HR: 11:05h
AN: GP52A-04    [Abstracts]
TI: Magnetostratigraphy of Cave Sediments
AU: Bosák, P
EM: bosak@gli.cas.cz
AF: Institute of Geology AS CR, Rozvojová 269, Praha, 165 00, Czech Republic
AU: * Pruner, P
EM: pruner@gli.cas.cz
AF: Institute of Geology AS CR, Rozvojová 269, Praha, 165 00, Czech Republic
AB: Karst and cave fills (karst sediments) are relatively special kinds of geologic materials, as the karst environment favors both the preservation of paleontological remains and their destruction: on one hand, karst is well known for its wealth of paleontological sites, on the other hand, most of cave fills are completely sterile (especially the inner-cave facies). Another specific feature is that karst systems can be „frozen" (halted) and then rejuvenated, often for several times. Reactivation processes may degrade the record into an unreadable form, often mixing karst fill of different ages (collapses, reworking, redepositions). Only the last accumulation phase has been dated in caves in most cases. The fossilisation of the cave (fill by deposits) and rejuvenation (exhumation of the fill) reflect changes in resurgence area. The rejuvenation of the karst process can excavate the previous cave fill completely, which is the most common case resulting from the polycyclicity and dynamics of cave environments. Under favorable settings, fills belonging to more infill phases and separated by distinct hiatuses (unconformities) can occur in one sedimentary profile. Such amalgamation is typical especially in ponor part of the cave. During our previous research we found, that owing to the specific character of karst sediments, the use of paleomagnetic methods can bring surprising result, without regards to the character of the method (correlated- ages not providing direct numerical output). 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. The analytical results confirmed that the complete step/field procedure offered by the alternating field and thermal demagnetization methods must be applied. The sampling for magnetostratigraphy analysis must be dense enough. Repeated sampling in some profiles from Slovenia have shown that only dense sampling (i.e. each 2-4 cm) can ensure reliable results. The application of complete analysis only to pilot samples and shortened, selected field/step approach, to other samples did not offer sufficient data set for reliable interpretation. Therefore the dating of cave sediments by the application of palaeomagnetic methods - magnetostratigraphy - represents a highly difficult and sometimes risky task. Case studies from the karst areas of Czech Republic, Slovakia, Hungary, Slovenia, Italy, proved that the correlation of obtained arrangements of normal and reverse polarized magnetozones with standard palaeomagnetic scales can be finish only with difficulties and with a high degree of uncertainty. Such reality can be finely exemplified nearly on all examined logs from Slovenia and some from Slovakia. The correlation of obtained magnetostratigraphic data with standard scales indicate, in most cases, substantially older age of cave filling than expected earlier: e.g., most of Slovenian results indicate even pre-Quaternary ages, in spite of former interpretation to mid-Pleistocene as maximum age expected.
DE: 1520 Magnetostratigraphy
DE: 1527 Paleomagnetism applied to geologic processes
DE: 9335 Europe
DE: 9604 Cenozoic
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly