Fig. 2. Map of the Za Hajovnou cave (modified after Kadlec et al., 2014; Lundberg et al., 2014; Musil, 2014). Locations marked by “Section No. 1” and “Section No. 2” are discussed in the text (Kadlec et al., 2005, 2014). The map shows the relation between Connecting Passage Corridor, Birthday Corridor, and Excavated Corridor (m a.s.l.: meter above sea level).
Upper sediments of the cave were dated by U/Th dating of flowstones from 118 ± 1 to 267 ± 3 ka and the sediment sequence below was inferred to contain the Matuyama-Brunhes boundary in Section No. 1 (Fig. 3) (Musil, 2005; Kadlec et al., 2005, 2014; Lundberg et al., 2014; Bábek et al., 2015). The sedimentation in the cave corridors was active from the Early Pleistocene until the beginning of the Middle Pleistocene. The sediment then consists of Pleistocene glacial time in north-western Europe called the Cromerian Interglacials complex (Muller, 1992), MIS19 (marine isotope stage) which is interglacial period ~780 ka (Pol et al ., 2019), and Matuyama-Brunhes reversal (Kadlec et al., 2005, 2014; Lundberg et al., 2014; Musil, 2014; Musil et al., 2014; Žák et al., 2018).
The Matuyama-Brunhes boundary (781 ka) was suggested in the upper part of the backwater fine sediments, deposited from suspension in the flooded cave. These sediments underlay mostly non-fluvial deposits which entered the cave through a steep passage and filled the Connecting Passage Corridor (Kadlec et al., 2014; Lundberg et al., 2014; Musil et al., 2014). The total thickness of the sediments, deposited from suspension, reaches up to 4.3 m.
Sedimentary sections retrieved in the Za Hajovnou cave by Kadlec et al. (2005, 2014) were composed of two parts. The first part (Section No. 1, in Fig. 2) was situated in the Excavated Corridor about 28 m from the cave entrance (Kadlec et al., 2005) (Fig. 2). It was intepreted to contain the magnetic transition from a reversed to normal polarity and inferred from the age dates of the overlying non- fluvial sediments, that our data indicate that the reversal is the actual Matuyama-Brunhes reversal. Sediment thickness in the Section No. 1 is 0.8 m. The second sedimentary section (Section No. 2) partially overlapped the Section No. 1 and was located in the Excavated Corridor (Kadlec et al., 2014) (Fig. 2). Kadlec et al. (2014) indicated that this section had sediment with just reversed polarity except for the upper part of the sediment where the magnetization was difficult to interpret because the sediments had weak magnetization for which the sensitivity of the Agico JR-5A spinner magnetometer was insufficient. Section No. 2 underlies the Section No. 1 and contains older backwater sediment with reversed magnetic polarity (age > 781 ka) (Kadlec et al., 2014).
The difficulties in the interpretation of the primary study by Kadlec et al. (2014) was the motivation for the presented research. Here we collected 44 oriented discrete sedimentary samples from the Excavated Corridor near the upper backwater sedimentary Section No. 1 (Fig. 2, 3).