Současný stav odkrytí fosiliferních karbonátů v Suchém Dole (Český masiv, lužická oblast, krkonošsko-jizerské krystalinikum)

Abstract

The collection of fossils in the abandoned limestone (? Lower Cambrian) quarry in Suchý Důl close to Dolní Albeřice (eastern margin of the Krkonoše Mts.) became practically impossible after the end of mining and modification of the pit for private recreational purposes. However, in the last phase of mining (2016), hundreds of blocks of these limestones with a weight of 200 to 500 kg were used to reinforce the banks of the Lysečiny and Albeřice brooks and the retention reservoir on the Lysečiny brook. After several years of natural weathering of the surfaces, the primary and secondary textures in the limestones are very well preserved and distinct when viewed with the naked eye or a hand lens. In the summer of 2022, I examined about 10 m2 of mostly oolitic limestone surfaces with a hand lens and found several structures which were probably of organic origin. The total area of studiable limestone is estimated at 100–250 m2. Due to weathering of the surfaces, the possibility of collection of fossils culminates in the current years and will become negligible within 5–10 years. Only cross-sections through the potential biogenic structures were observed. They have an ovoid, elliptical, or triangular shape. This could indicate their affiliation to the Archaeocyatha group, two individuals of which have been already found in the quarry. The comparison with the previous findings suggests that the findings depicted in Fig. 1A–1D can represent preserved fragments of Archaeocyatha. A little crest of an irregularly sinusoid shape in Fig. 1E shows preservation similar to the primary textures, i.e. ooids. This suggests its organogenic origin. The size and course of the crest indicates that it could represent, for example, a cross-section through a trilobite cephalon. For comparison, a usual way of weathering of veins of secondary origin in limestone block is shown in Fig. 1F. In addition to their sharp contact, they are tightly folded, which is typical of deformed rocks. On the other hand, during the field work in 2022, any deformed, flattened or elongated ooids (as known, e.g., from the Ordovician ferrite ooids of the Barrandian area) were observed. It is suggested that during deformation, the ooids (primarily build of magnesium carbonate) “floated” in the ductile matrix (primarily calcite) similar to, for example, raisins in leavened dough. Therefore, the deformation, judging from the shape of the ooids, is small or negligible; in reality, however, the shells of the vast majority of organisms were completely destroyed, “mixed up”. Acceptance of this idea then explains why the Albeřice carbonates are so poor in fossils, although the oolitic facies gives the impression of a rock that is not deformed or recrystallized at all and the carbonates point to a revived, climatically favourable, rather shallow marine environment. However, the results of a field research in August 2022 show that the search for fossils in the limestones around Albeřice is currently (and will be for at most 10 years) in a promising state, due to the use of the Albeřice blocks as a flood control measure in the villages and their gradual selective weathering. A very fragmentary preservation of fossils can be expected.