Sedimentological analysis of the Oligocene Conglomerate from the Doblič Section near Kamnik

V osrednji Sloveniji zahodno od Kamnika, v Tunjiškem gričevju izdanja zaporedje terciarnih klastičnih sedimentnih kamnin, katerih bazo predstavlja oligocenski konglomerat. Opravljena je bila sedimentološka analiza oligocenskega konglomerata, z namenom določitve potencialnega sedimentacijskega okolja in izvora litoklastov. Posnet je bil detajlni sedimentološki profil Doblič v dolžini 63,5 m. Na posnetem profilu je bilo odvzetih 22 vzorcev za nadaljno mikrofacielno analizo. Določeni so bili struktura in sestava kamnin ter standardni mikrofaciesi značilnih litoklastov. Profil Doblič sestavljajo debelejše plasti srednje do debelozrnatih konglomeratov, ki se izmenjujejo s tanjšimi plastmi drobno do srednjezrnatih peščenjakov, meljevcev, meljev in glin. Kamnine so slabo do srednje sortirane in imajo polzrelo strukturo. Plasti so največkrat normalno ali inverzno gradirane s posameznimi erozijskimi kanali. Sedimentacija je potekalo v fluvialnem okolju rečnega sistema. Deli, kjer prevladuje izmenjavanje plasti konglomeratov in peščenjakov z normalno in inverzno gradacijo ter erozijskimi kanali, predstavljajo sedimentacijo v prepletajoči reki, deli, kjer prevladujejo peščenjaki, meljevci in gline pa predstavljajo sedimentacijo v meandrirajoči reki. V profilu je vidno menjavanje različnih delov rečnega sistema v depozicijski coni, od bolj proksimalnih delov prepletajoče reke z večjimi nakloni, do bolj distalnih delov meandrirajoče reke z nižjim gradientom in obratno. Prisotni so trije progradacijsko-retrogradacijski cikli. Mikrofacielna analiza je pokazala, da raziskane plasti sestavljajo predvsem karbonatni litoklasti in podrejeno roženčevi litoklasti ter mineralna zrna z lokalnim oz. regionalnim izvorom. Med karbonatnimi litoklasti je najbolj zastopan dolosparit, pomemben delež pa predstavljajo tudi globokovodni mikritni apnenci Slovenskega bazena in ostalih globljevodnih območij Južnih Alp ter plitvovodni apnenci, za katere sklepamo, da izvirajo iz kamnin Dinarske karbonatne platforme in Julijske karbonatne platforme iz časa triasa do vključno spodnje jure.In central Slovenia west of Kamnik in Tunjice Hills lies an outcrop of Tertiary clastic sedimentary rocks in which the base of the succesion is represented by Oligocene conglomerate. Sedimentary analysis of the Oligocene conglomerate was made to determine the potencial sedimentary environment and provenence of the characteristic lithoclasts. A sedimentary analysis of the 63,5 m long part of the succession – Doblič section was performed at which 22 samples were taken for the microfacies analysis. Structure and composition of the samples was analysed and standard microfacies was determined for characteristic lithoclasts. The Doblič section is made of thick layers of medium to coarse graded conglomerates, which alternate with thiner layers of fine do medium coarse sandstones, siltstones, silt and clay. Rocks are poorly to medium sorted and have submature texture. In most cases, beds in the profile are normally or inversely graded. The sedimentation took place in fluvial environment of the river system. Parts where conglomerates alternate with sandstones prevail and where normal and inverse gradations, as well as erosion channels are present, represent sedimentation in the braided river system, whereas parts where sandstones, siltstones and clays prevail represent sedimentation in the meandering river system. In profile, we can see changing of different parts of the river system in depositional cone, from proximal parts of the braided river with higher inclination to distal parts of the meandering river with lower gradient and vice versa. Three progradation-retrogradation cycles were recognized. Microfacies analysis show that majority of the analysed rocks are mainly made of carbonate lithoclasts and more rarely from chert lithoclasts and mineral grains of quartz with local and regional provenence. Among carbonate lithoclasts, most abundant are dolosparite grains, while deep-water micritic limestones of the Slovenian basin and other basins of the Southern Alps and shallow-water limestones, for which we assume they derive from Dinaric and Julian carbonate platforms from Triassic to Lower Jurassic, also represent the important part