Evolution of Permo-Triassic Fluvial Systems in the Central Iberian Basin, Spain

Abstract

This study investigates Permo-Triassic syn-rift sediments deposited in the Central Iberian Basin (CIB), an intracratonic basin within the Iberian Massif that developed on the break-up of Pangea in the Early Permian. These sedimentary rocks crop out in a series of exposures along the Iberian Ranges (IR), north-eastern Spain. Field work was conducted at eight localities across ~80 km of the Castilian branch of the IR with datasets collected including sedimentary logs, digital photographs, samples for elemental chemostratigraphic analysis and LIDAR. The aim of this research was to establish if relationships could be developed between the controls that define large-scale depositional processes in the continental realm and the stratigraphic record of fluvial systems.
Five sedimentary lithologies were found to be present: conglomerates, sandstones, mudstones, marlstones and limestones/dolomites. They were classified into twenty-eight lithofacies elements before being combined into six architectural elements, representing four distinct depositional environments. The main focus was given to the Buntsandstein sedimentary rocks, a series of lateral basal conglomerates and axial alluvial red sandstone and mudstone beds that were deposited in sub-basins with episodically changing rates of basin-floor subsidence. Rates of continental sediment accumulation were calculated and ranged from 0.025 to 0.119 m/1000 years. The overlying Muschelkalk deposits record the Tethyan marine transgression, which entered the study area at the end of the Late Anisian and reached the maximum flooding surface in the mid-Longobardian (Late Ladinian). The transgression occurred at a rate of 0.017 m/year for the first 1.5 Ma and then 0.046 m/year for the latter 1 Ma. The sedimentary structures were codified to demonstrate the complexity of the internal heterogeneity of channel sand bodies. The grain-size data were codified and used to establish seven depositional phases. From this, seven 3-D block diagrams were used to illustrate how eustatic sea-level changes and localised tectonics governed the large-scale architecture of the fluvial system and its transition during a marine transgression. Finally, a novel approach was used to quantify the cross-sectional architecture of sand bodies using LIDAR as an attempt to overcome bias in qualitative interpretations.