Depositional environments and magnetostratigraphy of the Neogene Mogna sequence and its relation to the Andean Foreland Basin

Abstract

A precise history of accumulation has been depicted for the Mogna Neogene sequence, using magnetostratigraphy, absolute dating, vertebrate fossil determinations and comparisons with neighboring sections. A two-stage tectonic history was modulated by changes in the drainage basin configuration and, apparently, climate. Eight Neogene formations have been dated, surveyed and correlated, suggesting that most boundaries are diachronous, up to 106 yr . The magnetochronology reveals accumulation from 19 Ma up to 1 Ma, only interrupted for an interval with doubtful correlation to the MPTS between 15 and 12 Ma. The lower Río Salado Formation accumulated in a playa-lake (basin center) while the four following units (Quebrada del Jarillal, Huachipampa, Qa. del Cura and Río Jáchal Formations) were deposited by sandy ephemeral rivers, differing from one to another in the channel/floodplain ratio, channel morphology, degree of floodplain bioturbation, and sandstone composition. The upper two, Mogna and El Corral Formations are gravelly and they interdigitate laterally and vertically. The first one was deposited by a major gravelly braided river, while the second accumulated in an alluvial bajada. The entire sequence documents a large-scale progradation of an arid-system alluvial wedge related to the eastward advance of deformation. The correlation of the deformational stages recognized at Mogna suggests a variable basin rheological behavior along the paleo-basin axis. During the initial deformation stage, a southward reduction of thickness (2 km) was accommodated by synsedimentary faults exposed at Los Pozos range (Pie de Palo Norte alignment), while during the main stage of deformation, subsidence was similar along the strike.