P. Kossler et al., Magnetic records of climatic cycles from mid-Cretaceous hemipelagic sediments of the Vocontian Basin, SE France, CRETAC RES, 22(3), 2001, pp. 321-331
A study of mid-Cretaceous hemipelagic sediments from the Vocontian Basin in
SE France has been carried out in order to identify palaeoclimatic cycles
driven by changes in the Earth's orbit ('Milankovitch cycles'). The calibra
tion of cyclic behaviour of magnetic properties can provide sedimentary chr
onology on a finer timescale than biostratigraphy or magnetostratigraphy. T
he observed Aptian succession of marls and claystones with intercalated bla
ck shales and marly limestones shows a cyclic pattern of variation in magne
tic susceptibility. Constant values of isothermal remanent magnetisation ra
tios (S-ratios) indicate a constant composition of ferrimagnetic minerals a
nd thus continuous sedimentation without elastic components delivered from
secondary sources. The curves of isothermal remanent magnetisation (M-ir),
which is sensitive to ferromagnetic minerals, correspond only in part to th
e susceptibility, so that the susceptibility signal is mainly dominated by
fluctuations of the paramagnetic mineral content. While the wave length of
susceptibility variations expresses the precessional and eccentric cycles,
a second precessional cycle is identified in the frequency spectrum of the
M-ir. Our results demonstrate the suitability of rock magnetic properties f
or identifying and interpreting climatic cycles in these Mesozoic successio
ns. (C) 2001 Academic Press.