Naslov
Organski facijesi Dimić člana (Poštak,Hrvatska)
(Organic facies of Dimići member (Poštak Mt., Croatia))

Autori
Troskot-Čorbić, Tamara ; Veseli, Vladimir ; Vlahović, Igor ; Velić, Ivo ; Velić, Josipa Tomljenović, Bruno ; Cota Lilit ; Bejdić, Goran

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, prošireni sažetak, znanstveni

Izvornik
5. Hrvatski geološki kongres / Knjiga Sažetaka / Abstracts Book / Horvat, Marija ; Wacha, Lara - Zagreb : Hrvatski geološki institut, 2015, 268-269

ISBN
978-953-6907-50-2

Skup
5. Hrvatski geološki kongres

Mjesto i datum
Osijek, Hrvatska, 23.09.2015. - 25.09.2015

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
organski facijes, kimeridž, Poštak jedinica
(organic facies, Kimmeridgian, Poštak unit)

Sažetak
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, and 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. According to the detailed geochemical investigation the fine-grained, laminated limestones of Dimici member have an excellent generation potential and represent very good to excellent oil-prone source rocks. Organic matter content is between 0.35 and 11.9%, in average 2.97% TOC. Petroleum potential is good (between 1.17 and 67.00 mg HC/g rock, in average 18.25 mg HC/g rock). Organic facies AB/B (after Jones, 1984, 1987) is determined. It is characterized with high organic matter content, excellent hydrocarbon potential and high transformation ratio. Type II kerogen dominates. Occasionally, the type I kerogen is present as well. According to microsolubility and strong yellow to yellow-orange fluorescence effect, organic matter is a mixture of algal, bacterially degraded kerogen and migrated bitumen. Amorphous organic matter gradually changes from lamalginite to bituminite I and finally to organo-bituminous-mineral groundmass, i.e. to matrix bituminite. Solid bitumen is incorporated in all structural types. Phytoplankton is represented by prasinophycean phycomata and dinoflagellate cysts. Throughout the geological columns quantitative variations in cyanobacterial and liptinite/liptodetrinite content are microscopically observed. Variations in C27 and C29 regular sterane were documented as well. Bitumen reflectance is in the range from 0.15 to 0.30% Ro (asphaltite group). Organic facies AB/B is characterized with high organic sulfur content (up to 10%). Stable carbon isotope values are in range from 13C -27.79 to -24.65‰ in kerogen and from -29.53 to -25.55‰ in bitumen. According to maturity parameters (Tmax, PI, TAI, fluorescence color, vitrinite reflectance, bitumen reflectance, biomarker maturity parameters M/H, 22S/(22S+22R), Rc, Rm(MDR), 20S/(20S+20R), Ts/Tm), the organic facies AB/B, i.e. the source rocks of Poštak area generally reach onset of oil generation.

Izvorni jezik
Engleski

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