Abstract

The Permo-Triassic ‘Buntsandstein’ facies was studied from outcrops in the Aragonian branch of the Iberian range (Zaragoza, Spain). Using an integrated analysis of sedimentological and palynological data, we propose a sequence stratigraphic analysis for the Permo-Triassic deposits in this area. A comparison with the stratigraphical cycles of other parts of the Iberian domain and other peri-Tethyan basins enables us to specify the stratigraphic context of the Lower Triassic in the western European domain and characterize the importance of the Lower Triassic hiatus.

The ‘Buntsandstein’ deposits studied here from the Aragonian branch occur in three outcrop areas: Tabuenca-Rodanas, Morata de Jalón and Fombuena. The Permo-Triassic deposits lie unconformably on the Hercynian (Variscan) basement. The ‘Buntsandstein’ facies were attributed to the ‘Detrital Group’ by Arribas [1984], who recognized four informal formations: the Araviana, Tierga, Calcena and Trasobares formations, in ascending stratigraphic order.

Based on sedimentological criteria indicating fluvio-lacustrine environments for the Araviana cycle, the basal ‘Buntsandstein’ facies of this area can be attributed to the Permian. The discontinuity observed between the Araviana and Tierga formations, and the palaeobotanical evidence indicating an Anisian age (i.e. Middle Triassic) for the Tierga, suggests a hiatus during the Lower Triassic in this area. The Lower Triassic in Spain and many other parts of western Europe is characterized by fluvial sediments preserved in arid palaeoenvironments, associated with occasional aeolian deposits, and lacking any palaeoflora. At the scale of the western European domain, the onset of Triassic sedimentation would appear mainly in the Olenekian. Stratigraphic continuity between Permian and Triassic non-marine deposits, with sediments attributable to the Induan (lower part of the Lower Triassic), is only observed in the central Germanic Basin. In the Aragonian branch, the first Triassic sediments are dated as Anisian and characterize the retrogradational trend of a major cycle (Tierga – Calcareous Group major cycle) showing the vertical evolution from braided rivers to marine Muschelkalk deposits. At the scale of the west-European domain, the Anisian is always characterized by fluvial and fluvio-lacustrine deposits, with palaeosols, passing upward into marine deposits. Thus, the connection with the open sea was established during the Anisian. In this western Peritethyan domain, the diachronous nature of the Muschelkalk transgression is evident: the onset of Muschelkalk facies occurs during the lower Anisian in the Germanic Basin, the middle Anisian in the eastern part of the Paris Basin and the late Anisian in the Iberian Peninsula.

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