Sedimentary successions in the Middle–Upper Oxfordian reef deposits from the southern part of the Kraków–Częstochowa Upland (Southern Poland)

Abstract

The sedimentary succession of the largest example of an Upper Oxfordian reef located in the southern part of the Polish sector of the northern Tethys shelf is described. Detailed sampling of exposures enabled characterization of the full succession of facies and microfacies diversity, documenting the reef evolution. The succession studied represents the maximum development of Upper Jurassic reefs in Poland corresponding to the upper Transversarium and the Bifurcatus zones, and followed by drowning of the carbonate platform in the lower Bimmamatum Zone. The reef succession comprises three types of facies, in which eight important microfacies types were distingushed, reflecting several stages of reef development. Mid-ramp, microbial-sponge frame-reefs represent a transgressive depositional sequence, up to tens of metres thick. A microbial-Crescentiella-ooid and ooid-intraclast-bioclast facies form numerous, decimetre- to metre-scale, sequences corresponding to higher-order, transgressive/regressive sea level changes. These facies represent a mid-inner ramp setting when sedimentation was dominated by bioclasts and non-skeletal grains (mainly ooids, oncoids, aggregate grains and intraclasts). The grains were stabilized by microbialites and cemented in early diagenesis, which created grain-dominated, microbial-cement supported reefs. As a result, both the mid-ramp, microbial-sponge frame-reefs and the shallow-water, grain-dominated, microbial-cement supported reefs form extensive, strongly lithified Oxfordian reef complexes in the Kraków–Częstochowa Upland.