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Lithofacies, biostratigraphy and discontinuity surfaces recorded in deposits across the Pliensbachian–Toarcian transition (Lower Jurassic) in southern Lika and Velebit Mt. (Croatia)

The Pliensbachian–Toarcian transition and Early Toarcian are marked by major paleoenvironmental changes, which correspond to biotic crisis recorded in deep and shallow marine Tethyan environments, as well as in continental environments (MAILLIOT et al., 2009 and references therein). This study was carried out on two Lower Jurassic sections in southern Lika and Velebit Mt. in order to reconstruct the environmental changes occurred in a time interval spanning the Late Pliensbachian and the Early Toarcian. Carbonate deposits encompassing the youngest Lithiotid limestones and the oldest intensely bioturbated mud-rich limestones also known as “spotted limestones” have been studied for analyses of litho- and biofacies, biostratigraphy, and discontinuity surfaces. Two sections crop out in the areas of Southern Lika (Štikada quarry ; St section, 40 m thick) and Velebit Mt. (Mali Alan pass ; MA section, 46 m thick). The assemblage of benthic foraminifera indicates a latest Pliensbachian–earliest Toarcian age of the studied sections (cf. VELIĆ, 2007). Most important species are Pseudocyclammina liasica found throughout both sections and Socotraina serpentina with FAD 3.5 m below first beds of typical “spotted limestones”. Careful correlation with the data published in SABATINO et al. (2013) confirmed the established age. The recorded succession consists of several lithofacies units: (1) occasionally burrowed micritic limestones rich in benthic foraminifera, (2) dark grey micritic limestones with lithiotid shells and worn skeletal grains, (3) thin clayey–marly beds in alternation with argillaceous limestones with intensely burrowed horizons and pelagic influence, (4) micritic limestones with lithiotid shells, worn bioclasts and marine omission surfaces, (5) thin clayey–marly beds alternating with unfossiliferous argillaceous limestones, and (6) burrowed micritic limestones with worn bioclasts (typical “spotted limestones”). First unit is recorded only in Štikada quarry (coeval part at Mali Alan is covered) and fifth unit is present only in Štikada quarry. (1) The unit is built of thinly- and medium thickly-bedded occasionally burrowed micritic limestones (mudstone, wackestone and floatstone) rich in small (miliolids, glomospiras, valvulinids ; Agerina martana, Siphovalvulina variabilis, S. colomi, Nodosaria sp., Sieberina sp.) and larger (Amijiella amiji, Paleomayncina termieri, Planisepta compressa, Everticyclammina praevirguliana, P. liasica) benthic foraminifera, with common bivalve, brachiopod and echinoderm fragments, and rare fragments of alga Palaeodasycladus mediterraneus. Lithiotid shells are present in lower part of this 11 m thick unit. It contains several intensely burrowed surfaces, which coupled with dark grey peloids and mostly worn skeletal grains indicate deposition in low to moderate water energy subtidal open carbonate platform environments rarely shallowing to the shallow subtidal and intertidal zone (cf. FLÜGEL, 2004). (2) The 13 m thick unit is built of medium and thickly bedded micritic limestones with common lithiotid shells in MA section, and thinly to medium thickly bedded bioclastic wackestone with abraded and worn skeletal grains of echinoderms and bivalves (lithiotid shells are present only in the lower part of the unit) in Štikada quarry. Foraminifera are represented by the same species found in the underlying unit, but are less numerous. This unit records the last occurrence of alga P. mediterraneus. Depositional environment of low water energy protected carbonate platform interior is proposed for the Mali Alan area. However, worn skeletal grains, common small hyaline foraminifera and A. martana, common dark brown peloids, pronouncedly dark grey colour of the limestones, occasional burrowing, one bed surface with small erosional channels, and occasional intraclasts on bed contacts indicate deposition within low to moderate water energy deeper subtidal environments with occasional storm influence. Upper boundary of this unit in the Štikada quarry is marked by a brecciated surface with angular cm-sized intraclasts in micrite matrix with worn bioclasts and small subangular to rounded brown to orange intraclasts. (3) Third unit clearly differs from the underlying units: in the Štikada quarry it is 9 m thick and built of alternation of thin-laminated clayey–marly beds and pronouncedly dark grey laminated medium thickly bedded argillaceous limestones. Benthic foraminifera are almost completely absent. Filaments, rare ostracods and pelagic crinoids are found in the lower part of the unit, while bioclasts of bivalves, brachiopods, echinoderms, and pelagic crinoids are common in the upper part. Clayey–marly beds and argillaceous limestones are organized in sequences in which the limestone beds are thickening and the clayey–marly beds thinning upwards. Uppermost part of the unit is devoid of clayey–marly beds. Instead, it contains several intensely burrowed and bored surfaces pointing to several periods of non-deposition or low sediment accumulation rate. Upper unit part at Štikada correlates well with similar burrowed and to a lesser extent bored surfaces at Mali Alan. Lower part of the third MA unit is covered. Depositional environment of deeper, low water energy open platform interior with pelagic influence is proposed for Mali Alan, while alternation of laminated clayey–marly beds and argillaceous and bioclastic limestones, intense burrowing, worn bioclasts and intraclasts, pelagic fossils, small slump structures indicate mid-ramp environment for Štikada section (FLÜGEL, 2004). (4) The unit is built of micritic bioclastic limestones with lithiotid shells abundant in the whole MA unit and in the lower part of the St unit. Lithiotid limestones are very poor in other fossils. Upper parts of both units are characterized by resedimented peloidal-intra-bioclastic packstone-grainstone with worn bioclasts, fragments of echinoderms and pelagic crinoids, common small agerinas and hyaline foraminifera (generally with species similar to second unit), with addition of FAD of Socotraina serpentina, which was firstly described from the late Early Jurassic carbonates of Yemen and United Arab Emirates (BANNER et al., 1997). The upper boundary of this unit is placed at pronounced burrowed and bored surface (marine hardground) in Štikada section, which can be correlated with similar, but less intensely burrowed surface at Mali Alan. For Mali Alan depositional environment low to moderate water energy open platform interior with pelagic influence is proposed, while litho- and biofacies, and several brecciated fractured beds at Štikada point to deposition on the middle part of the carbonate ramp. (5 & 6) At Mali Alan fourth unit is overlain by burrowed peloidal microbioclastic wackestone with worn bioclasts (typical “spotted limestones”), while in the Štikada quarry fourth unit is in sharp contact with a 1.4 m thick unit of thin clayey–marly beds alternating with partly recrystallized laminated argillaceous mudstones with very rare filaments, which pass upwards into typical “spotted limestones” with peloids, filaments, echinoderms, and small foraminifera. Clayey–marly beds and argillaceous mudstones sharply overlaying marine hardground imply deeper marine carbonate ramp environments for Štikada comparing to Mali Alan, with periods of sea-floor exposure and condensed sediment accumulation before establishment of conditions favourable for the life of suspension and mud-eaters. Vertical upward change of litho- and biofacies points to general deepening of the depositional environment and possibly partial drowning of the studied part of the carbonate platform caused by synsedimentary tectonics, the relative sea-level rise during the early Toarcian and slowed carbonate production due to the biotic crisis. Interpretation of the middle carbonate ramp as depositional environment for Štikada section is also supported by common marine omission surfaces, which are typical for ramps (cf. HILLGÄRTNER, 1998 ; CHRIST et al., 2012). Thin interval of sedimentary record with discontinuity surfaces in the upper part of third unit recorded and correlated in both studied sections coincides with the position of the Pliensbachian–Toarcian boundary as proposed by SABATINO et al. (2013) and possibly indicates a hiatus around this boundary. The first record of foraminiferal species Socotraina serpentina and exact stratigraphic position of its first occurrence enabled correlation of the two studied sections. The results of this detailed litho- and biofacies study of carbonate deposits in the area of southern Lika and Velebit provided more detailed insight into the paleoenvironmental changes that occurred within the studied part of the carbonate platform during the Late Pliensbachian–Early Toarcian transition. BANNER, F.T., WHITTAKER, J.E., BOUDAGHER-FADEL, M.K. (1997): Socotraina, a new hauraniid genus from the Upper Lias of the Middle East (Foraminifera, Textulariina). Rev Micropaléontol, 40, 115–123. CHRIST, N., IMMENHAUSER, A., AMOUR, F., MUTTI, M., TOMÁS, S., AGAR, S.M., ALLWAYS, R., KABIRI, L. (2012): Characterization and interpretation of discontinuity surfaces in a Jurassic ramp setting (High Atlas, Morocco). Sedimentology, 59, 249–290. FLÜGEL, E. (2004): Microfacies of carbonate rocks, analysis, interpretation and application. Springer, Berlin Heidelberg, New York, 976 p. HILLGÄRTNER, H. (1998): Discontinuity surfaces on a shallow-marine carbonate platform (Berriasian, Valanginian, France and Switzerland). J Sediment Res, 68, 1093–1108. MAILLIOT, S., MATTIOLI, E., BARTOLINI, A., BAUDIN, F., PITTET, B., GUEX, J. (2009): Late Pliensbachian–Early Toarcian (Early Jurassic) environmental changes in an epicontinental basin of NW Europe (Causses area, central France): A micropaleontological and geochemical approach. Palaeogeogr Palaeoclimatol Palaeoecol, 273, 346–364. SABATINO, N., VLAHOVIĆ, I., JENKYNS, H., SCOPELLITI, G., NERI, R., VELIĆ, I., PRTOLJAN, B. (2013): Carbon-isotope record and palaeoenvironmental changes during the early Toarcian Oceanic Anoxic Event in shallow-marine carbonates of the Adriatic Carbonate Platform in Croatia. Geol Mag, 150, 1085–1102 VELIĆ, I. (2007): Stratigraphy and palaeobiogeography of Mesozoic benthic foraminifera of the Karst Dinarides (SE Europe). Geol Croat, 60, 1–113.

Upper Pliensbachian; Lower Toarcian; lithofacies; discontinuity surfaces; Socotraina serpentina; Lika

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