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Large benthic foraminifers as proxies of bathymetric changes in the middle to late Eocene Dinaric Foreland Basin (northern Dalmatia, Croatia)

This combined micropalaeontological and sedimentological study is based on the middle to late Eocene deposits in the Novigrad sub-basin, one of several SE-trending narrow synclinal troughs within the evolving thrust wedge-top Dinaric foreland basin of northern Dalmatia. Deposits of the subbasin SW limb indicate repetitive clastic and carbonate-ramp sedimentation, apparently driven by the emergence and submergence phases of the trough-bounding blind-thrust growth anticline.Phases of clastic sedimentation involved progradation of a wave-dominated gravelly shoreline (foreshore) facies over a sandy shoreface covering heterolithic sand-mud offshore transition and muddy offshore deposits. The offshore-transition and offshore deposits are commonly intercalated with foreshorederived, gravelly debris-flow and slump deposits indicating pulses of gravitational resedimentation, probably triggered by an excessive uplift and/or normal faulting of the anticline flank. Phases of biogenic carbonate sedimentation, attributed to the anticline submergence, were dominated by the accumulation of large benthic foraminifers (nummulitids, operculinids, orthophragminids, aborescent foraminiferids, asterigerinids, rhapydionids and alveolinids) and their fragmented tests along with bryozoans, echinoids, corals and red-algae debris. The composition of foraminifer assemblages, their detailed systematic and functional morphologic analysis, the preservation degree of their tests (varying from poor to good, including micritization and overgrowth by either foraminifers or algae) and the abundance of certain morphotypes (limestones, mostly wackestones and packstones, with a matrix composed of fine bioclastic hash) are thought to indicate a carbonate ramp depositional setting. The carbonate depositional system probably comprised an array of nummulite banks formed close to the fair-weather wave base, possibly with inner ramp lagoons and an open-marine middle/outer ramp zone. The foraminifers indicate a Priabonian age of the carbonate deposits in the Novigrad sub-basin, which highlights the high frequency of carbonate/clastic environmental changes. The demise of large benthic foraminifers in each carbonate unit seems to be a result of the environment shift from low-nutrient oligotrophic and euphotic conditions – favourable for shallow-water benthic foraminifera – to increasingly eutrophic conditions due to the increase of erosion and input of terrigenous nutrient associated with a relative sea-level fall. The carbonate units on the synclinal sub-basin flank are thus interpreted to be transient carbonate ramps developed during short-term relative sea-level rises and terminated by relative sea-level falls. The short-term changes between clastic and carbonate sedimentation on the sub-basin flank may be due to a combination of the 4th-order (Milankovitch) eustatic cycles and the tectonic pulses of structural evolution of the wedge-top foreland basin.

Large benthic foraminifers; middle to late Eocene; Dinaric Foreland Basin; Croatia

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