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

• Main Authors: Ervin Mrinjek, Vili Pencinger, Jasenka Sremac, Boris Lukšić
• Format: Article
• Language: English
• Published: Croatian Geological Survey 2005-12-01
• Series: Geologia Croatica
• Subjects: Shelf; Shoreface; Offshore transition zone; Tempestites; Hummocky cross-lamination; Wave ripples; Oscillatory waves; Combined-flow currents; Trace fossils; Braidplain delta; Sequence stratigraphy; Northern Dalmatia; Croatia
• Online Access: http://www.geologia-croatica.hr/ojs/index.php/GC/article/view/193

<div>The Upper Eocene Benkovac Stone Member of the Promina Formation</div><div>of northern Dalmatia, Croatia, is a thinly bedded succession of</div><div>alternating carbonate sandstones and calcareous mudstones, ca. 40 m</div><div>thick, exposed as a narrow, SE-trending outcrop belt near the town of</div><div>Benkovac. This unit occurs in the middle part of the Promina Formation,</div><div>which is a spectacular calciclastic succession of deposits of late</div><div>Middle Eocene to Early Oligocene age, about 2000 m thick, showing</div><div>an upward trasition from deep-marine turbidites to shallow-marine</div><div>and alluvial deposits.</div><div>The sheet-like sandstone beds of the Benkovac Stone Member</div><div>are mainly 1&ndash;25 cm thick and have been classified into 6 facies and</div><div>3 subfacies, differing in stratification or showing various internal</div><div>sequences of stratification types. The thicker and most common beds</div><div>show plane-parallel stratification passing upward into hummocky</div><div>cross-lamination and undulatory to flat parallel lamination (Facies</div><div>S1), or consist of only the latter two divisions (Facies S2). Subordinate</div><div>beds show convolute stratification (Facies S3), are amalgamated</div><div>(Facies S4), or are homogenized and merely graded (Facies S6). The&nbsp;thinner beds have more uneven boundaries and show translatory ripple</div><div>cross-lamination (Subfacies S5a), climbing ripple cross-lamination</div><div>(Subfacies S5b) or pinch-and-swell lamination attributed to starved</div><div>and rolling-grain ripples (Subfacies S5c). The intervening mudstone</div><div>beds (Facies M) are silt-streaked and bioturbated. Trace fossils indicate</div><div>a combination of Zoophycos and Cruziana ichnofacies.</div><div>The sedimentary succession was deposited in a microtidal offshore</div><div>transition zone characterized by muddy &ldquo;background&rdquo; sedimentation</div><div>punctuated by discrete storm events. The observed spectrum of</div><div>tempestite sandstone beds represents a wide range of storm events,</div><div>varying in magnitude and in the mode of sand dispersal &ndash; from the</div><div>pure action of oscillatory waves to pure geostrophic currents. The</div><div>majority of tempestites are attributed to a combination of these two</div><div>end-member factors, with the geostrophic currents often enhanced by</div><div>a high load of sediment suspension (density-modified currents).</div><div>The Benkovac Stone Member is underlain by muddy offshore</div><div>deposits (Debelo Brdo Member) and covered by sandy to gravelly</div><div>shoreface deposits (Otavac Member), which in turn pass upwards into</div><div>braidplain deltaic and alluvial deposits. This regressive succession is</div><div>considered to be a parasequence deposited as a highstand systems tract</div><div>during a gradual, stepwise rise of relative sea level. The thick parasequence&nbsp;consists of progradational and retrogradational sets of much&nbsp;smaller parasequences, the record of which differs markedly in the</div><div>shoreface and offshore transitional part. The difference is attributed to</div><div>the underlying contrast in the physical factors controlling the supply</div><div>of sand to these shallow shelf zones.</div>