On the radiometric significance of glauconies in studies of provenance: An example of the quaternary marine sequences of the Rio Grande Cone (Pelotas basin, southern Brazil).

The sediments of the Pleistocene-Holocene marine sequences of the Rio Grande Cone (RGC) (offshore Pelotas Basin, southernmost Brazilian Margin) are dominantly composed by muddy facies with thin interspersed sandy layers containing abundant very fine-to coarse green grains, heavy minerals assemblage, shards and planktonic foraminifera. These components contribute for a better understanding of the RGC filling during the Pleistocene-Holocene. However, the aim of this work was focused in the radiometric dating of the green grains as tool for provenance studies in marine sedimentary sequences. Based on Electron Microprobe, Scanning Electron Microprobe, Transmission Electron Microprobe and optical analysis the green grains occur as: (01) yellowish-green grains with smooth surfaces, low K 2 O and moderate Fe 2 O 3 contents, (02) pale green grains with surfaces slightly fractured with moderate K 2 O and Fe 2 O 3 contents, and (03) dark green grains characterised by fractured (cracked) surfaces and high of K 2 O and Fe 2 O 3 contents. These characteristics classified them as glauconitic minerals ranging from glauconitic smectite to glauconitic mica (glauconite) indicating the first (nascent) and intermediate (slightly-evolved and evolved) stages of the glauconitization process. K–Ar radiometric dating indicated ages from 33.2 ± 0.99 to 6.69 ± 0.54 Ma corresponding from Rupelian Stage (Oligocene) to Messinian Stage (Miocene) to these grains. Considering these glauconitic grains within a Quaternary sequence, an allochthonous (detrital) origin was attributed whose probable source areas were older sedimentary sequences exposed on the seafloor along fold-and-thrust belts related to the gravitational collapse of the RGC depocentre. After eroded the glauconitic minerals probably were distributed by countourite currents and deposited within the Quaternary deposits of the RGC. • K–Ar radiometric dating of glauconitic minerals to improve the knowledgment of deep-marine sedimentation processes. • The K–Ar ages obtained from the glauconitic minerals can explain the deposition of the Rio Grande Cone. • The radiometric results cleary show that the K–Ar in glauconites is a reliable technique to stratigraphic studies. [ABSTRACT FROM AUTHOR]