Heavy mineral response to the progradation of an alluvial fan: implications concerning unroofing of source area, chemical weathering and palaeo-relief (Upper Cretaceous Parkstein fan complex, SE Germany)

Abstract

During the Late Cretaceous, the NE Bavarian Basement was continuously uplifted along the deep-seated “Franconian Line” fault zone. As a consequence of this vertical displacement, a fan sequence was deposited in the adjacent lowlands. In this area Upper Triassic arkoses are unconformably overlain by a lignite-bearing arenaceous series, the Parkstein Formation, representing a meandering to anastomosing fluvial system. These deposits gradually pass upwards into the Red Claystone and Ferruginous Sandstone Member, deposited under strongly oxidizing conditions, as shown by the ubiquitous presence of trivalent Fe compounds and the lack of coal. The succeeding Friedersreuth Formation contains widespread debris flows and was deposited in an alluvial fan system. Alluvial fan progradation ended with the conglomerates of the Hesserberg Formation, which contains boulders as much as 0.5 m in diameter. The pronounced variation in depositional environment and grain-size is (from bottom to top) accompanied by a striking variation in heavy mineral assemblages. Minerals of high chemical and mechanical stability (zircon, tourmaline, rutile) prevail among the detrital heavy minerals of the basal formation, whereas younger formations bear less stable minerals such as members of the amphibole and epidote groups. These stable detrital heavy minerals are well-rounded to rounded. Provenance and palaeo-relief are most decisive for the coexistence of heavy minerals with different stabilities derived from a great variety of source rocks of different ages (Precambrian and Early Palaeozoic gneisses, late Variscan granites, Permo-Carboniferous volcanites, Triassic arkoses). Heavy mineral associations reflect the reverse lithological sequence of the adjacent source area, as a function of the interaction between uplift and erosion along the boundary fault (“unroofing story”). During slow uplift, chemical weathering operative in the peneplained hinterland and on the alluvial plain helped to release the minerals from the parent rocks and to remove labile constituents from the detritus of the clastic fan sediments. With increasing rate of uplift, the intermixing of minerals from the distal basement source and the proximal Permo-Mesozoic platform sedimentary source became less pronounced. Some authigenic minerals (anatase, “leucoxene”, Fe-oxides, Fe-sulphides, Fe-carbonates, Fe-phosphate) in context with the absence of some heavy minerals in the sand fraction that are present in rock fragments (e.g., apatite) help determine inferences about changes in pH values and may help constrain Eh values during fan progradation.