engleski

Facies Diversity and Environmental Significance of Aptian-Albian Platform Deposits of Mt. Svilaja (Dalmatia, Groatia)

Within the Lower Cretaceous (Aptian-Albian) carbonate platform deposits of Mt. Svilaja, two chronostratigraphically equivalent successions were analyzed – Milešina and Jarebinjak, situated on the southern and southeastern slopes, respectivelly (Cvetko Tešović et al., 2003). They are only 7 km apart from each other but differ in facies composition and in Aptian emersion features. Both localities reveal similar microfacies characteristic of the Lower Aptian interval, but differ from each other in thickness. Both are composed of wackestones, oncoid-bioclastic floatstones with Bacinella irregularis Radoičić and requieniid rudist fragments. Characteristic species include Praeorbitolina cormyi Schroeder, Palorbitolina lenticularis (Blumenbach), and Voloshinoides murgensis Luperto Sinni & Masse, as well as Novalesia distorta ArnaudD-Vanneau, Debarina hahounerensis Fourcade et al., Spiroloculina cretacea Reuss, Istriloculina cf. elliptica (Iovcheva), Rumanoloculina robusta (Neagu) and very rare Salpingoporella dinarica Radoičić. However, at the Jarebinjak locality, as distinct from the Milešina locality, at the beginning of the Lower Aptian a species-rich rudist assemblage occurs embedded within bioclastic floatstones. At weathered surface caprinid, caprotinid and probably monopleurid transverse sections can be recognized. Unfortunately, the caprinid thick inner shell layers that include diagnostic pallial canals were originally aragonitic and are thus frequently heavily recrystallized. Nevertheless, a few caprinid transverse sections could be referred to Offneria nicolinae (Mainelli), O. cf. murgensis Masse, O. cf. Interrupta (Paquier), caprotinids to Glossomyophorus costatus Masse et al., and monopleurids to Agriopleura indicating an Early Aptian age (Masse, 2003). The caprinids assemblages as well as the thicker Lower Aptian interval, indicates an influence of open sea circulation and a more pronounced accommodation space, respectively, probably as a result of synsedimentary tectonic. On the Milešina locality, several metres thick sequence of algal wackestones, peloid-miliolid packstone-grainstones and peloid-miliolid wackestones with very frequent Salpingoporella urladanasi Conrad et al., S. hasi Conrad et al., and rare S. dinarica Radoičić, as well as foraminiferal taxa ; Pseudonummuloculina aurigerica Calvez, Glomospira urgoniana Arnaud-Vanneau, Istriloculina granumtrici Neagu. Sabaudia minuta (Hofker), Vercorsella scarsellai (De Castro), V. immaturata He and V. laurentii (Sartoni & Crescenti), directly overlie certain Lower Aptian strata. Regressive trend and subaerial exposure features, which are usually characteristic for this horizon elsewhere in the Karst Dinarides area (Velič et al., 1989), occur here too. The regressive trend is marked by a thin layer of breccias and/or frequent charophytes, which reflect a change in environmental condition ; an increasing fresh water influence. Contrary to that, at the Jarebinjak locality that horizon exhibit more distinct regressive trends and 7 to 8 thin beds of clay and marl associated with subaeral exposure features. The limestone beds within this occasionally emergent horizon are characterized by algal wackestones with common to dominant Salpingoporella dinarica Radoičić, Orbitolina (Mesorbitolina) parva Douglass, O. (M.) texana (Roemer), Pseudonummoloculina aurigerica Calvez and Glomospira urgoniana Arnaud-Vanneau. Above the emergence horizon, the Upper Aptian? – Lower Albian succession at both localities comprise micritic limestones with variable amounts of calcareous mud, pellets, peloids and skeletal grains (rare miliolids and ostracods). They irregularly alternate with thin layers of peloid-intraclastic-skeletal packstone– grainstones and peloid wackestone– packestones with fragments of molluscs (usually gastropods) and benthic foraminifera ; Orbitolina (Mesorbitolina) texana (Roemer), Vercorsella scarsellai (De CAastro), V. cf. immaturata HE, Sabaudia minuta (Hofker), Cuneolina sliteri Arnaud-Vanneau, Istriloculina cf. granumtrici Neagu, Debarina cf. D. hahounerensis Fourcade et al., and others.

Facies; Lower Cretaceous; Caprinids; Aptian emersion; Mt. Svilaja; Croatia

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