Abstract
The up to 450 m-thick Upper Jurassic Lemeš Formation includes organic-rich deep-water (max. ~ 300 m) sedimentary rocks deposited in the Lemeš Basin within the Adriatic Carbonate Platform (AdCP). The Lemeš Formation was investigated regarding (1) bio- and chemostratigraphy, (2) depositional environment, and (3) source rock potential. A multi-proxy approach—microfacies, Rock–Eval pyrolysis, maceral analysis, biomarkers, and stable isotope ratios—was used. Based on the results, the Lemeš Formation is subdivided from base to top into Lemeš Units 1–3. Deposition of deep-water sediments was related to a late Oxfordian deepening event causing open-marine conditions and accumulation of radiolarian-rich wackestones (Unit 1). Unit 2, which is about 50 m thick and Lower early Kimmeridgian (E. bimammatum to S. platynota, ammonite zones) in age, was deposited in a restricted, strongly oxygen-depleted basin. It consists of radiolarian pack- and grainstones with high amounts of kerogen type II-S organic matter (avg. TOC 3.57 wt.%). Although the biomass is predominantly marine algal and bacterial in origin, minor terrestrial organic matter that was transported from nearby land areas is also present. The overlying Unit 3 records a shallowing of the basin and a return to oxygenated conditions. The evolution of the Lemeš Basin is explained by buckling of the AdCP due to ophiolite obduction and compressional tectonics in the Inner Dinarides. Lemeš Unit 2 contains prolific oil-prone source rocks. Though thermally immature at the study location, these rocks could generate about 1.3 t of hydrocarbon per m2 surface area when mature.
Highlights
The Jurassic-to-Paleogene Adriatic Carbonate Platform (AdCP: Vlahović et al 2005) is dominated by shallowwater carbonates
Rock–Eval pyrolysis was performed on a Delsi Rock Eval II + instrument to determine the amount of free hydrocarbons (HCs) (S1: mg HC/g rock), HCs generated during gradual heating (S2: mg HC/g rock), and the temperature of maximum HC generation (Tmax)
We propose the formation of the Lemeš Basin took place during Middle-to-Late Jurassic west-directed ophiolite obduction above the wider Adriatic plate (Gawlick and Missoni 2019 and references therein)
Summary
The Jurassic-to-Paleogene Adriatic Carbonate Platform (AdCP: Vlahović et al 2005) is dominated by shallowwater carbonates. Kimmeridgian-to-Tithonian deep-water (~ 200 to 300 m) carbonates, siliceous rocks, and black shales occur within the Lemeš and Gorski Kotar Basins (Fig. 1). The elongated southeast–northwest trending Lemeš Basin formed in the platform interior (see Fig. 1c) at the onset of intense block-faulting and subsidence at the northeastern AdCP margin during the early Kimmeridgian. This is believed to have been influenced by oceanic spreading in the Vardar Ocean (Bucković et al 2004, following Herak 1986). Normal-faulting and extensional subsidence resulted in the deposition of the deep-water bituminous limestones in this basin during a time of otherwise global eustatic sea-level fall (Bucković et al 2004; Haq et al 1988)
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