Pressure dissolution features in Oxfordian microbial-sponge buildups with pseudonodular texture, Kraków Upland, Poland

Jacek Matyszkiewicz, Alicja Kochman

Abstract


Part of the Oxfordian carbonate buildups in the southern part of the Kraków Upland is developed as pseudonodular limestones, which represent segment reefs. These limestones are composed of connected, rounded-oval to subangular carbonate pseudonodules. The pseudonodules, densely packed within the limestone, fall out easily under mechanical stress.

The recently observed texture of pseudonodular limestones resulted from two stages of chemical compaction. During the first stage, in the Late Jurassic, high-amplitude and low-amplitude stylolites and dissolution seams were formed. The sites particularly favourable for the development of high-amplitude stylolites were the boundaries between already lithified fragments of the laminar, rigid microbial-sponge framework. The low-amplitude stylolites formed mainly in the intercalated wackestone-packstone, which was lithified somewhat later; hence, the dissolution seams originated at the contacts between the rigid microbial-sponge framework and the wackestone- packstone.

After Early Cretaceous erosion, which decreased the burial load, Late Cretaceous sedimentation enabled the renewal of pressure dissolution. Thus, some low-amplitude stylolites evolved into dissolution seams. In stylolites composed of both low- and high-amplitude segments, dissolution proceeded at the bases of interpenetrating high-amplitude stylolite columns, with the simultaneous transformation of low-amplitude stylolite segments into dissolution seams. These seams, which formed at the initial stage of chemical compaction, were subjected in turn to further pressure dissolution, giving rise to the formation of horsetail structures.

The vertical stress field, which triggered the pressure dissolution processes, presumably resulted in the formation of high-angle and vertical incipient tension gashes. At the beginning of the processes, these gashes remained closed. In the Cenozoic, under the extensional regime generated by overthrusting Carpathian flysch nappes, some high-angle and vertical dissolution seams and low-amplitude stylolites opened up, forming deformed dissolution seams and deformed stylolites. Under the same conditions, the high-angle and vertical tension gashes opened up as well.

Subsequently, during the exposure period, unloading fractures developed, partly as a result of the opening of some subhorizontal and horizontal dissolution seams and stylolites. The unloading fractures, along with the already existing vertical and high-angle tension gashes, formed the network changing the limestone into pseudonodules of various shapes and sizes. The open spaces between the limestone fragments became local conduits for karst waters.

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