Classification of soft-sediment deformation in carbonates based on the Lower Cretaceous Vigla Formation, Kastos, Greece

Abstract

Soft-sediment deformation (SSD) structures are widespread within the Lower Cretaceous Vigla Formation (limestones and shales) that are exposed in a 7–10 km long and 100 m high section along the east coast of Kastos Island in the Ionian Sea. SSD was observed both in limestones and in shales, and formed during or immediately after deposition, during the first stage of sediment consolidation. They are classified into six (6) different types of deformations: (1) Thick synclines and anticlines, formed due to strong synsedimentary deformation, that were produced mostly from N-S directed normal faults or from the interaction between normal and transfer faults, and are situated between undeformed horizons. (2) Thin slumps ascribed to normal fault activity and especially slumping in the footwall of the active fault, prograding orthogonal to the fault direction. (3) Thin to medium slumps produced from the synsedimentary interaction of N–S normal faults and E–W directed transfer faults, showing multiply directions of progradation. (4) Thin SSD related to diapiric intrusions that also associated with fault activity. (5) Thick slump horizons displaying erosional contacts with the neighboring undeformed limestones in the same horizon, resulting from gravity-driven mass-transport deposits that flowed across inclined basin floors. (6) Thin slumps, with chert nodules, within a slumped horizon, resulting from a slumping phase both after the sedimentation and after early diagenesis in carbonates deposits, but during the sedimentation of the lower Cretaceous Vigla limestones. All the above categories were characterized as seismites or deformation due to fault activity. This tectonism resulting in tilted blocks, the creation of space in hangingwall downthrown blocks, and changes of inclination of the basin floor, outcrops with high lateral extent in different directions adds constraints to the geometry of slumps. The transition zone between Vigla limestones and shales showed particularly abundant deformation structures. Although the Vigla Formation limestones have low primary porosity, the presence of many SSD horizons could increase secondary porosity in this formation.