--> --> Reassessing the Late Cretaceous geological history of the Ionian Basin, western Greece

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Reassessing the Late Cretaceous geological history of the Ionian Basin, western Greece

Abstract

The Ionian Basin is part of the external Hellenides and is bounded by the Ionian thrust to the west and the Gavrovo Thrust to the east. The Ionian Basin follows the evolutionary stage of a stretched lithospheric setting and therefore, is filled with sediments that belong to the pre-, syn- and post-rift phases. The post-rift phase can be separated into two sub-phases, the Early Cretaceous sub-phase, with the Vigla limestones and shales, and the late Cretaceous to Eocene sub-stage, with breccia, microbreccia and calciturbiditic units. These carbonates are proven good reservoir rocks in the Katalokon oil field, as well as in the Ionian zone of Albania and in the central and southern Adriatic offshore of Italy. These Upper Cretaceous (Senonian) deposits are 200-400 m thick and consist of calciturbidites, with globotruncanids and rudist fragments, as well as micro brecciated intervals with rudist fragments. NNW-SSE directed normal faults are responsible for intense tectonic activity during the syn-rift stage. Further, younger synthetic and antithetic normal faults (NNW-SSE directed) that formed during the early stage of the post-rift phase produced active margins, horsts and asymmetrical half-grabens. Due to the asymmetry of the sub-basins and high variations in water depth, strong currents were produced developing chert condyles. The active uplifted shoulders of these half-grabens produced significant amounts of sediment available for accumulation, whereas the steep slope gradients led to instability and slumping. The Ionian Basin is clearly dissected into half-grabens that are dissimilar in terms of water depth and geometry. As such, they are filled with sedimentary piles of strikingly different thickness. The intense tectonic activity persisted during the late stage of the post-rift phase, but gradually this activity was reduced, from Paleocene to Eocene. During the late stage of the post-rift phase the syn-sedimentary synthetic and antithetic faults produced horsts (platforms) internally to the Ionian Basin, with shallow waters depths. As a result, reefs were developed and subsequent erosion part of these reefs led to the accumulation of microbreccia/breccia in the uppermost part of the upper Cretaceous formation. The upper Cretaceous deposits were also influenced by syn-sedimentary transfer faults. These transfer faults produced negative and positive flower structures, with strong deformation of sediments during the sedimentation. Roll-over structures with pseudo-anticline geometries were formed, internally to the sedimentary sequence. The regions influenced by the transfer faulted zones were strongly deformed, and also acted as the southern and northern boundaries of the NNW-SSE directed half-grabens and due to the strong dissecting of the pre-existing deposits act also as source-producing material areas. Turbiditic currents amanating from the shelves and the slopes of the margins, as well as from the platforms of the Ionian basin accumulated breccias and microbreccias in the channelized shaped sub-basins. Occasionally, the presence of chert intercalations and/or condyles indicates high water depths.which permitted the concentration and deposition of siliceous material. Incomplete turbiditic beds, with Ta to Tc of Bouma sub-divisions indicate a position closer to the source area (inner fan). Incomplete turbiditic beds suggest a more distal position from the source. The Upper Cretaceous studied deposits in Araxos area build up an up to 350 m thick sequence. This sequence is composed of interbedded calciturbidites and breccia deposits, forming more than 6 cycles. Calciturbidites range from 2-10 m thick whereas breccia thickness range from 2 to 50 m.