Hierarchical dynamic stratigraphy in various Quaternary gravel deposits,Rhine glacier area (SW Germany): implications for hydrostratigraphy

The approach of dynamic stratigraphy aims to understand genetic processes that form stratigraphic units in a hierarchy of spatial and temporal scales. This approach was used to investigate Quaternary gravel deposits in terms of their sedimentology and in order to characterize the various sedimentary units in terms of their hydrogeological properties. Facies analysis within 62 gravel pits, laboratory permeability measurements of field samples and geophysical surveys (3-D georadar, 2-D seismic reflection) led to the detection and classification of sedimentary heterogeneity according to the following six scales whereby each scale can be translated into defined hydrostratigraphic units. (1) Particles and pores (micro scale) that reflect depositional and diagenetic fluid dynamics as well as source material behaviour (e.g. grain-size, roundness, lithological composition). This was found to be important for the hydrogeochemistry of groundwater in gravel aquifers (e.g. higher sorption capacity of carbon-rich limestone particles for organic pollutants). (2) Strata (meso scale) contain the recognition of sorting, fabric, texture and stratinomic features, which can give an indication of transport and depositional dynamics. Five major lithofacies groups, for example, were distinguished within fluvial gravel-bed deposits. Their variable hydraulic properties led to their subdivision into 12 hydrofacies types. They form the smallest mappable hydrostratigraphic units, which may result in either preferred pathways for fluid flow or flow barriers. (3) Depositional elements (macro scale) enable reconstruction of sedimentary/geomorphic elements and their dynamics within a depositional system (e.g. gravel-bed braided river systems are dominated by gravel sheet, gravel dunes and scour pool depositional elements). Hydrostratigraphically, the architecture of depositional elements influences the hydraulic connectivity and local permeability structure/distribution within an aquifer body. Five types of depositional elements in fluvial gravel-bed deposits were distinguished and their geometries/dimensions quantified. (4) Facies bodies (mega scale) composed of a stack of depositional elements and strata recording distinct environmental systems and their dynamics (e.g. a coarse-grained prograding delta system). Hydrostratigraphically, facies bodies represent major compartments of an aquifer. Six major types of meltwater-controlled facies bodies were identified in the study area. (5) Genetic sequences (mega scale) reflect the shifts of depositional environments caused by allocyclic changes (e.g. glacial advance recorded by a coarsening upward sequence) or autocyclic changes of landscape shaping events. These sequences may form separate hydrostratigraphic units or aquifer storeys. (6) Basin fill (giga scale) comprising the lateral and vertical stacking of facies bodies and genetic sequences controlled by either long-term glacier dynamics or short term flood events. The regional distribution of permeable gravel units and, for example, less permeable diamicts builds the larger scale hydrostratigraphy.