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

Abstract

This paper focuses on the relationships between the large-scale stratigraphic architecture of the Almazán basin infill and the sedimentation rates (SR) calculated for precise time intervals. Our aim is 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 Gómara fluvial fan, the main sediment transfer system of the Almazán 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 2600 meters 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 Gómara 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 30-40 cm/kyr) are related to fluvial environments with low density ribbon-shaped channels, while low SR (around or below 10 cm/kyr) are related to high density sheet-like channels. Laterally, mud dominated environments with high SR (15-20 cm/kyr) grade into palustrine/lacustrine carbonated environments with low SR (around 9 cm/ky). The lowest SR (about 3 cm/kyr) are related to the development of stacked calcrete profiles in distal floodplain and in the connection of distal alluvial and palustrine/lacustrine units.