The Benkovac Stone Member of the Promina Formation: A Late Eocene Succession of Storm-Dominated Shelf Deposits

Abstract

The Upper Eocene Benkovac Stone Member of the Promina Formationof northern Dalmatia, Croatia, is a thinly bedded succession ofalternating carbonate sandstones and calcareous mudstones, ca. 40 mthick, exposed as a narrow, SE-trending outcrop belt near the town ofBenkovac. This unit occurs in the middle part of the Promina Formation,which is a spectacular calciclastic succession of deposits of lateMiddle Eocene to Early Oligocene age, about 2000 m thick, showingan upward trasition from deep-marine turbidites to shallow-marineand alluvial deposits.The sheet-like sandstone beds of the Benkovac Stone Memberare mainly 1–25 cm thick and have been classified into 6 facies and3 subfacies, differing in stratification or showing various internalsequences of stratification types. The thicker and most common bedsshow plane-parallel stratification passing upward into hummockycross-lamination and undulatory to flat parallel lamination (FaciesS1), or consist of only the latter two divisions (Facies S2). Subordinatebeds show convolute stratification (Facies S3), are amalgamated(Facies S4), or are homogenized and merely graded (Facies S6). The thinner beds have more uneven boundaries and show translatory ripplecross-lamination (Subfacies S5a), climbing ripple cross-lamination(Subfacies S5b) or pinch-and-swell lamination attributed to starvedand rolling-grain ripples (Subfacies S5c). The intervening mudstonebeds (Facies M) are silt-streaked and bioturbated. Trace fossils indicatea combination of Zoophycos and Cruziana ichnofacies.The sedimentary succession was deposited in a microtidal offshoretransition zone characterized by muddy “background” sedimentationpunctuated by discrete storm events. The observed spectrum oftempestite sandstone beds represents a wide range of storm events,varying in magnitude and in the mode of sand dispersal – from thepure action of oscillatory waves to pure geostrophic currents. Themajority of tempestites are attributed to a combination of these twoend-member factors, with the geostrophic currents often enhanced bya high load of sediment suspension (density-modified currents).The Benkovac Stone Member is underlain by muddy offshoredeposits (Debelo Brdo Member) and covered by sandy to gravellyshoreface deposits (Otavac Member), which in turn pass upwards intobraidplain deltaic and alluvial deposits. This regressive succession isconsidered to be a parasequence deposited as a highstand systems tractduring a gradual, stepwise rise of relative sea level. The thick parasequence consists of progradational and retrogradational sets of much smaller parasequences, the record of which differs markedly in theshoreface and offshore transitional part. The difference is attributed tothe underlying contrast in the physical factors controlling the supplyof sand to these shallow shelf zones.