Subsurface and outcrop characterization of large fluvial channel deposits of the Late Cenozoic Solimões Formation (Acre Basin, western Amazonia).

Cores recovered by the Trans-Amazon Drilling Project (TADP) in the Acre Basin provide an unprecedented opportunity to investigate the stratigraphic record in the Andean foreland in the western Brazilian Amazon. This study integrates subsurface data with sedimentological data of correlative units from outcrops along the Juruá River and roadcuts, providing key information on depositional geometry and architectural elements. The entire sampled interval of the core (870 m), except the uppermost 12 m,  is assigned to a single lithostratigraphic unit, identified as the Solimões Formation. The core succession comprises fine- to medium-grained sandstones, siltstones, and mudstones, exhibiting variable degrees of paleopedogenetic alteration. The sand-grade facies are rich in feldspar grains and lithic fragments, displaying a grayish coloration that contrasts with the reddish oxidized hues and locally green-mottled colors observed in more intensely paleoweathered profiles. Four facies associations (FA) are defined: sandy fluvial channel deposits (FA1 and FA2) and finer-grained overbank and floodplain deposits (FA3 and FA4). Facies associations FA1 and FA2 are best represented by deposits exposed along the BR-364 highway in the eastern Acre Basin, where they form channel-dominated sandy successions, with homogeneous successions up to 1.5 m thick. FA3 and FA4 are best represented by deposits exposed along the Juruá River riverbanks in the western Acre Basin, characterized by laterally extensive tabular beds intercalated with thin sandy bodies deposits forming fine-grained heterolithic deposits, generally with well-developed paleoweathering features and cemented by calcite. Subsurface deposits from the core record a phase of long-term aggradation associated with foreland basin subsidence. These facies are interpreted as bar top deposits transitioning into floodplain environments of a large alluvial system, with very limited occurrence of lacustrine sequences. The limited lateral accretion in floodplain or shallow pond environments was subject to repeated subaerial exposure with prolonged floodplain stability under waterlogging and/or seasonal discharge variability. Further refinement of these interpretations, together with ongoing geochronological and geochemical analyses, should help to improve our understanding of Amazonian basin development and its linkage to climate and tectonic change through the Late Miocene and Pliocene.