Faulting variability in the Kortrijk Clay Formation, Princess Elisabeth Zone, offshore Belgium: Implications for clay tectonics

Layer-bound faults in fine-grained sediments, observed in sedimentary basins worldwide, are frequently interpreted as the result of polygonal faulting. In clay formations, these faults are linked to clay tectonics—post-depositional processes that generate intraformational fault systems with no predominant orientation. As part of subsurface investigations for future wind farm development in the Princess Elisabeth Zone (PEZ), offshore Belgium, we analyse faulting observed within the Eocene-aged Kortrijk Clay Formation, a lateral equivalent of the London Clay Formation.

Using ultra-high-resolution seismic and acoustic reflection data with dense grid spacing (40-60 m), we identified two distinct fault groups that transition sharply between the southern and northern PEZ. The first group, dominant in the southern PEZ, comprises ENE-WSW oriented faults (N75°E-N85°E) with steep dips of 60°-70°, narrow spacing of 50-160 m, and fault lengths typically under 0.5 km. In contrast, the second group, prominent in the northern PEZ, features NNE-SSW faults (N355°E–N25°E) with shallower dips of 35°–45°, wider spacing of 100–500 m, larger displacements of up to 5 m, and fault lengths extending to 1.7 km. This group also includes a major fault with a displacement of up to 15 m, aligned parallel to the other faults. Unlike the southern faults, some of these northern faults propagate deeper, potentially reaching the top of the Cretaceous. Additionally, folding structures are also observed, with fold axes oriented both parallel and oblique to the fault directions.

The distinct partitioning of fault orientations and distributions between the two areas raises critical questions about the origin and controls of the faulting process. The presence of dominant fault orientations contradicts the diagenetic-related polygonal faulting model, which typically predicts faults lacking a preferred orientation. These dominant orientations, along with the occurrence of major faulting—particularly in the northern PEZ—point to the potential influence of regional tectonic stresses. However, the controls on the spatial partitioning of these fault groups remain unclear, highlighting the need for deeper seismic data that would allow further investigation into the deeper regional structure to better understand the faulting processes within this clay formation.