EVOLVING FLUVIAL ARCHITECTURE DURING A MARINE TRANSGRESSION - UPPER BUNTSANDSTEIN, TRIASSIC, CENTRAL SPAIN

The sedimentation in central Spain during the Late Anisian (Middle Triassic), (Limos y Areniscas de Rillo Formation), recorded the evolution from an alluvial environment into a coastal one with a diversity of environments. This evolution was the result of one of the episodes of the Tethys sea transgression over Iberia. During the first and second stages of this evolution, mixed-load, low sinuosity channels evolved upwards into a distal braidplain with ephemeral run-off and poorly defined channels. During a third stage, the characteristics of the sediments indicate the existence of both continental and marine waters in a transitional zone with seasonal hydrological fluctuations. The fourth stage records the sedimentation in an upper intertidal to supratidal environment in a semiarid coastline with the presence of thin cryptalgal sheets and chicken-wire textures. The final evolution consists of an environment with a high percentage of fluvial channel deposits, probably related to a meandering pattern. This evolution has been correlated with the depositional sequences established for the NE of Spain where the Anisian transgressive event resulted in the formation of an inter- to supratidal carbonate ramp. So, lowstand, transgressive and highstand system tracts from depositional sequence-1, have been recognized in the centre of Spain.