Petrography and provenance of Cretaceous fluvial sandstones of the Bajo Barreal Formation, San Jorge Basin, Argentina

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

Bariloche

Otro; XI Reunión Argentina de Sedimentología y IV Congreso Latinoamericano de Sedimentología; 2006

Asociación Argentina de Sedimentología

The lower member of the Bajo Barreal Formation (Late Cretaceous) represents the record of fluvial systems with abundant ash-fall events deposited during the sag stage at the western sector of the San Jorge Basin. The studied section is located north of Puesto Confluencia (45º 44’ 52’’ S; 69º 41’ 59’’ W), in the southern end of the San Bernardo Range, Chubut province. It is composed by channel-fill sandstones interbedded with floodplain deposits dominated by primary and reworked tuffs and fine-grained sandstones. The goals of this contribution are: 1) to make a petrographic characterization of the fluvial channel sandstones; 2) to define the succession of diagenetic processes; and 3) to infer the provenance and possible source rocks. Ten thin sections, each corresponding to different channel sandstone bodies, were studied using a petrographic microscope. The samples show framework grains that represent up to 91% of the rock volume, scarce ortomatrix (less than 2%) and variable proportions of cement (1% to 38%). The clastic fraction displays open packing dominated by floating grains with tangential contacts, along with subordinate long and concave-convex contacts. Commonly, this fraction is composed by volcanic lithic fragments with different textures (felsitic, felsophyric, pilotaxitic, trachytic and vitric-vitrophyric), plagioclase, orthoclase and monocrystaline quartz, in decreasing order of abundance. Nevertheless, some samples display abundant glassy shards, pumice fragments and pyroclastic fragments with eutaxitic texture. Accessory minerals are represented by hornblende, undetermined pyroxenes, biotite and opaque minerals. The principal cement types are clay and microgranular quartz, although some areas show abundant analcime or scarce calcite patches. The optical porosity ranges from 1% to 18%, with an average of 11%, and it is dominated by mesopores with occasional hydrocarbon impregnations. The porosity is mainly secondary, recorded by partial to total grain disolution or fractures, though primary intergranular porosity occurs subordinately. According to the Folk’s sandstone classification, the studied samples are mostly feldspathic litharenites and litharenites, although two lithic arkoses and one feldsarenite were identified. The recognized diagenetic signatures are related to processes that belong to the eodiagenetic zone. The precipitation of clay cement and alteration-solution of unstable grains, such as volcanic lithic fragments and plagioclase, were the first eodiagenetic processes. A continued alteration-solution of lithic fragments, mainly of vitroclasts, allowed the precipitation of microgranular quartz and analcime cements, as well as favoured the formation of secondary porosity. The sequence of diagenetic processes ended with the precipitation of calcite patches, weak mechanical compaction of framework grains and, secondarily, migration of hydrocarbons. Plotting of the studied samples in Dickinson’s provenance diagrams indicates a provenance from undissected-transitional arc for the lower member of the Bajo Barreal Formation at the studied locality. The abundance of volcanic lithic fragments, with the above-mentioned textures, suggests a source-rock lithology dominated by acid to intermediate volcanic rocks. Considering the inferred source rocks and the dominant paleotransport directions toward the E-SE as determined by field studies, the possible source area is located at the western sector of the Chubut province, where there are outcrops of Jurassic-Cretaceous volcanic successions.