AAPG member Claudio Bartolini, a senior geologist with Repsol YPF in Houston, was visiting northern Spain in June when he visited San Juan de Gaztelugatxe islet, a tiny island on the coast of Biscay belonging to the municipality of Bermeo, in Spain’s Basque Country.
The outcrops quickly caught his eye – and not just because it was a scenic vista.
Geologically, the island is made up of Albian limestones from the so-called Urgonian Complex, a biosedimentary system formed along the Paleotethys realm. At this location, this unit represents a carbonate platform margin dissected by synsedimentary faults associated with the opening of the Bay of Biscay in the Middle Cretaceous.
What Bartolini recognized were outcrops mapped and studied by his colleague, Jose C. Vicente Bravo, director of new business development with Repsol YPF in Madrid, Spain, as part of his dissertation research.
Vicente Bravo saw that these Urgonian facies assemblages and their associated paleogeographic setting are analagous to the petroleum-prolific El Abra and Tamabra formations (age-equivalent units) that characterize the Golden Lane and Poza Rica districts, Tampico-Misantla Basin, Mexico.
Who knew a vacation excursion could provide such a valuable setting?
According to the study, the footwall of this fault-controlled carbonate platform consists of base of slope breccias triggered by steep slopes and coeval synsedimentary faulting.
Breaking-up of the carbonate platform produced up to 500 meter-thick breccia deposits at the base of slope apron, and the overall drowning of the carbonate platform into a deepwater basin. The blackish layered sequence onlapping the carbonate breccias are thin bedded turbidites (strata dipping to the right), representing deepwater strata of the Black Flysch Group, a Middle Albian to Lower Cenomanian turbidite complex.
Incidentally, the island is connected to the mainland by a man-made bridge, Bartolini said. On top of the island – after 237 steps – stands an hermitage dedicated to San Juan.
Etymologically, the word gaztelugatxe comes from the Basque gaztelu (“castle”) and aitz (“rock” or “crag”), forming “Crag of the castle.”
The small church dates from the 10th century and seems to have come from the Knights Templar. The hermitage also houses various votive offerings from sailors who survived shipwrecks. Several legends surround this mystic landmark.