Petrography and chemistry of sandstones from the Swiss Molasse Basin: an archive of the Oligocene to Miocene evolution of the Central Alps

Abstract

Oligocene to Miocene fluvial sandstones from the Swiss Molasse Basin were analysed for sandstone framework composition, heavy minerals, whole-rock geochemistry and detrital chrome spinel chemistry. Samples were taken from the proximal part of the basin close to the Alpine main thrust and are chronostratigraphically calibrated between 31 and 13 Ma. Sandstone composition allows the identification of different source rocks, and their variation in time and space place constraints on the Oligocene to Miocene evolution of the Central Alps. In the eastern part of the basin, sandstones document a normal unroofing sequence with the downcutting from Austroalpine sedimentary cover into Austroalpine crystalline rocks and, slightly later at 21 Ma, into Penninic ophiolites. In the central part, downcutting into crystalline basement rocks occurred at 25 Ma, and the removal of the sedimentary cover was much more advanced than in the east. This may be interpreted as a first signal from the doming of the Lepontine area. At 20 Ma, extensional tectonics in the hinterland led to the first exposure of low-grade metamorphic rocks from the footwall of the Simplon Fault in the Central Alps. Erosion of these rocks persisted up to the youngest sediments at 13 Ma. In the western part of the basin, a contribution from granitoid and (ultra)mafic rocks is documented as early as 28 Ma. The source for the (ultra)mafic detritus is Penninic ophiolites from the Piemonte zone of the western Alps, which were already exposed at the surface at that time.

Modern sands derived from the Vertiskos Unit of the Serbomacedonian Massif (N. Greece): a preliminary study on the weathering of the Unit

“…This could indicate that the samples studied, due to their quartz content, may have been deposited under a wet climate where chemical weathering pervasive. The samples studied were deposited in a semi-arid climate and were coupled by the effects of strong topographic relief [57]. The presence of feldspar and heavy minerals in the samples supports the latter hypothesis (temperate and seasonable climate where erosion is primarily due to physical weathering).…”

Provenance of the Bosnian Flysch

“…this indicates that sediment recycling and, consequently, cr-spinel concentration is not a significant process in forming these sandstones (von Eynatten 2003). similarly, Zr/sc ratios (McLennan et al 1993) give no evidence for significant zircon concentration.…”

From Subduction to Collision in the Northern Tibetan Plateau: Evidence from the Early Silurian Clastic Rocks, Northwestern China

“…Three lines of evidence can be used to recognize ophiolitic detritus in sedimentary rocks: (1) serpentinite fragments (Arai and Okada 1991;von Eynatten 2003), (2) anomalously high concentrations of Cr and Ni (Hiscott 1984;Garver and Scott 1995;Garver et al 1996;Totten et al 2000 and references therein), and (3) the oc- currence of Cr-spinels (Stevens 1970;Dick and Bullen 1984;Cookenboo et al 1997;Ganssloser 1999;Hisada et al 1999;Lee 1999).…”

“…Pronounced sedimentary recycling could concentrate Cr-spinel relative to unstable lithic fragments and minerals, resulting in the high Cr/Ni ratio and the presence of detrital Cr-spinels but also the lack of serpentinite fragments in the studied lower Silurian sandstones (von Eynatten 2003;Yang et al 2012). Although no corresponding spinel chemical data from the Dachadaban ophiolite are available for comparison, this interpretation is consistent with the studied Cr-spinels falling in the SSZ peridotite field on the Al 2 O 3 -TiO 2 diagram ( fig.…”