Origin and paleoenvironmental significance of Al-rich glauconite in the Ediacaran/Cambrian deposits of the Lublin Basin, Poland (SW margin of Baltica)

Abstract

Geochemical and mineralogical analyses of glauconite from formations deposited at the Ediacaran – Cambrian boundary along the SW margin of Baltica, integrated with sedimentological characteristics of its host-rocks, are discussed in the study. SEM-EDS and XRD studies indicate evolved to a highly evolved stage of maturation of glauconitic grains, i.e. they are characterized by a high K2O content (>6,5%) and only a small amount of smectite within an ordered R > 1 glauconite-smectite (G-S) mixed layered structure. Despite its high K2O content, glauconite shows a low Fe2O3 and high A2O3 content. Such chemical composition is distinctive for Precambrian glauconites that originated from abiotic substrates. We have identified glauconite in fractures of K-feldspar and quartz, indicating that at least some of the evolved/highly evolved glauconite grains have been formed by the replacement of these minerals. We have also recognized a new type of substrate, i.e. clay coatings, which has not been reported in previous publications on glauconite. In our opinion, the green grains characterized by cleavage and higher-order birefringence, usually described as vermicular glauconite, result from glauconitization of clay coatings, rather than biotite or muscovite flakes as previously thought. Clay coatings composed of 2:1 clay minerals, mostly smectites, represent natural parent material for initiation of G-S evolution that does not require considerable structural changes as in case of biotite or muscovite.Results of glauconite investigation compiled with the sedimentary record of the Ediacaran – Cambrian succession of the Lublin Basin allow us to reconstruct the environment of glauconite formation and its subsequent history. Glauconite occurs in quartzitic sandstones that show evidence of deposition in shallow marine, high-energy environments (barrier islands, tidal flats and shoreface), but reveals widespread transport. It was formed in a lagoon environment, which, on the one hand, was an area with a relatively stable sedimentation rate compared to concurrent estuaries or tidal flats, and on the other hand, was close to the area of source material that provided detrital substrates, i.e., feldspars and clay coatings, formed in clay-rich soil horizons on river terrace or delta sediments. The latter substrate appears to be an essential indicator of the environment of glauconite formation, which may occur in close proximity to a river mouth or an actively eroded coastline.