engleski

SUBMERGED PALEOLANDSCAPES OF KARST RIVERS ZRMANJA, CETINA, NERETVA AND KOLOČEP IN THE EASTERN ADRIATIC COAST (CROATIA)

During the Last Glacial Maximum (LGM) lowstand, large areas of the Adriatic shelf were exposed. At that time, relative sea level (RSL) was at least 125m lower than present (FAIRBANKS, 1989 ; LAMBECK et al., 2011 ; BENJAMIN et al., 2017). This enabled formation of lakes, river valleys and their floodplains that were subsequently submerged during Holocene transgression (VACCHI et al., 2016 ; ANTONIOLI et al., 2009). Here we present a study of karst paleorivers Zrmanja, Cetina, Neretva and Koločep located along the Eastern Adriatic Coast (Figure). Well preserved submerged paleolandscapes are detected using high resolution acoustic geophysical methods (sub- bottom profiler) coupled with bathymetric data. Interpreted geophysical data are compared to RSL curve for the Adriatic region (BENJAMIN et al., 2017, LAMBECK et al., 2011). Seismic data for paleo-Zrmanja River shows channels incised into Pleistocene clastic rocks exposed during the early Holocene. Larger valley flowing from present Karin Sea area into present Novigrad Sea area joined river Zrmanja and continued into Velebit Channel. Numerous channels detected in seismic profiles point to the existence of braided river system. Western part of the depression in today’s Novigrad Sea hosted a small lake. As the RSL rose marine sediments overlie fluvial and terrestrial sediments at depth 35, 7 m below sea level (b.s.l.), an event dated at 11440 BP (HASAN, 2017). As the RSL rose, it caused a gradual formation of 27 m thick alluvial fan in the present Zrmanja River mouth located at the exit of the canyon. Present day floodplain of the Neretva River located at the river mouth extends landward over 10 km. Acoustic geophysical data shows similar floodplain formed approximately 60 km to the west, between islands Hvar, Šćedro and Korčula, at sea depth of 65-70 m b.s.l. Geological structures form acoustic basement of the sedimentary basin that predefined the accommodation space, as well as river path. Fluvial sediments with parallel to subparallel stratified reflectors onlap on the basin edges and overlie limestones. According to RSL curve (LAMBECK et al., 2011) fluvial sedimentation began approximately 13900 BP at 85 m b.s.l. After the basin was filled up to 70 m b.s.l., paleo-Neretva floodplain widened all the way to Korčula Island, creating a 10 km wide floodplain. Major erosional surface is visible, as Neretva River eroded its older fluvial sediments. Two meters of marine sediments overlie fluvial sediments. Cetina river mouth is located at the exit from the canyon in the town of Omiš. Seismic data reveal that during the LGM river did not flow in NW direction as a continuation of today’s flow but created a valley along the coast in SSE direction. In the postglacial period river eroded flysch sequences deposited on the steep hillslopes of Omiška Dinara mountain, and created over 15 m deep and 350 m wide river valley. Due to a Holocene RSL rise, channel was infilled with retrogradational fluvial sediments. Submerged bottom of the valley along Omiška Dinara mountain stands at 81 m b.s.l. Deposition of fluvial sediments started after 13800 BP (LAMBECK et al., 2011). As sediments gradually filled in the channel, Cetina created a 3 km wide floodplain. Thickness of fluvial sediment is 20 m. Fluvial sediments are overlain by 3-5 m thick marine sediment sequence. Nine acoustic units (marine, lake or fluvial) can be recognized on the geophysical data recorded in the Koločep Channel. During the LGM lowstand intensive erosion occurred in the central part of the basin. Sediments were transported by torrents, whose springs still exist today as submarine springs at depths of 45-50 m b.s.l. Torrents joined and formed a river flowing through Veliki vratnik passage in SW direction towards Mljet Island, and continued along it’s coastline towards the west. Eastern bank of the “Koločep” River was bounded by eolian sand bar. River ceased to exist as RSL rose to approximately 50 m b.s.l. Submerged valley between Pelješac and Mljet Islands is incised into sandy sediments and is well defined on bathymetry data. This work was supported by the Croatian Science Foundation Project “Lost Lake Landscapes of the Eastern Adriatic Shelf” (HRZZ-IP-2013-11-9419).

acoustic geophysical methods, submerged landscape, Holocene, Eastern Adriatic Coast

nije evidentirano