Provenance of the Sardona Flysch, eastern Swiss Alps: example of high-resolution heavy mineral analysis applied to an ultrastable assemblage

Abstract

The Sardona Flysch of eastern Switzerland was deposited in the northern sector of the embryonic Alpine Foreland Basin, the so-called North Penninic Trough, during the Late Cretaceous-Early Tertiary. The flysch sediments consist of quartz arenites, sublitharenites and arkosic arenites, plus polygenic, pebble to boulder conglomerates. The Sardona Flysch was sourced by a single petrologic province composed of plutonic, Variscan granitoids, gneisses and low-grade, quartzo-feldspathic schists, plus associated cover sediments throughout its sedimentary history. Its heavy mineral suite principally comprises zircon, tourmaline, rutile and varying amounts of apatite, whose distribution cannot be used to characterise the various lithostratigraphic units. However, recording the diagnostic varieties of ultrastable heavy minerals using a ‘high-resolution heavy mineral analysis’ (HRHMA) yielded conclusive results. The varietal distribution of zircon proved especially informative, demonstrating that changes in its distributional trends coincide remarkably well with lithostratigraphic subdivisions and reveal subtle variations in the provenance of the flysch. The limited number of heavy mineral species was predetermined by the low diversity of parent rock lithologies in the hinterland, rather than resulting from high sediment maturity, or progressive removal of unstable species during diagenesis. The basin was structurally confined by the South Helvetic and North Penninic Prättigau Swells, which defined its northern and southern margins, respectively; these structural highs may represent Mesozoic normal fault-blocks reactivated under compressive stresses within the European foreland. They provided detritus to the Sardona flysch basin and prevented the influx of material from the orogenic thrust wedge. Hence, garnet and unstable heavy minerals recycled from medium-grade metamorphic rocks within the mineralogically more diverse Austroalpine thrust wedge are absent from the flysch sediments.