DBPapers
DOI: 10.5593/SGEM2014/B11/S1.043

THE INTERNAL STRUCTURE OF THE THIRD TERRACE OF THE RIVER GAUJA AND IMPLICATIONS OF ITS INTERPRETATION ON PALEOHYDROLOGICAL RECONSTRUCTIONS

M. Krievans
Wednesday 1 October 2014 by Libadmin2014

References: 14th International Multidisciplinary Scientific GeoConference SGEM 2014, www.sgem.org, SGEM2014 Conference Proceedings, ISBN 978-619-7105-07-0 / ISSN 1314-2704, June 19-25, 2014, Book 1, Vol. 1, 325-332 pp

ABSTRACT
The main outlines of the drainage network in Latvia results from the retreat of the last Scandinavian Ice Sheet. Studies in evolution of the largest river valleys and their comprehensive geomorphological analysis started in sixties and early seventies of the previous century. Some later major focus was directed on modern alluvial processes. Geological and geomorphological research in smaller valley forms was not systematic and was paid less attention. Only in the last decade an analyses of smaller river terraces, channel profiles and litofacies in the lower reaches, particularly in their junction with a main river are proceed. This paper presents series of results which were obtained from several outcrops of the River Gauja valley span, known as the Gauja spillway. Field observations were combined with mapping of terrace levels, gullies and tributaries. In order to understand the internal structure of terraces hand-drilled boreholes were made, lithological composition and textures of sediments were studied, and lithofacies analysis of the sediment units was performed in outcrops. The detailed environmental reconstruction was based on all available data combined into lithofacies description and interpretation. Lithological composition, textures and facies analysis of some outcrops give evidence on accumulation of sediments in basin, supposedly in glaciolacustrine environment but not alluvial origin as it was interpreted in previous studies. Still uncertain is question about genesis of River Gauja terrace III. In previous studies both highest terraces of the lower complex (terrace III and II) were correlated to levels of the stage Bgl II and phase Bgl IIIb of the Baltic Ice Lake. Sediment depositional environment was interpreted as oxbow lake and floodplain members. The latest studies of the outcrop exposing internal structure of the “riser” of terrace III on the right bank of the river north of the farmhouse “Dukuļi” testifies sediment deposition in palaeobasin contacting with dead ice. Such interpretation is also supported by evidence of supraglacial till lenses located in the lower part of the outcropped section. Only upper part of the cross section testifies sediment accumulation environment as alluvial or alluvial-lacustrine which was produced as a result of the water drainage from meltwater basin. After palaeobasin leaking, the River Gauja valley cutting and erosion terrace formation started. According to geological and geomorphologic evidences, then river cut and its bed gradually narrowed.

Keywords:Fluvial architecture, Alluvial deposits, Late-glacial, geomorphology

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