engleski

Sedimentary dynamics in the central part of the Adriatic Carbonate Platform from the Late Oxfordian to the Earliest Tithonian (Poštak Mt., Croatia)

During the Late Oxfordian and Early Kimmeridgian compressional tectonics formed Lemeš intraplatform trough in the area of todays Lika, Western Bosnia and Herzegovina and Northern Dalmatia, which had continuous connection with the open sea. Deeper marine 'Lemeš Beds' were for the first time described in 1909 by Schubert in his famous book 'Geology of Dalmatia' from the Lemeš locality on Svilaja Mt., and their Kimmeridgian to Early Tithonian age was determined on the basis of a very rich ammonite assemblage. During recent investigations the most complete succession of Kimmeridgian deeper marine deposits has been documented at the slopes of Mt. Poštak, where it is visible that deposits from the Lemeš locality correspond to the upper part of the complete succession. Therefore new informal stratigraphic unit Poštak is proposed, composed of three members: Rastičevo, Dimići and Lemeš. Deposits underlying Poštak unit are of the Oxfordian age, and are composed of thick-bedded light brown oncoid-peloid-bioclastic limestones composed of algal-balls oncoids, rounded intraclasts, peloids, gastropods, bivalves, small benthic foraminifera and abundant micrite matrix. These limestones were accumulated in the low-energy shallow subtidal. (A) Lower part of the Poštak unit, the Rastičevo member, is composed of alternation of light-brown thick-bedded limestones with rare bivalve and echinoderm bioclasts and thin intraclastic-peloid-skeletal intercalations. These deposits of probably Lower Kimmeridgian age were deposited in newly formed intraplatform trough connected to the open sea. (B) Middle part of the Poštak unit, the Dimići member, is characterized by alternation of light grey and greenish-grey limestones with radiolaria and dark grey, thin-bedded to laminated, mostly recrystallized radiolarian limestones enriched in organic contents. They usually comprise numerous greybrown chert layers and/or nodules with numerous radiolarians infilled by microquartz. Frequent fractures, fissures and microstylolites are commonly infilled by dark organic matter. Laminated limestones have significantly higher total organic contents than interbedded thicker light grey and greenish-grey limestone beds. These middle Kimmeridgian rocks were deposited in deepest parts of the intraplatform trough, and therefore are spatially restricted. (C) Upper part of the Poštak unit, the Lemeš member, is composed of well-bedded light grey to light brown limestones with rare chert nodules. They contain calcitized radiolaria, ammonites, aptichi, pelecypods, gastropods, echinoids, planktonic foraminifera, rare belemnites and finely dispersed or within stylolites and fissures concentrated organic matter. Towards the younger parts of the unit there are gradually more and more layers comprising material transported from the contemporaneous shallow-marine environments, and these layers are becoming thicker. Limestones of Lemeš member were deposited during the gradual infilling of the intraplatform trough, but planktonic fauna, especially relatively frequent layers with ammonites of Upper Kimmeridgian to Lower Tithonian age, indicate still preserved connection with the open sea. Overlying Tithonian deposits are not present at the Poštak Mt., since Poštak unit limestones are in tectonic contact with the Lower Cretaceous deposits – wide contact zone is infilled by polymictic carbonate breccia corresponding to Velebit (‘Jelar’) breccia.

Jurassic, Adriatic carbonate platform, Lemeš

nije evidentirano