Coercitometric studies of the terminal Cretaceous deposits of the Volga region were carried out for the first time in the Bolshevik quarry located near the city of Volsk (Saratov Right Bank). The quarry is composed of carbonate rocks – chalk-like marls and writing chalk with varying degrees of clay content. A total of 99 stratigraphic levels characterizing 66 m of the section were sampled. As a result of previous studies [1], intervals with increased concentrations of magnetic minerals were identified in the section. Microprobe investigations confirmed that these intervals contain magnetite microspherules of cosmic origin ranging in size from 3 to 10 μm, suggesting an exogenic contribution to the formation of the magnetic properties of the rocks. Analysis of hysteresis loops showed an extraordinary homogeneity of the magnetic fraction in terms of coercive parameters. According to the Day-Dunlop diagrams [2], the particles are in a multidomain (MD) state, occupying a compact area, which contradicts the estimates of magnetic particle sizes obtained by electron microscopy.
The aim of this study was to determine the possible reasons for discrepancies the size of magnetic grain from the Day-Dunlop diagram and direct microscopic observations.
For this purpose, decomposition of the coercivity spectra (CS) based on the saturated remanent magnetization curve was performed for all samples using a program implementing the EM method [3]. According to [4], based on the positions of the coercivity spectrum maxima (PCSM), these correspond to components D (25–27 mT), Bs (36–47 mT), and Bh (59–62 mT). D corresponds to detrital grains, predominantly in the MD state; BS and BH are traditionally associated with single domain (SD) and pseudo single domain (PSD) grains of biogenic or chemical origin. In our case, it is most likely that the fine particles are of cosmic origin, which determines the small dispersion of PCSM values and the constancy of their contribution to the total CS and, as a result, the constancy of the Jrs/Js and Bcr/Bc ratios. The contribution of the coarsest-grained component D to the total CS does not exceed 26%, with the contribution of the fine-grained component Bh being ~60%, which cannot ensure the concentration of points in the MD region on the Day-Dunlop diagram. Such a discrepancy between the Day-Dunlop diagram and the real sizes of magnetic grains (given their identical composition), according to [5], may be conditioned by a mixture of several ensembles of magnetic grains of different sizes. Another explanation for such a discrepancy may be a special single-vortex or multi-vortex state of fine magnetic grains, in which points on the Day-Dunlop diagram shift into the MD region [6].
Undoubtedly, further studies are required for a final resolution of this complex issue, but the results for the Bolshevik quarry serve as an excellent illustration of the ambiguous interpretation of the Day-Dunlop diagram in the presence in the rock of several homogeneous groups of magnetic grains with different domain states.