Results of palaeomagnetic research of karst sediments in Slovenia

Category Geomorphology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Zupan Hajna, Nadja۱; Mihevc, Andrej۲; Pruner, Petr۳; Bosلk, Pavel۳
Holding Date 10 September 2008

Results of more than 10 years intensive study of palaeomagnetic properties and magnetostratigraphy of karst sediments in Slovenia are summarized. The research covered the most important karst regions, from lowlands to high mountains. It included both well-known and documented sites, and relatively unknown or newly found locations in caves and surface karst sediments. The territory of Slovenia, with its numerous karst regions, long history of karst evolution and relatively good knowledge of the karst sediments represents an ideal testing ground for comprehensive research on individual infilling processes, their stages and periods. The questions concentrated to the time span of karst evolution in Slovenia, age of karst surfaces, speleogenesis and rates of processes. The majority of karst sediment dating has been carried out in south-western Slovenia (in the north-western part of the Dinaric Karst, which is known as the Kras) where Eocene flysch is the last marine deposit preserved in the geologic record. The Oligocene to Quaternary period represented mostly terrestrial phase with prevailing surface denudation and erosion processes. Therefore only karst sediments preserved on karst surface and in subsurface can yield some facts and ideas of karst evolution and its age.
Dating of cave sediments by the application of the palaeomagnetic method is a difficult and sometimes risky task, as the method is comparative in its principles and does not provide numerical ages. Repeated sampling in some profiles have shown that only dense sampling (high-resolution approach with sampling distance of 2–4 cm), can ensure reliable results. Correlation of the magnetostratigraphic results we obtained, and the interpretations tentatively placed upon them has shown that in the majority of cases, application of an additional dating method is needed to either reinforce the palaeomagnetic data or to help to match them with the geomagnetic polarity timescale.
The most important result is the discovery that cave fills have substantially older ages than generally expected earlier (max. about 350 ka). Palaeomagnetic data in combination with other dating methods, especially biostratigraphy, have shifted the possible beginning not only of the speleogenesis but also of the cave filling processes in Slovenia far below the Tertiary/Quaternary boundary. Results suggest that there were probably some distinct phases of massive deposition in caves. The oldest one took place from about 1.8 to more than 5.4 Ma (with two phases at 1.8 - 3.6 and about 4.1 - 5.4 Ma). The data support and better define the estimated ages of the surface and cave sediments that were based on geomorphic evidences, especially from unroofed caves.
The evolution of the caves took part within one karstification period, which began with the regression of Eocene sea and exposing of limestones at the surface within complicated overthrusted structure, which formed principally during Oligocene to early Miocene.