STRATIGRAPHY AND PROVENANCE ANALYSIS OF THE ALAMOR-LANCONES BASIN, SOUTHERN ECUADOR: A CRETACEOUS FOREARC SYSTEM

The Cretaceous Alamor-Lancones Basin (ALB) of northern Peru and southern Ecuador is located north of the Huancabamba deflection, between the Amotape Massif and the Celica submarine arc. This basin contains a ~4 km-thick succession of sedimentary series. The lower part comprises continental to shallow-marine sediments of the Aptian-Albian Puyango-Progreso Group (PPG). Deep-water sediments of the Cenomanian-Maastrichtian Zapotillo Group (ZG) unconformably overlain the PPG and is coetaneous with the Celica arc. Prevailing models proposed an allochthonous origin for the basement rocks and its sedimentary infilling.

In this study we present a refined stratigraphy and a provenance analysis of the sedimentary deposits of the ALB. The latter includes heavy minerals abundance, sandstones petrography and detrital zircon U-Pb geochronology.

For the PPG, the provenance analysis of the basal Aptian Sábalos formation shows in its lower part material mainly derived from a continental crust source while the upper part shows a mixed volcanic and continental crust source. It suggests a material derived from both, basement crustal rocks and a volcanic arc. Detrital zircon U-Pb dating of the upper part of Sábalos formation indicate a youngest age of 122 ± 3.1 Ma. This age overlaps with biostratigraphic ages and suggest a coeval volcanism. Detrital ages also indicate zircons derived from a Precambrian source, likely derived from the South American craton.

For the ZG, the provenance analysis indicates a broadly mixed volcanic and continental crust source. The lower part of the group, including the Chaquino and Tronco Quemado formations, presents a volcanic dominated source, while the upper part, including the Mangahurco and Cañaveral formations, show a dominant continental crust source. Detrital zircon U-Pb dating of Tronco Quemado formation indicate a youngest age of 69 ± 1.4 Ma. A population ranging between ~90-69 Ma suggest the presence of an unreported Cretaceous volcanism coeval with the sedimentary series, that could represent the continuation of the Celica Arc. The presence of a continental crust source and a coeval volcanic input of the analyzed section suggest that the Alamor Lancones Basin was formed in a forearc position, receiving material derived from the South American continental margin and an active volcanic arc.