Tide-dominated Middle Devonian sequence from the northern part of the Holy Cross Mountains (Central Poland)

Abstract

The dolomitic Wojciechowice Formation distinctly differs from the remaining, mainly shaly Middle Devonian succession in northern part of the Holy Cross Mountains (Central Poland). The upper Member of the Formation (Crystalline Dolostone Mb.), in greater part dolomitized but also containing limestone beds, is composed of shallowing-upward cyclothems well exposed in Skaly quarry in the Bodzentyn syncline. The lower parts of the cyclothems, interpreted as subtidal facies, contain fossils characteristic for restricted environments. They are grouped into two assemblages. The first, with brachiopods (largeBornhardtina andEmanuella), massive stromatoporoids, and subordinate gastropods and amphiporoids is related to a deeper subtidal environment, while the second (mainly amphiporoids, gastropods, ostracodes and calcareous algae) is shallower subtidal. Towards the top of succession the fossil content radically decreases. The upper parts of cyclothems are composed mainly of different types of laminites. In these parts of the section, interpreted as intertidal/supratidal units, stromatolites, desiccation polygons, intraformational breccias, and common bioturbations are present. The whole succession was deposited in a low-energy environment, only intermittently affected by high-energy events. For their most spectacular example of this, aBornhardtina-coquinite, a tempestitic origin is proposed. The interval with cyclic sedimentation studied correlates with the dolomitized lower “Unit I” of the Stromatoporoid-Coral Kowala Formation from the southern part of the Holy Cross Mountains, which exhibits sabkha-type cyclicity. The differences in development of cyclothems in both regions resemble outer and inner part of an extensive platform, and correspond well with basic trends of the Lower-Middle Devonian transgression in the Holy Cross Mountains. The general succession of formations deposited during this process coincides with transgressive events on Johnson's eustatic curve for the Devonian.