Evolution of the Southern Margin of the Julian Basin with Emphasis on the Megabeds and Turbidites Seqence of the Southern Julian Prealps (NE Italy)

Abstract

From the Maaslrichtian up to the Early Eocene, the Julian (or Slovenian) Basin is characterized by a mixed siliciclastic/carbonale deposits system which exhibit complex sedimentalion patterns depending on various controls: sea level changes; tectonic movements in both carbonate platform and northern source areas; subsidence; proximity/distality to siliciclastic, calciclastic, allodapic (carbonate) sediment source areas; abundance of terrigenous detritus; paleomorphology of the slope of the Friuli Platform, the main source of carbonate detritus. The depositional sequences ofthe Julian Basin are the result of these controls and their interaction. Herein the examination of the stratigraphic sections is restrictedto the Middle Paleocene up to the Early Eocene ("Flysch di Masarolis"and "Flysch del Grivo"),where the most important carbonate megabedsare recognized. Fourteen stratigraphic sections of the "turbidites withmegabeds" sequence were examined (mainly along the N-S direction).All sections are located near the southern margin of the basin,at the foot of the slope of the north-eastern edge of the Friuli Platform.Comparison of log s revealed marked differenoes in the thicknessof the megabeds and of the interbedded calciturbidites and siliciclaslicturbidites. This could be ascribed to the proximity to the source areasof mega beds and to the abrupt stope morphology.By means of the litho-biostratigraphic analysis of the sedimeentswhich filled the Julian Basin, the provenances from the main sourceareas are outlined. The prevailing calciclastic detrilus (CE ,I) mixedwith allodapic carbonate sediments (CI) came from the South;siliciclastic detritus (N CE) mixed with other calciclastic detritus (CE,II) from N, NW as consistently indicated by paleocurrent directions;minor (and late) mixed carbonate sediments (CE, ill) came from NE.A rough sedimentary balance comprising megabeds, thick beds,multisource turbidites (siliciclastic, carbonate, mixed plus couplets)is calculated.An attempt to apply the eustatic sea level curve of Haq et al. (1987)to Maastrichtian-Paleocene and Eocene deposits of the Julian Basinis made. Lowstands of the Maastrichtian- Paleocene and of the LateYpresian are well recognizable. More problem s arise upon examiningthe "Flysch del Grivo" section which is slrongly controlled bytectonic mechanisms. The triggering mechanisms for mega bed emplacement in the Julian Basin is related to seismic activity along thesouthern border of the basin. In conclusion, the "turbidites withmegabeds" sequence is integrated within the framewot;k of the geological evolution of the Julian Basin.