Fluvial channel deposits and coeval paleosol development across the pre- and onset Paleocene-Eocene Thermal Maximum (PETM), Tremp-Graus Basin, Spanish Pyrenees

Criteria for determining whether river channel morphology and associated deposits vary as a direct or indirect function of climate factors remain poorly constrained. An effective approach to this problem is the integrated analysis of channel deposits and coeval interchannel paleosols. In the Esplugafreda and Claret Formations, an alluvial-plain sedimentary succession recording abrupt climate change across the Paleocene-Eocene transition (the Paleocene-Eocene Thermal Maximum - PETM), the paleoriver hydrodynamics were compared with the coeval paleosol processes to discriminate the impact of the climatic forcing factors on both. Channel deposits of the pre-PETM Esplugafreda Formation reflect intermittent, paroxysmal flooding, as indicated by: (i) interlayered sandy mudstone with evidence of subaerial exposure; and (ii) thin conglomerate and sandy beds with antidune bedforms suggesting shallow floodwater and irregular, rapidly fluctuating discharge. Coeval paleosols contain Bssk horizons with slickensides and hematite dominance, indicating a negative hydric balance, well-drained conditions, and episodic wetting consistent with limited seasonal precipitation. In contrast, channel deposits of the member 1 and 2 of Claret Formation (pre-PETM and PETM onset event, respectively) are characterized by: (i) sandy conglomerates with low-angle cross-stratifications and tabular beds corresponding to large lateral and longitudinal bars, respectively; (ii) an absence of fine-grained interlayers; and (iii) more homogeneous grain-size distribution. These features indicate more persistent flow, greater flow depth and more stable discharge conditions when compared to Esplugafreda Formation channels. Coeval paleosols exhibit abundant redoximorphic features, more developed slickensides, goethite enrichment, and locally organic-rich A horizons, reflecting a general increase in soil moisture in interchannel areas. Integrated sedimentological and paleopedological evidence documents a progressive increase of humidity and more regular precipitation across the transition into the PETM.