No abstract

No abstract

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. grainsize, roundness, lithological composition). This was found to be important for the hydrogeochemistry of groundwater in gravel aquifers (e.g. higher sorption capacity of carbonrich 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 permea...

No abstract

Closely spaced (1 m) ground penetrating radar (GPR) profiles were used for a three-dimensional characterization and comparison of glaciofluvial gravel-bed deposits in palaeodischarge zones of the Würmian Rhine glacier (southwestern Germany). Previous sedimentological outcrop investigations revealed three regionally reoccurring architectural styles of gravel bodies. For each of these styles a three-dimensional GPR dataset has been acquired in active gravel pits in order to calibrate the radar profiles with outcrop walls and to analyse, in three dimensions, the depositional elements and their stacking pattern in the subsurface.The GPR data were interpreted by mapping reflection terminations in order to delineate genetically related units. In particular, radar facies types and radar sequence boundaries were used to define and map depositional elements. Both accretionary and cut-and-fill depositional elements could be identified. Accretionary elements are characterized by horizontally to low-angle inclined (1–3°) and moderately continuous reflections (5–30 m) terminating on flat sequence boundaries; they represent the deposits of gravel sheets and traction carpets. In contrast, cut-and-fill elements are characterized by low to steeply inclined (3–25°), often discontinuous reflections terminating on concave to trough-shaped lower truncation boundaries; these are interpreted as scour-pool fills and small dissection elements (e.g. chutes and lobes).The three basic architectural styles of gravel bodies can be distinguished on the basis of the size and proportion of cut-and-fill elements mapped within the radar images. One type of gravel body is composed of an amalgamation of large cut-and-fill elements whereas the other two types are dominated by accretionary elements and differ by the proportion of smaller cut-and-fill elements.The results show that GPR is an adequate technique to illuminate the sedimentary architecture of the various types of gravel bodies. GPR data allow detailed three-dimensional reconstruction of depositional elements and their stacking pattern in the subsurface.