene evolution of climate and marine circulation: deep sea clay mineral evidence The Antarctic Paleoen vironment: A Perspective of Global Change Antarctic Research Series 56: 97 117 Rupert JP, Granquist WT, and Pinnavaia TJ (1987) Cata lytic properties of Clay Minerals In: Newman ACD (ed.) Chemistry of Clays and Clay Minerals, Mineralogical Society Monograph No 6: pp 275 318 London: Mineralogical Society Srodon J (2002) Quantitative mineralogy of sedimentary rocks with emphasis on clays and with applications to K Ar dating Mineralogical Magazine 66: 677 688 Velde B (1992) Introduction to Clay Minerals London: Chapman & Hall Vogt C, Lauterjung J, and Fischer RX (2002) Investigat ion of the clay fraction (