Palaeohydrogeological control of palaeokarst macro-porosity genesis during a major sea-level lowstand: Danian of the Urbasa–Andia plateau, Navarra, North Spain

An extensive palaeokarst porosity system, developed during a pronounced mid-Paleocene third-order lowstand of sea level, is hosted in Danian limestones of the Urbasa–Andia plateau in north Spain. These limestones were deposited on a 40–50 km wide rimmed shelf with a margin characterised by coralgal buildups and coarse-grained bioclastic accumulations. The sea-level fall that caused karstification was of approximately 80–90 m magnitude and 2.5 Ma in duration. During the exposure, a 450 m wide belt of sub-vertical margin-parallel fractures developed a few hundred metres inboard of the shelf edge. Most fractures are 90–100 m deep, average 1 m in width, and are associated with large erosional features created by collapse of the reefal margin. Inland from the fracture belt, three superimposed laterally extensive cave systems were formed over a distance of 3.5 km perpendicular to shelf edge, at depths ranging from 8–31 m below the exposure surface. The palaeocaves range from 0.3 to 2 m in height, average 1.5 m high. They show no evidence of meteoric processes and are filled with Thanetian grainstones rich in reworked Microcodium, a lithology that also occurs infilling the fractures. The caves are interpreted as due to active corrosion at the saline water–fresh-water mixing zone. Caves are missing from the shelf edge zone probably because the fractures beheaded the meteoroic lens preventing mixing-zone cave development beyond the fracture zone. Towards the platform interior, each cave system passes into a prominent horizon, averaging 1 m in thickness, of spongy porosity with crystal silt infills and red Fe-oxide coatings. The spongy horizons can be traced for 5.5 km inboard from the cave zone and occur at 10.5 m, 25 m and 32 m below the exposure surface. In the inland zone, two additional horizons with the same spongy dissolution have been recognised at depths of 50 m and 95 m. All are analogous to Swiss-cheese mixing-zone corrosion in modern carbonate aquifers and probably owe their origins to microbially-mediated dis