Fluvial-marine transitional depositional environment influencing the diagenesis in the buntsandstein of thuringia (German Democratic Republic)

Abstract

The Buntsandstein in Thuringia (German Democratic Republic) consists predominantly of sandy and gravelly fluvial sediments which in parts of the sequence pass into marine deposits. Extensive braided-river systems produced vast stream sand bar and sand sheet sediments which coalesced by lateral amalgamation and vertical stacking to persisting complexes that permit the stratigraphical connection of the Folgen megacycles. In the marginal parts of the basin, a higher amount of channel conglomerates occurs, whereas towards the centre of the depression, fine-grained overbank deposits become more abundant and thicker. Marine influences are frequent in the estuarine reach, and the fluvial magnacycle is terminated by the major transgression of the R'ot sea. The palaeocurrent directions reflect three main sources of sediment material, with the flows deriving from Gallian Massif, western part of Bohemian Massif and northern border of Bohemian Massif (Western Erzgebirge) and being directed towards northnortheast, northwest and westnorthwest, respectively. The infilling of the depositional area results in a seaward migration of facies being typical for progradation. The geological framework for the diagenetic alterations of the sediments comprises a large epicontinental basin with platform deposition, resulting in low total thickness of the sequence and shallow depth of burial due to thin overburden. Progressive burial diagenesis is terminated by uplift and weathering descending from the surface during Cretaceous and Tertiary, resulting in overprinting effects of a retrograde or diaphthoritic weathering diagenesis. The postsedimentary phenomena and processes in sandstones include formation of rim cements comprising hematite, feldspar, quartz and chalcedony; growth of basal cements being dolomite and calcite, gypsum and anhydrite, and kaolinite; phase changes and transformations representing mainly illitic, kaolinitic and carbonatic degradation of feldspars; and authigenesis of heavy minerals such as anatase, brookite, barite and garnet. The reconstruction of the temporal succession of diagenetic processes from intergrowth and contact relationships and from minus-cement-porosities shows that most of the phenomena are of an early nature thus being still controlled by the depositional environment and the palaeogeographical setting. The establishment of a more intense burial diagenesis was suppressed by the only shallow subsidence and by the inhibition of immigration of extraformational solutions due to the sealing effect of the underlying Zechstein and the overlying Röt halites, respectively. The influence of the fluvial-marine transitional depositional environment on the diagenesis is best reflected by the areal restriction of cement types fitting into the palaeogeographical framework. The transitional belt between the central and marginal parts of the basin is characterized by an evolution of diagenetic pore waters from concentrated sea water containing enriched amounts of Ca, Mg, CO₃ and SO₄ thus producing carbonate and sulphate cements, whereas the post-sedimentary history in the marginal areas is controlled by undersaturated fluvial fresh waters giving rise to precipitation of quartz and feldspar cements. In contrast to the progressive burial diagenesis, the retrograde weathering diagenesis is not related to the original depositional environment, but is only bound to the tectonical position of the sequence. Compaction is insignificant in sandstones, but is of considerable importance in mudstones. A large amount of the compaction finally resulting in reduction of the thickness to abt. one fifth already takes place in the first hundred metres of burial thus being almost in equilibrium with sedimentation. Differential subsidence due to compaction of mudstones in diversified milieu results in lateral differentiations between deposition and omission, with additional sediments forming in the muddy central part of the basin being equivalent to gaps in the sandy marginal belt. The increasingly marginal nature of the sediments from the bottom to the top and the increasing maturity in the same direction underline the progressive significance of the reworking of older deposits. The evolution from fluvial milieu into marine environment ideally takes place in cyclothems passing from fluvial via beach and lagoonal to tidal flat setting.