RECOGNIZING THE ROLE OF SALT TECTONICS ON THE GROWTH OF THE LATE CRETACEOUS BOIXOLS / SANT CORNELI ANTICLINAL STRUCTURE: EVIDENCE FROM RUDIST REEF FACIES DISTRIBUTION AND SYNDEPOSITIONAL SLUMPING

The timing of the origin of the southern Pyrenees and associated structural features has been a topic of debate within the geological community for decades. The Sant Corneli / Boixols anticlinal structure in the south-central Pyrenees, one of the earliest Pyrenees structures, is Late Cretaceous in age. Originally the growth of this anticlinal structure was attributed to compressional Alpine tectonics, which began immediately following the Santonian. Stratigraphic evidence, however, suggests that this structure was active during deposition of the middle and late Santonian Sant Corneli Fm, predating regional Alpine compression. Detailed mapping of complex facies relationships preserved withing the Sant Corneli Fm was used to evaluate the timing of the Sant Corneli / Boixols anticlinal structure relative to the start of Alpine compression.

Three lines of evidence within the middle to late Santonian Sant Corneli Fm indicate that the Boixols Anticline was active and generating seafloor relief prior to Alpine compression. First, shallow rudist reefs are isolated on the crest of the growing Boixols anticline. Second, syndepositional slumps within the slope strata of these reefs show the Boixols anticline was actively creating seafloor relief at the time of Sant Corneli Fm deposition. Third, northward prograding quartz-rich grainstone derived from the Ebro massif to the south was deflected around the Boixols anticline, but prograded unimpeded over the Sant Corneli anticline. This stratigraphic evidence, along with abundant local outcrops of evaporites and a salt dome beneath the neighboring Isona anticline, strongly suggests salt tectonics was responsible for the origin of the Sant Corneli / Boixols anticlinal structure.

The growth of the Sant Corneli / Boixols anticlinal structure resulted from salt tectonic activity during deposition the middle to late Santonian Sant Corneli Fm, prior to regional Alpine compression. Later compression overprinted the salt tectonics, masking the earlier salt tectonic event. A combination of structural geology and sedimentology was needed to resolve this long-standing debate regarding the timing of the origin of the Pyrenees.