Sequence stratigraphy of Eocene incised valley clastics and associated sediments, Island of Rab, northern Adriatic Sea, Croatia

Abstract

Clastic sediments of Middle–Late Eocene age were studied on the Island of Rab (northern Adriatic Sea, Croatia) in order to reconstruct their depositional history, depositional environments, and geometry of sandstone bodies. Detailed outcrop logging and mapping revealed the response of depositional systems to frequent relative sea-level changes, which initiated significant basinward and landward shifts of facies, respectively. Tidal sandstones are commonly underlain by shoreface sandstones, and overlain by offshore sandy marls, whereas the latter are again overlain by shoreface sandstones. Major relative sea-level falls initiated basinward shift of depositional systems and the incision of incised valleys or estuaries, and consequently truncated the underlying shelf sediments. In some cases, the accelerated sea-level fall caused rapid shoreface progradation which is interpreted as a forced regression. Relative sea-level rise caused flooding of the incised relief, and deposition of tidal sandstone bodies which overlie type-I sequence boundaries. The coarse lag sediment of these sequence boundaries locally disappears laterally, and the boundary is granulometrically less prominent. All of the major bounding surfaces have been recognized in the sections studied, although the maximum flooding surface is recognized as a thin “zone” instead of a single surface. Altogether, 28 complete sequences, and 15 parasequences are recognized in the informal unit of the Lopar sandstones, documenting the depositional response to high-frequency relative sea-level oscillations. They have so far not been recognized in the Eocene of the eastern Adriatic region.