engleski

Holocene sedimentation in front of Rijeka city

This paper deals with Holocene sedimentation and with coastal and sea-bottom changes in front of Rijeka city, northwestern coast of Rijeka Bay, channel part of the Adriatic Sea. Coastal area of Rijeka is a part of karst plateau which is mildly inclined toward the coast of Rijeka Bay. Bedrock is composed of Cretaceous and Paleogene carbonate rocks. The carbonate rock mass visible on the elevations is intensively fractured and karstified and therefore, in major part, highly water permeable. Low and even parts of the relief are covered by both sandy-gravely to sandy-silty alluvial deposits of Rječina river and rocky embankment. The sea-bottom of Rijeka Bay is mostly covered by Quaternary and recent deposits: closer to the coast with gravel and sand, and in deeper parts with quick mud. Coastal part of the relief was significantly changed by filling during construction of the port. Coastal area of Rijeka is abound in karstic springs. Groundwaters are flowing through deep karstic underground to spring zones by ways that are still not well known and are under influence of tidal changes. Medieval nucleus of the city is bordered by Potok valley with a stream that runs to the sewage system and Rječina river valley with a homonymous stream. Today, waters from springs and surface streams flow into sewage system or are freely infiltrated through embankment. Bottom of Rijeka Bay had become a local erosion base due to a sea-level lowering during the Würm. Channels of the ancient streams in Potok valley and on Rječina river- mouth, were cut in a carbonate bedrock up to 60 m deep. Surface streams were flowing toward lower situated ancient lakes. Because of an allocated coastal line, terrigenous sediments have been depositing far from the recent coastal line. That could explain the position of gravely alluvial fan in front of present Rječina river-mouth. Adriatic Sea started to flood Rijeka Bay again some 10 ka B.P. Lower parts of river valleys were submerged and karst estuary was formed due to a rapid sea- level rise. Marine sediments were found by drilling in Potok valley and Rječina river valley, several hundred meters from the recent coast. Continuous layer of fresh-water peat, found in Potok valley, is an indication of the maximum Holocene marine transgression. The age of 6400 ± ; ; yr B.P. was determined by radiocarbon analysis. Decelerated sea-level rise during the last 6000 years created conditions for filling of estuaries. Intensive sedimentation in microtidal estuary environment started from that period. Most of the sediments were brought over by erosion of Paleogene flysch rocks and Quarternary deposits from Grobničko polje. Minor part of sediments were brought over through a karst background and was deposited around springs that coastal part of Rijeka is abound with. Because of a small volume of sedimentation basin as well as intensive filling during the last millenniums, the estuary of Rječina river valley and Potok valley were completely filled up and sedimentation characteristical for delta had started. Transition from fluvial to marine environment occurred in relatively small area. In the area of the Rječina estuary, distribution channels with coarse grained sediments and swampy parts with fine grained sediments enriched with organic matter, were discovered. A greater granulometric diversity of particles can be seen in sedimentation body upstream from the present Rječina river-mouth and on the sea-bottom an enchanced uniformity. Currents along the coast moved sediments towards the north-west where they were more uniform comparing to the Rječina delta. Gravely fans discovered on the top of sedimentation bodies are proof of the periodical huge floods caused by changes in climatic conditions.

Holocene sedimentation ; Rijeka

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