Geologia Croatica (Nov 2010)
A Middle Jurassic Radiolarite-Clastic Succession from the Medvednica Mt. (NW Croatia)
Abstract
On the NW part of Medvednica Mt. radiolarites with carbonateolistoliths, shales and siltites, matrix-supported conglomerates andbasic volcanic rocks were investigated. This facies association isinformally named the Poljanica unit. Major element geochemical dataindicate deposition of radiolarites in the vicinity of the middle oceanicridge, while sedimentological data indicate deposition in an areacloser to the continent. Shales and siltites, as well as matrix-supportedconglomerates, were deposited in short periods characterised byincreased input of terrigenous material. Matrix-supported polymictconglomerates are composed of silicified shales, lithic graywackes,cherts and metabasalts, and were deposited by debris flow mechanismsas a consequence of synsedimentary tectonic activity. Carbonateolistoliths are composed of biomicrosparite, and jointly withdeformed radiolarian cherts compose an olistostrome. Basic volcanicrocks represent high-Ti tholeiitic basalts formed in the MORB realm.Micropalaeontological investigation of radiolarite samples provedthe Middle Jurassic (latest Bajocian - early Bathonian to lateBathonian - early Callovian) age of the Poljanica unit. Additionally, anew radiolarian species Theocapsomma medvednicensis n.sp. hasbeen described. Conodont analyses from carbonate olistoliths in radiolarites proved their Triassic age.The investigated radiolarite-clastic succession is the result of subductionprocesses. Further continuation of this process caused incorporationof these deposits into the accretionary prism, where theywere brought in direct contact with Triassic volcanic rocks and radiolarites(in the form of a tectonic mlange).Based on the lithological similarities with the Middle Jurassicturbidite-olistostrome successions in the Western Carpathians andNorthern Calcareous Alps, the study area is considered to be part ofthe Meliata-Hallstatt Ocean.
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