Zjawisko twardego dna w profilu kredy Mielnika

Abstract

PHENOMENON OF HARD GROUND IN THE CRETACEOUS SECTION OF MIELNIK ON THE BUG RIVER (EASTERN POLAND)

In Mielnik on the Bug river (Eastern Poland) exists an outcrop of the Cretaceous, with a visible boundary between the Maastrichtian and the Campanian. This outcrop has recently been described by E. Bieda (19518) who investigated its stratigraphy on the basis of its foraminifer fauna. The Mielnik Cretaceous is developed in a facies of writing chalk. On the boundary between the Maastrichtian and Campanian we observe the Hard Grounds (Fig. 1). The section of this outcrop, beginning with the top, is as follows:

1 m. - marly writing chalk, with phosphorites in the lower part

0.5 m. - marly writing chalk, with numerous phosphorites and with glauconite in its lower part

0.4 m. - white writing chalk, intergrown by marly chalk

15 m. -white writing chalk.

The surface of the layer of white chalk is furrowed by erosion. Into the depressions of its surface there penetrated glauconitic chalk which fills as crevices, penetrating down up to 0.4 .m. into the interior of the white chalk. The phosphorites contain practically on quartz nor glauconite grains, - proof that they developed on top of the hard ground before deposition of the marly chalk with glauconite. The hiatus in sedimentation on the boundary of the Campanian and Maastrichtian has been identified at many localities of Poland as well as in Belgium.