Minor pedogenesis and local aeolian influences in sandy to pebbly braidplain deposits of the solling-folge (upper buntsandstein) in the hessian depression (hessen and lower saxony, F.R. germany)

Abstract

The fluvial braidplain environment of the Solling-Folge (Upper Buntsandstein) in Hessen and Lower Saxony (F.R. Germany) is highlighted by associated minor pedogenesis and local aeolian influences and represents a milieu being generally unsuitable for the origin of coexisting non-alluvial products. The aeolian features in the Solling-Folge comprise aeolian sands, in situ and allochthonous ventifacts and granule ripples, whereas the millet-seed sand which occurs as well is interpreted as being of fluvial channel lag origin. Indirect evidence of aeolian influences on fluvial sedimentation is given by wave ripples and wrinkle marks. The pedogenic features in the Solling-Folge include Violette Horizonte palaeosols, carbonate precipitations and root tubes not accompanied by violet colour, traces of initial pedogenic overprinting and Brockelbank carbonate breccias, with the latter representing products of destruction and reworking of mature palaeosols. According to the distribution of aeolian and pedogenic features in time and space and in view of the facies variations of the fluvial settings, the braidplain is divided into proximal, marginal-medial and central-medial parts which are characterized by different importance of the associated non-alluvial environments. The medial part of the braidplain surrounding the Eichsfeld Swell is a special case of development deviating considerably from the general trend, with the protecting function of the high enabling accumulation and preservation of aeolian sands which are completely unknown in the open braidplain. In terms of relative amounts of aeolian and pedogenic features, the proximal and marginal-medial parts of the braidplain are characterized by more abundant occurrence of aeolian features and less frequent presence of pedogenic features, whereas the inverse relationship exists in the central-medial part of the braidplain. The termination of fluvial deposition in the sandy wedge of the Solling-Folge due to vertical evolution and longitudinal horizontal zoning into channel-dominated braidplain, overbank-dominated floodplain and playa mud flat also set an end to the possibilities of origin of both wind-created structures and palaeosols. The comparison of the Solling-Folge in Mainfranken, Hessen and Lower Saxony with the Zwischenschichten in Eifel, Saar area, Pfalz and Vosges being correlative braidplain systems, and the evaluation of various common and distinctive features of both sequences allow the definite assignment of the Solling-Folge to the Upper Buntsandstein. The main lines of evidence for assessing Solling-Folge and Zwischenschichten to represent equivalent formations derive from the vertical distribution of Violette Horizonte palaeosols and Brockelbank carbonate breccias being totally absent in the Middle Buntsandstein, and are further related to the rarely occurring to lacking aeolian features which are widespread in some parts of the Middle Buntsandstein. The correlation of Solling-Folge and Zwischenschichten and the comparative evaluation of the stratigraphic sequence into which these members are embedded enables a basinwide connection of the depositional history especially by recognizing an overall break in the evolution of the fluvial system along with a major change in palaeoclimatological conditions at the beginning of both successions. The general depositional model for Solling-Folge and Zwischenschichten comprises a braided river system with low-sinuosity channels enclosing overbank plains, with regional variations and diversifications being mainly linked with changes in spacing of watercourses, frequency of sidewards displacement of streams, and periodicity and extension of floods which account for varying importance of primary-depositional restriction of formation and secondary-erosional removal of calcrete palaeosols which are the typical associated non-fluvial features in both alluvial series.