Lower Kimmeridgian facies and sedimentary succession of a shallow-water coated-grain-dominated carbonate ramp of the northern peri-Tethyan shelf: an example from the Radomsko Folds (central Poland)

Abstract

An Upper Jurassic succession is exposed in the active Rogaszyn Quarry (Kodrąb area) located in the Radomsko Folds structure (central Poland). Six facies types were distinguished, comprising oolitic, oncolitic, biodetrital and pelitic limestones as well as marls and marly clays. Typical lithologies are limestones with coated grains deposited in shallow-water, fore-shoal, shoal and lagoonal parts of a carbonate ramp. Three types of ooid and three types of oncoid were identified, which display several shapes and sizes, and a complex structure of cortices. The ooid types show micritic, radial-fibrous and mixed, micritic/radial-fibrous cortices. Type 1 oncoids comprise micritic or bioclastic nuclei enveloped by non-laminated or laminated micritic cortices. Type 2 oncoids are composed of micritic or bioclastic nuclei and laminated, fossil-bearing cortices covered by microbial envelopes. Type 3 oncoids are the largest of the oncoids observed, attaining cm-scale dimensions. These oncoids are formed of a Bacinella-dominated meshwork incorporating smaller ooids and oncoids, or they have bioclastic or micritic nuclei covered by complex cortices with micritic, laminated, fossil-bearing, Bacinella-dominated fabric and microbial envelopes. Boundaries between the specific types of cortical envelopes are usually sharp and accentuated by organic encrustations, elongated bioclasts or borings. Commonly found are chambers of the ichnogenus Entobia representing the boring traces of carbonate-excavating sponges and the cryptoendolithic foraminifer Troglotella incrustants. The complex cortices indicate different growth stages of oncoids at different conditions of currents and microbial activity. Carbonate deposition was accompanied by mixed siliciclastic-carbonate sedimentation resulting in multiple intercalated layers of marls and pelitic limestones as well as marly clays. The deposits exposed in the Rogaszyn Quarry as well as in the Kodrąb area are compared with the neighbouring Upper Jurassic deposits of the Szczerców area and the SW margin of the Holy Cross Mts. (both central Poland). The successions from the Kodrąb area are generally similar to the Lower Kimmeridgian sequences known from adjacent areas, but with locally observed facies variability.