Variation in heavy mineral associations of Permo-Carbonif erous fan sequences (Southern Germany); Their implications concerning provenance and basin evolution

Abstract

Fan deposits from the Stockheim, Erbendorf, Weiden and Schmidgaden Basins in the immediate surroundings of the Mid-European Variscan basement contain translucent and opaque heavy mineral assemblages which have been strongly controlled by intrabasinal (volcanism, hydrothermal activity and weathering) and extrabasinal (provenance) parameters. Sediments of this marginal facies are less intensively altered by diagenesis than equivalent beds in the basin centre. Therefore, the amount and type of heavy minerals present in these depositional basins may contribute to basin and provenance analyses ofthese terrigenous fan sediments. Pyroclastic fan deposits contain smaller amounts of allogenic heavy minerals and in places they have a larger proportion of authigenic heavy minerals than found in alluvial fans. Crystal habits of zircon, apatite and anatase or mineral ratios (e.g. anatase/Fe silicate) are supplementary tools for the determination of sediment sources related to denudation of metamorphic rocks or bear a significant volcanic-derived influx. Deeply circulating waters and paleosols may be recognized on the basis of mineral transformation in the system Fe-Ti-O (giving rise to plates of anatase) and by a conspicuous depletion of particularly phosphates. Moreover, the heavy minerals in the fan deposits are an immediate response to the basement uplift in the hinterland. The history of basin subsidence, denudation of the basemement and even the P-T slope of the metamorphic source area may by inferred from the reverse order of heavy minerals discovered in the foreland. This will work well if metapsammopelitic rocks are present in the hinterland, but less well, if ultrabasic rocks occur in the source area. This procedure may successfully be used for samples from drill cores, cuttings and even samples from outcrop from fan deposits. Under these circumstances, the impact of modern weathering and soil forming processes on the heavy minerals suites has yet to be carefully established before discussing intra- and extrabasinal factors.