Zechstein (Ca1) limestone-marl alternations from the North-Sudetic Basin, Poland: depositional or diagenetic rhythms?

Limestone-marl alternations constitute a characteristic rock facies within the Zechstein Limestone (Ca1) deposits of the North-Sudetic Basin, Poland. Rhythmites of this type are exposed in a 10-metres section at Nowy Kooeciół, which from the beginning of the last century has been considered representative for this part of the Zechstein basin, and were formerly interpreted as calcareous storm sediments alternating with marls deposited during more quiet conditions. Our results of sedimentological, petrographical and chemical studies suggest that the Nowy Kościół section is a record of distal storm deposits, but that the present-day layering does not precisely reflect the depositional rhythm. Post-depositional changes have overprinted and blurred the original pattern.We propose a model of diagenetic alteration in which dissolution and reprecipitation of calcium carbonate in the shallow subsurface formed new layer boundaries; subsequent modifications during deeper burial followed these early diagenetic changes.