Sedimentary evolution of the continental Early–Middle Triassic Cañizar Formationud(Central Spain): Implications for life recovery after the Permian–Triassic crisis

摘要

The Permian–Triassic transition (P–T) was marked by important geochemical perturbations and the largestudknown life crisis. Consequences of this event, as oxygen-depleted conditions and the unusual behavior ofudthe carbon cycle, were prolonged during the Early Triassic interval delaying the recovery of life in both terrestrialudand marine ecosystems. Studies on Lower Triassic sediments of continental origin, as in the case ofudWestern Europe, are especially problematic due to the scarcity of fossils and absence of precise dating. TheudCañizar Fm. is an Early–Middle Triassic unit of continental origin of the SE Iberian Ranges, E Spain. A detailedudsedimentary study of this unit allows a shedding of light on some unresolved problems of the continental depositsudof this age.udThe top of this unit is dated as early Anisian by means of a pollen association, while the age of its base is hereudestimated as late Smithian or Smithian–Spathian transition. Different facies associations and architectural elementsudhave been defined in this unit. In the western and central parts of the basin, this unit shows sedimentaryudcharacteristics of fluvial deposits with locally intercalated aeolian sediments, while in the eastern partudthere is an alternation of both aeolian and fluvial deposits. Sedimentary structures also indicate changes inudthe climate conditions, mainly from arid to semiarid. Two marked arid periods when well-preserved aeolianudsediments developed during early–middle Spathian and Spathian–Anisian transition. They alternated withudtwo semiarid but more humid periods during the late Spathian and early Anisian. These conditions basicallyudcorrespond with the general arid and very arid conditions described for central–western European plate duringudthe same period of time. The Ateca–Montalbán High, in the northern border of the study basin, must haveudrepresented an important topographic barrier in the western Tethys separating aeolian dominated areas toudthe N and NE from fluvial dominated areas to the south.udThe Cañizar Fm. has been subdivided into six members (A–F) separated by seven (1–7) major boundingudsurfaces (MBS). These surfaces are well recognized laterally over hundred of km and they representud104–105 My. MBS-5 is considered to be of late Spathian age and it is a clear indication of tectonic activity,udrepresented by a mild unconformity. This event represents a change in the sedimentary characteristicsud(reactivation) of the unit and from here to the top of the unit are found the first signals of biotic recovery,udrepresented by tetrapod footprints, plants, roots and bioturbation. All of these characteristics and the estimatedudage represented by the MBS-5 event permit this surface to be related to the coeval Hardegsen unconformityudof Central–Western Europe. These first signals of biotic recovery can thus be related to anudincreased oxygen supply due to the new created paleogeographical corridors in the context of this tectonicudactivity. These biotic signals occurred 5 My after the Permian–Triassic limit crisis; a similar delay as occurredudin other coeval and neighboring basins.