Synsedimentary Polygonal Faults System in Tight Chalk - Multiscale Evidence from Outcrop, Flamborough Head (UK)

The Flamborough Head chalk cliffs (Yorkshire, UK) are an outstanding outcrop of once deeply-buried tight-chalk and are likely to be representative of sub-surface chalk reservoirs in nearby onshore and offshore regions, i.e. the North Sea. These chalk outcrops display a very complex fracture network, which has been widely described and which has been traditionally interpreted as the result of polyphased extensional tectonic events due to their random distribution. High-resolution drone imaging performed on the platform revealed the overall organization of the fault network displaying irregular polygonal cellular patterns. They likely correspond to polygonal fault systems (PFS) with an isotropic horizontal state of stress during early compaction. Field observations allowed to evidence the synsedimentary nature of these normal faults involved in the PFS, supporting the fact that faulting occurred when the sediment was in a gel-like state shortly after deposition. Different orders of magnitude of imbricate polygonal fault systems were identified, with nested polygon cells of centimeter to hectometer scale. The Oligocene transpressive tectonic events resulted in the reactivation of preferential orientations of the PFS as tectonic stylolites (with horizontal peaks), highlighting the importance of PFS in fluid transport.