Ammonites from the Kraków area
At the end of the Triassic and beginning of the Jurassic carbonate formations were being destroyed and removed from the land. The Middle Jurassic sea entered into the vicinity of Kraków in the Upper Bathonian and lasted till the Kimmeridgian. In the Bathonian, clay deposited locally. It was used to make fireproof material (Grójec clay). Numerous prints of plants have been found there. Poorly cemented sand are younger, widespread deposits of the Middle Jurassic–Callovian. Above them, there is a layer of sandy crinoid limestone containing numerous fossils: brachiopods, bivalves, belemnites, echinoderms and ammonites, the genera Macrocephalites and Phylloceras. In the upper part of the limestone, there is a nodular, bottom-like layer (fragments of Callovian rocks in iron-manganese cover), sometimes cut by an erosion plane as it is in Zalas near Krzeszowice. Above the profile there is a layer of limestone called Balin oolite containing numerous iron ooids. An oolite layer, or when it does not occur, a nodular one is covered with stromatolites – petrified results of algae or bacteria activity. Pink, yellow, red and grey marly limestone and grey marly poriferans, banded, bedded or rocky limestone build the Oxfordian formation. Numerous outcrops of Jurassic rocks from Kraków vicinity have been described in palaeontological and sedimentological literature. Balin near Chrzanów, Czerna near Krzeszowice and the Racławka valley are the best-known localities of Callovian fauna occurrence from the Balin oolite. The exposure of the cover in the porphyry quarry in Zalas near Krzeszowice and small limestone excavations at Podłęże near Brodła are the source where several dozens of species of ammonites have been collected. Among them, there are adult representatives of the species Creniceras crenatum (Brugière) reaching the maximum diameter of 20 mm and medium size (several dozens of centimetres) of the genus Macrocephalites, Phylloceras and Perisphinctes. Gregoryceras tenuisculptum Gygi and Bullatimorphites bullatus, found as single specimens, are collectors’ rarities. Among curiosities, it is worth to mention well-preserved and quite large specimens of Euaspidoceras paucituberkulatum Arkell and Peltoceratoides constantii (d’Orb.) obtained in 1998. Ammonites belong to an extinct group of cephalopoda living from Devonian till Cretaceous. They were undergoing quick evolution, therefore they are usually used in stratigraphy as index fossils to determine ammonite layers, which are characterised by one or a group of characteristic ammonites. Their common occurrence, easy gaining and beauty of shells, casts and external moulds cause them to be the most popular fossils among collectors. Secondary mineralisation of shells and camerae appears quite often. Calcite, barite, pyrite, chalcedony and quartz replace frequently aragonite shells of ammonites.
Elaborated by Remigiusz Molenda (The Geological Museum at the Faculty of Geology, Geophysics and Environmental Protection of the AGH University of Science and Technology)l, © all rights reserved