Coniacian-?Santonian paralic sedimentation in the Rakowice Małe area of the North Sudetic Basin, SW Poland: sedimentary facies, ichnological record and palaeogeographical reconstruction of an evolving marine embayment

Stanisław Leszczyński


The Coniacian-?Santonian siliciclastic succession outcropped in a sandstone quarry at Rakowice Małe (Żerkowice Member of the Rakowice Wielkie Formation, and the Czerna Formation including the Nowogrodziec Member; North Sudetic Basin, SW Poland) provides an interesting example of paralic deposits. Lithofacies and ichnofossil examination indicate coastal, lacustrine, paludal and lagoonal sedimentation. Valuable new data are supplied by trace fossils, a feature not considered yet in the literature on the Upper Cretaceous of the North Sudetic Basin. Trace fossils are overall abundant in the upper part of the Nowogrodziec Member and overlying part of the Czerna Formation. The following ichnogenera: Thalassinoides, Ophiomorpha, Asterosoma, Palaeophycus, Planolites, Skolithos, Teredolites, Chondrites, Cylindrichnus, Arenicolites, Rosselia, Teichichnus, Phycodes, Phycosiphon, and Schaubcylindrichnus are represented. The trace fossils represent the Skolithos, Teredolites and Cruziana ichnofacies. The Cruziana Ichnofacies is typified by the richest trace fossil assemblage characteristic of its archetypal, proximal and stressed expressions.

An upper shoreface to foreshore origin of these sediments is documented using lithofacies and the ichnofossils Ophiomorpha and Thalassinoides in the exposed part of the Żerkowice Member. Dominance of kaolinite, lack of burrows and upward passage into paludal deposits is interpreted to indicate a lacustrine origin of variegated clayey mudstone at the base of the Nowogrodziec Member. The changes of depositional environments are interpreted as resulting from separation of the area from the open sea by a sand barrier formed due to the termination of the forced regression. Siltstones containing plant roots and fragments of drifted wood showing the trace fossil Teredolites clavatus, together with coal-seams containing Thalassinoides isp., are assigned to indicate a coastal plain, paludal deposition of the overlying part of the Nowogrodziec Member and incursion of marine waters. The fining upward sequence constituting the top part of the Nowogrodziec Member and showing almost archetypal Cruziana Ichnofacies substituted by its expression indicative of highly stressed, brackish conditions are shown to indicate extensive drowning of the area and lagoonal sedimentation. Termination of the drowning, embodied in a maximum flooding surface, is indicated in a bed of coaly mudstone at the top of the Nowogrodziec Member. Sedimentation on a periodically prograded brackish bay shoreface is inferred from lithofacies, ichnofossils and body fossils for the deposits overlying the Nowogrodziec Member and topping the examined succession. The trace fossils indicate Cruziana Ichnofacies and Skolithos Ichnofacies in the expression of slightly stressed environ- ments. The whole examined part of the Czerna Formation is interpreted as a fifth-order transgressive-regressive cycle.

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