Lake Pannon sedimentation model in the Legrad field area (NW Croatia)

The Neogene to Quaternary Lake Pannon sedimentary successions of the SW part of the Pannonian Basin are characterized by three 2nd order megacycles divided by major regional unconformities. In here described sediments are related to the transtensional post-rift thermal basin subsidence and belong to the second megacycle between the Base Pannonian to the Base Pliocene unconformity For the interpretation of the genesis and evolu-tion of the wider depositional area around the recent gas field Legrad in the NW Croatia inter- disciplinary approach were used. Information on lithology, ichnology, sedimentary structures and structural elements were obtained from complete macroscopic examination of cores. Petrophysical and calcimetry measurements, detailed petrographical (and microporosity) and palynological analyses were done. Geochemical evaluations were considered. All available well data were correlated and calibrated with well log and seismic data. Paleorelief features (like channel position and their direction) are visible on coherency time-slices.Investigated deposits and related cores are presented by alteration of sandstone, siltstone and marls. Sandstones, generally lithic arenites, are very well cemented with intergranular porosity. They are moderately sorted, with grains in the range from very fine to medium sand size. Sandstones are sporadically rich in organic matter. Siltstone is also well cemented. Marls occasionally contain smaller amount of siliciclastic component and pyrite occurrences. Open and reduced microfractures are sometimes visible. Sandstones are largely homogenous and thick to several meters. Horizontal and cross- lamination are detected only in uppermost part of the studied sedimentary succession (Lower Pontian deposits). Soft sediment deformation structures like load casts, ball-and-pillow and flame structures are relatively frequent. Rip- up marl clasts are sporadically observed suggesting channel sedimentation. Subvertical bioturbation is common. Terrigenous organic matter (phytoclast debris) are ubiquitous. Palynological macerals of the cored sedi-ments in the lower part of the investigated interval are rich in organic contents. Most of the palynofacies (70–80 %) is amorphous matter. Liptinite kerogene, generally made of weakly diverse dinocyst assemblage, some pollen grains (mostly bisaccate) and a few green algae remnants (Botryococcus brauni), completed the organic residue (10–20 %). Lignohumine clasts are scarce. Macerates are biodegraded. Pyrite inclusions in palynomorphs are frequent. Index fossils Spiniferites bentorii and S. bentorii oblongus specify the Upper Pannonian age. By the biostratigraphical classification of the organic walled microphytoplankton, samples belong to Spiniferites bentorii main zone, to the Spiniferites bentorii oblongus subzone. Younger sediments, although somewhat different, are also rich in organic matter. Lignohumine clasts of variable sorts and size make 30–60 %, while amorphous matter makes 30–40 % of total organic matter content. Liptinite component, composed of dinocysts, some freshwater alga (Pediastrum simplex, Spirogyra sp.), various polled grains (chiefly bisaccate) and a few spores, make 10–20 % of palynofacies. The Lower Pontian age is confirmed by Spiniferites balcanica, S. bentorii coniunctus and S. validus. According to the dinocyst palynozonation, samples belong to Spiniferites balcanica main zone, to the Spiniferites bentorii coniunctus subzone. Sediments originate in lacustrine environment as channel (and interchannel) deposits and subaquatic fans caused by gravity flow mechanisms (mostly turbidites). The Late Pannonian sedimentation took place in slightly deeper environment of decreased salinity, moderately distant to shore and terrigenous influx. The Early Pontian sediments were created in also somewhat distal and deeper freshwater environment but in the zone of the significant terrigenous influence.