Integration of geochronological, lithofacial and paleontological data to refine the context of the Marifil Complex (Jurassic), Río Negro, Argentina

Abstract

In northeastern Patagonia, outcrops of the Marifil Complex located three km north of Mina Delta XXI expose a succession of volcaniclastic and sedimentary rocks 160 m thick. This succession, along with other correlatable deposits, were previously mapped as undifferentiated ignimbrites, tuffs, and sandstones in recent regional geological surveys. The Marifil Complex in the study area comprises ten distinct lithofacies, including paraconglomerates forming the basal layer of the volcano-sedimentary succession; coarse-grained arkosic sandstones; volcanic sandstones and siltstones containing fossil flora; mixed volcanic-clastic rocks; massive brecciated and bituminous limestones; massive lapilli-rich and crystal-rich eutaxitic tuffs; and porphyritic rhyolite intrusions that crosscut the other facies. The volcano-sedimentary succession studied is interpreted as having been deposited in a small lacustrine basin adjacent to an explosive volcanic center. The macrofloral assemblage includes equisetalean stems assigned to Equisetites sp., conifer vegetative and reproductive structures (Pagiophyllum spp., a probable bract/seed-scale complex and a probable pollen cone), and an incomplete leaf fragment with reticulate venation of uncertain affinity. Additionally, a gymnosperm pollen grain assigned to Inaperturopollenites indicus Srivastava was identified from the same levels. The age of this geological unit, determined by U-Pb zircon geochronology, is 189.5 ± 2.2 Ma, constraining the deposition and associated paleoflora to the Sinemurian-Pliensbachian boundary. These volcanic and volcano-sedimentary deposits are associated with the development of an extensional basin, possibly linked to the breakup of Gondwana.