MAGNETOSTRATIGRAPHY OF CAVE SEDIMENTS

Karst and cave fills are frequently complex assemblages of differently aged deposits. A specific feature of karst systems is that deposition can be “frozen” (halted) and then rejuvenated. Reactivation processes may degrade the record into an unreadable form, often mixing karst fill of different ages. Only the last accumulation phase has been dated in caves in most cases. The rejuvenation of the karst process can excavate the previous cave fill completely, or under favorable settings, fills belonging to more infill phases and separated by distinct hiatuses can occur in one sedimentary profile.

The use of paleomagnetic methods can bring surprising results, The method can serve as helpful tool to interpret not only the age of cave sediments but also to understand the evolution of karst landscape and tectonic history of the region. To obtain more precise results, it is necessary to combine paleomagnetism with other methods of numerical-, relative- or correlated-dating. Analytical results confirm that the complete step/field procedure offered by the alternating field and thermal demagnetisation methods must be applied. The sampling for magnetostratigraphy analysis must be dense. Repeated sampling in some profiles from Slovenia have shown that only dense sampling (i.e. each 2-4 cm) can ensure reliable results. Work with the analysis of pilot samples and a shortened, selected field/step approach did not offer sufficient data set for reliable interpretation.

Therefore the dating of cave sediments by the application of palaeomagnetic methods - magnetostratigraphy - represents a difficult and challenging task. Case studies from the karst areas of Czech Republic, Slovakia, Hungary, Slovenia, and Italy, demonstrate that the correlation of arrangements of normal and reverse polarised magnetozones with standard palaeomagnetic scales can be achieved only with difficulty and with a high degree of uncertainty. Such realities can be seen in all examined logs from Slovenia and some from Slovakia. The correlation of magnetostratigraphic data with standard scales indicate, in most cases, substantially older cave fill ages than previously supposed: e.g., most of Slovenian results indicate pre-Quaternary ages, in spite of former interpretations of mid-Pleistocene maximum ages.