Stratigraphic and provenance analysis of the Alamor–Lancones Basin: Implications for the Cretaceous–Paleocene tectono–magmatic evolution of the northern Andean forearc system

Abstract

The Cretaceous–Paleocene Alamor–Lancones Basin is a well-preserved and long-lived forearc basin in southwestern Ecuador and northwestern Peru. It occupies a key morphotectonic region of the northern Andes, where the forearc transitions from continental basement in the south to accreted oceanic terranes in the north. This study investigates the stratigraphic evolution of the basin through provenance analysis of siliciclastic units and their response to fluctuating tectonic regimes that influenced the region. The oldest basin fill corresponds to the fluvial–alluvial Sábalos Formation, which unconformably overlies marine Paleozoic metasedimentary rocks of the El Tigre Unit. Whereas the lower Sábalos Formation was derived from exhuming crustal rocks, the upper levels record increasing input from coeval Aptian–early Albian volcanism. A shift to an extensional tectonic regime led to mid-Cretaceous lithospheric thinning, normal faulting, and submarine mafic volcanism of the continental Celica–Lancones Arc (∼105–91 Ma). Extension-related subsidence resulted in a transition to marine sedimentation, as recorded by the Puyango and Ciano formations during the middle Albian–Turonian. Following a Santonian compressional event (∼85 Ma), deep-marine deposition of the Zapotillo Group in the basin documents pre-, syn-, and post-accretion of the Caribbean Large Igneous Province (CLIP). Pre-accretional forearc sedimentation, as represented by the Santonian–late Campanian Chaquino and Tronco Quemado formations, records magmatic arc provenance with minor continental basement input. A latest Campanian–Maastrichtian shift to dominantly crustal provenance within the Mangahurco and Cañaveral formations provides evidence for exhumation of forearc basement rocks north of the basin in response to accretion of the CLIP at ∼75–70 Ma. Post-accretional accumulation of the Cazaderos and Casanga formations (∼66 Ma) recycled late Maastrichtian–Paleocene magmatic arc detritus from the emerging continental Sacapalca Arc. Overall, the evolution of the Alamor–Lancones Basin highlights how temporal shifts in the regional tectonic regime exerted first-order control on forearc deformation (basin subsidence and inversion), sediment provenance, and coeval arc magmatism, the latter also recorded by detrital zircon Th/U ratios.