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Middle Devonian Skały Formation in the Holy Cross Mountains (Poland) : formal description and subdivision based on new field data

Racki Grzegorz, Wójcik Krystian, Halamski Adam T., Narkiewicz Marek

Annales Societatis Geologorum Poloniae

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2022

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Vol. 92, No. 4

425--444

EN

The well-known fossiliferous and lithologically variable clay-carbonate series in the Łysogóry Region (northern part of the Holy Cross Mts, central Poland), enclosed between the Middle Devonian Amphipora dolomites and limestones (Kowala Formation) and siliciclastics (Świętomarz Beds), is defined formally as the Shaly-Calcareous Skały Formation. This Upper Eifelian to Middle Givetian, ca. 250–280 m thick unit, consists of marly and clay shales, interbedded many times with various limestone types (including encrinite and biohermal varieties), as well as with marls and siltstones. Its diagnostic feature is the presence of variable skeletal accumulations, formed by exceptionally numerous, well-preserved and diverse macrofauna (including brachiopods, corals, crinoids, bryozoans), described since the 19th century. The stratotype is located in the eastern slope of the Dobruchna stream near the Skały village and belongs to the Silurian to Upper Devonian Grzegorzowice-Skały section. Compared to the previously used term, Skały Beds sensu Pajchlowa (1957), the lower boundary is redefined, owing to a new exposure in the active Skała Quarry, and placed higher, at the base of the famous brachiopod shales (set XIV of Pajchlowa), instead of the formerly accepted lower boundary at the base of set XIII. Set XIV is formally distinguished as the Dobruchna Brachiopod Shale Member. The higher part of the Skały Fm (sets XV–XXVA) is not subdivided further, as the poorly exposed succession, including in particular the type area, precludes a more accurate recognition of lithological variability. The upper boundary of the Skały Fm is placed at the top of set XXV sensu Pajchlowa (1957), corresponding to the boundary between subsets XXVA and XXVB sensu Malec and Turnau (1997). A hypostratotype of the upper boundary is selected in the outcrop M0 at Miłoszów, 2.5 km westwards from the type section, allowing recognition of the diachroneity of lithological change defining the transition from the Skały Fm to Świętomarz Beds. A borehole situated in a key location would be an obvious next step in the further elucidation of the stratigraphic sequence of the Łysogóry Region.

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Habitats in the Pre-Taghanic (Givetian, Middle Devonian) muddy carbonate ramp at Miłoszów (Holy Cross Mountains, Poland) : geochemical and microfacies evidence

Pisarzowska Agnieszka, Racki Grzegorz, Rakociński Michał

Annales Societatis Geologorum Poloniae

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2022

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Vol. 92, No. 4

381--409

EN

The well-known fossiliferous and lithologically variable Middle Devonian Shaly-Calcareous Skały Formation in the Łysogóry Region (northern part of the Holy Cross Mountains, central Poland) was studied for the first time in terms of elemental geochemistry, carbon isotope stratigraphy and limestone microfacies. Three Lower to Middle Givetian marly-limestone successions, exposed at Miłoszów, represent middle to outer facies belts of the vast carbonate ramp, characterized by very rich epifaunal and infaunal benthic life in muddy, oxic, eutrophic, and photic zone habitats. Brachiopods and occasionally corals (in mesophotic association), erect branching bryozoans, and tiny crinoids played a leading role among flourishing sessile suspension-feeders. High-energy storm events, possibly even a tsunami, during the brief Early Givetian time strengthened a prolific carbonate ooze delivery system from shallow ramp areas, including restricted back-ramp lagoons and a variety of organic buildups, populated by corals and stromatoporoids. The ecologically mixed skeletal grain association is characterized by the rich occurrence of a typical lagoonal biota, calcispheres and amphiporoids. The effective carbonate factory declined stepwise regionally during the Middle Givetian because of an intermittent progradation of the deltaic system of the Świętomarz Beds, linked with climate cooling and the activation of block movements. The regional carbonate crisis resulted in the demise of diverse benthic life, including the prolific calcified microbiota. The higher Skały Formation succession, deposited between the important Kačák and Taghanic bioevents, is noticeable for a background carbon-isotope pattern in carbonate and organic matter signatures, with the baseline with the baseline δ13 Ccarb values between 1‰ and 2‰. The microfacies and chemostratigraphic data confirm that at least the lower pumilio bioevent was not recorded in the Łysogóry Region.

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Cyclic ecological replacement of brachiopod assemblages in the top-Eifelian Dobruchna Brachiopod Shale Member (Skały Formation) of the Holy Cross Mountains (Poland)

Woźniak Paweł, Halamski Adam T., Racki Grzegorz

Annales Societatis Geologorum Poloniae

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2022

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Vol. 92, No. 4

445--463

EN

The palaeoecology of fossiliferous shales, belonging to the upper part of the Dobruchna Brachiopod Shale Member (= set XIV) of the Skały Formation (northern Holy Cross Mountains), was studied quantitatively in a succession in the transient (1989) trench A, 5.6 m thick, near the village of Skały. The top-Eifelian strata, recording the carbonate crisis during the global Kačák Bioevent, are well known for having a particularly diverse brachiopod fauna. The four brachiopod assemblages, recognised herein, were mainly controlled by the evolving bottom-sediment properties of the outer carbonate ramp basin. Soft, unstable substrates were inhabited by the poorly-diversified Poloniproductus assemblage, associated with a distinctive, ‘incumbent’ set of largely semi-infaunal, generalist species. The pioneer community, as a result of progressive consolidation of bioclast-enriched sediment, evolved toward a more diverse biota. This consequent stabilisation of the substrate resulted in the progressive growth of crinoid thickets or bryozoan-dominated biostromes and patches, associated with rich, subordinate, sessile and vagile benthos. In this stage, diverse brachiopod assemblages were dominated by the pedunculate, eurytopic, ribbed spiriferide Eleutherokomma or specialized orthides (Aulacella, Costisorthis) in the Dobruchna Mbr, and by the expansive, large, free-lying orthotetide Xystostrophia in the overlying set XV of the Skały Fm The cyclic ecological replacement, with the characters of ecological succession in the final phase, was evidently stimulated by an irregular transition from soupy muds to a mosaic of bioclast-rich and firmer, biogenic sediments, within the cyclic pattern of distal tempestite sedimentation. The three episodes of variously reduced deposition rate, recorded in the more diverse benthos, culminated in the pioneer bryozoan/coral reef growth and abundance of epibionts, alternating with times of destructive storm activity and deposition from suspension clouds in the muddy habitats.