Linking sedimentation rates and large-scale architecture for facies prediction in nonmarine basins (Paleogene, Almazan Basin, Spain)

This article focuses on the relationships between the large-scale stratigraphic architecture of the Almazan basin infill and the sedimentation rates (SR) calculated for precise time intervals. Our aim was to improve the understanding of the timing and causes of the architectural changes, their significance in terms of accommodation space and sediment supply and their relationship with climate and tectonics. The study area includes the Gomara fluvial fan, the main sediment transfer system of the Almazan basin during Paleogene times. Its large-scale architecture shifted through time between a stacking pattern of low density ribbon-like and high density sheet-like channel fills. Laterally to the fluvial system, mudstone and evaporitic mudstone units represented evaporitic mudflats which passed laterally into palustrine/lacustrine limestone units interpreted as lakes and ponds. Stacked calcretes occurred in distal alluvial and distal floodplain settings. A magnetostratigraphy encompassing 2600m guided by available fossil mammal biochronology has provided a temporal framework that spans the complete Paleogene infill of the basin, from Late Lutetian to Late Oligocene, filling a gap in the Cenozoic chronostratigraphy of Spanish basins. This permits to constrain the kinematics of the structures both in the basin and in its margins, and to provide the timing for the depositional sequences. These data, combined with a magnetostratigraphic map, where magnetic reversals were traced through the Gomara monocline, allow a detailed analysis of the SR variability across the fluvial system and its adjacent depositional environments. The results show that high sedimentation rates (around 30-40cmkyr(-1)) are related to fluvial environments with low density ribbon-shaped channels, while low SR (around or below 10cmkyr(-1)) are related to high density sheet-like channels. Laterally, mud dominated environments with high SR (15-20cmkyr(-1)) grade into palustrine/lacustrine carbonated environments with low SR (around 9cmky(-1)). The lowest SR (about 3cmkyr(-1)) are related to the development of stacked calcrete profiles in distal floodplain and in the connection of distal alluvial and palustrine/lacustrine units.