engleski

The provenance of Drava alluvial terrace sediments in the area of Bilogora

Bilogora is a Plio-Quaternary transpressional morphostructure of Dinaridic orientation situated in the southwestern part of the Pannonian Basin. Its tectonic origin is associated with Neogene-Quaternary evolution of the Drava Depression, i.e. Pliocene-Quaternary tectonic inversion and structural reactivation of Neogene faulted structures. This hilly terrain occupy Drava Depression and Bjelovar Subdepression being composed of thick Neogene and Quaternary sediments succession (KRANJEC et al., 1971 ; SAFTIĆ et al., 2003). Sediment deposition during the Quaternary was associated with the hydrodynamic system of paleo-Drava River. The Drava River created four alluvial terraces through Quaternary, two of them exposing Pleistocene and two Holocene clastic succession (ŠIMUNIĆ et al., 1990). The oldest of the four Drava terraces exposed in the Bilogora area consists of cyclic coarse grained gravel succession (sub to well rounded pebbles) altering with subordinate sand and silt layers that are covered by Holocene aeolian fine grained sediments like loess. The total thickness of Pleistocene alluvial sediments is up to 80 m (ŠIMUNIĆ et al., 1990). In this study principal research objectives were to constrain the source area of the Pleistocene alluvial sediments in the area of Bilogora. The studied alluvial sediments were previously dated by luminescence dating technique that reviled a Middle Pleistocene age (WACHA et al., 2018). Quantitative heavy mineral analysis was conducted on a 25 samples from eight different localities along the Bilogora. In the northwestern part of Bilogora ten samples were collected, four samples were collected from Mučna Reka, four samples from Novigrad Podravski and two from Selinec. Three samples were collected in Sveta Ana and two from Črešnjevica, both located in the central part of the Bilogora. Final ten samples, six from Cabuna, two from Špišić Bukovica and two from Rezovac were collected in the southeastern part of the study area. Results show that samples from the southeastern part are dominated by garnet that makes approximately up to 50% of all translucent heavy minerals, along with epidote/zoisite, rutile and tourmaline (Fig.1). This suggests a dominant Alpine source of the generated sediment (MUTIĆ, 1975). The transported material probably had arrived directly from NW and was deposited by the paleo-Drava River, or it may have been originated from recycled uplifted Neogene strata. The sand from the central part of the Bilogora Mt. contains mostly of epidote/zoisite, followed by garnets, amphiboles and rutile. The proportion of garnet appears to decrease towards the northwest. At Mučna Reka site, the samples are almost entirely depleted of garnet what is quite unusual as Holocene sands of the Drava River and most of the loess and older Neogene sediments in the area are rich in garnet of Alpine provenance. This could indicate a local origin of the detritus, possibly from pre- Neogene basement rocks being exhumed in the nearby Kalnik and Ivanščica mountainous area to the west, or/and Slavonian mountains to the southeast. On some of the studied localities conspicuous pebbles of light coloured tuffs with biotite and other volcanic lithologies have been found. Similar volcanic rocks were deposited on the Mt. Kalnik during the Lower Miocene. U(Th)/He dating on apatite grains from the tuffs has shown an average age of 19 Ma. Since the collected pebbles were very soft and easy to disintegrate, it is highly improbable that they were transported over long distances. This possibly pinpoint to the local origin of at least part of the detritus in the Bilogora area. Due to ambiguity of the results and the fact that the composition of heavy mineral assemblages in sands may be influenced by many processes during transport, deposition and diagenesis, geochemical analysis of detrital garnet and rutile grains on several representative samples provide additional information on the provenance, because varietal characteristics of individual mineral species are usually inherited directly from the source area.

Bilogora, Pleistocene, Drava alluvial terrace, provenance, heavy minerals

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