engleski

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

The Upper Eocene Benkovac Stone is an approximately 30 m thick thin bedded succession that outcropps in a very narrow and long zone in the vicinity of Benkovac town. This succession is a part of the Promina beds, an aproximately 2000 m thick carbonate-clastic succession of late Middle Eocene to Early Oligocene age with a clear shallowing-upward trend - from deep-marine to shallow-marine and alluvial deposits. Benkovac Stone is characterized by the intimate interbedding of sandstones and fine– grained sediments. Detailed study of logs identified 6 sandstone facies: parallel laminated to hummocky-cross and flat laminated sandstone, hummocky cross to flat laminated sandstone, convolute laminated sandstones, amalgamated sandstones, normal graded thin sandstones, wave - rippled sandstones with 3 subfacies (sandstones with migrating wave ripples, sandstones with climbing wave ripples and sandstones with pinch and swell lamination). Fine-grained facies consist of mudstones and thin, from few mm to 1 cm thick silty streaks. The streaks form vary from flat to undulatory and lens- or pinch-swell like. They were deposited under the influence of waves, geostrophic currents and density-induced flow which have been changing and have been various during predepositional and depositional phase in the shore area, above and below the storm wave base. Due to the great numbers of available processes and factors, simple and straightforward interpretation of facies origin is not possible. However, on the basis of detail outcrop analysis and interpretation founding on complex hydraulic setting it is possible to achieve the following conclusions: (1) there are no features that can infer deposition from pure processes. On the other hand all sandstone beds (except thin normal graded beds) display features that suggest more or less influence of wave oscillations. Therefore various intimately combined currents have been involved in sands deposition ; (2) deposition in an open and broad shallow-marine (shelfal) micro-tidal environment was strongly influenced by relatively frequent storm processes ; (3) the major part of Benkovac Stone layers was deposited in offshore-transitional zone (between fairweather wave base and stormweather wave base), whereas considerably smaller part of layers (moderately bioturbated mudstones interbeded by thin normal graded sandstone beds and silty streaks) was sedimented in offshore zone (below storm wave base) ; (4) shelf environment was not amply, but steadily fed by sands and finer material.

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

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