Provenance of Late Cretaceous fluvial deposits of the Bajo Barreal Formation (San Jorge Basin, Argentina) revealed through combined petrography and geochemistry

UMAZANO, A.M.; BELLOSI, E.S.; VISCONTI, G.; JALFIN, G.; MELCHOR, R.N.

Buenos Aires

Otro; XII Reunión Argentina de Sedimentología; 2008

Asociación Argentina de Sedimentología

The Upper Cretaceous Bajo Barreal Formation (San Jorge Basin, Argentina) is a fluvial succession mostly composed of channel sandstones interbedded with thicker floodplain deposits dominated by tuffaceous strata (Sciutto, 1981). The goal of this contribution is to infer the provenance of the unit at two selected localities within the San Bernardo Range, Chubut province: Sierra Nevada Anticline (SNA; 44º 50’ 50’’ S, 69º 25’ 21’’ W) and Puesto Confluencia (PC; 45º 43’ 33’’ S, 69º 41’ 11’’ W). Provenance was assessed using petrographical descriptions and sandstone detrital modes (n = 38), as well as geochemical analyses (n = 40) including major oxides and trace elements, both from channel sandstones and floodplain tuffaceous deposits. Channel sandstones are dominantly litharenites and feldspathic litharenites with abundant participation of volcanic lithic fragments showing different textures (porphyritic, eutaxitic, felsitic, pilotaxitic, trachytic and vitric-vitrophyric), pumice and plagioclase. K-feldspar, quartz and sedimentary rock fragments are scarce. Accessory minerals are represented by hornblende, pyroxenes, biotite and opaque minerals. QFLu diagram and several petrographic provenance indicators including dominance and textural types of rock fragments, K-feldspar/plagioclase ratio and inclusion-free quartz grains indicate a provenance from acid to intermediate, pyroclastic-rich, arc-related volcanic rocks. This interpretation agrees with the values of various elemental ratios of trace elements such as La/Sc, Th/Sc, Cr/Th, Eu/Eu* and (La/Lu)N and discriminant functions based on major elements. CIA values  indicate that original material experienced a low degree of chemical weathering at the source region and during transference stage. Tuffaceous floodplain strata are commonly vitric tuffs dominated by pumice fragments with rounded to ellipsoidal vesicles. A few vitric tuffs show a high content in glass shards with cuspate or platy morphologies. Subordinate components include volcanic lithic fragments, quartz, orthoclase, plagioclase, opaque minerals, biotite and hornblende. Zr/Ti versus Nb/Y and SiO2 versus K2O diagrams, as well as a MORB normalized multi-element diagram of selected trace elements shows that the pyroclastites were expelled from a medium-K content, intermediate, arc related, magmatic source. Considering the palaeoflow direction towards the east and south-east and an active subduction along the western margin of Patagonia during the Late Cretaceous, it is interpreted that Bajo Barreal Formation constitutes the sedimentary record (distal facies of volcano-flanking fan or apron) of a coeval volcanic arc. Channel sandstones were probably mostly derived from the Divisadero Formation volcanic rocks because of similar petrographical features and chondrite normalized REE pattern. In contrast, pyroclastic tuffaceous floodplain deposits would be derived directly from vents located over the actual outcrops of the Patagonian Batholith, which are currently located 120 km from the studied outcrops.