Carnets de Géologie (May 2010)
Traces de volcanisme explosif dans le Campanien pyrénéen aux alentours du stratotype de limite Campanien-Maastrichtien à Tercis (SO France, N Espagne). Repérage biostratigraphique avec une étude particulière du foraminifère Radotruncana calcarata [Traces of explosive volcanism in the Pyrenean Campanian around the Campanian-Maastrichtian boundary type section at Tercis (SW France, N Spain). Biostratigraphy with emphasis on the foraminifer Radotruncana calcarata]
Abstract
Pyroclastic material including idiomorphic crystals of sanidine, biotite, and apatite, first identified in the type section at Tercis are contemporaneous with the Radotruncana calcarata (planktonic foraminifer) total range zone. A search was undertaken to define the palaeogeographical extent and to determine the importance of these remains of a previously unknown Campanian regional alkaline volcanic episode. A preliminary biostratigraphical search was carried out in the Tercis area and in the western Pyrenees followed by detailed micropalaeontological studies in southwestern France and northern Spain. The findings from five upper Campanian platform and flysch facies are reported in detail and deductions from three others are summarized. First, The microfaunal count of the boundary stratotype at Tercis has been increased by the use of acetolytic techniques to disaggregate indurated carbonates. So isolated tests of some key biostratigraphic markers are documented for the first time. The paucity in specimens of Radotruncana calcarata previously seen in thin sections from the type outcrop is not due to a scarcity of this particular taxon but to a general dearth of globotruncanids in the platform facies deposits of the Aturian Basin. The study shows that acetolysis multiplies the power of investigation of indurated limestones. Thus, the abundance and diversity of microproblematica discovered and studied in the stratotype section since 2006 has been confirmed and two new forms of incertae sedis are reported, one of which is called here Velafer ovatus. Today, 70 species of microproblematica are recognized at Tercis. These microproblematica allow a chronologic calibration of the deposits like or even more precise than those obtainable using ammonites or planktonic foraminifers. Sections sampled in the western Pyrenean flysch facies include the stratigraphic interval comprising the total range zone of Rd. calcarata for which we suggest here a duration of 0.80 ±0.05 Ma. No microproblematica were found in these deep detrital facies. All sections reaching the Campanian-Maastrichtian boundary contain a Contusotruncana (foraminifer) of which the morphology, illustrated anew from 3 discrete sections, is consistent with that of the specimens identified at this level in the Tercis type-section; they were called C. contusa or C. contusa?/C. patelliformis? These micropalaeontological studies substantiate the choice of Tercis as the best section in the world for location of the stratotype of the Campanian-Maastrichtian stage boundary; they show again its striking regional and global correlative power thanks to the unique diversity of its biological record of the late Cretaceous. In addition, it justifies the choice of a boundary level selected using the near-coincidence of a variety of key events bracketting a guide-event; 12 were clustered around the mid-level 115.2 of the type section where the GSSP of the Campanian-Maastrichtian boundary has been recognized internationally. Consequently, the selected level does not depend on a unique "magic marker" with its inherent uncertainty caused by difficulty in identification and scarcity locally and in other localities and environments. This may well be the case for the guide-ammonite Pachydiscus neubergicus as commonly used as a marker of the Campanian-Maastrichtian boundary.The biostratigraphical study precisely locates the pyroclastic material observed at Tercis as within the range of Rd. calcarata, immediately above the last occurrence of Tubellus hunzikeri, the first occurrences of Lucernellus aubouini and Aquilegiella varia, 3 microproblematica of which the distribution is well documented at Tercis. Previously reported evidences of volcanism at Tercis: the presence of kaolinite in the clay size fraction and of microcrystals of sanidine, biotite, zircon, apatite are supplemented here by the discovery of slightly calcic and sodic siliceous spherules with traces of magnesium, aluminum and potassium. Kaolinite and sanidine have also been found in contemporaneous levels at Peyrata, in the vicinity of the type section. Farther away in the flysch facies, the contemporaneous deposits contain pyroclastic biotite mica flakes, idiomorphic zircon crystals and, more rarely, apatite crystals of which the morphology is consistent with a volcanic origin. The presence of traces of a volcanic episode in Campanian levels representing about 1 Ma of deposition are thus confirmed but in the Pyrenees no information is available concerning the site of its origin. In fact, the explanations for the presence of the pyroclastic material involves two possibilities: either there was Campanian alkaline volcanic activity in the Pyrenees similar to that associated with subduction (which has not been identified and is not consistent with the accepted geodynamical model of the area) or the pyroclastic material comes from North America, distant some 5000 km. Such long distance transport would involve an west-east eolian conveyance of mm sized volcanic elements for 1 Ma (a hypothesis to our knowledge undocumented).
