Changing depositional environments in the semi-restricted Late Jurassic Lemes Basin (Outer Dinarides; Croatia)

The up to 450 m-thick Upper Jurassic Lemes Formation includes organic-rich deep-water (max. similar to 300 m) sedimentary rocks deposited in the Lemes Basin within the Adriatic Carbonate Platform (AdCP). The Lemes 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 Lemes Formation is subdivided from base to top into Lemes 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 Lemes Basin is explained by buckling of the AdCP due to ophiolite obduction and compressional tectonics in the Inner Dinarides. Lemes 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 m(2) surface area when mature.

Automated summary

The Upper Jurassic Lemes Formation in the Adriatic Carbonate Platform is a 450 m-thick organic-rich deep-water sedimentary rocks deposited in the Lemes Basin. The formation was studied using various proxy methods to analyze its bio- and chemostratigraphy, depositional environment, and source rock potential, revealing different units representing different depositional environments and oxygen conditions. The evolution of the Lemes Basin was attributed to tectonic events in the Inner Dinarides, with Unit 2 identified as a prolific oil-prone source rock.