Paleocene coralgal reefs of the western Pyrenean basin, northern Spain: New evidence supporting an earliest Paleogene recovery of reefal ecosystems

The rate of recovery of photic reefal ecosystems after the Cretaceous-Tertiary crisis is controversial. Most reports from earlier authors concluded that Paleogene reef systems did not completely recover from that crisis at least until Oligocene-Miocene times. Several other authors, however, have pointed out that such conclusion was biased by poor preservation and/or inaccessibility of many early Paleogene successions and that the recovery of reefs might have been a faster process. The latter possibility is here strongly supported with data from the Pyrenean basin, where Paleocene shallow-water carbonate deposits are thickly developed and outcrop widely. Five growth phases of reef development have been recognized and age dated with calcareous nannofossils, which together define a complete sequence of expansion-reduction of reef ecosystems during the Paleocene epoch. Reef growth phase 1 (early Danian, lower-middle NP3 calcareous nannofossil Zone), the initial step of that sequence, demonstrates that coral-dominated reef systems came back into existence less than 2 Ma after the K/T boundary, a comparatively short time considering the magnitude of the end-Cretaceous biological crisis. The climax of the sequence is recorded by growth phase 3 (late Danian, NP4 Zone), when a thick barrier-reef complex was developed, which was at least 200 km in length (but probably much larger) and made up by a well diversified coralgal assemblage. Reef growth phases 4 and 5 (early and middle Thanetian, NP6 to NP8 Zones) were characterized by low-relief reef banks dominated by encrusting calcareous algae and had less well-diversified coral assemblages, features that point to a deterioration of environmental conditions, tentatively interpreted in terms of a climatic cooling. Reef bioconstruction almost disappeared in the Pyrenean basin during late Thanetian times, although in all likelihood this is a regional effect of no evolutionary relevance. The new data suggest that the Cenozoic recovery of reefs was rapid but punctuated, with a phase of important reef expansion already in the late Danian.