The Influence of Preexisting Structure and Halokinesis on Organic Matter Preservation and Thrust System Evolution in the Ionian Basin, Northwest Greece

Abstract

The opening of the Ionian basin and its internal differentiation is attested to by lateral facies and thickness variations of the formations deposited during the Pliensbachian and Tithonian (synrift formations). The beginning of the synrift sequence is represented by the Siniais Limestones (Pliensbachian) and their lateral equivalent, the Louros Limestones. The geometric characteristics of the extensional basin depend on both extension related to the latest opening of the Tethys ocean and halokinesis of the Ionian zone evaporitic substratum. The orientation of the extensional regime is deduced from the direction of stratigraphic pinch-outs of the synrift formations, directions of transport of slump features, and direction of movement on synsedimentary faults observed in t e base of these formations in the half grabens. The postrift period is marked by an unconformity at the base of the Vigla Limestones, which represents the early Berriasian breakup during which sedimentation was synchronous in the entire Ionian basin. The accumulation of organic matter in the Lower and Posidonia beds of the Ionian zone during the Toarcian and Tithonian is directly related to the geometry of the synrift period of the Ionian basin. Restricted subbasins were formed where the geometry of the basin favored stagnation and consequently the locally euxinic conditions of bottom waters. Anoxic conditions persisted locally to the postrift period in areas where the Upper Siliceous Zone (Albian-Cenomanian) of the Vigla Limestones is well developed; these areas probably represent subbasins that were preserved by the continuation of halokinetic movements during the postrift period. During the early Miocene Alpine orogeny, collision-related compressive stresses on the margin induced the reactivation of preexisting fractures, which were responsible for the inversion tectonics that affected the Mesozoic basin. The geometric characteristics of the inverted basin were dependent on lithology (evaporites), geometry of the extensional structures, and direction of the compressional phase. The observed geometries do not always correspond to the classical scheme of inversion tectonics. The geological evolution in the Ionian basin is an example of inversion tectonics of a basin with an evaporitic substratum. The opening of the Ionian basin and the inversion tectonics influence both the source rocks and the probable hydrocarbon traps of the Ionian zone.