Sudden changes in fluvial style across the Permian-Triassic boundary in the eastern Iberian Ranges, Spain: Analysis of possible causes

摘要

The sedimentary record of the Late Permian and Early Triassic of the eastern Iberian Ranges shows four major, sudden, or very rapid, vertical changes in fluvial style. The Late Permian sedimentary cycle starts with the Boniches Formation, of alluvial fan-braided fluvial origin, which grades vertically over within a few metres into the Alcotas Formation, deposited by low to high sinuosity, avulsion-prone rivers with extensive floodplains. The Alcotas Formation contains calcimorphic soils, plant remains and pollen and spore assemblages. However, the upper third of the unit is devoid of all organic remains and soils and is characterized by a dominant red colour, the sandstone levels were deposited by high-sinuosity, meandering rivers. This major change took place during the Late Permian and is probably coeval with the emplacement of the Emeishan basaltic Large Igneous Province (LIP) in SE China. Rocks of the Boniches and Alcotas Formations are separated by an angular unconformity from the overlying strata, which consist of the Late Permian conglomeratic Hoz del Gallo Formation, of alluvial fan-gravel braided fluvial origin and the sandy Canizar Formation, of low-sinuosity sandy river origin. The Permian-Triassic boundary lies, probably between the upper part of the Hoz del Gallo Formation and the first metres of the Canizar Formation. Late Permian pollen and spore assemblages have been found in the Hoz del Gallo Formation but the Canizar Formation is barren, with the exception of an Anisian (Middle Triassic) assemblage at the top. Tectonic extensional pulses in the Iberian Basin caused the changes observed between the lower and upper parts of the Boniches Formation, at the base of the Hoz del Gallo Formation and between the lower and upper part of this Formation. The changes observed in the uppermost part of the Alcotas Formation are not easily explained by tectonic causes, nor those in the passage from the Hoz del Gallo Formation to the Canizar Formation. Similar sedimentary characteristics of the sandy Canizar Formation such as amalgamated sandstone bodies, erosion and reactivation surfaces, dominant trough cross-stratification, tabular geometry, absence of plant remains and pollen and spores, and absence of silts and clays to those of coeval formations in places as far away as Australia, South Africa and Brazil suggest a global rather than local cause for these abrupt changes in fluvial style. This global cause was probably die-off of plant cover over extensive areas of the catchment, related to the end of the Permian mass extinction and possibly related to the emplacement of the West Siberian basaltic Large Igneous Province (LIP), responsible for drastic atmospheric and marine changes. (c) 2005 Elsevier B.V All rights reserved.