engleski

A deep structural borehole on Medvednica hilly zone - review of new data and possible reinterpretation of existing geological model

Location of new deep structural borehole was planned with aim to cover all lithostratigraphical units, from the youngest to the oldest one. According to preliminary geological profile, made for the purpose of Detailed Engineering Geology Map of Podsljeme Urban Area in scale of 1:5000, the thicknesses and depths of sediments were estimated as follows: Quaternary sediments (Q1) on 45 m, the upper part of Upper Pontian sediments (2M7) on 60 m, the lower part of Upper Pontian sediments (1M7) on 145 m, the Lower Pannonian sediments (M61) on 160 m, the Upper Pannonian (M62) sediments on 20 m, sediments of Sarmatian (M5) on 20 m and of Badenian (M4) on 110 m. The real thicknesses of deposits were measured after drilling and they differ in some way from those that have been estimated. Core inspection revealed all litostratigraphic units mentioned above, but with different thicknesses: Qaternary sediments with 10.2 m, the upper part of Upper Pontian sediments with 42.8 m, the lower part of Upper Pontian sediments with 127.0 m, the Lower Pontian sediments with 50.0 m, the Upper Pannonian sediments with 86.3 m, the Sarmatian sediments with 14.7 m and Badenian sediments with thickness of 19.0 m. Quaternary sediments are characterized by alteration of clayey silts and silts enriched on Fe-Mn concretions. At the depth of 9.6-10.1 m a presence of quartz pebbles was noted. The contact with sediments in the base is sharp and erosional. Similar sediments were found in the wider area of Medvednica hilly zone and their stratigraphical position was based on the analyses of pollen content. The upper part of Upper Pontian sediments is represented by yellowish to reddish brown and grey to dark gray clayey and silty sands. They are enriched on micas, and in places with Fe-Mn concretions. Within a certain intervals limonite (28.1-28.2 m) and calcite crust (13.0-16.6 ; 33.5-33.7 m) were noted. These sediments are usually horizontally laminated, but in places they are massive. They belong to the environment of delta which intruded into the shallow brackish lake. Stratigraphical age has been determined on the basis of fossils among which the most abundant were molluscs and ostracods. According to the name of most abundant mollusc, these sediments are also known as the “Rhomboidea deposits”. Lower part of Upper Pontian sediments is composed of silts rich with calcite and silty marls with typical grey or greyish colour. They are mostly massive although at the end of this interval they are characterized with horizontal bedding and in alteration with thin layers of sands. They are the result of sedimentation which took place in border conditions, deep to the shallow environment. Sedimentation circumstances were proved by the existence of certain macro and microfossils. At the boundaries between Lower Pontian and Upper Pannonian there are transition deposits which consist of dark gray massive marls. They were deposited in deep, brackish lacustrine environment and contain a rare plant particles and shell fragments. Sarmatian deposits are characterised as medium sorted grainstone. Except carbonates, the presence of quartz, quartzite and cherts with numerous fossil particles (benthic foraminifera, fragments of seaweeds, ostracods, molluscs, shells, etc.) was noted. A detailed mineralogical analysis revealed a large amount of smectite clay minerals. As a result of their specific crystal structure they are very sensitive to shrinking and swelling. Badenian sediments are represented with heterogeneous lithotamnium limestones which are predominantly composed of peloids with presence of biofragments, such as: shells of molluscs, ostracods, particles of seaweed and benthic foraminiferas. The size of the fragments is between 0.2-0.4 mm. Limestones are poorly sorted, recrystallised with micritic to microsparitic matrix. New structural borehole revealed real thicknesses of geological units and this new data is important in better understanding of the geological structure of Medvednica hilly zone.

Engineering geology map; Podsljeme urban area; litostratigraphy

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