Marl smear evolution in carbonate marl successions: An outcrop investigation from the Maltese Islands

: Fault zone architecture and fluid flow behaviour in mixed carbonate-marl successions remain poorly constrained relative to siliciclastic and pure carbonate systems, limiting our ability to predict fault sealing behaviour in heterogeneous sequences. Here, we present outcrop data from a faulted Oligocene - Miocene succession in the Maltese archipelago, where aquifer bounding normal faults juxtapose bioclastic and micritic limestones against carbonate marls. Detailed stratigraphic logging and field mapping of five fault zones reveal systematically developed marl smears in mixed carbonate - marl successions, forming the dominant component of the fault core; the first quantitative characterisation of this feature in such systems. Faults evolve from thin, localised marl smears at low (< 2m) offsets to thick (up to 33 m), laterally extensive, internally heterogeneous zones at higher displacements (125 m +)). Marl smear scales positively with both host bed thickness and fault displacement (r 2 = 0.71), although scatter (±15 m) reflects structural complexity and strain partitioning. Deformation is preferentially localised within the marl unit, producing fault architectures comparable to those observed in siliciclastic systems, but with different scaling behaviour. Existing siliciclastic derived fault seal algorithms systematically underestimate fault core thickness in carbonate -marl successions by up to an order of magnitude, likely reflecting the mechanical contrast between the competent carbonate beds and fissile phyllosilicate rich marls. These findings advance our understanding of fault controlled fluid flow in mixed carbonate-marl successions and provide a robust new predictive framework for aquifer compartmentalisation and connectivity in faulted groundwater systems, improving groundwater resource management in carbonate systems.