Carboniferous “green shales” of the Solimões Basin: volcanic ashes (K-bentonites)

Abstract

Several K-bentonite beds have been found intercalated with sedimentary rocks of the Juruá Formation (Namurian) in the Solimões Basin, northern part of Brazil. These beds are only 1 to 15 cm-thick, but are spread over a wide area, comprising Rio Urucu, Leste de Urucu and Juruá oil and gas fields. They exhibit sharp contacts with the adjacent layers (carbonates, shales or even sandstones) and can be easily identified by their typical apple green color, homogeneous texture and smooth silky touch.
These K-bentonites are composed almost exclusively of either a type of ordered illite-smectite mixed-layer or of discrete illite (1M polytype) - both clay minerals formed from the alteration of Carboniferous volcanic ashes. Unknown previously, such beds point out to an additional clastic source to the basin at this time. Due to their specific origin, they can therefore become stratigraphic markers; which can be useful for local and regional correlations along of the Solimões Basin.