Depositional history and clay minerals of the Upper Cretaceous basin in the South-Central Pyrenees, Spain

An ordered sequence of well-defined sedimentary environments reflects the deepening and shallowing stages in the depositional history of the Upper Cretaceous basin in the South-Central Pyrenees, Spain. The sequence, which has a Santonian age at its base, starts with a calcarenite barrier system on which occurs a coral reef showing well-differentiated fore- and back-reef facies (Congost Limestone Formation). Marked deepening of the basin occurred during the deposition of the overlying glauconitic nodular limestones, as is evidenced by a predominantly planktonic fauna, consistent fine grain size, a very large planar slump scar, and frequent other slump structures (Anserola Formation). Sedimentation continued in a deep marine, carbonate turbidite facies (Vallcarga Formation). An olistostrome at the top of the turbidites contains dislocated masses of the same turbidites, an indication of the extreme mobility of the basin. The sequence is then regressive, via marls, which in part constitute the turbidite basin slope facies, into shallow-neritic to coastal cross-bedded calcarenites (Arén Sandstone Formation), and, via lagoonal and coastal-swamp deposits, ultimately into fluvio-lacustrine red beds (Tremp Formation; Upper Maastrichtian/Lower Paleocene). The total of added maximum thicknesses is approximately 3000 m. The depositional history of the basin demonstrates the dominating influence of tectonic movements on the type of sedimentation. In the initial stages, the rate of subsidence exceeded the rate of sedimentation, and the basin consequently deepened (Anserola to Vallcarga formations). The regressive development is due to a slowing down of the general subsiding movement and continued sedimentation (Vallcarga to Arén formations). Many features in the turbidite sequence are directly related to tectonic movements. The clay-mineral assemblages consist of illite, montmorillonite, chlorite, kaolinite, and mixed layers in varying proportions. Poorly crystallised, degraded illite occurs in the Congost and Anserola formations; well crystallised, fresh illite was introduced into the basin with the start of turbidity-current deposition. Montmorillonite is absent in the high-energy deposits (barrier calcarenites in Congost limestones, Arén sandstones), which is attributed to the relatively slow settling rates of this clay mineral. The absolute vitrinite reflectance as determined in the Vallcarga turbidite beds is 0.96% (fixed carbon 67%). Except for possible authigenesis of chlorite, no significant diagenetic clay-mineral transformations have taken place.