engleski

The Age of the Đurđevac Sands

The Alpine orogenesis resulted in intense erosion of the elevated mountains and gravitational transport of the material. During the Pleistocene glaciation, an increase in the volume of ice resulted in intense crushing and grinding of rocks. During interstadials, the ice was melted and the grained material was transported and additionally fragmented by newly formed rivers (as the Drava River) to lowlands and formed floodplains. A lot of material in suspension and reduction of the potential energy of the flow caused meander development and periodical floods followed by deposition of fine-grained sediments in the Podravina region. Also, during the cooling periods and freezing of the vegetation, the northwest winds blew away silty and clayey material. These natural separation processes resulted in accumulation of medium-sorted sand with a high percentage of heavy mineral fraction (HMF) (7-54%) (GALOVIĆ & POSILOVIĆ, 2017). Light mineral fraction (LMF) is dominated by quartz and HMF by garnet (GALOVIĆ & POSILOVIĆ, 2017). Based on HEĆIMOVIĆ (1987), during the Holocene this sands have been repeatedly resedimented by winds and formed dunes (the Đurđevac Sands) that are discordantly situated on the Pleistocene loess of the Bilogora Mt. slopes, and in the Drava River Valley on the Holocene sands and gravels of the second alluvial terrace and partly on the marshy sediments. Mineral composition of sands and transport by saltation mechanism disabled preservation of potential fossils. Based on Mohs scale of mineral hardness, the sand grains are almost exclusively harder (>6) than calcite/aragonite mollusc shells and dentin (3) or even tooth enamel (5). Thus the age of the Đurđevac Sands was determined by the superposition principle. During dry years, the sand was unsuitable for rooting. In history, the Đurđevac Sands were a real threat to agricultural cultures and settlements in this area due to their instability. Settling of sands at the end of the 19th and early 20th centuries was achieved, mainly by afforestation. Recent forest arenosol has an organic horizon (Oh) 1-3 cm thick and a humus horizon (Ah) up to 10 cm. At control sites, without affecting forest vegetation, arenosol is low in humus (up to 2 cm) (VRBEK et al., 2017). Generally, arenosols are typical soils developed on recent dunes from arid to humid and per humid, and from extremely cold to extremely hot climate. The vegetation ranges from a desert over scattered vegetation (mostly grassy) to light forest (IUSS WORKING GROUP WRB, 2015). The climate in the area is continental, with springs warmer than autumns. According to Lang's rainfall factor, the area of the Đurđevac Sands, during the reference period 1961-1990 had humid, and during the modern period 1991-1996 semi-humid climate, indicating climate warming (BILANDŽIJA et al., 2017). Within the CSF project SAPIQ, in 2016 the Đurđevac Sands were preliminarily investigated in the sandpit Draganci. Within the sand dunes sets, two palaeosoils, type arenosol, were discovered (Figure 1). Sedimentological analysis and sampling of palaeosoils and overlying and underlying sands were performed. It was intended to correlate the Holocene warm humid periods documented by pedogenesis (charcoal pieces extracted from the palaeosoils dated by the 14C method) with the historical data during which the dunes did not travel. Unexpectedly, the analysis showed that these two palaeosoils developed just before and at the very beginning of the Holocene. Preserved bioturbations confirm that the palaeosoils are in situ. The older palaeosoil (12.483±53 a BP) is the thinner (about 15 cm thick) and the degree of pedogenic development indicates semi- arid climate with annual rainfall less than 300 mm and predominantly covered by grass (IUSS WORKING GROUP WRB, 2015 ; PEYRAT, 2007). It is similar to the modern forest soil in the area. The younger palaeosoil (11.705±55 a BP) is about 40 cm thick and pedogenetically well- developed. The upper border is erosional, so the original thickness cannot be assumed. However, it can be assumed that the climate was warmer and more humid, vegetation lusher than today and/or it can be assumed that this pedogenesis lasted longer. A lamination of sands underlying those palaeosoils is not noticeable. However, 20 meters laterally, the metric sets of the dune laminas beneath the palaeosoils were recognised. Based on superposition, it can be concluded that those dune laminas are older than dated palaeosoils, and were formed by the end of the Pleistocene. During that dry and cold period, the Drava River has provided enough sandy material (HEĆIMOVIĆ, 1987) and aeolian resedimentation could form the dunes. Thus, it is possible that older dune sediments of the Đurđevac Sands were formed before the end of the Pleistocene, and palaeosoils formed just before and at the beginning of the Holocene preserved them from erosion.

Đurđevac Sands ; palaeosoil ; dune ; Holocene ; Pleistocene

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