Uwagi o stratygrafii, sedymentacji i paleogeografii dolnego i środkowego oksfordu okolic Krakowa

Abstract

Przedstawiono spostrzeżenia dotyczące stratygrafii, wykształcenia litologicznego oraz skamieniałości oksfordu dolnego i środkowego okolic Krakowa. Podkreślono małą miąższość osadów tego wieku, zjawiska kondensacji stratygraficznej warstw, występowanie skamieniałości na wtórnym złożu, a także osadów typu twardego dna. Na podstawie spektrów rodzin amonitowych, spektrów wszystkich grup skamieniałości oraz wykształcenia litologicznego podjęto próbę batymetrycznej interpretacji zbiornika morskiego dolnego i środkowego oksfordu okolic Krakowa.SOME REMARKS ON STRATIGRAPHY, SEDIMENTATION AND PALEOGEOGRAPHY OF THE LOWER AND MIDDLE OXFORDlAN IN THE VICINITIES OF CRACOW With reference to earlier papers by the Author (R. Tarkowski, 1983a, b), there are emphasized some more important data on stratigraphy, lithological development of rocks and fossils of the Lower and Middle Oxfordian in the vicinities of Cracow. The rocks are characterized by small thickness, generally increasing upwards: the Quenstedtoceras mariae zone is here some tens cm thick, the Cardioceras cordatum zone - up to 3 m thick, and the Perisphinctes plicatilis zone - up to 10 m thick.Some of the studied sections are characterized by the lack of fauna and rocks of certain zones or subzones. This is the case of the section K at Podłęże and the section A at Zalas (Fig. 2). Rocks of the Quenstedtroceras mariae zone are missing in the majority of the studied sections. At Szklary (section Y, beds 1 and 2) and Zalas (section C, bed 3), there has been found as assemblage of ammonites typical of different subzones which indicates stratigraphic condensation. In the former (Szklary locality, section Y, beds 1 and 2) there were also found some redeposited ammonites. The above mentioned lack of fossils and rocks of certain zones and subzones in the Oxfordian sections from the vicinities of Cracow may be explained in terms of some phenomena of sedimentary character - a crisis in carbonate sedimentation at the turn of the Callovian and Oxfordian and in Early Oxfordian. Sediments of the hardground type (R. Bromley, 1975, fig. 18 A3) have been found in some sections at Zalas. The sediments display some traces of browsing and features indicative of two stages in cementation. This implies temporary breaks in this already slow sedimentation (Zalas, section C, bed 3). There were also found some other sedimentary structures: ferruginous encrustations, intraclasts of Callovian and Oxfordian material, glauconite occurring in the form of fine grains or lumps up to a few cm in size. Such features are most common in the Lower Oxfordian in the vicinities of Cracow. Ammonites predominate in macrofauna in the Lower and Middle Oxfordian in the vicinities of Cracow (Fig. 3). Belemnites and terebratulids appear less common, and rhynchonellids, bivalves, gastropods and echinoids - sporadical. The recorded changes in faunal spectra, along with those found in ammonite spectra (Fig. 4), and Iithological development of the strata in the studied zones and subzones of the Oxfordian gave the basis [or reconstruction of bathymetry of the marine basin (Fig. 5). In the Early and Middle Oxfordian, sea was not deep (40 to 120 m deep) in the Cracow area. II was the deepest in the Cardioceras cordatum zone, which is shown by the highest share of ammonites in faunal spectrum (Fig. 3) and the lowest share of benthic, fauna (bivalves, gastropods and echinoids). Upwards in the section the strata indicate decrease in depth of the basin and, at the same time, more and more uniform environmental conditions (see R. Tarkowski, 1983b). This conclusion is further supported by the nature of ammonite spectra (Fig. 4). The analysis of ammonite spectra obtained for the Lower and Middle Oxfordian from the vicinities of Cracow (Fig. 4) shows that the recorded fauna is representative for the Submediterranean faunal province. Differences in the spectra reflect temporary Subboreal influences. The influences were, however, to weak to allow Boreal or Subboreal fauna to predominate in the ammonite assemblages.