The Pozalagua Quarry in the Basque–Cantabrian Basin of northern Spain exposes a unique set of fault-associated dolomites that can be studied on a decametre scale. The dolomites developed along the Pozalagua Fault system in
slope-deposited limestones of Albian age. Following marine phreatic diagenesis, the limestones were subject to meteoric karst formation. The resulting cavities were filled either by angular limestone fragments in a black
clay-rich matrix, or by cave floor/pond (now dolomitized) sediments. The subsequent diagenetic history reflects repeated periods of fracturing, fluid expulsion, dissolution and cementation. Contrasting fluid pulses resulted in the formation of a network of hydrothermal karst and the subsequent development of coarse-crystalline calcite cement, zebra dolomite, recrystallized coarse-crystalline dolomite, elongated blue–grey coarsecrystalline
dolomite cement in the open fault and, finally, coarse-crystalline saddle dolomite. Decimetre-size reworked host-rock fragments present in the latter two dolomite phases probably reflect roof collapse fragments of a cave
system that developed along the Pozalagua Fault system. However, there are also metre-scale host-rock fragments that apparently ‘float’ in the coarsecrystalline
saddle dolomites, implying that either fragment assimilation was a widespread process or violent expulsion of fluids occurred along the Pozalagua Fault system. The presence of pre-dolomite and post-dolomite stylolites, parallel to bedding, supports a linkage between the diagenetic events and the Late Albian tectonism that affected the region.